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.

Higher energies for laser-accelerated particles possible

The use of compact laser accelerators for cancer therapy with charged particles such as protons could become possible in the future if scientists succeed in generating protons with very high energies. 

High-performance lasers are very promising and compact proton sources which could be used, for example, in future cancer therapies. With the DRACO laser, HZDR physicists demonstrate for the first time ever that protons are able to absorb energy very efficiently during the first acceleration phase.

Physicists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) directed the light of the high power laser DRACO perpendicularly and obliquely onto a thin metal foil; thus, permitting them to demonstrate for the first time that accelerated protons follow the direction of the laser light. By incorporating this new data into a conventional model describing the laser particle acceleration, high proton energies which have not been realized so far might become achievable. The results have been published in the scientific journal Nature Communications.

The intense and ultra-short light pulses of the high power laser DRACO can be thought of as disks of about 10 centimeters in diameter and being as thin as a normal sheet of paper. If one of these disks of light is focused onto a thin metal foil, the extreme high electric and magnetic forces will pull negatively charged electrons out of the foil. These electrons will then accelerate positively charged protons away from the foil's surface. To date, many experts have thought that commercially available laser systems would not be suitable for future cancer therapy applications because they have such short laser pulses, and the energy which is achieved by the accelerated protons is correspondingly too low. The results published by the HZDR group demonstrate for the first time that proton energies needed for cancer therapy could, in principle, also be generated from such a short pulse laser. This prospect motivated the Dresden researchers to study the particle acceleration process very closely.

New Two-Phase Model for Laser Accelerated Particles

A light pulse coming from the DRACO laser and directed perpendicular onto a thin metal foil accelerates electrons, and thus also protons, perpendicularly to the foil's surface, just like previous models predict. But that is not the case with a tilted laser pulse. If the angle of the thin light disk is slightly tilted with respect to the axis of propagation, something unexpected happens during the first phase of the particle acceleration. The electrons feel the rotation of the light disk and follow the direction in which the light hits the foil. Moreover, protons are accelerated along this direction as well and, in contrast to the electrons, maintain their direction. This novel observation of the directional dependence permits the Dresden physicists to also look directly at the underlying acceleration process.

"During the first acceleration phase, the distance between the electrons and the foil is extremely small. Once the short laser pulse has pushed them through the foil, they immediately swing back again because the foil has a positive charge. That is one reason why we were very surprised to discover that not only the electrons follow the motion of the laser light, but also the protons exhibit this previously unknown directional dependence," notes the doctoral candidate and main author of the current publication, Karl Zeil. He managed to detect another particular feature which only occurs with ultra-short laser pulses: The initial phase is decisive for the entire acceleration process. During the first 30 femtoseconds – that is, one millionth of one billionth of a second, and equal to the length of the laser pulse – the acceleration is very efficient. The short and efficient acceleration phase is followed by a longer expansion phase, during which a uniform and symmetrical plasma cloud is formed. The protons, however, gain so much energy during the first phase which, in turn, makes them so fast that they finally can reach higher energies than conventional models would predict.

Precisely how the fast electrons oscillate around the foil, and thus, accelerate the protons, is investigated by the HZDR scientists also with the help of simulations. Karl Zeil: "Experiments and simulations agree quite well with each other. With the newly obtained data we can now extend the presently existing models. This essentially means that ultra-short pulsed lasers like our DRACO laser could potentially be capable of generating protons with sufficiently high energy so that they can be used in future cancer therapy. That we were successful in obtaining these results is both very pleasing and very motivating."

DRACO Being Expanded, PENELOPE Newly Added

The DRACO laser currently reaches a peak power of 150 terawatts – this translates into the output of all power plants in the world – albeit only for a period of 30 femtoseconds at a time. The laser physicists at the Helmholtz-Zentrum Dresden-Rossendorf want to expand DRACO to 500 terawatts and are currently building a petawatt laser system called PENELOPE. As a modern accelerator technology, particle acceleration with laser light provides considerable advantages when compared to conventional systems: The acceleration distance is much shorter and the costs for such systems are potentially lower. Currently, the OncoRay center, which is jointly supported by the cooperation partners HZDR, University Hospital, and TU Dresden, is building a modern proton therapy facility on the University Hospital's campus. The new facility will be used for cancer research and therapy. For the first time ever, the prototype of a high performance laser will be operated here in addition to a conventional proton accelerator.