Analysis of the beam loading effect in a compact linac with long bunch tails

Beam loading effect is an important issue in all kinds of accelerating structures, especially in a high current case, which might result in a significant difference between the actual output energy and the designed energy. In this paper, we perform simulations and experimental studies on the beam loading effect in an S-band traveling wave (TW) linac with a maximum acceleration gradient of 19.4 MV/m for an electron bunch larger than 200 pC. The TW linac is designed to drive a terahertz free electron laser (THz-FEL) and can produce beam energies varying from 8 14 MeV. The main challenge of the beam loading analysis come from the long bunch tail produced by the thermionic cathode gun. The long bunch tail causes considerable beam loss in the linac and brings great difficulties in the beam commissioning, especially in the beam energy tuning. So, the macro-particle tracking (MPT) algorithm is proposed to accurately analyze the beam loading effect of the long tail beams. The MPT algorithm uses energy conservation in each cell of the TW linac to take into account the particle loss and phase shift during the acceleration process. Based on the THz-FEL test facility, beam energies were measured under different loading cases. The experimental results confirmed that the new algorithm could predict the beam energy with a relative error less than 1.4%, while that of the traditional method is 4% 6%.