I think at those power levels you start vaporizing significant amounts of metal - the walls were heating up a fair bit at current power levels. You might be abl

I think at those power levels you start vaporizing significant amounts of metal - the walls were heating up a fair bit at current power levels. You might be able to plate to increase reflectivity at GHz (gold, maybe something else is better up there, could do multi-layer metamaterial). But if you're vaporizing metal on the walls that's going to act like a propellant, so it will be very hard to distinguish what's causing the thrust (since thrust via propellant can be much stronger per Watt than the proposed propellant-less technique).