A Study of the Variation of Maximum Beam Size with Quadrupole Gradient in the FMIT Drift Tube Linac

The sensitivity of maximum beam size to input mismatch is studied as a function of quadrupole gradient in a short, high-current, drift-tube linac (DTL), for two prescriptions: constant phase advance with constant filling factor; and constant strength with constant-length quads. Numerical study using PARMILA shows that the choice of quadrupole strength that minimizes the maximum transverse size of the matched beam through subsequent cells of the linac tends to be most sensitive to input mismatch. However, gradients exist nearby that result in almost-as-small beams over a suitably broad range of mismatch. The study was used to choose the initial gradient for the DTL portion of the Fusion Material Irradiation Test (FMIT) linac. The matching required across quad groups is also discussed.