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.