The pure fusion devices in my novel use a series of X-ray lasers to ignite the fusion fuel (combination of lithium 7 and deuterium which is what made the real life Castle Brovao test so much more poweful than the scientists prdicted). For a eight femtoseconds, the lasers are releasing energy on the terrawatt scale. Ths takes a LOT of energy and so the bomb's capacitors are charged before bomb release. The bomb's yield is variable. The fuel is also loaded prior to launch depending on the desired yield. Maximum yield is 35 megatons. As a result it is more of a tactical weapon than a strategic weapon (strategic bombs are anti-matter "planet killers" which render planets uninhabitable).