Traveling wave prebunching of electron beams for free electron masers

Abstract

We describe a direct, broadband e-beam prebuncher, based on the use of a traveling wave type prebuncher, as a means for producing dense, modulated e beams, suitable for use with broadband free electron masers, to enhance their radiation emission process. The capability to density-modulate an e beam in a range from zero to near unity in a frequency range from 3 GHz to 12 GHz are among the outstanding features of this prebuncher. The density modulation (prebunching level) of the e beam exiting from the prebuncher is determined as a function of the RF power input to the prebuncher and as a function of frequency and beam current. This determination is made on the basis of performances measurements on a traveling wave tube (TWT) before its output section was severed (the TWT gun section and the first two of the three-section TWT were subsequently used as the prebuncher), and on the basis of existing information on current modulation of "spent" beams in the nonlinear region of TWT operation. The debunching occurring in a drifting prebunched e beam is estimated, based on the velocity spread of electrons in the prebunched beam and on space charge forces. The bunching level is reduced, but not destroyed by the debunching, in the case of our free electron maser.