Certainly looking at Safire/Aureon work (and contacts with the boss) suggest (but only suggest) that mixed-gas plasmas can have remarkable self-sustaining behaviour. Reading between the lines of their published work indicates that a low-pressure mix of say Argon and Hydrogen can self-organise and self sustain, the mix of light and heavy ions perhaps causing some interesting ricochet effects. Of course, their plasmas are radiative so not conservative which means they are too 'leaky' to work as a useful storage mechanism. Unless they are generative of course.
What I was proposing was that whatever mix of elements and charged particles are trapped in the capacitor would be arranged in order to have an optimum reaction environment. A reactive metal substrate either a powder or some hydrogen permeable wafer/anode would be inside the capacitor. The dense "entangled" electron plasma would surround the bonding of hydrogen isotopes with metal atoms. I am hoping for high frequency light and energized electrons (heV-keV scale, below gamma) as the overpoweringly main energetic products. A vacuum capacitor or super capacitor like device has potencially plenty conversion.
Why not - but the material tested must be perfect A) stable in vacuum B) resistant against free electrons and their reducing effect C) perfect insulator or it would affect vacuum and/or distribution of electrons and most probably ruin it. And size of sample must indeed remain miniscule. There is still question, whether the vacuum capacitor could work with other particles than electrons, for example hydrogen nuclei, i.e. protons - you'll need the source of free protons after then.
Some sort of high temperature ceramic container probably, with a well placed fuel wafer coated in a high surface area hydrogen/metal reaction interface. Remember hydrogen is technically a metal to, so suncell like *H2 chemistry should work as would denser *H inside Fe and/or Ni (possibly as a steel alloy) for example.
