Observation of plasma wakefield acceleration in the underdense regime

N Barov,M E Conde,J G Power,J B Rosenzweig,W Gai

doi:10.1103/physrevstab.3.011301

Abstract

Initial experiments which have explored the physics of the underdense (blowout) regime of the plasma wakefield accelerator (PWFA) at the Argonne Wakefield Accelerator facility are reported. In this regime, the relativistic electron beam is denser than the plasma, causing the beam channel to completely rarefy, and leaving a high quality accelerating region which also contains a uniform ion column. This ion column in turn allows the drive and accelerating beams to be well guided over many initial beam beta-function lengths. The results of these experiments, which have taken place over several years, are reviewed. Notable achievements in the course of these studies include the creation and measurement of drive and witness beam generated in an rf photoinjector, as well as previously published studies on drive beam guiding in the underdense regime. In addition, these experiments allowed measurement of both beam energy loss and gain, at a maximum average rate of 25 MeV/m in this regime of the PWFA, which is consistent with a peak acceleration gradient of 62 MeV/m in the excited waves. Difficulties associated with this type of experiment are discussed, as are prospects for mitigating these difficulties and achieving high gradient acceleration in planned future experiments.

Highlights

Much progress has been made in recent years in the experimental demonstration of acceleration in plasmas
Operation of the plasma wakefield accelerator (PWFA) [3] in the extremely nonlinear (“blowout,” where the beam is denser than the plasma) regime was originally proposed by Rosenzweig, Breizman, Katsouleas, and Su in 1991 [4]
This round of experiments at the Argonne Wakefield Accelerator (AWA) facility gave the first measurements of plasma wakefield acceleration in the blowout regime

Summary

INTRODUCTION

Much progress has been made in recent years in the experimental demonstration of acceleration in plasmas. 1 and 2, the fields approach or exceed the nonrelativistic pplasma wave-breaking limit [8,9,10,11,12], eEWB ෇ mec2kp Х n0 ͑cm23͒ ͑eVcm This allows for high gradient operation at longer wavelengths, due to lower (relative to the linear regime) plasma densities and relativistic lengthening of the plasma oscillation period. This ratio is not necessarily greater than unity for all conditions of blowout— it is, approximately 0.1 in the experiments described in this paper This situation, in which the beam is denser than the plasma, but does not drive longitudinal wakes near to the wave-breaking amplitude, is encountered when the beam radius or length is too small, so that even though the beam is dense enough to achieve blowout, it does not have enough charge to drive large wake fields. The inequality given in Eq (6) indicates that the Argonne Wakefield Accelerator (AWA), which was not originally designed for low emittance operation, can drive a plasma wake field in the blowout regime, but with a very small margin of error

PRELIMINARY EXPERIMENTAL WORK AT ANL

ACCELERATION AND DECELERATION IN THE BLOWOUT REGIME

15.6 MeV 18 6 5 nC

Findings

CONCLUSIONS