MODELING THE APLE INJECTOR SOLENOID MAGNETIC-FIELD WITH THE BIOT-SAVART LAW

The APLE (Average Power Laser Experiment) photoinjector uses four cavi ties to accelerate electrons to 5 MeV. A solenoid placed between the f irst two cavities focuses the low energy (1.2-MeV) electron beam. Once the beam profile at the photocathode is determined, the beam transpor t and the beam quality of the injector depend on the detailed field pr ofile of the solenoid. Previous study showed that the stacked current loops with a canted common axis could not describe the measured magnet ic field profile. In this paper, we modeled the fields accurately by i ntegrating the fields from each section of the solenoid using the Biot -Savart law. By comparing the measured profiles with the calculated pr ofiles of transverse magnetic fields on the beam axis z, we determined the angle of the solenoid axis by a best match of transverse field co mponents. We then implemented the Biot-Savart algorithm in the particl e simulation code PARMELA and calculated beam qualities at optimized i njector conditions. This study shows that the transverse fields calcul ated from the Biot-Savart law for a canted solenoid can describe the a symmetric profiles. Also the degree of sensitivity to the transverse f ield from the misaligned solenoid was revealed through the calculation s.