Low frequency noise in a coupled-cavity traveling wave tube

Summary form only given, as follows. Coupled-cavity travelling wave tubes (CCTWT) are devices for producing high power amplification of microwave radiation. A large fraction of these devices suffer a problem with low frequency noise, close to the carrier frequency, which limits their performance. Current understanding of this phenomenon is that ions produced by collisions between the beam electrons and residual gas in the tube are trapped in potential scallop along the beam line, which are caused by overfocusing of the beam. Relatively low energy electrons produced in the ionization collisions also build up in the drift tube. Once sufficient ion density has built up in the beam region, the potential collapses and the ions escape in a burst. Regular build up and loss of ions in this fashion is assumed to produce the observed noise. The ion behaviour is obviously strongly influenced by the electrostatic potential in the tube. In standard systems the beam potential is negative with respect with the walls and the beam collector, so that ions can only escape back to the cathode. However, at sufficiently high ion densities the beam potential may increase enough for ions to escape to the tube walls. In some systems the beam collector is biased negatively to improve the tube efficiency, which could confine the less energetic ionization electrons, boosting plasma production and increasing the noise. The 2D PIC-MCC code XOOPIC is used to perform a series of simulations with different values of the collector potential to characterize tire mechanisms by which ions are produced, trapped and then escape from the tube, in order to confirm that ions are in fact the source of the noise, and possibly how to ameliorate the effect.