The beauty of inertial confinement was supposed to be its freedom from the instabili­ties in magnetic confinement. No such luck: there are new instabilities! First there is an old one, the Rayleigh-Taylor instability (Chap. 5), which occurs whenever a light

fluid pushes against a heavy one. The expanding plasma pushes against the capsule with a huge force. If there is any deviation from smoothness in either the capsule or the laser light, small ripples will grow and destroy the compression before it gets very far. Figure 10.40 shows what can happen.

Parametric instabilities are a new class of instabilities caused by laser radiation [41]. In Fig. 10.41, a laser ray enters the blown-off plasma from the right and gener­ates a wave in the plasma shown by the curly line. This wave has maxima in plasma density at the vertical bars. The laser ray reflects off these density stripes coher­ently, as if they were a diffraction grating. The reflected ray goes off to the right. The incoming and reflected waves interfere constructively to strengthen the plasma waves, which then reflect more strongly yet. The net result is that much of the incoming light is reflected back toward the laser. Less light reaches the capsule, but that is not the worst part.

Extreme care must be taken to prevent the reflected beam from being amplified as it goes back through the laser. Otherwise, it will fry the laser. There are two kinds of plasma waves that can be generated in a parametric instability. One is an

ion acoustic wave, in which case the instability is called stimulated Brillouin scattering (SBS). The other is an electron plasma wave, in which case the instabil­ity is called stimulated Raman scattering (SRS). The worst part about SRS is that the plasma wave accelerates a beam of electrons. This can preheat the DT fuel so that it cannot be compressed to the required size. All this happens in a very small space, so the beam of electrons is very narrow, forming a pinch. The magnetic field generated by this beam is measured in megagauss (100s of tesla). It is not true that inertial fusion avoids magnetic fields! The instabilities, however, are not those in magnetic fusion.

The higher the frequency of the laser light, the higher are the densities where SBS and SRS occur. Higher frequencies will penetrate more deeply into the plasma corona and minimize these instabilities. This is the reason the NIF laser will use the third harmonic (“3w”) of its fundamental frequency even though almost half the light intensity will be lost in the conversion.