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December 2000 (Volume 9, Number 11)
Lasers converge on a fusion target at the University of Rochester's OMEGA facility.
Advances in Tabletop Laser Accelerators
Donald Umstadter of the University of Michigan's Center for Ultrafast Optical Science reported 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 mere microns. Not only is this acceleration rate up to a thousand times larger than in conventional accelerators, but Umstadter's lab has just shown that the brightness of the tabletop particle beam is roughly ten times higher than that produced by conventional accelerator technology. This is because, in part, laser accelerators can produce extremely narrow particle beams. Another reason is that tabletop lasers can now exert light pressures of gigabars, the highest ever achieved, approaching that of the thermal pressure of the Sun. Umstadter and colleagues have also 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.
Researchers are now considering using such a tabletop device as an injector for coherent x-ray sources, such as the Linac Coherent Light Source facility proposed at the Stanford Linear Accelerator Center. 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 are used, the electrons might be accelerated by a novel mechanism, in which the laser light directly accelerates the electron oscillations of the plasma.
Improvements in "Direct Drive" Fusion
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 have used "polarization smoothing" to significantly improve 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, 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 by 40-70%. The results bode well for direct-drive implosions of targets on OMEGA and Livermore's planned National Ignition Facility.
Fusion in a Beer Can?
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.
The National Spherical Torus Experiment
Researchers presented some of the first physics results from the National Spherical Torus Experiment (NSTX), the new magnetic fusion device at the Princeton Plasma Physics Laboratory. In addition to the 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. 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.
The current loops formed during CHI have similarities to the coronal loops seen on the sun's outer surface during solar flares. 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 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.
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