Plasma Light as Diagnostic for Wakefields Driven by Developing Self-Modulation of a Long Particle Bunch

We outline plans to use plasma light emitted as atomic lines radiation as a diagnostic for wakefields driven in plasma by a self-modulating, long proton bunch. This diagnostic is built into the design of a new vapor/plasma source that will also allow for imposing a plasma density step of various height at various locations. Such a step of a few percent in relative density placed at a location where the self-modulation process grows is predicted by numerical simulations to make the bunch drive wakefields with GV/m amplitude over hundreds of meters of plasma.