This is a high power microwave emitter whose very high frequency (33GHz, IIRC - home µwave ovens are 2.45GHz) limits its energy deposition to a thin tissue layer. It's very effective, but big and cumbersome because of the high power requirements. At these frequencies, components are tiny. If one needed less than ten Watts, this could be a box the size of a desktop PC, maybe a shoebox.

I'm fairly sure this could be (is being?) scaled down to something resembling the pain box used on Paul Atreides in Dune. An efficient "enhanced interrogation" technology is just too tempting for too many in power.

(edit: omitted word)