Abstract

We have investigated the viability of using plasmas formed by ionization of high Z, low ionization potential element rubidium (Rb) for beam-driven plasma wakefield acceleration. The Rb vapor column confined by argon (Ar) buffer gas was used to reduce the expected limitation on the beam propagation length due to head erosion that was observed previously when a lower Z but higher ionization potential lithium vapor was used. However, injection of electrons into the wakefield due to ionization of Ar buffer gas and nonuniform ionization of ${\mathrm{Rb}}^{1+}$ to ${\mathrm{Rb}}^{2+}$ was a possible concern. In this paper we describe experimental results and the supporting simulations which indicate that such ionization of Ar and ${\mathrm{Rb}}^{1+}$ in the presence of combined fields of the beam and the wakefield inside the wake does indeed occur. Some of this charge accumulates in the accelerating region of the wake leading to the reduction of the electric field---an effect known as beam loading. The beam-loading effect is quantified by determining the average transformer ratio $⟨R⟩$ which is the maximum energy gained divided by the maximum energy lost by the electrons in the bunch used to produce the wake. $⟨R⟩$ is shown to depend on the propagation length and the quantity of the accumulated charge, indicating that the distributed injection of secondary Rb electrons is the main cause of beam loading in this experiment. The average transformer ratio is reduced from 1.5 to less than 1 as the excess charge from secondary ionization increased from 100 to 700 pC. The simulations show that while the decelerating field remains constant, the accelerating field is reduced from its unloaded value of 82 to $46\text{ }\text{ }\mathrm{GeV}/\mathrm{m}$ due to this distributed injection of dark current into the wake.

Highlights

  • In this paper we describe experimental results and the supporting simulations which indicate that such ionization of Ar and Rb1þ in the presence of combined fields of the beam and the wakefield inside the wake does occur. Some of this charge accumulates in the accelerating region of the wake leading to the reduction of the electric field—an effect known as beam loading

  • The beam-loading effect is quantified by determining the average transformer ratio hRi which is the maximum energy gained divided by the maximum energy lost by the electrons in the bunch used to produce the wake. hRi is shown to depend on the propagation length and the quantity of the accumulated charge, indicating that the distributed injection of secondary Rb electrons is the main cause of beam loading in this experiment

  • The first phase of research on plasma wakefield accelerators (PWFAs) at the Final Focus Test Beam (FFTB) of the SLAC National Accelerator Laboratory demonstrated the ability of a PWFA to achieve ultrahigh acceleration gradients of over 50 GV=m [1] in a meter scale plasma

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Summary

INTRODUCTION

The first phase of research on plasma wakefield accelerators (PWFAs) at the Final Focus Test Beam (FFTB) of the SLAC National Accelerator Laboratory demonstrated the ability of a PWFA to achieve ultrahigh acceleration gradients of over 50 GV=m [1] in a meter scale plasma. The ionization potential (IP) of the first electron of Rb is 4.4 eV (compared to 5.4 eV for Li), so the SLAC electron beam is expected to propagate 40% further (and give a correspondingly higher energy gain) in the beam-ionized Rb plasma compared to the previously used Li plasma according to Eq (3) This choice of Rb is expected to reduce the problem of emittance growth of the accelerating beam due to ion motion [8,9] in future PWFAs. In the present experiments, the plasma ions are assumed to remain stationary on the scale of the bunch length. The ionization of the Rb1þ ion within the wake occurs as a result of the combined effect of the wakefields and the beam field even if the beam field alone is below the threshold value for field-induced ionization This injection of charge may be quite localized if the drive bunch is initially not matched to the plasma. Other alkali elements with lower ionization potential considered to replace lithium in plasma wakefield acceleration face the same problem

BEAM LOADING AND TRANSFORMER RATIO
IONIZATION INJECTION INTO THE PLASMA WAKEFIELD
EXPERIMENTAL SETUP
PHYSICS OF DISTRIBUTED INJECTION
EVIDENCE FOR DISTRIBUTED INJECTION
BEAM LOADING DUE TO DISTRIBUTED INJECTION
VIII. SIMULATIONS
Findings
CONCLUSIONS
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