Direct drive is what we have pictured so far: a spherical target is compressed by laser light impinging on it uniformly from all directions. The main problem is that the laser beams must have no hotspots that can cause Rayleigh-Taylor instabilities to develop. Research on direct drive is the main mission of the Omega laser in the Laboratory for Laser Energetics in Rochester, New York. After years of trials, optical tricks have been devised to produce beams of the required uniformity.

Indirect drive is considerably more complicated. The laser is first fired into a cylindrical cavity called a hohlraum, German for “hollow space.” Upon striking the inside wall of the hohlraum, which is usually made of gold, the laser light generates intense X-rays. The capsule in the center is bathed in a sea of X-rays, which com­press it uniformly. Because of their high frequencies, X-rays are not subject to parametric instabilities. However, the laser beams must enter the hohlraum through a small hole in either end. Any stray light that hits the side of the holes will generate plasma and excite parametric instabilities there. Figure 10.47 is a view of a gold hohlraum, and Fig. 10.48 is an artist’s rendering of laser beams entering a hohlraum with a capsule in the center.

Indirect drive, the main emphasis of the programs at Livermore, is known to work well in bombs, but it is much more complicated for fusion than direct drive is. The hohlraums are hard to make, and the capsules have to be suspended at the center. (For this, there has been talk of using spider-web strands, for which there is no man-made replacement.) The hohlraums have to be shot to the center of the target chamber because the DT would melt if they were dropped slowly. Even then, cooled holders, shown in Fig. 10.49, have to be used to keep the hohlraum at cryo­genic temperature during its transit through the chamber. The holders also help protect the hohlraum from the force applied to accelerate them. Fast ignition is a new complication. To achieve better efficiency, this new method uses a very short prepulse focused with a cone (Fig. 10.50) to ignite the DT gas at the center of the pellet. The fusion energy from that burn helps to ignite the main fuel.

Imagine the sequence of each shot. A laser pulse is generated in an oscillator and is divided into 196 beamlets, each of which is passed through numerous amplifiers and optical switches in a 300-m path until the total energy exceeds 1 MJ. These beamlets form 48 beams, which the switch yard sends into the target chamber, shown in Fig. 10.51. Each beam is focused onto the target with micron accuracy in space and nanosecond accuracy in time. For indirect drive, the beams are divided

Fig. 10.50 Diagram of a fast-ignition hohlraum [44]

into two bunches, each entering the hohlraum at one end. The beams must not spill over onto the edges of the holes, or else they would make plasma and block the entrance. The cylinder must be aligned perfectly with the beams. In fast ignition, the hohlraum must also be in the right rotational position for the cones to be

aligned. After the shot, everything is vaporized, and the chamber has to be cleared for the next shot. In experiments, the targets are rigidly held by an arm, and successful implosions of the pellet have been achieved.