TWT with propeller

TWTs were conceived four years after the invention of the klystron (1942) by Austrian Rudolph Kompfner, a researcher during the Second World War at Clarendon Laboratories (Oxford). But it was John R. Pierce, another talented researcher at Bell Labs, who first understood how these devices worked and established the theory between 1944 and 1947: the beam couples with an electromagnetic wave whose phase velocity is close to, but very slightly lower than, that of the electrons. This gave rise to a physical principle known as the Cherenkov effect: a charged particle with a speed barely greater than the speed of light in this medium radiates energy.

This is a general phenomenon in wave physics: we all know that an aircraft flying slightly above the speed of sound gives up part of its kinetic energy in the form of an acoustic shock wave (the "double bang"). The originality...