Labview blcok for SDG II control box

Most functions of the SDG II can be controlled by any computer with a standard RS-232 serial port. After connecting the SDG II with computer RS-232 through data transmission cable, it's easy to control the SDG II by sending a RS-232 commands. Valid commands return the string "Ok", but invalid commands or queries return the string "Bad".

I can run a program under Labview to send or query RS-232 commands only by pushing buttons. The front control panel and LabView block diagram are shown here.

Set up timing delay for new designed nozzle

In order to synchronize the old small nozzle with laser pulses, I have added about 1 millisecond delay between the laser seed pulses and the final laser pulses. However, this delay is not enough for the new big slit nozzle, which needs at least 3 or 4 milliseconds to be operated. So I think it's better to trigger the nozzle with the first laser pulse and fire with the second laser pulse. Because the Prometheus runs at 0.2 Hz, there is 5 seconds delay between two continuous pulses. It's free to tune the delay time during this 5 seconds, the main point is how to synchronize the gas nozzle with the second laser pulse. A digital delay box was used, this box can provide the timing delay from seconds order to microsecond order. After several trials, the delay was set up at about 4 s 913 ms.

Changed resistors for 40184 spark gap

I found the resistors connected to the 40184 spark gap was broken, which caused the misfire for the Prometheus. This 40184 spark gap(SG1) was installed inside the 50kV trigger generator box. SG1 is a three electrode, 2:1 gap-space, irradiated-gas-plasma switch. The switch operates in the swinging cascade mode in which the intermediate electrode is biased at one-third of the working voltage.

Testing NI GPIB-232CV-A

We received the National Instruments GPIB-232CV-A today, I installed the converter and tested it. The interesting thing was that I could use the mouse or keyboard plug, rather than a AC/DC power supply to provide the power for this board.

In order to make sure the board worked, I connect a TDS2042 oscilloscope with GPIB cable to the converter. Before testing, it's necessary to configure the converter parameters. The DIP switches were used to set up the parameters, the setup of SW1 was shown as followed:

Indication	GPIB address					Termination		Mode
Switches	1	2	3	4	5	6	7	8
ON/OFF	OFF	OFF	OFF	ON	OFF	OFF	ON	ON
Note	This address matched the scope GPIB address					LF termination		C

The setup of DIP switch SW2 was shown as below:

Indication	bits per second			Parity	Check	Stop	Data	Flow
Switches	1	2	3	4	5	6	7	8
ON/OFF	ON	OFF	ON	OFF	OFF	OFF	ON	OFF
Note	9600 baud			Odd	Disable	1 bit	8	Disable

After several times to change the DIP switches setup, the scope data was successfully transfered to the computer through scope GPIB port, converter and serial port.

Filaments and self-focusing of UV laser beam

Yesterday we found the small mirror surface of the beam expander was damaged, we figured out that it was caused by the laser beam filament or self-focusing. We improved the spatial filter vacuum, covered the optical tables, cleaned the windows, and filled the new gases into TWINAMP. All these behaviors might increase the laser output energy, which finally provoked the strong nonlinear effect such as filament or self-focusing.

We used the photo papers to imprint the laser beam patterns, one was put before the beam expander, the other one was put in front of the convex mirror in the beam expander box. Each print recorded about 300 laser shots at ~24 mJ. It's obvious that the beam print before the telescope was smoother than the one in front of the convex mirror. Some dark spots were recorded at the edge or in the bottom of the laser beam, which almost matched the damaged spots on the convex mirror.

Beam Profiles of Tripler and after 1st sptial filter

We measured the beam profile and wavefront of tripler (see the right figure). M²(X)= 1.58, M²(Y)=1.56; Beam width(X)= 3.3 mm, Beam width(Y)= 2.9 mm.

After the first beam expander, the beam size was enlarged about 10 times. In order to measure the whole beam, we used a lens to concentrated the beam into the CCD camera. The beam profile is shown in the below. Only the beam size was changed to bigger, the other beam characteristics were similar to the tripler one(see the bottom figure).
M²(X)=1.89, M²(Y)=1.72; Beam width(X)=20.5 mm, Beam width (Y) = 23.4 mm.

X-Ray Rainbow

via Physics News Update #805

In 1670 Isaac Newton demonstrated the composite nature of sunlight when he sent a carefully collimated sunbeam through a prism, which spread out the light into a rainbow of colors; by sending a beam of single color through a second prism (with no further spreading) Newton showed that the color was not being imposed by the prism but was intrinsic to the light itself. Now physicists using the Advanced Photon Source at Argonne National Lab, in Illinois, have spread out a beam of X-rays (which are, after all, just a more energetic version of visible light) into a rainbow of colors.

Trying to reflect X-rays from a surface is difficult because X-ray wavelengths are some 10,000 times shorter than those for visible light. Glancing reflection of only a few tenths of a degree is normally possible, and even then the beam of X-rays will suffer very little wavelength-dependent spreading. However, another phenomenon, Bragg diffraction, allows for scattering of X-rays from a crystal through large angles; in this case the incoming X-rays scatter not merely from a top layer of atoms in the crystal but from numerous atomic planes. Furthermore, if the atomic planes are not parallel to the crystal surface the diffracted X-ray beam will be spread out prismatically into a range of component wavelengths (or colors).

In the Argonne experiment an incoming beam of 9-kiloelectronvolt X-ray photons with angular spread of only 1 micro-radian (two-tenths of an arcsecond) was backwards scattered and spread out into an X-ray rainbow with an angular dispersion of 230 micro-radians (see figure).

Argonne physicist Yuri Shvyd'ko (shvydko@aps.anl.gov, 630-252-2901) says that his rainbow is not just a novelty but will have many practical applications in X-ray optics. These include compression of X-ray pulses in time and the development of X-ray monochomators (which fashion X-ray beams of pure wavelength, or color) and much higher-resolution X-ray spectrometers.

Shvyd'ko et al., Physical Review Letters, 8 December 2006

1st Spatial Filter leakage

Mike helped us fix the vacuum pump for the first Spatial Filter of TWINAMP, however we found there was a big leak from the vacuum pipes. We checked the pipes step by step, firstly found one of the O-ring was broken, then small holes around the vacuum pipes. After replacing the O-ring and gluing the leaks, the vacuum reached about 10 mTorrs.

Supervision software operates and maintains large laser systems

I dreamed I could control the large laser system by computer someday, fortunately I found the Thales Laser company already achieved it.

Supervision software operates and maintains large laser systems

Auther: Fabien Ghez

Large laser systems, such as terawatt- and petawatt-class ultrafast amplifiers or Nd:glass fusion beams, are complex to use and maintain, primarily because they are large systems that comprise several smaller ones. A terawatt ultrafast laser, for instance, consists of several optical elements such as an oscillator and its pump, a set of amplifiers, many pump lasers, beam-shaping devices, a stretcher-compressor pair, timing electronics, cooling units, and systems that interface to user experiments.

So in addition to operating the overall system, a user must also bring a capacity for tuning and maintaining all of the components individually and collectively. Laboratories often employ engineers or Ph.D. scientists whose sole or primary responsibility consists of maintaining and operating such highly complex experimental tools. To ease this burden for the average user while enabling broader application of large laser systems, computerized supervision tools have become available that combine hardware and software dedicated to system operation, tuning, and maintenance. The goal of such processing capabilities, of course, is to enable the user to spend time on the application rather than on tweaking the laser. The primary processing tasks can be divided into two areas.

The first of these areas, synchronizing laser pulses of femtosecond duration to single-shot experiments with negligible jitter, is one of the toughest tasks in laser-driven experiments. Supervision software enables the laser to be slaved to external clock references, or to be used as a main system clock. In both cases computer-driven synchronization signals set the pace for both the laser and the experiments. X-ray generation in connection with a linear electron accelerator offers a classic example of this sort of application: the laser beam collides with an electron beam and subpicosecond accuracy is required despite the very noisy environment.

In terms of the second control area, laser operation and maintenance, supervision software and hardware enable automated start-up of all pumps and other devices, monitors operation of all systems, including motors and beam analyzers, and diagnosis malfunctions, thus enabling rapid preventive maintenance while minimizing the need for system tweaking. If a large laser system must operate in an environment that would be unsafe for human operators, such as a room exposed to harmful radiation, supervision software enables remote control of operational tasks such as start-up, shutdown, the control of energy levels, and the optimization and alignment the beam, all through a fiberoptic link.

System architecture

In the Thales laser-control architecture, supervision software controls, through an RS232 connection, a laser-control system that drives all individual control units and drivers; a femtocontrol unit that drives the diagnostics; one or several synchronization units, CCD cameras and diagnostics; and all connections between computers, power supplies, electronic racks, and breadboards (see Figure).

The masterclock (synchronization unit) generates all signals to trigger pump lasers, Pockels cell, and CCD cameras in synchronization with the master RF of the oscillator. This provides the aforementioned system synchronization with the laser oscillator, reduction of jitter, and improvement of stability, whether within one laser system or among several. Of course, if the laser is only one part of a global facility, the computer can be linked to global supervision software via network connections and protocols to be determined by the user.

The femtocontrol unit is a single electronic rack that can simultaneously drive and control stepper motors, motorized mounts (x and y axis), and pump-laser interlocks. It can also make some spectral and power measurements using integrated photodiodes. Those integrated and calibrated photodiodes can be implemented in breadboards to control the power levels of different amplifiers and pump lasers.

The compressor can also be adjusted through the software, enabling pulse duration to be adjusted remotely without removing system covers (which can prove particularly useful with vacuum-chamber compressor systems, or pulse propagation in air). The software adjusts the distance between gratings by means of a translation stage.

The laser-beam profile can be monitored by the user on the computer screen through a Firewire CCD camera, which enables saving of the profile in jpeg format. This camera also enables control of beam profiles in intermediate amplifiers. And when coupled with an alignment system it can also control beam pointing.

In some experiments, precise pulse energy is required to improve efficiency. This can be achieved via an attenuation process that reduces output power without degrading other performance parameters (such as polarization, beam focusing, energy, stability, and contrast). The easiest way to implement such a solution is to modify the delay on pump lasers to reduce the gain of some amplifiers. The drawbacks of this solution are that it can degrade some parameters of the beam in a way that invalidates the experiments, and that a precise attenuation within the required range may not be possible. An alternative software-enabled solution avoids such problems by using rotating waveplates coupled to polarizers that can vary the output power from 15% to 100% without affecting other specifications.

The Thales supervision software is based on Labview 7.0 (National Instruments; Austin, TX), which in addition to user-friendliness enables compatibility with a broad range of user software systems. So when laser and experimental protocols are compatible it may even become possible to integrate laser control with experimental software, and to operate the laser and experiment as a single system.

From Laser Focus World November, 2006

On the node of a wave

A compact electron accelerator can be made by the cunning use of laser pulses to let electrons 'surf' on a plasma wave. The problem has been controlling exactly how much the electrons are accelerated.
Jerome Faure and Victor Malka at the ENSTA/CNRS laboratory near Paris injected electrons into a plasma wave created by a single intense laser pulse.

a, In Faure and colleagues' experimental scheme, the primary laser wakefield pulse ionizes helium gas to a plasma. If the parameters of pulse and plasma are chosen appropriately, the electrons of the plasma oscillate about a fixed spot. b, If a second 'tow-in' pulse travelling in the opposite direction crosses the first, a standing wave forms. Electrons are pushed left and right in the standing wave from the antinodes to the nodes. c, Some electrons — those pushed to the right — gain enough speed to get caught up in the following wave crest and are accelerated forwards. The energy gain of the electrons is determined by how far they have to surf through the plasma, and so by where exactly along the plasma the two laser pulses cross.

From Nature 444, 688-689 (7 December 2006)

2D imaging conical x-ray spectrograph

We are using a Von Hamos x-ray spectrograph to observe the x-ray imession from laser cluster interaction, which only provides the spectral information. I think we can use this novel x-ray crystal spectrograph to measure the spectral resolved 2-D image in our experiment.

Extreme luminosity imaging conical spectrograph

S. A. Pikuz, T. A. Shelkovenko, M. D. Mitchell, K. M. Chandler, J. D. Douglass, R. D. McBride, D. P. Jackson, and D. A. Hammer
Laboratory of Plasma Studies, Cornell University, 439 Rhodes Hall, Ithaca, New York 14853

A new configuration for a two-dimensional (2D) imaging x-ray spectrograph based on a conically bent crystal is introduced: extreme luminosity imaging conical spectrograph (ELICS). The ELICS configuration has important advantages over spectrographs that are based on cylindrically and spherically bent crystals. The main advantages are that a wide variety of large-aperture crystals can be used, and any desired magnification in the spatial direction (the direction orthogonal to spectral dispersion) can be achieved by the use of different experimental arrangements. The ELICS can be set up so that the detector plane is almost perpendicular to the incident rays, a good configuration for time-resolved spectroscopy. ELICSs with mica crystals of 45×90 mm2 aperture have been successfully used for imaging on the XP and COBRA pulsed power generators, yielding spectra with spatial resolution in 2D of Z pinches and X pinches. ©2006 American Institute of Physics

Rev. Sci. Instrum. 77, 10F309 (2006)

Catching the wave

In recent years the use of high-order harmonic radiation to create and control events on attosecond timescales has grown at a phenomenal rate. With the use of carrier-envelope phase (CEP) stabilization and few-cycle laser systems it is possible to probe physical processes on unprecedented timescales. Here, we report the first experimental observation of high-harmonic emission at individual half-cycles of a laser pulse. We show that these half-cycle emissions are extremely sensitive to the CEP, providing a route to a new single-shot measurement technique of the CEP. We use this technique to measure the CEP of an 8.5 fs pulse at a centre wavelength of 800 nm with an accuracy of 20 as (1 as=1x10^-18 s). With appropriate spatio-spectral filtering of the harmonic spectra, our calculations show that we can isolate emission from an individual half-cycle cutoff, which corresponds to a single isolated attosecond pulse of duration less than 300as.

Citation: "Half-cycle cut-offs in harmonic spectra and robust carrier-envelope phase retrieval", Nature Physics, Sunday 3 December 2006.

related link: Attosecond Technology

Timing delay jitter problem

Unfortunately, we could not resynchronize the TWINAMP using the two separated trigger pulses. I think the large RMS jitter from BNC 7010 digital delay generator makes the synchronization failure. So I replaced the BNC generator with SRS DG-535, the TWINAMP ran steadily. DG-535 has only 50 picosecond RMS jitter. The best way to eliminate this jitter is to control 4 channels of DG-535 by computer. If we buy a GPIB-RS232 converter from NI, it's easy to communicate with only one DG-535 for switching on or off the trigger signals("out 1" and "sync out").

Lasers Break an Optical Barrier -- Stimulated Emission Depletion (STED)

This year's 10th annual German Innovation Award has been awarded to Stefan W. Hell, the director of the Max Planck Institute for Biophysical Chemistry, for his project "Light Microscopy with Unprecedented Resolution".

More than 100 years ago, Ernst Abbe laid down the law that the wave nature of light limits the smallest resolvable spot size to one-third of a wavelength in diameter, or approximately 200 nm. He predicted that any features smaller than this must always be blurred by diffraction and hence be indistinguishable by focusing light microscopes. Professor Hell resolved a spot that was 30 percent smaller than Abbe's minimum using a focusing fluorescence microscope and a pair of laser beams operating at different wavelengths. One beam -- a frequency-doubled UV-pulse -- caused excited molecules to be distributed by the fluorophore, while the other, slightly red-shifted fundamental beam quenched them through stimulated emission.

The researchers generated the two pulses with a Mira 900 Ti:sapphire laser, pumped by an Innova 400 argon-ion laser, both from Coherent Inc. of Santa Clara, Calif. The Ti:sapphire operated at a wavelength of 766 nm and a 120-fs pulse length. The outgoing beam was divided into an excitation beam and a fundamental. While the pulses of the first beam were frequency-doubled with a nonlinear optical crystal made by Fuzhou, China-based Casix Inc., the pulses of the fundamental were stretched to 40 ps by a Coherent grating compressor/decompressor. The technique requires pulsed lasers because, to eliminate fluorescence efficiently, the fundamental pulse must do its job in a much shorter time -- a few picoseconds --than the fluorescence lifetime of the dye -- a few nanoseconds.

related link: STED microscopy

(from Photonics.com)

New timing setup for beam stabilization system

The beam stabilization system needs control signals from SDG II "out 1" and "sync out", so I have to change the timing again. I already modified the timing for the front end before (on July 7th), but I did not consider to trigger the Pockcel and TWIN AMP separately. Now I have to insert one more delay box such as BNC delay generator in order to trigger the TWIN AMP.

The beam stabilization system includes one CCD camera and one chopper, both of them also need the TTL pulses to trigger. The pulses from BNC #1 to trigger CCD, #2 to trigger the beam chopper.

JLab FEL Breaks Power Record

The most powerful tunable laser in the world has shattered another power record. Officials report that the free-electron laser (FEL) at the Thomas Jefferson National Accelerator Facility produced a 14.2 kilowatt beam of laser light at an infrared wavelength of 1.61 um on Oct. 30.

Heater and reservior voltage for TWINAMP

The TWINAMP could not be triggered by internal pulse correctly but only by external signal normally yesterday. By my early experience, the problem was caused by the thyratron heater and reservoir voltages. We measured the heater voltage(V_H) from the power supply output, which is about 10.2 VAC. When I decreased this voltage down to 9.4V (the minimum adjustment), the TWINAMP ran well. We also measured the voltages from thyratron side.

	Power Supply Output	Before filter	Before Thyratron
V_Heater(VAC)	9.30	7.80	7.50
V_Resevior(VAC)	7.78	7.48	7.48

On May 1st, I set the Heater and Reservoir voltages: V_H=11.35V, V_R=8.35V. On Oct. 18, 2005, we set the voltages: V_H=7.05V, V_R=7.49V.

Ultrafast, Intense Laser Captures Nanoscale Images

HAMBURG, Germany, Nov. 14, 2006 -- Using a single, extremely short and intense x-ray laser pulse, an international team of scientists have, for the first time, taken a high-resolution diffraction image of an object such as a protein before the intensity of the radiation destroyed the sample. The experiment was the first successful application of "flash diffractive imaging" and begins a new era in structural research.

The new method will be applicable to atomic-resolution imaging of complex biomolecules when even more powerful x-ray lasers, currently under construction, are available. The technique will allow scientists to gain insight into the fields of materials science, plasma physics, biology and medicine.

The scientists, part of an international collaboration led by Lawrence Livermore National Laboratory's (LLNL) Henry Chapman and Janos Hajdu of Uppsala University in Sweden, achieved the feat using the world's first soft x-ray free-electron laser, located at the FLASH facility at Deutsches Elektronen-Synchrotron (DESY) in Hamburg. Their work will appear on the cover of the December issue of the journal Nature Physics (12 November 2006 | doi:10.1038/nphys461).

The experiment suggests that in the near future, images from nanoparticles and even large individual macromolecules -- viruses or cells -- may be obtained using a single, ultrashort high-intensity laser pulse before the sample explodes and turns into a plasma. This means that scientists could better understand the structure of macromolecular proteins without crystallizing them, which is required in conventional x-ray structure analysis, and have the ability to rapidly study all classes of proteins.

(From Photonics.com)

Low water pressure alert system

The water pressure alert system has been used in our lab for many years. When the city water pressure is low, the system will buzz loudly, then the 24V control signal will be shut down. This alert system looks simple but very useful. It is used to protect the diffusion pump water cooling for x-ray and YAG laser water cooling system.

From last week, the alarm kept buzzing. I had to set the alarm to silent. I opened the circuit box to check today and found the problem came from the reset button. When I pushed the button, the reset could not be short. So even the water pressure sensor operated well, the 24V power could not be applied to the relay coil. The alarm was recovered after the reset button was replaced.

Replaced Hurricane compressor grating

Due to the air humidity, the compressor grating absorbed some dust, this finally decrease the reflection efficiency. We replaced the old grating with the brand new one ordered from Spectra-physics. The measured power before compressor is about 500mW, after the compressor, the power is 300mW, which is bigger than 260mW using the old grating.

The power was decreased to 250mW when I added the 1.02ms delay for diode pump laser. After the delay time adjusted accurately, the power recovered to 300mW. The new delay time is 1020.2us.

Leak from Alumnium box

We cleaned all TWINAMP windows, filled 400mbar F2 + 1400mbar He for leakage detection last night. We found the pressure was dropped by 50mbar for the long tube, and we could smell pungently. Because the o-rings for window and PVDF plate were changed last month, maybe the gas leak came from the aluminum box connected to the window frame. We found some black grain in the aluminum box. The o-ings were changed immediately.

O-ring sizes for TWINAMP

#1. between PVDF frame and Al box
#2. between Al box and 2nd plate
#3. between 2nd plate and PVDF plate
#4. between PVDF plate and window

For Short tube: window size 75 x 40 x 5.1
#1. 60 x 2
#2. 120 x 2
#3,4. 64 x 2 (if the opening on the end plate and PVDF is 65)
60 (or 62) x 2 (if the opening on the end plate and PVDF is 60)

O-ring between reservoir and PVDF fram: 410 x 4
O-ring between PVDF reservoir and upper ground plate: 400 x 4

For Long tube: window 100 x 50 x 10

#1. 85 x 2
#2. 125 x 2
#3,4. 85 x 2

Repolished the TWINAMP long tube windows

We removed t he winodws from TWINAMP lone tube, and found they were very dirty. The small black grains on the surface like last time. There was a thin gray film on the inside surface when we looked the windows at the grazing angle. This film could not be removed by normal optical cleaning method. So we tried to repolish it using 1 micro power. After repolishing, the surface looked very smooth.

Pulse measurement for final amplifier

The Prometheus gases has been changed.

We calibrated the energy meter for large target chamber, it's about 30 times between the outside meter and the real energy. The output energy from the front end is about 16 mJ, the high voltage of Prometheus is 22 kV. The measured energy before the target nozzle is ~ 220 mJ.

The time duration and the phase were measured by FROG. The raw data and the retrieved data looks similar. Compared the front end measurement, the pulse was enlarged because of the dispersion. The pulse width is around 190-220 femtoseconds, the phase kept flat.

Laser Focus World introducing my blog

The column writer, Ms. Gail Overton introuded my blog on Laser Focus World in October.

Snapshots of laser wakefields

Electrons, protons and even ions can be accelerated using the extreme electric fields generated when a high-power laser is focused into a plasma. But the structure of the so-called laser wakefields that are driven through the plasma at almost the speed of light — analogous to the wake produced behind by a boat as it travels on water — has until now only been discernable through simulations of the process. Nicholas Matlis and colleagues have produced the first direct images of a laser wakefield, by using a holographic technique that reconstructs an image of the wake structures from the way in which they perturb the interference of two coherent light beams passing through the plasma. The technique provides a new tool for studying laser–plasma interactions and potentially improving the performance of laser-driven particle accelerators.
Nature Physics Published online: 2, 749 - 753 (October 2006)

TWINAMP window holder design

Because the thickness of the MgF2 window for long tube was different, the old holder can not be used. Excepting the thickness, the other dimensions should not be changed anymore. The material: Aluminum Plate (Al Zn Mg Cu 1.5). Surface: good quality (milled profiles)

Leybold TRIVAC D16B pump

We used the rotary vane pump to vacuum the first spatial filter for TWIN system, this pump could not work two weeks ago. At first, we guessed the start capacitor was broken, however the pump did not work any more even we changed a new capacitor. Mike helped to separate the motor and the rotary vane, he found the motor worked well. The problem was the spider coupling was broken.

FROG measurement using new CCD camera

We used the new FLI back illuminated CCD to take the FROG picture. I found this new CCD camera very sensitive to the UV light compared with our old CCD. For the old one, we had to expose about 5 seconds to take Hg I 248 nm line, for this new one, it's only about 30 ms.

When we were taking the FROG images, the signal was very strong. I had to use a ND filter (~0.005 transimission) to attenuate the light. The retrieved data is almost match the raw data. But for the old CCD camera, it's not well to match these data.

MaxCam CM 1-1

In order to measure the UV beam effctively, we bought a low cost, high QE, back-thinned CCD camera from Fingre Lakes Instrumentation.

Camera Serial Number: 2357 04 Sensor Type: Class 1 / 512 x 512 / 24um pixels E2V CCD 77-01-1-390 Lab Ambient Temperature: 23 C Setup measurement Temperature sensor ok: checked Shutter opens fully: checked Shutter closes fully: checked Window clean on both surfaces: checked CCD free of dust: checked Purging 0.2 torr rise time > 90 seconds: checked Fresh desiccant: checked Argon filled: checked Mechanical/Cosmetics No scratches or marks on Case: checked Shutter test properly: checked Serial number label is present: checked

Image Quality CCD test temperature: -15 C Camera achieves T greater than: 37 C Mean bias level of bias frame: 2367 Standard deviation @ test temp, small area: 3.3 Mean saturation level: 65535 Noise distribution is random: checked Bias frame histogram is Gaussian: checked Standard test target appearance ok: checked Bias over scan frame saved as 2357 04 bias OS -15C. fts Light frame saved as: 2357 04 lgt -15C.fts Flat field frame saved as: 2357 04 ff -15C.fts Software A/D serial Number: #3329 Final Check, no frost visible@ -15 C Tested by: Dave Johnson October 13, 2006

Vacuum pump for first spatial filter

We found the vacuum pump for TWIN first spatial filter does not work. I guessed it's caused by the motor start capacitor, unfortunately this kind of cap was from Europe.

Driving electron beams to 1 GeV

Progress continues apace in the development of laser wakefield accelerators, which produce high-energy beams of electrons, atoms and molecules using the extreme fields generated in a plasma by high-power lasers. But for the potential of these devices to be realized — in practical applications that range from studying the behaviour of matter under extreme conditions to proton therapy for the treatment of deep-seated tumours — the energy of the particle beams they produce must reach the giga-electronvolt range. Initial estimates had suggested that reaching such energies would require lasers capable of producing pulse powers of the order of petawatts. But Wim Leemans and colleagues have proved the predictions wrong by producing 1-GeV electron beams using only 40 terrawatts of laser power — the trick being to focus the laser into a 3.3-cm-long gas-filled capillary discharge tube.

Nature Physics 2, - pp696 - 699 (2006)

Uranium Beam-Pumped UV Laser

Lasers consist of an active medium of excitable atoms, a pumping mechanism for exciting those atoms, and a cavity for building up a pulse of coherent radiation. At the Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung, or GSI) in Darmstadt, Germany, scientists have succeeded for the first time in using a beam of uranium ions as the pump for producing ultraviolet laser light.

It works like this: the uranium beam ionizes argon atoms, which ionize krypton atoms, which in turn form excited molecules with fluorine. The krypton fluoride molecules are the excited entities which emit coherent light at a wavelength of 248 nanometers. A laser that uses this rare gas-halide mixture is called an excimer (excited dimer) laser.

This is not the shortest laser wavelength ever achieved, and the uranium pumping scheme is not all that energy efficient. So why then use this approach to producing laser light, especially when electrically pumped commercial krypton fluoride lasers are available? Because this was a test run for producing laser light in excimers that can't be electrically pumped.

According to Andreas Ulrich of the Technical University of Munich (andreas.ulrich@ph.tum.de), the goal is to excite excimers of pure rare gases for producing radiation in the VUV (vacuum ultraviolet) and soft X-ray region of the spectrum. Only now have uranium beams at GSI been powerful enough to provide the pumping power for lasers in this wavelength region. Being so heavy, uranium atoms deposit their energy into a gas much more efficiently that lighter particles such as electrons.

Ulrich et al., Physical Review Letters, 13 October 2006
Contact Andreas Ulrich
Technical University of Munich
andreas.ulrich@ph.tum.de

Copied from Physics News Update 796 #2, October 11, 2006 by Phil Schewe and Ben Stein

Compound Refractive x-ray Lenses (CRLs)

The compound refractive x-ray lenses (CRLs) are used to focus and collimate hard x rays, and image objects using hard x rays. The lenses have been experimentally shown to operate in energy ranges from 4 to 80 keV, at focal lengths as small as 5 cm, with numerical apertures of 10-3 or better. These operational ranges can be further extended.

The main principle behind compound refractive lenses (CRLs) is an ability of a group of thin lenses to greatly shorten the focal length. A series of N lenses with small apertures is used to achieve both one- and two-dimensional focusing and imaging at x-ray photon energies, where refractive imaging has previously been thought to be impossible due to the weak refraction of x rays. Note that the refractive index for x-rays is less than 1.

Therefore, unlike visible light optics which will cause visible rays to diverge, the concave lens will focus x-ray photons. Among the lenses we offer are cylindrical, spherical and 1- and 2- dimensional parabolic units.

From Adelphi Technology Inc.

Relay K4 on South side replaced

After running one hour, the Prometheus system was shut down. The Filament light was on and the south bank was totally off. The system was recovered just after changing the relay K4, which is the power controller.

The calibrated Xe spectrum

Obtaining the first order spectrum

We tried to record the first order spectra many times, however we only got a single spectral line. Today we reduced the gas pressure and moved the focal spot a little behind the nozzle center, we also increased the MCP voltage(~1 kV) and phosphor voltage (~4 kV). We observed the soft x-ray spectrum successfully, we will calibrate these spectra soon.

Bright zeroth order spectrum

We installed a new parabolic reflect mirror to focus the laser beam and adjusted the alignment of the spectrometer. In order get the signal, we changed the nozzle repetition rate to 0.5 Hz, the gas pressure decrease around 100 psi. Compared the last zeroth order spectrum, this looks very bright and sharp. The line became very weak when the nozzle was not triggered, it means that there was only 248 nm laser hit MCP.

Unfortunately, we could not find any interesting spectra when letting the spectrometer scanned in the whole range.

Leak from the McPhonsen spectrometer

We tried to vacuum the McPhonsen 247 over night, however there was a big leak from the bellow connector.

Ping thought the leak from the bellow, so he changed a new bellow. After than, he found the threads of two screws for mounting the bellow were badly damaged, which mainly caused the leakage.

After he put two new screws to tighten the bellow flange, vacuum of the pump system reached 3*10^-5 in 20 minutes.

Zeroth order spectrum form VUV spectrograph

We are using a 1-meter 86 degree grazing incidence model 247 McPherson spectrometer to measure the soft x-ray emission from laser plasma interaction. The detector is a single stage 2 inch diameter MCP with a potential of between of 200V to 1kV across it. The electron burst from the MCP is accelerated by another potential of 3 kV to 5 kV into a phosphor screen. The green photons from the phosphor is collected by a camera lens and sent to CCD camera. By adjusting the spectrograph, we obtained the 0-order spectrum.

That's no laser, it's a particle accelerator

That's no laser, it's a particle accelerator

Geoff Brumfiel

Israeli physicists have turned a laser into a particle accelerator. Dubbed a paser — for particle acceleration by stimulated emission of radiation — the device accelerates bundles of electrons using the same principle as a laser.

At present it can only accelerate electrons by about 0.15% of their initial speed, but it could lead to compact particle accelerators and tabletop X-ray devices, according to Samer Banna of the Israel Institute of Technology in Haifa. He and his colleagues will publish their work in Physical Review Letters.

Conventional lasers exploit the quantum properties of atoms. An energy source is used to boost the electrons in a group of atoms into an elevated energy state. Passing light in the form of photons stimulates the atoms and causes the electrons to fall back to the lower energy level, emitting more photons in the process. These in turn stimulate more atoms and so on, so that a large number of photons are emitted. The photons are all identical, which makes the beam of light uniform.

Pasers work on a similar principle, but the output is accelerated electrons. Packets of electrons are fired into a cloud of excited carbon dioxide gas. As in a laser, the gas releases a large number of identical photons. But those photons are instantly absorbed by the passing electrons, which get an energetic kick, and leave the device moving more quickly than when they came in.

The fact that the paser uses atoms to speed up electrons sets it apart from other particle accelerators. "This is unlike anything that's come before," says Eric Colby of the Stanford Linear Accelerator Center in California. The unique mode of action makes the paser far more efficient than current machines, which achieve acceleration by generating enormous electric fields inside huge cavities. Colby is optimistic about the paser's potential. "It's a pretty small effect now," he says, "but there are strong technical reasons to believe that a very significant gain in acceleration is possible."

Levi Schächter, of the Israel Institute of Technology, believes that the paser could also make its mark as a source of X-rays. If the high-speed electrons have their paths bent after they leave the device, they will release a laser-like beam of X-rays that could be used for medical or nanotechnology applications. But Schächter is reluctant to guess exactly what may come of the technology. "In Hebrew we say, 'It's difficult to make predictions, particularly regarding the future'."

It wouldn't be possible to produce the exact equivalent of a laser beam with electrons — the Pauli exclusion principle states that electrons cannot exist in the same energy state at the same time. But laser equivalents can in theory be created for other types of particle, such as gravitons (which carry the gravitational force), phonons (packets of vibration) or some nuclei — if a system can be found that emits them. In June, for example, a group of researchers reported building a sound laser, or saser, that uses semiconductor technology to create a uniform beam of phonons (A. J. Kent et al. Phys. Rev. Lett. 96, 215504; 2006).

Colby says that these new systems show that it is easier than one might think to generate laser-like behaviour. "If you can store energy in a material," he says, "a great many things can be done."

Grating Cleaning Procedure

David found the compressor grating became dirty, that caused the output energy lower. He suggested to order a new grating or clean the grating. We ordered a new grating from Spectra-Physics, the model number is 9800-2680, the size is 30x110x16. The grating cleaning procedure is listed below.
----------------------------------------
Direct Contact Method:

Spectra-Physics RGL provided the direct contact method and they have used it on gratings with
minor dirt or oil contamination.

1. Select a pan large enough to accommodate the grating as well as being able to allow unrestricted movement around the grating. A 20cm x 20cm x 5cm (8" x 8" x 2") aluminum baking pan was used here.

2. Heat water temperature to 32 C (90F) +/-1C.

3. Mix together Woolite 2 capfuls of (or equivalent) with water.
http://www.fabriclink.com/Woolite/Home.html in case one is not familiar with Woolite.

4. Place grating in the soapy water pan.

5. The water should cover the top surface of the grating by 10-12mm.

6. Adjust water level in pan accordingly and maintain water temperature of 32-34 C.

7. Grating is to be lightly stroked with a cotton ball in the direction that is parallel to the groove.

7.1. Note: "Stroking" means to drag the cotton ball across grating, applying no physical pressure, allowing just the weight of the wet cotton to contact the grating surface.

8. Remove the grating from the soapy water pan and rinse with distilled water. Make sure that the water is flowing down the grooves, not across the grooves.

9. Rinse the grating with isopropyl alcohol (rubbing alcohol 70% by volume) with the alcohol going down the grooves not across. Be careful for the alcohol will dissolve the ink on the grating. I would advise that one write the groove density on the grating with a pencil.

10. Blow dry the grating with nitrogen (or clean "air") starting at the top allowing the alcohol to run down the grooves. As with the rinsing, blow down the grooves not across them. When testing this method at SPMV, dry nitrogen at a pressure of 50 psi through a 6mm dia. nozzle was used. This dried the grating well.

Bake Out Method

The bake out method is generally used when the grating exhibits a film or 'fog' on the surface of the grating. The cause and/or composition of this film is unknown. However, baking out the grating usually results in the elimination of the film.

Place gratings in the oven with the gold surface up. Take care when placing the gratings so that the gold surface does not come in contact with anything.

Ramp up oven over at a 2 hour rate to 105 C.

Bake the gratings for 2 hours at 105 C.

Ramp down the oven at a 2 hour rate to room temperature.

Hurricane Compressor alignment procedure

The compressor consists of one grating, one horizontal retromirror set, and one vertical retro-mirror.
(1). Adjust the grating position horizontally and rotate the grating so that the input beam on the right bottom. The first order diffracted beam out of the grating should be centered on the horizontal retro-mirror without clipping of the spectrum.
(2). Morve the horizontal retro-mirror so that the spatially dispersed spectrum on the grating left bottom. The spectrum on the grating should be the same height as the input beam.
(3). Move and rotate the vertical retro-mirror so that the beam is retrace back to left top of the grating. Make sure the vertical retro-mirror is positioned as close as possible to the input beam without clipping the input beam.
(4). The beam coming out the compressor should be centered to the output port of the Hurricane with beam height equals to 4".
(5). If the beam height is not centered to the output port, one can adjust the y-axis of the vertical retro-mirror to center it.

How to adjust the Hurricane regen

(1). Blcok the Mai-Tai seed beam.
(2). Disable the Pockel Cell trigger.
(3). Taken the half-wave plate from PC1 postion, put it behind PC2, then rotate this plate to maximumize the output power. (in our case, ~550mW@745nm in free running).
(4). Tune the Evolution green beam last steering mirror to peak the output power.
(5). The output power should not be decrease when centering the iris.
(6). Adjust the half-wave plate to reduce the power minimum.
(7). Mark half-wave plate, then move it back to PC1 postion.
(8). Adjust the seed beam steering mirrors to make the time gap between free runing pulse and seed pulse around 70 ns.

Replaced X-ray gun ion gauge

Because the old ion gauge for x-ray gun did not work stably, I ordered a new ion gauge from MDC. The old gause was replaced as soon as I got this new gauge. The vacuum reached around 3x10^-5 Torr before I left the lab.

For LLG-TWIN, I have to change the gases twice to make sure the timing trigger steadily. The laser energy would be decreased in a couple of minutes after first gas filling, then it's hard to synchronize the seed beam and the TWIN. After evacuating the chamber and refilling the gases, the TWIN became better.

Timing delay for target nozzle

In this week, we tried to focus the laser beam to the Xe cluster for measuring the Xe(L) emission. Because the nozzle has been changed, the timing delay between laser pulse and nozzle trigger must be adjusted. On Monday this delay time was about 478 us, however it changed to 410 us next day, on Friday the dealy was 491us. The range is around 10 to 80 us, beyand these values, we saw nor Xe(M) light on the top CCD neither any singals from crystal spectrometer (for measuring L-shell light).

Trigger signals from frequency divider

Sometimes the signal from the frequency divider was strange. We used two frequncy divider, No.1 is 2 times divider, No. 2 is 10 times. The seed pulses with 4 Hz repitation rate was divided to 0.2 Hz by these frequncy dividers. I found the No.1 sent disordered signals. After moved this divider from one rank to the another, the problem was disappeared. So I used the same DC power supply to energize these dividers now.

The Relay K4 for cooling fans could not run well, previously it became well after only tightening the connectors. Yesterday I had to tighten many times, I replaced it this morning.

Windows cleaning and O-rings changed

I removed the windows of both tubes. The windows of the long tube are a little bit dirty, maybe the energy dropping was caused by the leakage. The most possibility is the leak from the windows. Firstly it's diffcult to take the window from the holder, I dropped some ethonal into the gap between window and the holder, the window became loose and was easy to remove. I cleaned windows carefully. Unfortunately I found there was a light breakage inside the window, we should order new window for the long tube.

I installed all windows in the afternoon, and made the passivation for both tubes. Before my leaving I filled the Helium into the chambers by 1500 mbar for leakage checking. I will continue to passivate the tubes.

TWIN windows became dirty again

After the labor day, we tried to continue the experiments. So I filled the new gases into the TWIN excimer amplifier this morning. The output energy of the front end was 34.7 mJ, Alex already used this beam to align the target and x-ray spectrometer. Unfortunately, the energy was dropped to 7.7 mJ suddenly in the afternoon, I found there were small dust grains on the long chamber windows, which was similar to the status two weeks ago (on Aug. 22th). I cleaned the same windows on May 10th (http://xysong.blogspot.com/2006/05/twin-dirty-windows.html). It is very strange that the windows became dirty frequently. I guess the filter was dirty or something wrong inside the long tube.

Obtaining the Xe(L) spetra successfully

The laser system ran very well today. The front end output a laser beam with about 26 mJ energy. After the Prometheus amplifier, the energy reached about 360 mJ as energizing 25 kV high voltage. Today the total shots is 1218, no misfire happened. The filament was shut down in the beginning, I could hear the click from the filament relay, after turn on again, the problem disappeared.

Because the nozzle was changed, I adjusted the timing delay for optimizing the Xe(L) outputs. Alex took many spectra from the target side.

Seed beam focal spot measurement

The seed beam was sent to the target chamber after expanding beam size from 3 cm diameter to around 10 cm diameter, then it was focused to the smaller spot by a Parabolic mirror. We are using the 248 nm coated paroblic mirror with 20 cm focal length, so the focal spot under the diffraction limitation is about 1.2 um.

In order to measure the focal spot, we put a 40X microcope objective lens behind the spot. A CCD camera without a window to record the spot image, the CCD size is 510x492, the pixel size is 9.6 um x 7.5 um. After calculation, the focal spot is about 2.9 um x 3.8 um.

Thanks, Randy and John

Tightening LLG-TWIN thyratron plate and ground plate

Last month, we found the arcing problem when running the LLG-TWIN excimer amplfier, I put a teflon plate for preventing the sparks. Today the arcing happened again, I am afraid the isolation plate could not really avoid the arcing. With Randy and John's helps, we removed the alumina plate and found the spark traces on plate of thyratron side.

We polished the plate surface and punched a new hole for conbine the plates tightenly. A wire braid was inserted between the plates in order to avoid the arcing.

Finally we fixed the arcing problem, however the output energy was still low and the beam quality was very bad. When I checked the lone tube windows I found there were some dark grains on the window surface. I want to open the chamber to clean them tomorrow.

Front-illuminated CCD or Back illuminated CCD?

A conventional CCD consists of a sandwich of semiconductor layers overlaid with a network of "gates", electrodes which serve to transfer charge from one pixel to another as the device is read out. At low energies (E<1>5 keV). due to photon loss through the front of the back-illuminated device.

Comparison of QE of front- and back-illuminated CCDs. The lack of gate structure at the exposed surface of the back-illuminated device, combined with its reduced physical thickness, results in improved QE for energies <3>5 keV), due to photon loss through the front of the back-illuminated device.

copied from http://cxc.harvard.edu/newsletters/news_05/node11.html

CCD detectors are not able to convert all of the photons that strike the surface into electrons for a variety of reasons. Quantum efficiency (QE), which describes the ability of the CCD to turn photons into a useful form of output, is basically the ratio of incoming photons to those photons actually detected by the CCD. The typical range of efficiency is from a few percent up to 90%. Efficiency will also vary with the frequency of light (color) observed. Front-illuminated thick chips are not as sensitive to blue light as they are to light with longer wavelengths.

Interested link:
CCD glossary

Changed the H.V. cable

I found a spare cable in the lab, so I just replaced the burnt cable with it. But the plug body connected with the power supply was borken, so I replaced it with the plug body of the burnt cable.

TWIN H.V. cable burnt

In the afternoon, the TWIN excimer amplifier suddenly stopped firing after running for half hour. I found the power supply light became red, the light was off after pressing the reset button. Then I pushed the H.V. button and Laser button, I heard a buzz sound inside the excimer chamber. I energized gradually to the amplifier, the voltage reached 12 kV, the buzz appeared again then the system shut down immediately.

I opened the chamber, and turn on the system to check where the buzz came from. I found the high voltage cable was burn, the buzz was from the broken cable. The arcing from the cable caused the fail in high voltage increasement.

Arcing from LLG-TWIN excimer inside

When I ran the TWIN excimer amplifier, I could hear the crack sound inside the chamber. This phenomenon was happened before, which was caused by the spark from the Thyratron plate and ground plate. I put two picece of plastic sheet to isolate them. Probably the sheet became dirty or they were too thin. The Thyratron plate should be tightened with the real ground plate in order to avoid the spark, however it will take several days to remove the plate to drill the holes for tightening. The simple way is to replace the isolating plate with a new teflon plate. After replacing, the system ran well.

Modification of front end timing

In order to synchronize the seed pulse and Prometheus ASE, I tried many ways without using more delay control equipments. But all the attempts were failed, I have to consider change the front end timing. By studying the old system timing, I found a 10ms delay was added to the CAP pump Q-switch laser, I could copy this setup for our new system. So I inserted a 1ms delay by using BNC delay controller, then replaced the original delay controller from LLG with SRS DG-535. The channel A of DG-535 was used to switch Pockell in and channel C for Pockell switch out. The output of channel D was used to control the excimer control box.

We will have extra 1 ms time delay for the Prometheus trigger and target nozzle controller.

Overlapping seed beam and Prometheus ASE

I tried to match the seed laser beam and the Prometheus ASE. The Prometheus ran in the good condition when Thyratron trigger and X-ray Anode trigger was set by 1.78 us and 1.22us earlier than Railgap trigger respectively. So the time delay between seed pulse and Prometheus could not be set less than 1.78us. I tried to adjust the time delay from 1.8us to 1.5 us (as shown in the pictures). I found when time delay was 1.5us, two pulses were overlapped, but the thyratron current wave forms indicated the Thyratron had no time to totally turn off.

The best solution is to add an extra time delay in the front end.

Timing Diagram

We simplified the timing setup when we installed the new High Voltage power supply. The prevent circuit has been removed, the Thyratrons were triggered directly by the signal pulses from SRS DG-535. The repetition rate was decreased by 0.2 Hz for Prometheus, the 4 channels were set as A=T+1 ms, B=A-1.75 us, C=B+30 ns, D=A-1.21 us.

Replace Main Charging Power Supply for Thyratron Capacitor Banks

The SCR Controller, SCR Power Block, HV Transformer with current limiting resistor, HV rectifier diodes, charging and dump resistors, LED status lights, Trigger Inhibit boxes, and all control and coaxial cables that went to the main control rack were removed. These functions were replaced by a new dual charging board and two Gamma Power supplies as shown the right picture.

Each side of the dual charging board (see schematic on bottom of previous page) utilizes ten 10K, 100W Ohmite wirewound resistors that serve to isolate the Gamma power supplies and additionally provide energy dumps for both Thyratron Banks via the common Ross relay. Each charging side of the board is the same; only the North side will be described herein but the South side functions in the same manner. Resistors R3 through R10 are used to dump a maximum 675 joules of stored energy in the five 0.30 uF capacitors per bank when charged to 30 kV. Resistors R1 and R2 can be used to set the constant current charging current limit of the Gamma power supply when the Ross relay is left de-energized. Resistors R21 through R25 and monitor resistor R31 provide a 10,000:1 voltage divider to be terminated into 1 Megohms or greater for power supply calibration purposes. This divider is not resistor-capacitor compensated and is not intended to observe fast discharge pulses. The 50-Megohm-divider string also provides a slight bleed for the power supply to help hold the charge voltage accurately. The North and South Banks were made completely independent by removing the 3K, 250W Ohmite wirewound cross-connecting resistor and associated hardware in the top oil tank. The output of each new charging resistor string was connected to the North and South capacitor banks via a Dielectric Sciences coaxial, silicone based, semicon graded, high-voltage cable rated at 60 kV DC.

Both the North and South Banks were operated at a constant charging current of 12 ma and a charge voltage of 28 kV; this operation was successfully achieved only after much troubleshooting discussed later in this report. The constant current charging time can be estimated to be T = CV/I = (1.5uF)(28kV/(12ma) = 3.5 secs. In actuality, the charging time is a little greater due to the slight voltage drop across the charging resistor string as the power supply switches from constant current to constant voltage operation as the final charge voltage is reached. This power supply portion of Prometheus is only compatible with 0.2 Hz operations unless reduced power operations are invoked. The scope trace on the top of the next page shows the North and South Bank secondary voltages as well as the Thyratron primary currents.

Referring to the scope traces on the top of the next page, at first glance it appears that the transfer time of the North Bank (Red) appears to be faster than that of the South Bank (Blue) causing the Rail Gap switchout to be late by a few 100 ns for the North Bank. On the other hand, the North Thyratron primary current is slightly slower than that of the South. Both the North and South Thyratron secondary voltage traces start out somewhat different but are repeatable. The triggering of the North Thyratron Bank could be set to occur slightly later than the South Bank if so needed. The operation of the South Bank appears to be optimum. The flattening of the peak of the North Bank trace just before Rail Gap switchout is not seen on every trace; its cause is not explained at this time. The system was operated for about one hour at 0.2 Hz until a failure of the North Thyratron Bank occurred. The fault was traced to a single Thyratron on the North Bank and will be discussed in more detail later in this report.

Operational conditions for Prometheus at 28 kV (Full Power):
X-ray Anode Gamma Supply: 27 kV and 5 ma
North/South Gamma Supplies: 28 kV and 12 ma
Repetition Rate: 0.2 Hz
Maxwell 40168 Spark Gap Pressure: 35 psig
Maxwell 40161 Spark Gap Pressure: 64 psig
Maxwell Rail Gap Pressure: 34 psig

(Written by Randy Carlson)

Replaced X-ray Gun Anode Power Supply Successfully

Compared the schematics of old X-ray anode, we replaced the Hiptronics 30kV 5mA power supply with GAMMA 40kV 15mA power supply.

Replace X-ray Anode Power Supply and Upgrade High Voltage Tank

The Hipotronics Power Supply and most all components in the X-ray Anode High Voltage Unit (M05A) were removed except those concerned with the operation of the Ross relay in the HV Tank. A better layout of the capacitors in the HV Tank improving and eliminating suspect connections, a lower inductance and more robust ground, and routing all cables through a slotted hole in the corner of the HV Tank lid were provided. The previous voltage monitor, charging resistors, and dump resistors were removed. These functions were replaced by components mounted on a Lexan board that can be easily removed if a failure occurs. These actions enable the lid to be secured during operation and, if necessary, readily removed for component inspection during operation. The HV Tank lid interlock was made active; it had been previously bypassed. A schematic of the new X-ray Anode HV Tank charging board is shown below and now discussed.The X-ray anode charging board utilizes eight 10K, 100W Ohmite wirewound resistors that serve to isolate the Gamma power supply and additionally provide an energy dump via the Ross relay. Resistors R3 through R8 are used to dump about 110 joules of stored energy in the three 0.08uF capacitors when charged to 30 kV. Resistors R1 and R2 can be used to set the constant current charging current limit of the Gamma power supply when the Ross relay is left de-energized. Resistors R21 through R25 and monitor resistor R31 provide a 10,000:1 voltage divider to be terminated into 1 Megohms or greater for power supply calibration purposes. This divider is not resistor-capacitor compensated and is not intended to observe fast discharge pulses. The 50-Megohm-divider string also provides a slight bleed for the power supply to help hold the charge voltage accurately.

The X-ray Anode unit was operated at a constant charging current of 5 ma and a charge voltage of 27 kV giving a dose rate of about 0.4 mR/hr from the X-ray Anode electron source. The constant current charging time can be estimated to be T = CV/I = (0.24uF)(27kV)/(5ma) = 1.30 secs. In actuality, the charging time is a little greater due to the slight voltage drop across the charging resistor string as the power supply switches from constant current to constant voltage operation as the final charge voltage is reached. This power supply portion of Prometheus is compatible with 0.4 Hz operations. The scope trace as shown right picture compares the previous X-ray anode results with those of the new power supply system. The X-ray pulse is slightly higher and faster due to the lower inductance associated with a better layout and grounding of the capacitors in the X-ray Anode HV Tank.
(Written by Randy Carlson)

Install Gamma High Voltage Power Supplies in Main Control Rack

In preparation for installing the three gamma power supplies in the main control rack, the diagnostics patch panel was moved to be at the top of the rack containing the two Tektronix 2024 scopes. The System Control Unit (R01), Gas Processor Control Unit (R04), and the Vacuum Gauge Controller Unit (R05) were moved toward the bottom of the main control rack. The High Voltage Control Unit (R03) was removed along with its associated shielded “gray cube” box that contained the previous Main Charging control electronics. All unused control and coaxial cables were removed between the main control rack, the Pulse Modulator HV Power Supply (M03), the X-ray Anode High Voltage Unit (M05A), and the X-ray Anode HV Tank (MO5B). Three Gamma power supplies were then mounted in the main control rack and additionally supported on rails. Although not necessary, a space of 1.75” was left between the power supplies for more than adequate cooling and ease of installation. The X-ray Anode Gamma power supply was connected to the X-ray Anode HV Tank via a 100-foot Dielectric Sciences (DS-2124) coaxial charging cable. This cable is polyethylene based with a graded semicon center shield robustly rated for 100 kV DC. The other two Gamma supplies were connected to a new dual charging board (described later) via similar 34-foot long coaxial cables. All three power supplies are supplied by 120 VAC power from three unused legs of the 208 three-phase Main Power Interconnect Box (M00). The power requirements of each supply are 7 amps at 120 VAC. Appropriate circuit breakers are now on hand and need to be installed in the future. The fourth Gamma power supply that had damaged current and voltage meters during shipment to UIC was repaired with parts from Gamma High Voltage Research and tested; this power supply should be used as a direct replacement spare.
(Written by Randy Carlson)

Install new H.V. power supply for X-ray Anode

Randy and John came to help us to install the new High Voltage power supply for x-ray anode. This will simplify the system.

When we tried to apply voltage up to 4kV, we found there was a big current leak. We traced the whole circuit including Thyratron, charging resistors, Ross relay and tranformer connectors. The problem is from the resistor plate, there was a screw hole, which was drilled very deep to cause an arcing when applied high voltage. Randy decided to replace them with the Nylon screws.

June 12~16, Great Lakes Photonics Symposium

Great Lakes Photonics Symposium

Photonic Frontiers: Laser Acceleration: Short pulses speed particles

Intense femtosecond pulses can accelerate electrons, protons, and ions to high energies over very short distances, and could lead to a new class of compact, high-current accelerators.

Jeff Hecht, contributing editor

The concept of laser acceleration of charged particles dates back to 1979 when Toshi Tajima and John M. Dawson predicted that intense laser pulses could create a wake of plasma oscillations that could accelerate electrons to high energy.1 Their computer simulation of the effect attracted considerable interest because it held out the prospect of useful acceleration over much shorter distances than conventional particle accelerators. However, the short-pulse lasers of the day could not generate the peak powers needed for useful laser acceleration.

That changed with the development of chirped pulse amplification, which can generate extremely high peak powers in ultrashort pulses. The past several years have seen remarkable experimental progress, first with electrons and more recently with protons and heavier ions. Early experiments produced particles over a wide energy range, but recent results have narrowed the range of energies, a crucially important feature for applications that require precise control over particle energies.

Particle acceleration basics

Traditionally, charged particles have been accelerated by passing them through long metal tubes in which alternating electromagnetic fields were applied to a series of segments. The applied fields reverse as the particles pass through the segments, so the fields always accelerate the particles. The longer the tube, the more acceleration is applied to the particles, and the higher their energy. Laboratory-scale accelerators have meter-long tubes, but the accelerators used in cutting-edge high-energy physics can be several kilometers long. The maximum acceleration possible depends on the accelerator structure and the power of the alternating field. The upper limit is acceleration that increases energy by tens of megaelectronvolts per meter of tube length, so very high energies require huge accelerators.

Laser acceleration can generate much higher fields, so acceleration distance can be much shorter-typically a millimeter to drive electrons to 100 MeV. Firing powerful ultra-short pulses into a plasma or solid generates extremely intense electric fields, which can reach teravolts per meter at the instant of peak intensity. These fields overwhelm the electric attraction between the positive nucleus and the negative electrons, freeing both electrons and positive ions. The process also generates intense fields that accelerate the particles to high energies over short distances.

“Electron acceleration and proton acceleration are fundamentally different,” says Thomas Katsouleas of the University of Southern California (Los Angeles, CA), so different approaches have been developed for the two. The original laser-wake-field approach proposed by Tajima and Watson deposits energy in a plasma and works best for electrons. Protons and ions are much heavier and better accelerated by firing laser pulses that explode thin-film targets, freeing bursts of charged particles.

Laser-wake-field acceleration

Wake-field acceleration is often compared with surfing. When an intense laser pulse hits a plasma, it creates a density wave of free electrons. The electrons, in turn pull positive ions-protons or heavier nuclei-along with them, creating a density wave as they pass through the plasma (see Fig. 1). The density wave carries the free electrons with it, and the electrons can reach 100-MeV energies within a millimeter, roughly 1/5000th the distance needed in a conventional accelerator.

Like catching an ocean wave to surf, coupling a laser pulse into a plasma is tricky, and the process took time to perfect. The first experiments showed very fast acceleration, but it took time to increase the number of accelerated electrons and focus them in a narrow beam. In 2002, researchers accelerated a burst of 100 million electrons that spread within an angle of only three degrees, but their energy diverged widely.2 That was a concern. “For a lot of applications it’s the ‘holy grail’ to get a monoenergetic beam,” Katsouleas says.

A major advance came in 2004, when three groups reported much narrower ranges of electron energy in papers that appeared in the same issue of Nature.3, 4, 5 The key to their success was finding ways to inject a clump of electrons into a small part of the plasma so the electrons can be accelerated collectively to nearly the same energy. The researchers fired pulses with peak powers of 10 to 30 TW and lengths of 30 to 55 fs into gas jets 2 mm long. By creating plasma channels or adjusting the laser beam to guide the density waves through the jets, they managed to constrain energy spread to no more than 24% for up to a few billion electrons (see Fig. 2).

Laser ion acceleration

Protons and positive ions are too heavy for the wake-field approach to accelerate them effectively. Instead, researchers blow them away by hitting a thin, dense foil with a pulse reaching higher than 1018 W/cm2. The electric field is so much stronger than the nuclear attraction so it blows electrons out the back of the exploding foil at relativistic speeds. The electric charge of the accelerating electrons pulls protons or heavier ions along behind, accelerating the positive ions over micrometer-scale distances. “This is a pure one-stage process,” says Juan Fernandez of the Los Alamos National Laboratory (Los Alamos, NM).

Exploding foils were long known from inertial-confinement fusion experiments. However, they were not seriously considered for laser acceleration until the Lawrence Livermore National Laboratory (Livermore, CA) unexpectedly generated a well-controlled proton beam by firing its Petawatt laser at gold foils.6 Analysis showed that the protons came from impurities in the foil. The beam was intense, but ion energy was distributed over a wide range, and many applications require mono?energetic beams. Medical therapy, for instance, requires uniform energy to ensure the particles all penetrate the same depth.

A pair of experiments reported in January in Nature took a big step toward that goal, using different approaches to generate beams of protons and carbon ions with limited ranges of energy.

A team at Friedrich Schiller University (Jena, Germany) produced the proton beam by focusing 10-TW, 80-fs pulses from a Ti:sapphire laser to an intensity of 3 × 1019 W/cm2 on a 5-?m titanium foil. On the other side of the foil was an array of polymer dots 0.5 ?m thick and 20 ?m across. When a laser pulse hit the metal side of the foil behind a polymer dot, it blew off a cloud of hot electrons on the side of the dot, which in turn pulled protons from the polymer behind them (see Fig. 3). A plot of energy of the ?roughly 100 million laser-accelerated protons showed a narrow peak at 1.2 MeV, which was matched in simulations. The group calculates it could accelerate all 800 million protons in the polymer dot to an energy peak of 173 MeV-suitable for treating deep-seated tumors-if it had a laser that could deliver peak intensity of 10^21 W/cm^2.

Fernandez’s group at Los Alamos concentrated on the more difficult problem of accelerating heavier ions. It focused 30 TW, 600-fs pulses onto 10-um spots on a 20-um palladium foil with a thin graphite layer on the back. The intensity of 10^19 W/cm^2 blew relativistic ?electrons off the surface of the rear of the foil, which in turn accelerated highly ionized carbon atoms. The group found a 17% spread in the mean energy of about 36 MeV for the most abundant ions, C+5.

A key advantage of the laser approach is the ability to generate much higher ion currents than can conventional accelerators. Mutual repulsion of ions limits current in a conventional accelerator, but the laser accelerator can produce multiple kiloampere pulses because it produces a neutral beam containing electrons as well as ions, says Fernandez. “You’re shooting a plasmoid” that doesn’t want to fall apart.

Outlook

Laser accelerators are not going to replace the gigantic particle accelerators used in ?particle physics research. In principle, wake-field accelerators might be extended to produce high-energy electrons-but not protons. “You’re never going to make very high-energy proton beams this way,” says Fernandez.

But laser acceleration has two ?other big strengths. Because they use very intense fields to accelerate particles over short distances, they can be made small enough to fit in a laboratory, especially as the size of high-power, short-pulse laser comes down. Laser accelerators can also produce higher-?power beams. This combination makes them attractive for a wide range of applications. One is treating tumors with heavy ions, which deposit little energy until their velocity slows, zapping cancer cells deep inside the body without killing tissue along its path. Laser-accelerated beams might be used for fast ignition in inertial-confinement fusion. And once laser accelerators become available, more applications seem sure to appear.

REFERENCES

1. T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (July 23, 1979).
2. V. Malka et al., Science 298, 1996 (2002).
3. S.P.D. Mangles et al., Nature 431, 535 (Sept. 30, 2004).
4. C.G.R. Geddes et al., Nature 431, 538 (Sept. 30, 2004).
5. J. Faure et al., Nature 431, 541 (Sept. 30, 2004).
6. R. A. Snaveley et al., Phys. Rev. Lett. 85, 2945 (2000).
7. H. Schwoerer et al., Nature 439, 455 (Jan. 26, 2006).
8. B.M. Hegelich et al, Nature 439, 441 (Jan. 26, 2006).

Laser Focus World April, 2006