The cavity is closed on both ends, and is resonant like a bell at a few Ghz. There is a dielectric disk capped on one end, while the whole enclosures is sealed in metal, basically a Faraday cage. A signal of a few hundred watts was fed into the cavity, and then it was tapped out, recirculated through and RF amp, and pumped back in,, its a feed back circuit. Think of a hot mic feeding back in an auditorium, then re amplifying and tuning the whole EQ to the resonant ringing tone,, that's what's going on inside. At one end there is a dielectric disk which for all practical purposes is the equivalent of placing a pizza on the bottom of a microwave oven, and turning it on, then telling everybody the oven is lifting off the table..... the paper says there could be errors in the measurement of the lift because the thrust measured is extremely small.... nobody is really reading this paper, everyone is just parroting the press releases , and the fact that it was “peer”reviewed means what ? Consider man made global warming is also peer reviewed..... I'm still skeptical for example the thrust measured could be just simple thermal expansion from the release of the heat cooking the pizza puck in the cavity..... read for yourself here
http://arc.aiaa.org/doi/pdf/10.2514/1.B36120