So why haven't heavy ion accelerators been used to the same extent as neodymium-glass lasers in IFE research? Aside from the additional role of the lasers in defense research, a role that is not shared by heavy ion accelerators, the use of ion beams poses its own technical difficulties. High beam power is traditionally achieved with high currents. Obtaining adequate power requires total currents greater than 10,000 amperes. Induction linacs, linear accelerators that induce an electromotive force on ions by rapidly changing the strength of a magnetic field inside a cavity, have proven capable of producing the necessary current. However, transporting and focusing such intense beams is another story. Problems arise as a result of "space-charge" forces¬the mutually repulsive forces between so many positively charged ions.
Since the early 1980s, researchers with Berkeley Lab's Accelerator and Fusion Research Division (AFRD), have been demonstrating that heavy ion beams can be transported and focused at currents several times higher than was once believed possible in beams dominated by space-charge forces. After an initial effort working with a single high current beam, Berkeley researchers began accelerating and transporting a number of independently focused, less intense beams. The idea was that the combined energies of these multiple beams, when overlapped on a target, would be more effective than a single high energy beam. This work culminated in MBE-4, the world's first induction linac capable of accelerating and focusing four parallel beams simultaneously to an energy of nearly one million electron volts (1 MeV).
Deuterium and tritium are plentiful isotopes of hydrogen that can fuse to form helium and a neutron. The D and T reactants come in at 20,000 electron volts (20 KeV) of energy, and the He and n products exit at 3.5 and 14.1 million electron volts (MeV) respectively.
Figure courtesy Contemporary Physics Education Project
While impressive by the standards of what had been done in the past, this was still quite small by the standards of a commercial-scale driver. Now, in conjunction with the proposed experiments at NIF, the Berkeley Lab researchers, in collaboration with IFE researchers at LLNL and elsewhere, are ready to take their next step. They have proposed a new experiment to answer the question that NIF cannot: namely, can heavy ion beams be used to directly heat a fusion target?
This new proposal calls for the design and construction of an induction linac that would produce multiple beams of heavy ions (probably potassium because it can provide the needed data for a fraction of the cost of heavier ions) at 100 million electron volts and a current of several thousand amperes. The goal would be to deliver beams in pulses of about 10 nanoseconds that would pack from 3 to 30 kilojoules of energy. This is not enough to ignite a fusion fire, but it is enough to test the physics of directly heating a target with heavy ion beams. The proposal is essentially an update of one that was first described in 1981 by a physicist at Los Alamos National Laboratory named Roger Bangerter. He was at the time a strong believer in utilizing heavy ion beams as a driver for IFE.
"Given a long-established accelerator technology history that can guarantee repetition rate, efficiency, and reliability, accelerator-based inertial fusion should emerge as a fusion power system for the next century," Bangerter wrote.
Today, Bangerter heads the IFE program at Berkeley Lab, and his belief in fusion energy and the need for a heavy ion driver is strong as ever.
"As the world contemplates its dwindling fossil fuel supplies and the environmental costs of energy production, fusion looks ever more appealing," he says. "This accelerator we are proposing will enable us to evaluate the target physics for high-intensity heavy ion beams. Added to the data from the studies at NIF, we should emerge with a clear idea as to the best way to develop fusion energy technology."
Bangerter and other IFE experts have acknowledged that the technical challenges of heavy ion drivers probably do not pose as large an obstacle as does holding down the costs. Thanks to advances in accelerator technology, many made during the past 20 years of the IFE program at Berkeley, Bangerter and his AFRD colleagues believe they can build their new accelerator for $150 million. This represents roughly a 10-percent increase above the federal investment proposed for NIF¬a relatively small increase in expenditures for a disproportionately large payoff in essential technical information.
According to AFRD accelerator designer Andy Faltens, among the top cost-cutting technological improvements are better electrostatic and magnetic beam-focusing systems, new techniques for beam manipulation, and the use of improved ferromagnetic and other more cost-efficient materials.
"While all of the ingredients of an accelerator will continue to evolve, enough time has passed to see tangible improvements," Faltens says. "Our new design will be able to reach many more joules of energy per dollar than ever before."
If the U.S. Department of Energy approves the new proposal, Bangerter and his AFRD team, working in collaboration with a team of researchers at Livermore, aim to have a conceptual design completed by 1998. Construction could start in the year 2000 and take approximately three years. Experimentation could then begin at about the same time as fusion energy experiments begin at NIF.
In an Inertial Fusion Energy system, powerful beams of either ions or photons are directed onto a target of nuclear fuel, causing an implosion that ignites the fuel and releases an enormous quantity of heat.
Photo courtesy Laboratory for Laser Energetics, University of Rochester
"Experiments with a heavy ion driver will have to be done eventually to answer the questions that NIF cannot," Bangerter says. "From a scientific as well as a cost-effective standpoint, it makes sense for the heavy ion and laser experiments to coincide."
The inevitable exhaustion of fossil fuel supplies is no laughing matter. Those who would shrug this eventuality off as something that won't happen for another hundred years should remember another saying that has been making the rounds far longer than any jokes about fusion energy: If you don't learn to crawl, you'll never learn to walk.
Continue to part 3 of this story.