You are correct that the largest piece is the capacitors. Super capacitor technology is advancing quickly but they are the wrong kind of capacitors. The capacitors for FoFu require low inductance, fast discharge current and high voltage. Super capacitors tend to be low voltage, slow discharge capacitors. It is challenging to reduce the size of high voltage capacitors that can discharge as needed in FoFu. Other technologies may replace capacitors but a FoFu reactor will still need shielding. I can imagine a 5 MW reactor with everything fitting in a 1 car garage but much small than that is unlikely.
