Well, I know that you can slow down fission with carbon rods, which is part of how nuclear reactors work, maybe there's something that works for fusion? I'd expect it would need to be able to absorb a lot of gamma rays, which is what keeps fusion moving (well, that and the crazy heat). It's a lot harder than stopping a fission reaction, that's for sure. Especially with such a small warhead... wait, did we ever get a frame of reference? For all we know, couldn't that Nomad ship be 40x the size of the Harbinger?