Flash X-Ray (FXR) Accelerator Optimization Injector Voltage-Variation Compensation via Beam-Induced Gap Voltage

Lawrence Livermore National Laboratory (LLNL) is evaluating design alternatives to improve the voltage regulation in our injector and accelerator cells of our Flash X-Ray (FXR) machine. The operational peak electron beam current and energy at the X-ray generating target are 3.2 kA and 17 MeV. The goal is to create a more mono-energetic electron beam with variation of less than 1%-root-mean-squared (rms). This would allow the beam to be focused more tightly and create an X-ray source with a smaller spot-size. Our injector appears to have significant voltage-variation, and this report describes a technique to appreciably correct the deviations. When an electron beam crosses the energized gap of an accelerator cell, the energy increases. However, the beam with the associated electromagnetic wave also loses a small amount of energy because of the increased impedance seen across each gap. The phenomenon is sometimes called beam loading. It can also be described as a beam-induced voltage at the gap which is time varying. The polarity of this induced voltage is the opposite of the voltage in the injector. The time varying profiles of the injector and induced gap voltage are related through the beam current. However, while the change in magnitude is similar, they are not exactly the same. With the right choice of cell and pulse-power system impedance, the injector variations can be greatly reduced by cancellation, but not totally eliminated. The FXR injector voltage is estimated to be 2.5 MV-peak. The variation is estimated to be about 3.0%-rms for an interval of 60 ns. A simplified mathematical explanation of voltage compensation is given, and an idealized injector profile is used to quantify the effectiveness in a computer simulation. The result calls for a constant cell and pulse-power system impedance of 12.1 Omega. For this impedance, the compensated injector voltage-variation is less than 0.1%-rms.