The physical problem is that it is very difficult to drive the fuel capsule into a state where the energy produced by fusion exceeds the losses due to,for example, radiation. In the simplest case these conditions are described by the famous Lawson criteria, which for inertial fusion says that capsule must achieve a minimum temperature and areal density, in order to produce and then trap fusion alphas and bootstrap to ignition. There are several potential reasons why current experiments fail to reach these conditions. The recent experiments were designed to control one of the possible causes, the growth of Rayleigh-Taylor instabilities in the core of the capsule, which potentially injects cold material into the core, cooling it down. Fusion capsules are unstable to this and so it's an important effect to understand: the recent results have shown that by using designs that reduce RT growth capsules can get closer to ignition than ever before.

Jim, I understood that these targets being used in the recent high yield shots cannot be scaled to high gain. Is that true? Are there designs that can control the RT instabilities yet achieve high gain?

It's a little ironic, you say the instability cools the target; in the early days the worry was always preheating the target and stopping compression.

Preheat is certainly still a big concern! The high yield targets are in some ways similar to a preheated target, as they are designed to sacrifice convergence for stability. Right now there are only a few points on the graph- high convergence, unstable at one end and lower convergence, more stable at the other. It's very interesting to investigate the region in the between- perhaps there is a good compromise that gives even better results. It is also conceivable to use different materials in the targets to prioritize stability if that turns out to be the most important factor.