Experimental study of rf pulsed heating on oxygen free electronic copper

When the thermal stresses induced by rf pulsed heating are larger than the elastic limit, microcracks and surface roughening will occur due to cyclic fatigue. Therefore, pulsed heating limits the maximum surface magnetic field and through it the maximum achievable accelerating gradient. An experiment using circularly cylindrical cavities operating in the TE011 mode at a resonant frequency of 11.424 GHz was designed to study pulsed heating on oxygen free electronic (OFE) copper. An X-band klystron delivered up to 10 MW to the cavities in 1.5 mus pulses at 60 Hz repetition rate. One run was executed at a temperature rise of 120 K for 56 x 10(6) pulses. Cracks at grain boundaries, slip bands, and cracks associated with these slip bands were observed. The second run consisted of 86 x 10(6) pulses with a temperature rise of 82 K, and cracks at grain boundaries and slip bands were seen. Additional information can be derived from the power-coupling iris, and we conclude that a pulsed temperature rise of 250 K for several million pulses leads to destruction of copper. These results can be applied to any mode of any OFE copper cavity.