Plasma Source with Density Step for Proton Bunch Self-Modulation

Numerical simulation results\footnote{A. Caldwell and K.V. Lotov, Phys. Plasmas 18, 103101 (2011)} suggest that placing a positive plasma density step along the self-modulation process of a long proton bunch leads to wakefields that remain at near saturation values over long plasma distances, instead of rapidly decaying after saturation. % We describe the development of a plasma source based on a laser-ionized rubidium vapor\footnote{E. Oz, P. Muggli, NIMA 740(11), 197 (2014)} that allows for imposing a density step in the 0 to 10\% range at different locations, every 50\,cm over the first 4\,m. % This source will be used to optimize the effect of the step on wakefields' amplitude at the end of the 10\,m-long plasma. % The source thus includes ports for a THz plasma density diagnostic\footnote{A. Gopal, private communication} to directly image the plasma electron density perturbation that sustains wakefields. % The source design and parameters for the AWAKE experiment\footnote{P. Muggli (AWAKE Collaboration), accepted for publication in J. of Phys.: Conference Series (JPCS) arXiv:1911.07534 [physics.acc-ph]}