A high gradient plasma wakefield accelerator using two subpicosecond electron bunches

A high gradient plasma wakefield accelerator was tested at the Accelerator Test Facility of Brookhaven National Lab. Two $\sim $100fs electron bunches with total charge of 0.5nC separated by $\sim $500fs were fed into a 6mm long high density (1e14/cc to 1e17/cc) plasma generated by an ablative capillary discharge. The drive bunch created a $\sim $300MV/m wakefield that was sampled by the short witness bunch. The relative position of the witness bunch with respect to the drive bunch wakefield could be adjusted by varying the plasma density, thus allowing controllable energy loss or energy gain with small energy spread. The experimentally observed energy shifts are in good agreement with 2D model predictions.