Currently, lasers like the 50,000 joule NOVA laser at Lawrence Livermore National Laboratory provide the primary tool for ICF research, and the 1.8 MJ National Ignition Facility, now under construction, is anticipated to be the first driver to achieve ignition.

While currently providing the best research tool, lasers face a significant techical challenge as drivers for IFE power plants. Continuing advances in laser technology have begun to address questions related to efficiency and high repetition rates. Additioanl technical development continues on bringing laser energy into a target chamber requires a transparent vacuum window, which must be protected from the neutrons, heat and debris generated from exploding targets. The potential for laser drivers to be relatively inexpensive, particularly if fast ignition can be demonstrated at NIF, provides further motivation to study new laser optics protection methods, like liquid metal mirrors, for IFE. While noting the potential provided by laser drivers, for simplicity this chapter focuses an alternate driver concept, that provides different and potentially less challenging final-focus requirements.

An alternative method of heating materials to high temperatures involves smashing high-velocity ions into a stopping material like lead, so that the ion kinetic energy deposits in a very small mass of material. Magnets can focus beams of ions onto targets, and target-chamber designers can easily shield final-focus magnets from the heat, neutrons and debris from exploding IFE targets. Ion accelerators also have already demonstrated high efficiency, reliability and repetition rate capability, making accelerators the most attractive candidates for driving IFE power plants.

At the far end of an ion accelerator, away from the target chamber, the first piece of equipment is an ion source. The ion source generates pulses of ions, each with either one or two electrons stripped away, and with all of the ions possessing almost identical velocities. Figure 1 shows the major components of potential IFE accelerator system. In this case the ion source produces ions of cesium with 2-million eV energy (2 MeV), injecting the ions down 64 beam lines. With the slight positive charge on each ion, electric fields can accelerate them, and this acceleration occurs in many stages to clouds of ions traveling through kilometers of vacuum tubes.

In the example in Figure 1, in the first stage of acceleration the ions reach energies of 100 million eV (100 MeV), and then the ions from 64 beam lines are combined into 16 beam lines. The length of the cylindrical clouds of ions traveling down the beam lines remains relatively constant during this acceleration process, but because the ions now have higher velocities, the clouds pass by a stationary observer in only 5 millionths of a second (5 microseconds), rather than the 37 microseconds pulse length at the entrance of the first accelerating section. Because the accelerator contains moving charged particles, an electrical current, the power of the beams at each location is simply amperes per beam line. At the ion sources the current is not particularly impressive--0.4 amperes would not even trip a typical 15 ampere circuit breaker found in a house. However, as the charged particles move faster, and as beams are combined, the currents increase accordingly, so that when the ions deposit in the target, the power is enormous--400-trillion watts.

Fig. 1 - Major systems in a simple induction-type heavy-ion accelerator (Credit LBNL).

For IFE, ion-accelerator designers must meet two simple but challenging goals: first, to deliver the pulse of energy in a very short time, around ten billionths of a second; and second, to deposit the ions in the smallest possible volume of absorber material, because the absorber must be heated to around a million degrees Celsius before it can begin heating the inside of the target hohlraum effectively, and the more absorber mass that must be heated, the less energy remains for driving the capsule.

Because the ions all have positive charges that repel each other, the cylindrical clouds of ions tend to expand while traveling down the beam lines, and must be recompressed frequently by electrical fields or by magnets. The control of this electrical repulsion also requires keeping the length of the cylindrical ion clouds relatively long to keep the ion density sufficiently low. This reduces the current, since more time is required for the cloud to pass a given location. To obtain very rapid energy deposition, however, requires then that the clouds be compressed in length just before they reach the target. In Figure 1, compression by a factor of 10 reduces the final pulse duration at the target to 10 billionths of a second (10 ns). This compression process, which requires giving the ions at the tail end of the cloud a small additional push to catch up with the leading ions, provides an active topic of research for IFE accelerator designers who await accelerator facilities large enough to study beam compression experimentally under IFE-scale conditions.

Designers must also minimize the volume of absorber they heat. They do this two ways--by controlling the depth that the ions penetrate, and by striving to minimize the beam diameter where it strikes the target. Accelerator designers control the ion penetration depth by selecting the ion mass and energy. Even though accelerators use ions with only one, or at most two, electrons removed, the initial collision of high-velocity ions with the target strips away many of the remaining electrons. Because heavier ions lose more electrons and end up with larger positive electrical charges, heavier ions can stop more rapidly and deposit more energy over a given distance than lighter ions.

In general, target designers prefer to heat absorbers with thicknesses equivalent to around a tenth of a millimeter of lead--around the thickness of one, up to a few, sheets of writing paper. Figure 2 shows that a heavy ion, with an atomic number ranging from 36 to 82, can deposit between 1 and 10 billion eV (1 to 10 GeV) in this depth, compared to only around 50-million eV for a light ion like lithium (Li). Much smaller numbers of heavy ions, compared to light ions, are then needed to heat an IFE target. Heavy ions thus have less total positive charge, which simplifies focusing the ions down to a small spot size. (In fact, for light ions like lithium, so many ions are needed that some method of charge neutralization--allowing each ion to pick up an electron after final focusing--is required to obtain acceptable spot sizes. Research at Sandia National Laboratory has focused on this goal.)

Fig. 2 - Stopping distance for ions of various energies in lead (density = 11.3 gram/cm³). Heavier ions with atomic numbers A between krypton (Kr) and lead (Pb), like cesium (A = 55) can deposit much more energy in a given depth than light ions like lithium (Li).

Besides the repelling effect of the ions' positive charge, accelerator designers face another challenge in focusing beams to the smallest possible spot size. Unless ions are traveling in precisely the same direction, with precisely the same speed, ions will not focus to the same point after passing through a final-focus magnet. To the degree that ions have random motion before reaching final focus magnets, the focus spot will be smeared out to a larger diameter. Accelerator designers use a parameter called emittance to quantify the magnitude of this undesirable random motion. They take pride in having already designed ion sources that provide ions with sufficiently low emittance for IFE accelerators, and actively pursue methods to minimize emittance growth along the length of the accelerator.

Just due to their long length, and the large quantities of iron, copper, copper, and other materials required for construction, heavy-ion accelerators provide the most expensive component of a heavy-ion-driven IFE power plant. However, a single heavy-ion accelerator can produce enough pulses--a hundred per second or even more--to power a large number of separate target chambers. By driving large numbers of target chambers even the most expensive accelerator can begin to look attractive. However, this also points to the most important conclusion about heavy-ion accelerators: when NIF demonstrates the scientific feasibility of ignition, then the most important way that engineers and scientists can contribute to the future economic viability of IFE will be by applying their best talents to develop innovative approaches to accelerator design. Our best estimates for the amount of land, steel, concrete, copper, and equipment needed to build an IFE-class accelerator suggest that this goal can be achieved, even for a single 1,000 to 2,000 MWe power plant.

For more information about heavy-ion accelerators, visit the tutorial at the Lawrence Berkeley National Laboratory's Heavy-Ion Fusion Group web site.