UCLA Evidence for beam loading by distributed injection of electrons in a Plasma Wakefield Accelerator. Presented by Navid Vafaei-Najafabadi Advisor: Chan Joshi On behalf of E200 Collaborators

UCLA Authors (UCLA) N. Vafaei-Najafabadi, K. A. Marsh, C. E. Clayton, W. An, W. Lu, W. B. Mori, C. Joshi (SLAC) E. Adli, S. Corde, M. Litos, S. Li, S. Gessner, J. Frederico, M.J. Hogan, D. Walz, J. England, J. P. Delahaye P. Muggli

UCLA Why Rubidium Advantage: avoiding – Head erosion – Ion motion Disadvantage: – Secondary ionization

UCLA Transformer Ratio Beam loading can occur because of a trailing beam or captured background electrons

UCLA Required Charge for Beam Loading n b >n p Pencil beam Long beam Λ~1 Src: M. Tzoufras, PRL (2008) Our beam’s Characteristics n p =2.7x10 17,N=1.3x10 10,σ r =3 μm, σ z =35 μm Q>110 pC

UCLA Experimental Setup

UCLA Betatron Oscillations in Plasma X-ray yield depends on the number of oscillations If the trapped charge is a function of interaction length, we are prominently trapping Rb II. Ar RbII ArII RbI ArI Dark Current Region Rb RbII/ArII RbII Ar Evolution of beam envelope

UCLA Excess Charge Correlates with X-ray Yield

UCLA Absolute Calibration of Length Length is absolutely calibrated using experimental results from two ovens

UCLA Beam Loading with the Same Current Profile

UCLA Raw Data and Transformer Ratio Transformer ratio reduces with excess charge ÷6 x2 Foil3Foil2Foil1 No Foil

UCLA Transformer Ratio and Excess Charge

UCLA Simulations Show Beam Loading Simulation parameters: 2.7x10 17, ε N =250 mm-mrad Simulations where Rb II or Ar are turned off, showed little to no beam loading.

UCLA Reduction in Transformer Ratio E - drops because of head erosion E + drops at a faster rate than the E - because of beam loading

UCLA Conclusions Distributed injection of electrons occurs due to beam induced ionization of Rb + These electrons can beam load the wake and reduce the accelerating field. Quantified this effect by correlating the increase of Rb II electrons with energy gain and loss of the different slices of the drive electron bunch as the plasma length is varied. PIC simulations shows that this correlation is indeed the cause of the reduction of the transformer ratio due to the beam loading effect.

UCLA Future Work This has given rise to mixed gas experiment to produce decent quality and high energy beam Will write a long paper once this is accepted.

THE END

UCLA Data Corroboration for the Two Ovens Trapped charge has the same form as X-ray Yield

UCLA

Dipole radiation on X-rays Actual blocked radiation Schematically shown blocked radiation