In the discussion up to this point, little reference to the specifics of beams or target design has been made. As indicated, however, the requirements of pellet compression and beam-target coupling impose some very stringent demands on beam energy and the details of pellet composition.

High powered glass lasers have been used most frequently in inertial confinement fusion (ICF) research. The available beam intensity, focusing capability, state of technological development, and general availability are responsible for this popularity. Table 11.1 contains some properties of current lasers and, for comparison, also lists estimated requirements for actual fusion reactors.

Concerns about the eventual applicability of lasers to inertial confinement fusion has prompted considerable research on the potential role of ion

accelerators for such purposes. The prospects of enhanced localized beam energy deposition and control over beam energy while avoiding the previously mentioned undesired preheating by "hot" electrons produced in the laser light absorption process makes this alternative very promising. However, accelerators do introduce another set of problems among which are beam focussing for high- current accelerators as well as the need for large high-vacuum ion transport facilities. Table 11.2 lists some ion accelerator characteristics.

The composition of a pellet constitutes some interesting analysis and design problems. The existing types can be loosely grouped into three categories: (1) glass microballoons, (2) multiple shell pellets and (3) high-gain ion beam pellets.

Glass microballoons consist of thin walled glass shells containing a D2-T2 gas under high pressure, Fig. 11.3. The incident beam energy is deposited in the glass shell causing it to explode with part of its mass pushing inward and the remaining mass outward. Though microballoons are widely used in experiment, more efficient designs will eventually be needed for power plants.

Multiple-shell pellets contain an inner deuterium-tritium solid fuel core
surrounded by a high-Z inner pusher-tamper. Next is a thicker layer of low density gas surrounded by a pusher layer. Finally, a low-Z ablator material forms the outer layer, Fig. 11.3. This complex layer structure is designed for very specific functions. For example, the outer layer is to ablate quickly and completely when struck by the incident beam; the inner high-Z pusher-tamper is to shield the inner core region against preheating by hot electrons and X-rays.

Glass Microballoon Pellet

Low-Z Ablator Multiple Shell

Outer Pusher Pellet

Void

High-Z Pusher-tamper D-T Fuel

High-Gain Ion Beam Pellet

Fig. 11.3: Cross section of selected pellets for inertial confinement fusion.

More recently, heavy ion beam-pellets have been developed which depart in significant ways from the microballoon and layer shell design. In these designs, a vacuum sphere is surrounded by a D2-T2-DT fuel shell which is then surrounded by tamper-pusher materials, Fig. 11.3. These pellets are designed specifically for ion beam inertial confinement fusion with the thickness of the tamper-pusher materials carefully matched to the type and energy of the incident beams.

Major objectives of these designs are to optimize energy transfer, minimize hot electron production, and reduce requirements for symmetric beam energy deposition.