Thursday, May 22, 2008

Rochester's Omega Laser Receives 50-Fold Power Increase to Become 'Petawatt' Laser

The University of Rochester will mark another important step in the effort toward attaining sustainable fusion, the ultimate source of clean energy, Friday, May 16.

University President Joel Seligman, along with special guests, who include U.S. Senator Charles Schumer, U.S. Representative Thomas Reynolds, and Undersecretary and National Nuclear Security Administration Administrator Thomas D'Agostino, will dedicate the new Omega EP (Extended Performance) laser facility at the Robert L. Sproull Center for Ultra High Intensity Laser Research at the Laboratory for Laser Energetics (LLE).

The Omega EP comprises a new set of four ultra-high-intensity laser beams that will unleash more than a petawatt—a million billion watts—of power onto a target just a millimeter across. Working in conjunction with LLE's original 60-beam Omega laser, the Omega EP will open the door to a new concept called "fast ignition," which may be able to dramatically increase the energy derived from fusion experiments and provide a possible new avenue toward clean fusion power. If successful, fast ignition could lead to the highest energy densities ever achieved in a laboratory.

"I look forward to the profound scientific contributions the Omega EP extension will bring to the University and to the world," says Seligman. "It is a vital component of our nation's scientific capital and leadership, a key to strategic work on an independent energy future, and a vital part of the local economy, including $44 million in local expenditures just last year."

"Over the years, the University of Rochester's Laboratory for Laser Energetics has consistently brought Upstate New York's high-tech sector to the forefront of energy innovation," says Schumer. "It is a vital national resource as well as an economic boon to Rochester and to the entire Finger Lakes region. I was proud to secure over $61 million to support their efforts last year and will continue to look for ways in which the federal government can further collaborate with this dynamic laboratory in the future."

"Employing more than 500 Western New Yorkers, the Laboratory for Laser Energetics of the University of Rochester is essential to the growth of our community and ensures Rochester is on the cutting edge of technology," says Reynolds. "The new Omega EP laser is truly remarkable and serves as a clear demonstration of how our region remains a leader in world-class innovation. The Omega EP's success is a testament to the scientists, engineers, technicians, and students who made the project possible."

The original Omega laser fires multi-trillion watt bursts of energy—more powerful than the entire electrical generating capacity of the United States—making it among the three most powerful lasers in the world. Yet Omega will become approximately 50 times more powerful still with the inclusion of Omega EP. Such incredible intensities are necessary because creating electricity from fusion means heating the target fuel to a high temperature and confining it long enough so that more energy is released than is supplied to sustain the reaction. To release energy at a level required for electricity production, the fusion fuel must be heated to about 100 million degrees, more than six times hotter than the interior of the Sun.

Fusion, nuclear fission and solar energy, which includes biofuels, are widely seen as the only energy sources capable of satisfying the growing need for power for the next century without the harmful environmental impacts of fossil fuels. In a fusion power plant, one gallon of seawater would provide the equivalent energy of 300 gallons of gasoline; fuel from 50 cups of water contains the energy equivalent of two tons of coal. A fusion power plant would produce no climate-changing gases, as well as considerably less environmentally harmful radioactive byproducts than nuclear power plants currently do. And there would be no danger of a runaway reaction or core meltdown in a fusion power plant.

Beyond clean energy production, Omega and Omega EP will facilitate research impossible to attempt almost anywhere else on Earth. The way matter behaves in stars can be replicated on a small scale inside Omega's target chamber. Laser and materials technologies, electro-optics, and plasma physics will also be able to be studied under conditions never before possible.

Source: University of Rochester

Tuesday, May 20, 2008

Femtosecond laser delivers breakthrough performance

A Ti:Sa laser that emits sub-50 fs pulses at repetition rates of 10 GHz offers unique advantages for applications in spectroscopy.

Scientists in Germany and the US have built a passively modelocked Ti:Sa laser that ahieves an unbeatable combination of high bandwidth, high average power and repetition rates of up to 10 GHz. The laser, which delivers pulses of down to 42 fs, is intended to increase the signal-to-noise ratio of spectroscopic measurements that require a laser frequency comb, and could also be used in the development of optical clocks.

"The combination of high repetition rate and large fractional bandwidth is a measure for the merit of a frequency comb for spectroscopy purposes," said Albrecht Bartels of Gigaoptics, a German company that already markets femtosecond lasers operating at 1 GHz and 5 GHz. "The fractional bandwidth is typical for other Ti:Sa lasers, but very large compared with other 10 GHz sources — which usually deliver picosecond pulses."

The crucial advantage of such high repetition rates is that for the first time it allows the individual modes of the femtosecond laser — in other words, the "teeth" of the frequency comb — to be separated with a simple grating spectrometer.

"The spacing between the frequency comb modes depends on the repetition rate," explained Bartels. "Most applications of femtosecond laser frequency combs only require a single or a few specific modes out of the many available. Now we are able to isolate these modes and individually direct them to an experiment, while unwanted modes that only create additional noise are excluded."

According to Bartels, the laser supports around 500 modes, each separated by precisely 10 GHz. And because the spacing between the modes is larger than at lower repetition rates, the output power from the Ti:Sa laser is spread between fewer modes.

As a result, each mode delivers power levels of more than 1 mW, which is more than enough for most spectroscopic applications. "Some applications require only nanowatts per mode, but more power means more signal-to-noise ratio and thus quicker measurements."

The new laser design, which was unveiled in a post-deadline paper at the Conference for Lasers and Electro-Optics (CLEO) in May, was developed by Bartels in collaboration with researchers at the University of Konstanz in Germany and the US National Institute of Standards and Technology (NIST) in Boulder, Colorado. Bartels told that a commercial version of the laser is due to be launched within the next six months.

According to Bartels, the key parameter for achieving high repetition rates is the peak intracavity intensity, which is increased by tightly focusing the pump laser to a 10 µm spot within the Ti:Sa laser. The ring cavity design also supports higher repetition rates for a given number of cavity mirrors, in this case a minimum of four.

The use of a Ti:Sa crystal also ensures a broad gain bandwidth, as well as efficient pump light absorption and high gain over a short length — which is essential to achieve the 10 GHz repetition rate. The output wavelength of 783 nm is also useful for many applications.

"Most importantly, it matches the resonances of useful atomic systems, such as rubidium and caesium, which will allow the 10 GHz frequency comb to be locked to such atomic references for precision spectroscopy purposes," said Bartels.

The next stage, says Bartels, is for his co-workers at NIST to demonstrate the use of the laser in applications such as direct frequency comb spectroscopy, waveform generation, and astronomical spectrograph calibration. Further work on the mechanical packaging of the laser will also be needed before a commercial device can be released.

Friday, May 16, 2008

New technique measures ultrashort laser pulses at focus

Lasers that emit ultrashort pulses of light are used for numerous applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. But the quality of the results is limited by distortions caused by lenses and other optical components that are part of the experimental instrumentation.

To better understand the distortions, researchers at the Georgia Institute of Technology developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus. Measuring the pulse at the focus is important because that’s where the beam is most intense and where researchers typically utilize it. Knowing how the light is distorted allows researchers to correct for the aberrations by changing a lens or using a pulse shaper or compressor to manipulate the pulse into the desired form.

The device was described in a presentation at the Conference on Lasers and Electro-Optics on May 8. This research was funded by the National Science Foundation and published in the August 2007 issue of the journal Optics Express.

It is difficult to measure ultrashort pulses because they typically last between a few femtoseconds and a picosecond, which are 10-15 and 10-12 of a second, and faster than the response time of the fastest electronics. To achieve the highest possible intensity of the laser, the pulse must be as small as possible in space and as short as possible in time. However, focused pulses nearly always have distortions in time that vary significantly from point to point in space due to lens aberrations in focusing optics. To address those issues, the new device, called SEA TADPOLE (Spatial Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields), allows researchers to measure complicated ultrashort pulses simultaneously in space and time as they go through the focus.

The research team – which also included former graduate students Pablo Gabolde and Selcuk Akturk – used the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, were sent through optical fibers. The fibers were mounted on a scanning stage so that the pulses could be measured at many locations around the focus.

The pulses were crossed and an interference pattern was recorded for each color of the pulse at each location with a digital camera. The patterns were used to determine the shape of the unknown pulse in space and time and to create movies showing how the intensity and color of the pulse changed in space and time as it focused.

he researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, Bowlan performed simulations of pulses propagating through the experimental lenses. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.

Spherical aberrations occur when the light that strikes the edges of the lens gets focused to a different point than the light that strikes the center, creating a larger, inhomogeneous focused spot size. Chromatic aberrations occur because the many colors in the laser travel at different speeds and do not stay together in space and time as the pulse passes through glass components in the experimental setup, such as lenses. As a result, each color arrives at the focus at a different time, creating a rainbow of colors in the electric field images.

Aberrations can drastically increase the pulse length, which decreases the laser intensity. A lower intensity forces researchers to increase the power of the laser, increasing the possibility of damaging the sample. Aberrations can also yield odd pulse and beam shapes at the focus, which complicate the interpretation of the experiment or application.

Saturday, May 03, 2008

Faster than a Speeding Bubble

X-ray scattering images (above) and corresponding 3D depictions (below) of nucleation events, or "bubbles," forming in the semiconductor Indium Antimonide in the first instances after being hit with a laser pulse.

What do melting chocolate and bubbles in a champagne glass have in common? Besides being treats one might sample at a sophisticated soiree, they are both handy examples of first-order phase transitions in which a material transforms from one phase to another—that is, atoms changing from an orderly arrangement into a more chaotic arrangement.

Now, in an experiment led by Aaron Lindenberg, an international collaboration of scientists has uncovered new clues about the first instants of that process. The results are published in the April 4 edition of Physical Review Letters.

"We did not at all expect to see what we saw," said Lindenberg, "although in the aftermath we can go back and realize perhaps we should have. What's amazing about the process is that it spans such a huge range of time scales."

The process of melting, or in the case of champagne, of bubbling, has long been of interest to scientists. Phase transitions take place in the tiniest fraction of a second. In the case of Indium Antimonide (InSb), a semiconductor used by scientists to study such processes, the first steps in melting take a few hundred femtoseconds, a quadrillionth of a second. But no one knew what happened after that.

In the current study, the group used a laser to excite the sample and then measured the structure of the disordered liquid using X-rays, a technique called "pump-probe." Critical to the experiment is timing the initial laser used to pump the sample with energy, and the X-ray beam used to probe the results, to within mere femtoseconds. The resulting diffuse pattern of scattered X-rays from the disordered sample is used to map out where the atoms are at a given instant. Subsequent repeats of the pumping and probing at different relative delays between the laser and X-ray beam enables the researchers to reconstruct how the material evolves over time.

Lindenberg and colleagues found that the structure of the disordered liquid was far different from what one would have expected. Tiny atomic-scale bubbles, called nucleation events, form first and seed the process, a unique transient state of matter in which large fluctuations dominate the response of the material.

The group captured the process on a timescale 100 times shorter than any other previous X-ray study. The results give scientists a deeper understanding of how disordered materials behave on short timescales, and could lead to improved materials processing techniques, such as electronics manufacturing.

The current study also represents the last scientific paper to come from SLAC's Sub-Picosecond Pulse Source (SPPS) collaboration, led by Jerry Hastings, which was undertaken to study very fast atomic scale processes using ultra short pulses of X-rays. The work at SPPS presages the science to come from SLAC's Linac Coherent Light Source (LCLS), now under construction, which will create coherent X-ray laser pulses that are even shorter.

"SPPS was a remarkable success," said SSRL Director Jo Stohr. "It was great to see prominent X-ray scientists from all over the world coming to SLAC to participate in this unique experiment. It is an indication of what is yet to come with LCLS."

Source: by Brad Plummer, SLAC Today

Laser experiments offer insight into evolution of 'gas giants'

By shooting the high-energy Omega laser onto precompressed samples of planetary fluids, scientists are gaining a better understanding of the evolution and internal structure of Jupiter, Saturn and extrasolar giant planets.
The properties of dense helium (He) — which happens to be a principal constituent of giant gas planets like Jupiter — at thermodynamic conditions between those of condensed matter and high-temperature plasmas are theoretically challenging and unexplored experimentally.

Laboratory scientists collaborating with researchers at the Laboratory for Laser Energetics, CEA France and UC Berkeley were able to determine the equation of state (EOS) for fluid He at pressures above 100 GPa (one million times more pressure than the Earth’s atmosphere — one GPa (gigapascal) equals 10,000 atmospheres).

The only previous high temperature and pressure He EOS data available for constraining planetary models was performed at LLNL by Bill Nellis and his team using a two-stage gas gun. However, those earlier experiments used cryogenic techniques at ambient pressure so their densities were significantly lower than those achieved with the precompressed samples. Also, the final pressures, 16 GPa for a single shock, were significantly lower than the new laser shock data.

Theoretical research points out that material deep within a planet’s interior could exhibit unusual characteristics, such as high-temperature superconductivity, superfluidity and Wigner crystallization.

“The state of materials in the center of a giant planet are difficult to observe and challenging to create or predict,” said Gilbert Collins of the Physical Sciences Directorate. “Defining the equation of state of helium at these pressures is a first step to deepen our understanding of these massive objects.”

Jupiter is thought to contain matter to near 100 Mbar (100 million atmospheres of pressure.)

The LLNL team of Jon Eggert, Peter Celliers, Damien Hicks and Collins, together with several university collaborators from UC Berkeley, the Carnegie Geophysical Institute, CEA, Princeton, Washington State and the University of Michigan, plan to conduct experiments at the National Ignition Facility. There they will be able to recreate and characterize the core states of solar and extrasolar giants, as well as terrestrial planets, such as the recently discovered “superEarths,” to better understand the evolution of such planets throughout the universe.

Using the Omega laser at the Laboratory for Laser Energetics at the University of Rochester, the team launched strong shocks in He that was already compressed to an initial high state of pressure and density in a diamond anvil cell. Precompression allows researchers to tune the sample’s initial density and the final states that can be achieved with strong shocks.

Quartz was used as a reference material, allowing shock velocities to be determined just before and after the shock crossed the quartz-He interface. This technique reduced the measurement uncertainty as compared to previous studies.

“By applying a strong shock to a precompressed sample,” Collins said, “we can re-create the deep interior states of solar and extrasolar giant planets.”

The diamond anvil’s thickness determines the initial precompressed pressure. To prevent the sample from being heated before the shock, a preheat barrier was used to absorb the high-energy X-rays. An ultrafast diagnostic called VISAR (Velocity Interferometer System for Any Reflector), which works like a speedometer for shocks, recorded the shock velocity of the sample and reference material. From these data, the team determined the density and pressure of the shocked precompressed helium.

A pre-compressed helium sample is shown prior to shot in diamond anvil cell. The square is quartz reference, the circle is a gasket containing high-pressure fluid helium. After the shot, all that remains is a 2 mm hole in the target.

By applying laser-driven shocks to statically compressed samples, equation of state data for fluid He have been obtained with sufficient accuracy in the 100 GPa pressure range to test theoretical predictions.

They also discovered that near 100 GPa, the shock-compressed He transformed to an electronically conductive state and the shock front reflects the 532-nanometer probe laser beam of the VISAR.

The research also has other applications in the national security arena because the extreme conditions in a planet’s deep interior also occur during a nuclear weapon detonation. Plans are under way to significantly extend these research results with experiments at the National Ignition Facility.

The research appeared in the March 28 edition of Physical Review Letters.