Both magnetic and inertial confinement fusion involve two key processes for the attainment of a viable fusion energy system:

1. heating and ionization of the fuel to high temperature to achieve a favourable fusion reaction rate density, and

2. confinement of the fuel for a sufficiently long time to yield a net energy gain.

The particle density in a magnetically confined plasma is expected to not significantly exceed 1021 m’3, which constitutes a very low density gas when compared to atmospheric particle densities of about 1025 m’3 at standard temperature and pressure. In contrast, inertial confinement typically requires fuels compressed to densities that are several orders of magnitude greater than solid-approaching number densities of 1031 to 1032 rn3-exceeding even densities found in stars. A major feature of this high density is that since the fusion reaction rate is proportional to the density squared, the inertial confinement time required for a net energy gain will be significantly smaller than those in magnetic confinement devices. These points can be put into perspective by comparing the Lawson criterion-like requirements for inertial and magnetic confinement. For magnetic confinement, magnetic field limitations typically restrict ion densities to the order of 1021 m’3 with a confinement time necessary for energy break-even of about 1 s. On the other hand, for inertial confinement, the compressed density can be 1031 m’3 over a time interval of the order of 10’9 s.

Interest in inertial confinement fusion energy emerged later than that in magnetic confinement fusion. Its relevance to power production became apparent when it was recognized that concentrated beams from powerful pulsed lasers could be used to initiate a compressive process in a small solid or liquid target pellet, possibly resulting in a sufficient number of fusion reactions to yield a net energy gain.

The sequence of events in inertial confinement fusion can be briefly described as follows. A small pellet, with a radius less than ~5 mm and containing a mixture of fuel atoms, is symmetrically struck by energetic pulses of electromagnetic radiation from laser beams or by high energy ion beams from an accelerator, Fig. 11.1a. Absorption of this energy below the surface of the pellet leads to local ionization and a plasma-corona formation, Fig. 11.1b. The important consequences of these processes are an outward directed mass transfer by ablation and-by a rocket-type reaction-an inward directed pressure-shock wave leading to compressing and heating of the target, Fig. 11.1c. A follow-up shock wave driven by the next laser or ion beam pulse will then propagate into an already compressed region where it travels faster than its predecessor. Subsequent shock waves can thus propagate even more quickly. Tuning the beam’s pulse repetition rate such that the shock waves arrive at the pellet’s core simultaneously will provide for an adequately compressed state possessing temperatures suitable for initiating a substantial fusion bum. With the temperature and fuel density sufficiently high, the fusion reactions will occur until the pellet disassembles in a micro-explosion due to its excessive energy content, Fig. 11.Id. The disassembly typically takes place in a time interval of about 10’8 s, corresponding to the propagation of a pressure wave across the pellet with sonic speed vs.

Experience with inertial confinement has shown the importance of several processes and phenomena. For example, it is essential that the incident laser or ion beam strike the pellet symmetrically and that efficient energy coupling between the beam and the target be attained. The inner core should reach a high density very quickly before thermal conductivity heats the central region causing an internal pressure build-up that opposes high compression. A substantial fraction of the nuclear fuel should also bum before pellet disintegration.

We note that laser beams can penetrate to deeper layers of the pellet when they possess a higher frequency. Hence, intensive short-wavelength lasers are sought as drivers for inertial confinement fusion, evidently with a reasonable efficiency also required. A phenomenon of concern is that very high energy electrons generated in the initial laser light absorption process will penetrate into the centre of the target before the arrival of the dominant pressure wave thereby causing an undesirable preheating of the central core region resulting in an outward force effect to retard compression. Accelerators, on the other hand, transfer the beam energy more directly to ions in the target and can therefore be significantly more efficient; this provides some appeal for the use of light or heavy ion accelerators for such purposes.