Self-modulation of Long Particle Bunches in Plasmas at Slac

Long (σz ∼ 500 μm) SLAC electron and positron bunches sent into dense plasmas (ne = 0.6 − 2.3 × 10 cm−3) are subject to a transverse two-stream instability called the self-modulation instability or SMI. We use numerical simulations to show that when the SMI saturates, the wakefield excitation reaches the nonlinear regime of the plasma wakefield accelerator (PWFA). We show that defocused particles lead to changes in the transverse bunch size downstream from the plasma. Measurement of that size change is an indirect observation of the SMI occurrence. We also show that the initial plasma ramp expected in the experiment does not significantly change the SMI growth or the peak wakefield amplitude.