Abstract
Strategies for mitigating ionization-induced beam head erosion in an electron-beam-driven plasma wakefield accelerator (PWFA) are explored when the plasma and the wake are both formed by the transverse electric field of the beam itself. Beam head erosion can occur in a preformed plasma because of a lack of focusing force from the wake at the rising edge (head) of the beam due to the finite inertia of the electrons. When the plasma is produced by field ionization from the space charge field of the beam, the head erosion is significantly exacerbated due to the gradual recession (in the beam frame) of the 100% ionization contour. Beam particles in front of the ionization front cannot be focused (guided) causing them to expand as in vacuum. When they expand, the location of the ionization front recedes such that even more beam particles are completely unguided. Eventually this process terminates the wake formation prematurely, i.e., well before the beam is depleted of its energy. Ionization-induced head erosion can be mitigated by controlling the beam parameters (emittance, charge, and energy) and/or the plasma conditions. In this paper we explore how the latter can be optimized so as to extend the beam propagation distance and thereby increase the energy gain. In particular we show that, by using a combination of the alkali atoms of the lowest practical ionization potential (Cs) for plasma formation and a precursor laser pulse to generate a narrow plasma filament in front of the beam, the head erosion rate can be dramatically reduced. Simulation results show that in the upcoming ``two-bunch PWFA experiments'' on the FACET facility at SLAC national accelerator laboratory the energy gain of the trailing beam can be up to 10 times larger for the given parameters when employing these techniques. Comparison of the effect of beam head erosion in preformed and ionization produced plasmas is also presented.
Highlights
Earlier experiments at the Final Focus Test Beam facility at the Stanford Linear Accelerator Center (SLAC) [1] have shown that a plasma wakefield accelerator (PWFA) can sustain an accelerating gradient exceeding 50 GeV=m over almost a meter long distance
The large-amplitude plasma wave or wake was excited by a high energy electron beam operating in the so-called ‘‘blowout’’ regime [2,3], where the electron beam density nb is much larger than the plasma density np or the beam current exceeds a few tens of kA
The head erosion affects the drive beam’s ability to ionize the neutral gas and form the wake and terminates the acceleration process even though the drive beam still contains much energy. This problem is not significant in a fully preformed plasma it has proved difficult to generate large volume, high density, uniform plasmas needed for PWFA
Summary
Earlier experiments at the Final Focus Test Beam facility at the Stanford Linear Accelerator Center (SLAC) [1] have shown that a plasma wakefield accelerator (PWFA) can sustain an accelerating gradient exceeding 50 GeV=m over almost a meter long distance. Because of the excessive beam head erosion, the acceleration distance and energy gain can be terminated if the drive beam is no longer able to create the plasma (and the wake) through the field ionization process. In this case the energy gain of the particles in the trailing bunch is not limited by the energy depletion of the drive beam (pump depletion) but by head erosion.
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