ELECTRON BUNCH COMPRESSION DUE TO RF FORCES IN AN RF PHOTOINJECTOR FOR SMALL RF LAUNCH PHASES

The compression of a short, low-charge electron bunch in the first cel l of an rf photoinjector is calculated for start phases of the electro n bunch well before the peak of the accelerating electric field. This bunch compression results from a larger rf acceleration of the rear of the bunch than that of the front of the bunch due to the sinusoidal r ime dependence of the rf fields. The calculation in this paper shows t hat compressions of 50%, and more, of the initial bunch length are pos sible for early start phases, if the space-charge forces are negligibl e. Both a very accurate integral representation and a fairly accurate approximate formula for the compression are presented, which are valid for start phases from zero to about pi/2 radians, and for acceleratin g gradients ranging from those in on-going experiments to gradients se veral times larger.