Plasma Accelerators at the Energy Frontier and on Tabletops

Chandrashekhar Joshi, Thomas Katsouleas
2003 Physics today  
E xperiments using charged-particle accelerators have led to remarkable discoveries about the nature of fundamental particles and the behavior of nuclear matter. These breakthroughs have been made possible by dramatic advances in our understanding of the physics and technology of particle acceleration. 1 Accelerator beam energies increased exponentially-by an order of magnitude every decade-for half a century after the pioneering days of John Cockcroft and Ernest Lawrence in the early 1930s, as
more » ... the early 1930s, as established technologies were pushed to their limits and superseded by new ones (see figure 1) . The present state of the art for proton synchrotrons is the Large Hadron Colllider (LHC), under construction at CERN on the French-Swiss border. Its 7-TeV proton beams will, in effect, provide experiments with constituent quark energies on the order of 1 TeV. Looking further ahead, particle physicists are already planning for an electron-positron collider with 250-GeV beams, a neutrino factory, and even a TeV muon collider. These energies represent the so-called energy frontier, where particle physicists confidently expect to discover fundamentally new phenomena. Although the accomplishments of accelerator-based physics have been spectacular, there is much more to do. The accelerators now in operation or contemplated are expected to lead the way beyond the present manifestly incomplete standard model of particle physics. In particular, they should unearth new classes of particles and enhance our understanding of the asymmetry between matter and antimatter, the masses of the quarks and fundamental leptons, and the transition to the primordial quark-gluon plasma. We must ask, however, whether the rapid pace of discovery can continue without further breakthroughs in accelerator technology. Irrespective of accelerator topology or the types of particles being accelerated, the fact remains that high-energy accelerators that rely on radio-frequency technology are simply getting too big and too expensive. (See the article by Maury Tigner in PHYSICS TODAY, January 2001, page 36.) Is there a different paradigm for building particle accelerators at the energy frontier while dramatically reducing their size-and hopefully their cost? The impact that much smaller low-energy particle accelerators have had on other branches of science and technology has been equally impressive. Accelerators are being used, among other applications, for materials science, structural biology, nuclear medicine, fusion research, food sterilization, transmutation of nuclear waste, and cancer therapy. The requirements for such machines are very different from those of high-energy accelerators, and they vary from one application to another. Nonetheless, many low-energy applications would benefit from extremely compact "tabletop accelerators" that could provide beams of GeV electrons, protons, or ions with energies of a few hundred MeV per unit charge. In this article, we survey new approaches to chargedparticle acceleration by collective fields in plasmas. These approaches show considerable promise for realizing plasma accelerators at the energy frontier as well as tabletop electron and ion accelerators. The plasmas would not only provide unprecedentedly high acceleration gradients; they would also serve to focus the accelerated beams down to very small spot sizes. Instead of using RF waves to accelerate charged particles, as conventional accelerators do, plasma accelerators use plasma-oscillation waves excited by lasers or by "driver beams" of charged-particles. The accelerating gradients and focusing strengths that have been demonstrated in plasma experiments have been orders of magnitude greater than those achieved thus far by rf accelerators. The greater the accelerating gradient-if it can be maintained over sufficient distance-the shorter would be the accelerator required to reach a given energy. The impressive plasma-acceleration results already demonstrated in experimental structures raise hopes that this revolutionary technology may miniaturize future accelerators in the same way that semiconductor processors miniaturized electronics. In addition to the customary "Livingston curve" that charts the progress of working particle physics accelerators over the decades, figure 1 also indicates the highest energies achieved in various plasma acceleration experiments over the past 10 years. Admittedly, plasma schemes have a long way to go before they can produce beams with sufficiently high intensity and low energy spread and "emittance" (that is, angular spread) to be useful for doing high-energy physics. Nevertheless, one can see from figure 1 that the peak energy gain in plasma experiments has been increasing by an order of magnitude every five years. We describe here the ideas and developments that have led
doi:10.1063/1.1595054 fatcat:6vs5hbniwrbmtbmkofplfrz3lu