Abstract
We describe an external electron injection scheme for the AWAKE experiment. We use scattering in two foils, that are necessary as vacuum window and laser beam dump, to decrease the betatron function of the incoming electron beam for injection and matching into plasma wakefields driven by a self-modulated proton bunch. We show that, for a total aluminum foil thickness of ~ 280 μm, multiple Coulomb scattering increases the beam emittance by a factor of ~ 10 and decreases the betatron function by a factor of ~ 3. The plasma in the accelerator is created by a ionizing laser pulse, counter-propagating with respect to the electron beam. This allows for the electron bunch to enter the plasma through an “infinitely” sharp vapor-plasma boundary, away from the foils.
Highlights
During its first experimental run (2016-2018), AWAKE [1] reached two important milestones: the demonstration of the seeded self-modulation of the 400 GeV/c proton bunch delivered by the CERN Super Proton Synchrotron [2][3], and the acceleration of externally injected electrons from 19 MeV up to 2 GeV [4]
We use scattering in two foils, that are necessary as vacuum window and laser beam dump, to decrease the betatron function of the incoming electron beam for injection and matching into plasma wakefields driven by a self-modulated proton bunch
In this paper we study the incoming electron beam parameters to achieve matching to the plasma focusing force
Summary
During its first experimental run (2016-2018), AWAKE (the Advanced WAKEfield experiment) [1] reached two important milestones: the demonstration of the seeded self-modulation of the 400 GeV/c proton bunch delivered by the CERN Super Proton Synchrotron [2][3], and the acceleration of externally injected electrons from 19 MeV up to 2 GeV [4]. According to Liouville’s theorem, the incoming emittance is preserved if the transverse focusing force acting on the witness beam increases linearly with the distance from the axis This is achieved by fully blowing out the plasma electrons from the plasma cavity [7]: the system enters in the so-called blowout, non-linear regime. To maintain blowout and beam loading, the electron bunch charge density may not oscillate while propagating along the plasma This is satisfied by matching the electron beam to the plasma ion column focusing force. The beam is matched to the plasma (and its envelope does not oscillate along the plasma) when it is injected at the waist (σ (zinj) = 0, where zinj is the longitudinal position of the injection point, i.e. the plasma entrance) and the term in parenthesis in Equation 1 vanishes, i.e. the focusing force exactly balances the divergence of the beam Equation 2 defines the matching condition for β∗, and for the transverse beam size and emittance,
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