Speculation but...Im wondering if the clamps are pressing on the electrodes which actually slide in the tube thereby compacting the powder between them? Otherwise I can't see why such clamps were used. Over centre clamps can produce a lot of force.
I don't think the system uses any powder other than perhaps a tiny bit of LiAlH4 or LiH that may be vaporized.
I'm actually wondering if the electrodes may be inserted into the tube in a glove box filled with hydrogen to prevent atmospheric contamination. But there must be some way to then lower the pressure in the reactor so that a discharge can take place at lower voltages.