BELLA laser achieves world record power at one pulse per second

 On the night of July 20, 2012, the laser system of the Berkeley Lab Laser Accelerator (BELLA), which is nearing completion at the Lawrence Berkeley National Laboratory, delivered a petawatt of power in a pulse just 40 femtoseconds long at a pulse rate of one hertz – one pulse every second. A petawatt is 10^15 watts, a quadrillion watts, and a femtosecond is 10^-15 second, a quadrillionth of a second. No other laser system has achieved this peak power at this rapid pulse rate.

"This represents a new world record," said Wim Leemans of Berkeley Lab's Accelerator and Fusion Research Division (AFRD) when announcing the late-night success to his team. Leemans heads AFRD's Lasers and Optical Accelerator Systems Integrated Studies program (LOASIS) and conceived BELLA in 2006.

"My congratulations to the BELLA team for this early mark of success," said Berkeley Lab Director Paul Alivisatos. "This is encouraging progress toward a future generation of smaller and far more efficient accelerators to maintain our nation's leadership in the tools of basic science."

"Congratulations to all of you on this spectacular achievement," said Stephen Gourlay, Director of AFRD. "It doesn't seem that long ago that BELLA was just a dream, and now there is even more to look forward to. Thank you all for the hard work and support that made this a reality."

Leemans says, "BELLA will be an exceptional tool for advancing the physics of laser and matter interactions. The laser's peak power will give us access to new regimes, such as developing compact particle accelerators for high-energy physics, and tabletop free electron lasers for investigating materials and biological systems. As we investigate these new regimes, the laser's repetition rate of one pulse per second will allow us to do 'science with error bars' – repeated experiments within a reasonable time."

The BELLA design draws on years of laser plasma accelerator research conducted by LOASIS. Unlike conventional accelerators that use modulated electric fields to accelerate charged particles such as protons and electrons, laser plasma accelerators generate waves of electron density that move through a plasma, using laser beams to either heat and drill through a plume of gas or driving through plasma enclosed in a thin capillary in a crystalline block like sapphire. The waves trap some of the plasma's free electrons and accelerate them to very high energies within very short lengths, as if the accelerated electrons were surfing on the near-light-speed wave.

LOASIS reported its first high-quality electron beams of 100 million electron volts (100 MeV) in 2004 and the first beams of a billion electron volts (1 GeV) in 2006 – in a sapphire block just 3.3 centimeters long. Planning for BELLA began shortly thereafter.

The BELLA laser is expected to drive what will be the first laser plasma accelerator to produce a beam of electrons with an energy of 10 billion electron volts (10 GeV). Before being converted to other uses, the Stanford Linear Accelerator Center achieved 50 GeV electron beams with traditional technology, but required a linear accelerator two miles long to do it. By contrast, the BELLA accelerator is just one meter long, supported by its laser system in an adjacent room.

"LOASIS know-how in assembling our own laser systems allowed us to specify the laser requirements and specifications we'd need to achieve reliable, stable, tunable 10 GeV beams with short warm-up time," Leemans says. "U.S. Secretary of Energy Steven Chu said that new tools lead to new science, the kind BELLA is specifically designed to do. "

The BELLA laser system has already demonstrated compressed output energy of 42.4 joules in about 40 femtoseconds at 1 Hz. Its initial peak power of one petawatt is twice that of lasers recently said to produce pulses more powerful than that consumed by the entire U.S. "at any instant in time." "Instant" is the operative word, since the BELLA laser's average power is just 42.4 watts, about what a typical household light bulb uses. The enormous peak power results from compressing that modest average power into an extremely short pulse.

Developed by Thales of France, whose team at Berkeley Lab was led by Francois Lureau, and installed in facilities constructed at Berkeley Lab, the BELLA laser system is fully integrated with Berkeley Lab equipment and personnel protection systems. It is expected to rapidly improve upon its first record-breaking performance and will soon be able to deliver the powerful pulses needed to create 10-billion-electron-volt electron beams in an accelerator just one meter long. Experiments to demonstrate BELLA's ability to attain 10-GeV beams will begin this fall.

Electron self-injection into an evolving plasma bubble

Just five years ago, experimentalists finally demonstrated that such laser-plasma accelerators could produce monoenergetic, collimated electron beams with quality comparable to conventional accelerators. The secret was for the laser to produce a "bubble" almost completely devoid of electrons in its immediate wake that captured electrons from the surrounding plasma and accelerated them in an exceptionally uniform way. Yet the precise mechanism by which the bubble captured these electrons and accelerated them with such uniformity has remained one of the outstanding mysteries of this field.

Now new theoretical work by scientists from the University of Texas and Commissariat à l'Énergie Atomique (CEA, France), to be reported at the 2009 APS Division of Plasma Physics Annual Meeting, has shed light on this mystery. Formation of the exceptional quality electron beam is attributed to the evolution of the bubble shape which, in turn, is directly associated with the nonlinear evolution of the driving laser pulse (nonlinear focusing and defocusing).

The basic premise of this work is that the size of the bubble—the cavity of electron density traveling over the positive ion background with nearly the speed of light—is determined by the spot size of the driving laser pulse. Plasma nonlinearities cause the laser to focus and defocus in the course of propagation. Once the laser diffracts, the bubble expands. Electrons that constitute a dense electron shell surrounding the bubble move with relativistic speeds and thus have high inertia. As a consequence, some of them become too heavy to follow the expanding shell; they fall inside the bubble, stay inside till the end of the plasma (i.e. get trapped) and finally gain multi-GeV energy.

The trapped charge is proportional to the bubble growth rate. Once the laser becomes self-guided, and the spot size oscillations saturate, the injection process clamps. Simultaneously, longitudinal non-uniformity of the accelerating gradient equalizes the trapped electron energy. This scenario of self-injection and monoenergetic bunch formation is discovered and explored in fine detail in the 3-D particle-in-cell simulations. This is fundamentally different from the previous work which concentrated on either one-dimensional models of electron trapping or on the reduced description of transverse plasma wave breaking in planar 2-D geometry.

The discussed mechanism of electron self-injection is very robust in experiments with the high-power laser (tens of terawatts to petawatt). In addition, an appropriate modification of the plasma density (e.g. using a thin dense slab as a nonlinear lens for the laser) may cause the laser to self-focus and defocus faster, which results in a single self-injection event. This kind of laser beam manipulation may lead to the generation of a 2.5 GeV mono-energetic (~1% energy spread) electron bunch containing ~1010 electrons in a future experiment with the recently commissioned Texas Petawatt (TPW) laser - the most powerful laser in the world. Electrons with 2.5 GeV of energy are traveling at 99.999998% of the speed of light. Electron beams with such unique properties are clearly beneficial for medical applications, radiation physics, material science, and homeland security.
Source: American Physical Society

Laser recreates X-rays emitted by a black hole

Physicists have used high-power lasers to recreate X-ray spectra emanating from some black holes and neutron stars. Conclusions drawn from the experiment appear to conflict with previous interpretations of astronomical data, suggesting that we may have to rethink our view of the structure surrounding black holes and neutrons stars.

Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.

Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a "photo-ionized" plasma here on Earth because it requires an extremely hot source of radiation.

But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.

The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.

"We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively]," explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.

The beams are fired at a tiny plastic capsule, causing it to implode. "As it shrinks, a hot and dense plasma core forms inside the capsule," says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.

Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.

Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions. However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.

The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having "great potential for further development," because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.

Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.

Europe's big lasers: the exawatt roadmap

Europe is thinking big – big lasers, big science, big budgets. Over the next decade, a trio of planned pan-European research facilities will give scientists access to unprecedented laser powers and intensities, opening the door to exotic science that will shed light on the origins of the universe and, it is hoped, provide the foundations for a sustainable energy future.

The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.

"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.

Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (10^15 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."

This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)

Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 10^18 W) and intensities (10^24 W/cm2).

Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10^–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).

The European X-ray Free Electron Laser (European XFEL) is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.

DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."

Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

Big lasers, big science, big questions

Leading optical scientists agree that research and industry stakeholders need to do more if Europe is to maximize the benefits from a planned new generation of high-power laser facilities. That was one of the headline messages from the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the SPIE Europe conference in Prague, Czech Republic, last week.

Marking 50 years since the invention of the laser, the workshop was intended to open debate among senior figures from planned pan-European petawatt laser facilties (1015 W and beyond). Among the "blue-ribbon" initiatives under discussion were projects like HiPER (the High Power Laser Energy Research project), ELI (the Extreme Light Infrastructure), and the European X-Ray Laser project (XFEL).

"International infrastructures attract the best research scientists," Christian Kurrer, research programme officer at the European Commission, told delegates. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

With access to unprecedented laser power and scientific expertise, it is easy to see why large-scale science facilities are attractive to users. In fact, some might argue that they are too good and that they will pull in users (and resources) simply because they can guarantee results where smaller national institutions can't. "Industries want facilities for reproducible, reliable results and 100% service," was the opinion of Mike Dunne, HiPER project director.

At the same time, workshop participants agreed that there's plenty of work to do to ensure that stakeholders in research and industry are in position to maximize their interactions with "big science". "They [the laser facilities] have the scientific experts and we bring the industrial methods where the networks can really make a difference," said Federico Canova of Amplitude Technologies, a French laser manufacturer.

New European Union member states might also question the economic returns on their investment in big science, not least because the planned locations for all of these big laser facilities are in western Europe. Kurrer, however, prefers to view such challenges as opportunities. "While distribution [of projects] may never be good, the key will be to break down the borders. Europe is all about talking to each other and overcoming barriers."

Europe's new generation of high-energy laser facilities form part of an ambitious big-science roadmap coordinated by the European Strategy Forum on Research Infrastructures (ESFRI). The ESFRI roadmap covers capital and operational investments running to tens of billions of euros in strategic research areas like energy, environmental science and advanced materials.

World's largest laser gears up for ignition experiments

(PhysOrg.com) -- Construction of the National Ignition Facility (NIF), the world's largest and highest-energy laser system, was essentially completed on Feb. 26, when technicians at Lawrence Livermore National Laboratory (LLNL), where the laser is located, fired the first full system shot to the center of the NIF target chamber.

The test was the first time all 192 laser beams converged simultaneously in the 10-meter-diameter chamber. NIF has met all of its project completion criteria except for official certification of project completion by the U.S. Department of Energy, due by March 31.

An average of 420 joules of ultraviolet laser energy, known as 3-omega, was achieved for each beamline, for a total energy of more than 80 kilojoules (a joule is the energy needed to lift a small apple one meter against the Earth's gravity).

The energy level will be increased during the next several months, and when all NIF lasers are fired at full energy, they will deliver 1.8 megajoules of ultraviolet energy to a BB-sized target in a 20-nanosecond shaped laser pulse, generating 500 trillion watts of peak power -- more than the peak electrical generating power of the entire United States. This is considered more than enough energy to fuse the hydrogen isotopes of deuterium and tritium in the target into helium nuclei (alpha particles) and yield considerably more energy in the process than was required to initiate the reaction.

The last of NIF's 6,206 various optical-mechanical and controls system modules, called "line replaceable units" or LRUs, was installed on Jan. 26. The first LRU, a flashlamp, was installed on Sept. 26, 2001.

Workers have aligned and tuned NIF's final optical assemblies, which focus and convert the frequency of the project's 192 laser beams as they enter the target chamber and converge on the tiny target. Experimental systems and diagnostics are also being installed. Software for the integrated computer control system, which handles shot automation, has been completed.

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

Petawatt Power Peak Reached

The Texas Petawatt laser produced a petawatt of peak power on March 31, making it the highest powered laser in the world, said Todd Ditmire, a physicist at the University of Texas at Austin.

There has only been one petawatt laser in the US history, the Nova laser at Lawrence Livermore Laboratory (LLNL, operated by the University of California for the energy department). Nova, which took up a football field in space, is now defunct. In the past eight or so years, there has been a worldwide push to achieve petawatts (10 to the 15th power). Terawatts (10 to the 12th power) were produced by short pulse lasers in the late 1980s using chirped pulse amplification, the method Ditmire is using.

Other US petawatt projects include the OmegaEP laser at the University of Rochester, The Ohio State University petawatt, and the Z-Beamlet project at the Sandia National Labs Z-Petawatt Laser Facility. Projects are also underway in the UK, France, Germany, Japan, China, and other countries.

The challenge for researchers is to produce a lot of energy in a little time, and a petwatt can be the result if enough energy can be produced in a short enough pulse. The Hercules laser at the University of Michigan, for example, is only 0.3 petawatts, but it focuses to an incredibly tiny spot. For sheer power -- energy divided by pulse duration -- the Texas petawatt laser now leads the way in the US.

The laser produces a very short duration, very low-energy pulse, and this pulse is stretched in time to a very long pulse, is amplified to huge energy, then finally is compressed to a high-energy, super-short-duration pulse. One of the critical aspects of the system is the diffraction gratings used to compress the pulse; these were made by Jerry Britten's group at LLNL, and they are some of the most difficult-to-manufacture optics in the world.

Related Link: Texas High Intensity Laser

via: photonics.com

Petawatt laser approaches diffraction limit

A French team has combined adaptive optics (AO) with an elaborate alignment system to effectively correct wavefront aberrations in a high peak-power laser, achieving focal spots close to the diffraction limit. "The optimization procedure produces a considerable improvement in focal spot quality with a Strehl ratio of 0.7 for full-energy kilojoule shots," Ji-Ping Zou of the LULI laboratory told optics.org. "The procedure, once integrated into our control system, is straightforward and there are no operational penalties." (Applied Optics 47 704.)

Spatial phase and focal spot measurements using a low-energy pulsed probe before the fourth shot of a kJ shot sequence (5 shots, one shot every hour): a) and b): before and after the closed-loop convergence. c) Focal spot measurement during the fourth shot.

The LULI (Laboratoire pour l'Ulilisation des Lasers Intenses) laser delivers kilojoule pulses in the nanosecond range at 1053 nm, and is capable of reaching the petawatt regime through chirped pulse amplification.

The first category of aberrations is minimized by precise beam realignment between two successive shots, combined with a closed-loop AO system employing a bimorph deformable mirror with 32 actuators. An additional semi-automatic realignment of beam pointing and centring between shots controls the second category, while the AO system tackles the third group. The right is the schematic of the four amplification stages of the LULI2000. A bimorph deformable mirror is implemented between the second and the third stages. A wavefront sensor is positioned at the chain output.

The result has been reproducible focal spots close to the diffraction limit for full-energy kilojoule shots fired at one shot per hour. Zou's group has achieved a focal spot with a Strehl ratio - a measure of the fractional drop in light intensity as a function of wavefront error - of 0.7. The focal intensity can therefore reach 2.2 x 10¹⁸ W/cm² in the kilojoule per nanosecond range, and intensities as high as 10²¹ W/cm² are foreseen by Zou. Shot-to-shot reproducibility of the focal spot is said to be excellent, which is very important for laser-matter interaction experiments.

via Optics.org

Surface heating of wire plasmas using laser-irradiated cone geometries

It's reported on the recent issue of Nature Physics.

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2–4 keV. The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MégaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.

LSP modelling of the azimuthal magnetic field structure at the cone tip, 600 fs after the main interaction. A reversed field can be seen on the inside of the wire surface corresponding to the ohmic return current, which is shown on the right picture.

Laser-generated radiation for cancer therapy?

Will laser-generated radiation one day prove useful in cancer therapy? A UK collaboration aims to find out. Reported from oprtics.org:

A consortium of UK-based scientists has secured £5 million ($9.9 million) in research funding to turn the concept of laser-generated radiation into a robust, ready-to-go technology. The four-year project, involving researchers from nine separate institutions, could lead to cheaper, simpler solutions for proton and ion radiotherapy. Laser-energized radiation sources could also cut the cost of research into cosmic-radiation exposure from frequent air travel and manned space missions.

The project named LIBRA, which means Laser Induced Beams of Radiation and their Applications. Development of the technology will require access to a very high-powered laser with a rapid-fire repetition rate. One such system is the GEMINI laser, due to come on-line at the Rutherford Appleton Laboratory (RAL) near Oxford, UK, later this year. GEMINI is expected to deliver 1 PW (10^15 W) pulses every 20 seconds.

related links: LIBRA, GEMINI

PHOTONIC FRONTIERS: PETAWATT LASERS

Squeezing energetic laser pulses down to ultrashort durations can generate tremendous peak powers. A decade ago the Lawrence Livermore National Laboratory (LLNL; Livermore, CA) blazed the trail by modifying one arm of its Nova fusion laser to create the Petawatt Laser, which delivered pulses exceeding 1015 W (1 PW). Now some 20 petawatt lasers are in operation or development around the world, and European planners are aiming for an exowatt (1018 W) laser (see table).

Some major petawatt laser projects

Name	Site	Timetable	Parameters	Web site Link
Advanced Radiographic Capability	Livermore	2009	10 kJ, 10 ps
Extreme Light Infrastructure	Laboratoire d’Optique Appliquée, France	Proposal	10 kJ, 10 fs	ELI
Firex-1	ILE, Osaka, Japan	Under construction	10 kJ, 10 ps
GEKKO Petawatt Module	ILE, Osaka, Japan	In operation	500 J, 500 fs
Laser Megajoule	University of Bordeaux	Proposal	2 MJ, 300 ps-10 ns	lmj
LULI 2000	LULI, Paris	Under construction; completion 2006	200 J, 400 fs
Omega EP	University of Rochester	2007	2.6 kJ, 1 ps	omegaep
Petawatt Laser (original)	Livermore	1996-1999	1.3 kJ, 800 fs	MPerry
Phelix	GSI Darmstadt, Germany	Under construction, with heavy-ion beam	500 J, <500 fs	phelix
Polaris	University of Jena, Germany	Development	120 J, 120 fs	ultraphotonics
Texas Petawatt Laser	University of Texas, Austin	Late 2007	130 J, 150 fs	petawatt
Titan	Livermore	In operation	400 J, 400 fs or long-pulse	JLF
Vulcan Petawatt	Rutherford Appleton Lab, UK	In operation	400 J, 400 fs	vulcan
Z-beamlet	Sandia National Laboratory	Under construction	2 kJ, 1-10 ps ultimately	z-beamlet

The first petawatt laser

Livermore’s Petawatt Laser used a chain of Nd:glass lasers from one beam of the Nova fusion laser to amplify nanosecond pulses to the kilojoule range. Pulses were expanded and compressed with high-efficiency 75 cm gratings. Amplifier output of 1.3 kJ in an 800 ps pulse could be compressed down to a 430 fs pulse with peak power of 1.3 PW, which in turn could produce power density approaching 1021 W/cm2. The system generated its first petawatt pulse on May 23, 1996, and ran for three years until Nova was dissembled in 1999.

The Livermore experiments demonstrated the potential of petawatt lasers to concentrate tremendous energies into small volumes, opening a new regime of high-temperature and high-pressure matter for study. The intense fields could accelerate both electrons and positive ions to high velocities over short distances (see www.laserfocusworld.com/articles/252490). Experiments generated bright beams of high-energy x-rays and gamma rays. And Livermore also showed that firing petawatt lasers into a laser-heated fusion target produced a powerful shock wave that helped ignite the fusion fuel.

Second-generation petawatt lasers

The second generation of petawatt lasers is already operating. The Rutherford Appleton Laboratory (Didcot, England) uses a Ti:sapphire oscillator and an optical parametric amplifier to preamplify pulses which then pass through a beam of the lab’s Vulcan Nd:glass laser, and three additional 208 mm Nd:glass disks salvaged from Nova (see Fig. 2). Commissioned in 2002, it initially produced 800 fs pulses with peak power of 500 TW. Further refinements ramped up power, which reached the petawatt level in October 2004, delivering 423 J onto the target in a 410 fs pulse.

Livermore has built a second-generation petawatt laser called Titan around the old two-beam Janus Nd:glass laser used in fusion target experiments back in 1975, says Andrew Ng of Lawrence Livermore National Laboratory (LLNL). Overhauled with better glass, the system has two independent beam lines for chirped-pulse amplification and a new generation of pulse-compression gratings. The first experiments in June 2005 generated 400 J in 400 fs to reach petawatt peak power focusable onto an 8 µm spot. It also can operate in long-pulse mode, generating 1 kJ in less than 3 ns or 140 J in 250 ps. Titan can fire long and short pulses simultaneously from its two arms. Ng says that firing long pulses to create a plasma and short pulses to probe the plasma is a very effective way to study high-energy states.

Livermore is also planning a big step up in energy with a second long-pulse system for use with the National Ignition Facility. Called the Advanced Radiographic Capability, it initially will fire 1 kJ pulses to record multiframe x-ray movies of NIF targets, says lead scientist Chris Barty of LLNL. By combining four NIF beams, he hopes to generate 13.2 kJ in a 10 ps pulse. The first beamline is to be commissioned in spring 2009.

Most other systems in operation, construction, or planning stages are either long-pulse systems based on Nd:glass or Ti:sapphire systems generating pulses as short as 20 fs. The main exception is the $15 million Texas Petawatt Laser, which will use parametric amplification to raise the 1 J output of a Ti:sapphire oscillator to 250 J, which they hope to deliver in 150 fs pulses. Project director Todd Ditmire hopes to produce his first petawatt pulses late in 2007.

Laser Focus World August, 2006
Author: Jeff Hecht