Yes you could make photovoltaic systems that convert x-rays but there efficiency and lifespan would be very questionable. It seems that Focus fusion relies and not one but two major breakthroughs: Focus fusion and high efficiency x-ray photovoltaic. Either that or you going to need both heat engine cooling and a decelerator just to break even which would make a DPF fusion reactor much more costly. Although even if DPF does end up needing a steam engine and cooling towers it would still be economically competitive with coal and even nuclear power, just not by as much.
