High power X-Band microwave amplifiers and their application for particle acceleration

We report results from experiments studying X-Band amplification in both cylindrical traveling wave tube (TWT) and coaxial drift tube geometries. In the TWT geometry 800–900 kV, 1–2 kA, 50–100 ns electron beams are injected into cylindrical guides containing slow wave structures. The amplifiers operate in the TM01 mode. With single stage ripple wall slow wave structures gains of order 30 dB have been obtained with output powers of 100 MW. Using a dielectric loaded TWT amplifier gains of 30 dB have also been measured, although at lower output powers of 35 MW. In the coaxial geometry a 9 cm diameter annular, 400 kV, 7 kA, 50 ns electron beam is injected through a 16 cm dielectrically loaded cavity, and an X-Band magnetron provides frequency locking of the interaction. The interaction processes involved in these devices have been studied analytically and also simulated with the MAGIC code. These studies show significant electron energy spreads result from the amplification process in the TWT devices. A magnetic spectrometer has been built to analyze the output energy spectrum of the accelerated electrons. Preliminary results from this experiment show evidence of a change in the electron distribution at the amplifier output when the r.f. power is present.