It’s actually frozen deuterium tritium. Inside a small pellet blasted by X-rays which compress the pellet until it’s hot and dense enough for the fusion energy produced to continue burning and heating the deuterium/tritium until much more energy is produced than the energy of the X-rays.

It all happens in an instant. The pellet’s structure doesn’t survive. But it happens fast enough that just the inertia of the pellet (turning to a gas and then a plasma) keeps things confined for long to fuse a significant amount of the deuterium/tritium.