Plasma-based Particle Accelerators

Professor Chandrasekhar Joshi, Director
NEPTUNE Laboratory for Advanced Accelerator Research

In the highly sophisticated technological world that we live in today, examples of paradigm shifting breakthroughs that revolutionize entire fields by introducing truly substantial benefits are relatively infrequent. More than two decades of painstaking work by Professor Chand Joshi and his colleagues, is on the threshold of introducing such a paradigm shift in the extremely successful yet mature field of high energy particle accelerators. Accelerator based experiments have produced many key breakthroughs in our understanding of the physical world in the past 50 years. However, machines needed to explore the so-called Terascale Physics are extremely large, costly and time-consuming to build. Professor Joshi's research promises to miniaturize these gargantuan machines much in the same way as the introduction of integrated circuits miniaturized vacuum electronic devices.

The NEPTUNE Laboratory for Advanced Accelerator Research is a flagship research laboratory funded by the U.S. Department of energy for exploring high-risk, highpayoff concepts for accelerating charged particles. At the outset it was recognized that developing an entirely new concept for charged particle acceleration, would be a multi-disciplinary endeavor that would require a sustained research effort of several decades to bring to fruition. The Neptune Laboratory is therefore operated collaboratively by several faculty members from both the Electrical Engineering and Physics departments at UCLA.

The research being done brings together fields of high power lasers, plasma physics and microwave engineering. Starting from scratch in the late 1980s, the UCLA group has developed the Plasma-based particle acceleration scheme to a point where it has now become an internationally recognized field of research with many leaders of the field having received their training as students or postdocs at UCLA.

Particle accelerators are enormous in size because the microwave field that is used to accelerate charged particles can break down the walls of the accelerating structure as its intensity is increased. This limit is expressed as the accelerating gradient, which is typically 50 million volts per meter. In a total break with the present microwave based technology, UCLA researchers instead proposed to use the electric fields of a space charge density wave in plasma to accelerate particles. The phase velocity of the wave has to be at the speed of light so that the accelerating particles interact with the wave over a long distance and therefore gain a great deal of energy. The accelerating gradient of such a wave can be three orders of magnitude greater than that in a typical microwave powered structure.

This means that for the same final energy the accelerator can be a thousand times smaller. Experiments carried out in the Neptune laboratory show that intense laser pulses propagating through a plasma can excite plasma waves suitable for accelerating externally injected electrons. The laser used for this purpose is currently the highest power carbon dioxide laser in the U.S.. The electrons have to be pre-accelerated to relativistic energies (or moving close to the speed of light) so that they can "catch and surf the wave" and gain energy from it. The work sheds new light on wave-particle and wave-wave interaction in plasmas and gives stimulus to the birth of a new subfi eld "nonlinear-optics of plasmas". On a grander scale, the work being carried out in the Neptune Laboratory has spawned a much larger experimental effort on plasma particle acceleration that is being carried out at the Stanford Linear Accelerator Center (SLAC) by Professor Joshi's group in collaboration with groups from U.S.C. and SLAC. This work has recently led to the demonstration of energy doubling of 42 billion volt electrons from the SLAC linear accelerator in a plasma device less than one meter long.