Observation of wakefields in a beam-driven photonic band gap accelerating structure

Wakefield excitation has been experimentally studied in a three-cell X-band standing wave photonic band gap (PBG) accelerating structure. Major monopole (TM_{01}- and TM_{02}-like) and dipole (TM_{11}- and TM_{12}-like) modes were identified and characterized by precisely controlling the position of beam injection. The quality factor Q of the dipole modes was measured to be ∼10 times smaller than that of the accelerating mode. A charge sweep, up to 80 nC, has been performed, equivalent to ∼30 MV/m accelerating field on axis. A variable delay low charge witness bunch following a high charge drive bunch was used to calibrate the gradient in the PBG structure by measuring its maximum energy gain and loss. Experimental results agree well with numerical simulations.