Design and characterization of the DC acceleration and transport system required for the 1 MW Free Electron Maser experiment

A Free Electron Maser (FEM) has been constructed and is soon to be tested at the FOM Institute (Rijnhuizen) Netherlands with the ultimate goal of producing 1 MW long pulse to CW microwave output in the range 130 to 250 GHz. The DC acceleration and beam transport systems is eventually to be used in a depressed collector configuration requiring 99.8% beam transmission in order that the high voltage 2 MV supply be required only to supply 20 mA of body current. A relativistic version of the Herrmann optical theory originally developed for microwave tube beams is used to take into account thermal electrons far out on the Gaussian distribution tail which can translate into beam current well outside the ideal beam edge. This theory is applied to the FOM beamline design and predicts that the beam envelope containing 99.8% of the current can be successfully transported through the undulator for the expected range of assumed emittance values.