High-current beam transport in the FXR injector

The FXR injector is designed to generate a nominal 4-kA, 1.5-MeV electron beam which represents the central portion of a 15 to 20 kA beam emitted from the cold cathode. Because of difficulties in reaching the 4-kA level repeatably and with good beam quality during the early running-in experiments, it was decided at that time to develop an accelerator tune based on a 2.5-kA injected beam that was better behaved. This beam adequately satisfied the overall design requirements for FXR, with 500 Roentgen of bremsstrahlung produced at 1 meter from the target, and with a radiographic spot size of 3 to 5 mm. More recently, we have begun to develop a 4-kA tune. During preliminary tests carried out in September and October, 1982, we found that in order to extract a 3.8-kA, 1.65-MeV injector beam, the earlier, axial-magnetic-field profile had to be modified and strengthened considerably, and the observed quality (pulse shape) of the emerging beam was not as good as before. This note will describe the modeling effort that was carried out to explain these results