Longer answer: When I first read it, I didn't think that the limitation that he had proposed applied to the type of device that we were making. He was really criticizing a similar but-not-identical type of fusion concept, and I clung to the differences. However, as our work progressed, I saw that the basic concept applied, which is that the scattering which would occur in a plasma (or a beam) would dissipate the energy concentration faster than the fusion rate would compensate. In short, a beam would thermalize with its surrounding plasma at an energy rate faster than the fusion rate.
We looked at using van de Meer beam cooling to try to keep the beam in a highly collimated state which would reduce the thermalization rate, but this wouldn't work. We also tried using Landau damping to make self-reinforcing waves that could, in theory, keep the energy concentration, but this really didn't work.