<FONT face=Verdana>The main reasons of output microwave multi-frequency oscillation on S-band long pulse Relativistic Klystron Amplifiers were analyzed in this paper. Multi-frequency oscillation of RKA was suppressed evidently with PIC after some modifications on electron beams

bunching length

loaded Qvalue of output cavity

and leading magnetic field. According to the PIC results the long pulse RKA experiment was developed and a favorable experimental result was obtained

which the output microwave power increased from 300MW to 540MW

the output microwave pulse width increased from 50ns to 120ns. The efficiency was about 30%

the operation repetition was 20Hz.