An investigation of beam loading in a cavity

Summary form only given. Beam loading remains a problem of primary importance in high power vacuum electronics. The DC and AC space charge alters the resonant frequency, the quality factor Q, and the gap transit time factor. These effects need to be accurately accounted for in the evaluation of the intermodulation products in a klystron driven by a multi-frequency signal. In high power microwave sources, they are known to critically affect the device performance. These concerns prompted us to initiate a systematic study of beam loading of a cavity by an electron beam using the simulation code MAGIC [1]. Emphasis will be placed on the loading at various combinations of DC and AC beams. Our preliminary data show that a cavity with a cold Q of Q/sub c/=145 is reduced to a hot Q of Q/sub h/=130 for a 40 kV, 3 A beam at 5 GHz. This DC beam loading leads to Q/sub b/(DC)=1257 whereas the addition of just 0.3 A of AC current leads to Q/sub h/=122 with an additional Q/sub b/(AC)=1982. Here, Q/sub b/(DC) and Q/sub b/(AC) are defined by 1/Q/sub h/=I/Q/sub c/+1/Q/sub b/(DC)+1/Q/sub b/(AC). Our simulation results also show that, for the DC beam loading alone, Q/sub h/ is roughly a constant, at a value of 130 when the beam is fixed at 0.375 microPerveance, with the DC beam currents varying between 3 and 12 A in this example. Further data will be presented, showing beam loading as a function of beam current and voltage, of the fraction of the AC current in the beam, and of the degree of beam neutralization. Time frequency analysis is applied to the results.