[SLAC Pubs and Reports]
RF Pulsed Heating
RF pulsed heating is a process by which a metal is heated from magnetic fields on
its surface due to high-power pulsed RF. When the thermal stresses induced are
larger than the elastic limit, microcracks and surface roughening will occur due to
cyclic fatigue. Pulsed heating limits the maximum magnetic field on the surface and
through it the maximum achievable accelerating gradient in a normal conducting
accelerator structure. An experiment using circularly cylindrical cavities operating in
the TE011 mode at a resonant frequency of 11:424 GHz is designed to study pulsed
heating on OFE copper, a material commonly used in normal conducting accelerator
structures. The high-power pulsed RF is supplied by an X-band klystron capable of
outputting 50 MW, 1:5 mus pulses. The test pieces of the cavity are designed to be
removable to allow testing of different materials with different surface preparations.
A diagnostic tool is developed to measure the temperature rise in the cavity utilizing
the dynamic Q change of the resonant mode due to heating. The diagnostic consists
of simultaneously exciting a TE012 mode to steady-state in the cavity at 18 GHz and
measuring the change in reected power as the cavity is heated from high-power pulsed
RF. Two experimental runs were completed. One run was executed at a calculated
temperature rise of 120 K for 56 x 106 pulses. The second run was executed at a
calculated temperature rise of 82 K for 86 x 106 pulses. Scanning electron microscope
pictures show extensive damage occurring in the region of maximum temperature rise
on the surface of the test pieces.
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