Accelerator Driven Fusion

Fusion is the melding together of light atomic nuclei, two isotopes of hydrogen, to form heavier nuclei, an action that releases roughly one million times the energy released by the burning of oil. Unlike fission, where atomic nuclei are split, fusion cannot sustain a chain reaction, and with proper design, does not produce high level radioactive by-products that must be carefully stored for thousands of years. Presently, fusion experiments consume more energy than they produce, but research on two fronts is being pushed forward to solve this problem and eventually move toward practical electricity generating plants.


These two main areas are inertial confinement, also known as inertial fusion energy (IFE), and magnetic fusion energy (MFE). In MFE, thermonuclear fuel, in the form of an ionized gas or "plasma," is contained by a powerful magnetic field and heated to ignition. With IFE, no confinement system is required, even for atomic nuclei have been heated to the surface temperature of the sun. This is achieved by using  the fuel's inertia; a pea sized spherical capsule made of plastic or a light metal encases a hollow shell of frozen deuterium and tritium which is filled with deuterium and tritium. The outer layer of the sphere is rapidly heated so that it vaporizes, producing high pressure that implodes the frozen fuel with enormous force, increasing its density about a thousand times and causing the fuel in the center to ignite. 

In contrast to MFE, the capacity of IFE to produce far more energy than is consumed has already been vividly demonstrated. The question is whether the technology can be scaled down to a size useful for a power plant. One of the most critical issues is how best to ignite the thermonuclear reaction. Should the fuel be heated directly or indirectly? And what should be the source of this heat: a laser or a particle accelerator that produces beams of heavy ions? 
To be usable for power production, the energy must be supplied by a system which is far enough away from the fusion region that it can survive the blast and neutrons from the fusion burn. A beam of heavy ions has the requisite properties: it is focused onto the capsule by magnetic fields which are unaffected by the burn and which are generated by superconducting quadrupole magnets which can be situated about 7 meters from the capsule and be well shielded from the heat and radiation in the reaction chamber.

While the conditions near the reactor all favor a heavy ion driver, such an accelerator is well outside of present day experience. Most of the overall system cost and effort are devoted to the accelerator. A typical beam requirement for fusion might be 50,000 amperes peak at a kinetic energy of 10 GeV and a pulse duration of 10 ns. The peak current is very high, but the average current is just a few mA. Heavy ions have been accelerated to much higher kinetic energies, 100's of GeV for nuclear research, but at tiny currents. Likewise, heavy ion accelerators have had much higher average currents, amperes for ion thrusters for space travel, but at only about 10 kV acceleration voltages. The task for accelerator fusion is to increase the kinetic energy while maintaining the intensity and the beam quality required for focusing.

Ancient Cave Paintings 

The Research Laboratory of the French Museums,
located within the Louvre in Paris, has installed an accelerator for studying works of art by nuclear techniques. Museum researchers have studied 12,000 year old cave paintings. Their analyses of milligram size samples showed that the paleolithic artists prepared their paints by complex recipes which included a pigment, a binder, and an extender. By categorizing the paintings according to the paint recipes, they established a more accurate chronology of the cave art.