Detailed Analysis of the Interception Current Predicted by the Large-Signal Code TESLA for an Experimental Multiple-Beam Klystron

We present the results of detailed modeling of an experimental eight-beam four-cavity multiple-beam klystron (MBK) with an emphasis on the effects related to the circuit interception current. The parallel version of the 2.5-D large-signal code TESLA was used as the principal analysis tool. The primary MBK model used eight nonidentical beams/beam tunnels run in eight parallel processes using the measured values of R/Q on each gap of each beam tunnel. The simulation results are compared with the available experimental data and show good agreement between the calculations and measurements, including the predicted level of threshold RF drive power at the first appearance of body current. Additional modeling was performed using a less accurate approximation where all eight beams/beam tunnels were assumed to have identical properties. The results of the two different modeling approaches are analyzed in detail and compared with each other to determine the limits on the accuracy of the respective models.