Traveling wave prebunching of electron beams for free electron masers

We describe a direct, broadband e-beam prebuncher, based on the use of a tr aveling wave type prebuncher, as a means for producing dense, modulated e b eams, suitable for use with broadband free electron masers, to enhance thei r 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 GH z are among the outstanding features of this prebuncher. The density modulation (prebunching level) of the e beam exiting from the p rebuncher is determined as a function of the RF power input to the prebunch er and as a function of frequency and beam current, This determination is m ade 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" beam s in the nonlinear region of TWT operation. The debunching occurring in a drifting prebunched e beam is estimated, base d on the velocity spread of electrons in the prebunched beam and on space c harge forces. The bunching level is reduced, but not destroyed by the debun ching, in the case of our free electron maser.