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Donald Umstadter of the University of Michigan's Center for Ultrafast Optical Science (734 764-2284) will report on advances at his lab and elsewhere in tabletop laser accelerators, devices that use light to accelerate beams of electrons and protons to energies of a million volts in distances of only millionths of a meter (microns).

This acceleration rate or "gradient" is up to a thousand times larger than in conventional accelerators, because the tabletop laser light can now exert pressures of gigabars, the highest ever achieved, approaching that of the Sun. Not only that, but Umstadter's lab has just shown that the brightness of a tabletop particle beam is roughly ten times higher than that produced by conventional accelerator technology.

This is because laser accelerators can generate very narrow particle beams by focusing light on an extremely tiny spot in a gas target. Additionally, researchers have demonstrated a thousand-fold improvement in repetition rate, which is how often bursts of electrons can be accelerated with these devices. Tabletop accelerators now have a repetition rate of 10 Hz (corresponding to 10 electron bursts per second), compared to previous tabletop acceleration rates of one burst per ten minutes.

A couple of caveats: laser accelerators produce particles over a spectrum of energies, and only a few particles are at the highest energies. But even the lower-energy particles are useful for applications, Umstadter says, and there are enough of them within a narrow energy range to extract a monochromatic beam.

In fact, researchers are now considering using the tabletop accelerator as an injector for coherent x-ray sources, such as the LCLS (Linac Coherent Light Source) facility proposed at the Stanford Linear Accelerator Center (SLAC). The natural shortness of the tabletop pulses makes it potentially possible to eliminate the usual requirement for magnetic beam compression, in which an elaborate series of magnets causes the charged particles of a conventional injector to travel different distances so that they pile up in time.

Preliminary experiments (from three different countries) indicate that when ultrashort light pulses (less than 40 quadrillionths of a second) are used, the electrons might be accelerated by a novel mechanism, in which the laser light directly accelerates the electrons rather than indirectly through oscillations of the plasma. Previous experiments were done with laser pulses that lasted greater than 400 quadrillionths of a second, or approximately ten times longer. Umstadter will review the year's many advances in this emerging discipline, which has been termed "high-field science." (Paper LR1.001)

Plasma researchers have made the first use of a technique for improving a major form of laser-induced nuclear fusion known as "direct drive." In direct-drive fusion, lasers from many directions deposit energy directly on a shell containing fusion fuel; the light causes the shell to implode and trigger fusion reactions. Traditionally, direct drive has suffered from serious limitations, mainly because non-uniformities in the laser light's intensity cause the shells to implode in a less than optimal fashion. At the University of Rochester's 60-beam OMEGA laser system, researchers (David D. Meyerhofer, 716-275-0255) have utilized a method, known as "polarization smoothing," for significantly improving the laser beam uniformity. In a large laser such as the ones at OMEGA, each beam typically has unavoidable spatial fluctuations in intensity. To reduce these intensity fluctuations, researchers split each beam into two parts, each containing complementary or "orthogonal" components of the beam's electric field. Each of the polarized beams fluctuates independently of the other, so overlapping them averages or smooths out such intensity modulations. When such beams were used to induce fusion reactions (with the fuel shell imploding to about 7% of its original radius or 1/3000 of its original volume) the primary neutron yield from deuterium or deuterium-tritium filled plastic shells increased by about 70% compared to similar implosions without polarization smoothing. The emission of neutrons is generally proportional to the fusion reaction rate. At the same time, the smoother beams increased the compressed shell's "areal density" (density times radius) by 40-70%. Maximizing the areal density is a major factor for eventually achieving self-sustaining fusion reactions with laser fusion because it increases the opportunity for alpha particles, created as a result of fusion reactions inside the shell, to deposit their energy and heat the plasma further. Theoretical models predict additional improvements. These results bode well for direct-drive implosions of targets on OMEGA and Livermore's planned National Ignition Facility. This work will be described in papers BI3.003, HO2.001-004 and H01.007-008.

Researchers have discovered a powerful tool for creating and manipulating coveted "internal transport barriers" which prevent unwanted heat leakage from magnetically confined fusion plasmas. At the Alcator C-Mod National Tokamak Facility, located at the MIT Plasma Science and Fusion Center, researchers are developing a technique known as "off-axis ion cyclotron radio frequency" (ICRF) heating. C-Mod is a tokamak, a doughnut-shaped device which uses magnetic fields to confine plasmas. Normally, ions in these plasmas circle around the magnetic fields at different rates; the ions' resulting "cyclotron frequencies" vary according to their positions with respect to the tokamak's many fields. And for reasons not completely understood, the overall plasma rotates around the tokamak. In traditional techniques for heating the plasma with radio waves, researchers send in waves with a frequency that matches the cyclotron frequency of ions at the center of the plasma. However, MIT researchers studied the effects of moving the resonance location for the ICRF heating; in other words, they applied a radio frequency that matched the cyclotron frequency of ions at a location elsewhere in the plasma. When this resonance location was moved sufficiently far away from the center of the plasma, the overall rotation of the plasma was significantly slowed, or even reversed, and simultaneously with this change, a clear internal transport barrier developed, resulting in an extraordinary peaking of the plasma density, one that was at least two times greater than before. Internal transport barriers have been created before, but they often require the introduction of neutral atom beams which could not be feasibly placed in the designs envisioned for commercial fusion reactors. This new approach of creating internal barriers could prove to be extremely important, as it is potentially attractive for reactor applications. Paper HI2.001 - contact Catherine Fiore, 617-253-8440; John Rice, 617-253-5395.

Recent experiments in Germany and the United States have shown that fusion reaction rates and other properties in magnetically confined plasmas can be significantly improved by a relatively small amount of microwave power, applied at precisely the right location in the plasma. Tokamak plasmas and indeed most magnetically trapped plasmas are subject to the growth of "magnetic islands." These islands break up the smooth magnetic field surfaces that confine the plasma, leading to more rapid loss of heat from the plasma and making it more difficult to reach the high temperatures and pressures needed for nuclear fusion. Experiments first carried out in the ASDEX Upgrade tokamak (Max- Planck Institute, Garching, Germany) and, more recently, in the DIII-D tokamak (General Atomics, San Diego, CA) have confirmed theoretical predictions that islands due to high plasma pressure can be eliminated by adding a small amount of added electrical current at the island location. A narrow beam of microwaves can drive the desired current, with surgical precision, by interacting with electrons at the appropriate location. In experiments to be reported at the meeting, a magnetic island degraded the plasma pressure by about 20%. Adding one megawatt of microwave power, about one-tenth of the total power needed to heat the plasma, drove enough current to suppress the island. This allowed the plasma pressure to recover, resulting in a 35% increase in the fusion reaction rate at DIII-D. These pioneering experiments show the feasibility of improving the performance of fusion plasmas by small, precisely controlled modifications of their internal structure. Similar experiments have been carried out in tokamaks in the UK and Japan.

Paper GI1.001 - Hartmut Zohm
Max Planck Institut fur Plasmaphysik
011-49-89-3299-1925

Robert La Haye
General Atomics
(858) 455-3134

Temperature measurements show an island structure in a magnetically confined plasma (left, center of figure). After microwave "surgery" (right) the island has been eliminated. (ASDEX Upgrade tokamak, Germany). A temperature of 1000 electron volts is 11 million degrees centigrade.

Researchers are investigating an approach that offers the possibility of creating fusion energy in a small, inexpensive device. Known as Magnetized Target Fusion (MTF), the approach can potentially be developed on a short time scale because of its low cost. The MTF technique preheats and injects magnetized fusion fuel into an aluminum cylinder the size of a large beer can. Then the "beer can" is rapidly compressed by driving a giant electrical current along the wall of the cylinder. The compressed high-density plasma fuel burns in a few millionths of a second. The fast-moving solid metal wall, which compresses the fuel, has been developed for defense programs. The fuel-compression region implodes at pressures millions of times greater than that of the Earth's atmosphere. The process is analogous to that of a diesel engine, which compresses fuel to conditions where it more readily burns. The essential advantage of MTF is its potential to be tested for scientific feasibility and even developed up to the prototype stage using apparatus that costs a fraction of conventional approaches. Last fall, several components of MTF technology were demonstrated. Los Alamos, in collaboration with the Air Force Research Laboratory, now leads a project to develop the preheated plasma needed for MTF. Researchers subsequently hope to conduct an experiment that will test this preheated plasma along with components of the implosion system. Papers GP1.074, MP1.119, MP1.120, others - Contact Glen Wurden, Los Alamos (505-667-5633), Richard E. Siemon, Los Alamos; see http://fusionenergy.lanl.gov).

Hector Baldis of Livermore (925-422-0101) will show that plasmas can focus high-density, high-energy (30 GeV) electron and positron beams 1000 times better than the magnetic quadrupoles used in conventional accelerator technology. In the E150 experiment (http://www.slac.stanford.edu/exp/e150/) carried out at the SLAC Final Focus Test beam, a plasma could focus an electron beam to one third of its original diameter in just 2 centimeters. In addition, the researchers demonstrated plasma focusing of high-energy positron beams for the first time. Technologies have existed for focusing MeV electron beams, but not for the GeV beams that will be used in future accelerator experiments. This work demonstrates a potentially promising technique for focusing those GeV beams. The plasma's focusing effect was anticipated in earlier theoretical and experimental research, but not demonstrated until now. How does a plasma focus particle beams so well? To understand this effect, it is important to realize that electrons, or other electrically charged particles, in a beam experience two competing forces: a repulsive "Coulomb" force which tries to make the beam blow apart, and magnetic forces which push the electrons together. As it passes through a plasma, the high energy beam will redistribute the electrons so that the net Coulomb force is decreased but the magnetic force is not affected; this serves to pinch the beam closer together. Conventional plasmas seem to focus the beams very well; no exotic plasmas must be prepared. (Paper BO2.002)

Researchers will present some of the first physics results from the National Spherical Torus Experiment (NSTX), the new magnetic fusion device at the Princeton Plasma Physics Laboratory. It is called a "spherical torus" (ST) because the surface of the plasma in it is shaped like a sphere with a narrow hole through the center. To maintain plasma confinement in an ST and to help heat the plasma, a strong electric current, encircling the central hole, must be driven in the plasma. In December 1999, NSTX reached a primary design goal by operating with one million amperes of current induced in the plasma by a solenoid (a spool-shaped coil) passing through the central hole. In addition to this traditional way of driving the plasma current, the researchers are developing a new method for producing this current. Known as coaxial helicity injection (CHI), this technique involves injecting an electric current directly from coaxial circular electrodes inside the plasma chamber, in the presence of an applied magnetic field. A picture of the plasma inside NSTX during CHI is shown in Fig. 1.

The magnetic field causes the injected current to wrap many times around central column in its passage between the electrodes, so the current can be many times that injected, as shown in Fig. 2.

The current loops formed during CHI have similarities to the coronal loops seen on the sun's outer surface during solar flares. Just as in the solar corona, these loops can become unstable and relax to a lower energy state through a process known as magnetic reconnection. In the case of the ST, this lower energy state is one in which some of the current flows on field lines which close on themselves inside the vessel to form a confined plasma core. Whereas the traditional technique of inducing the current with a solenoid can only produce brief bursts of plasma current in an ST, the CHI technique holds promise for helping them to operate continuously, as needed for a future fusion reactor. The NSTX experiments build on earlier work by Prof. Tom Jarboe and his team at the University of Washington in the Helicity Injected Torus (HIT). Roger Raman, Dennis Mueller, and Dave Gates led the CHI experiments on NSTX. For details see paper BO1.004 by R. Raman and others. Contact Martin Peng, NSTX Program Director (1-609-243-2305) for information concerning the research program on the NSTX.

A movie of a plasma during coaxial helicity injection (CHI).

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A meeting that occurs every two years, the International Congress on Plasma Physics is organized under the authority of a 55-member International Advisory Committee and is also associated with IUPAP (International Union of Pure and Applied Physics).

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