Speaker
Description
Strongly nonlinear plasma wakefield accelerators (PWFAs) have achieved > 10 GeV energy gain along with important beam quality milestones for electron beams, but they defocus positron beams, resulting in comparatively low energy gain and beam quality. This asymmetry presents a long-standing challenge for plasma-based positron-electron colliders. Nonlinear PWFAs generated in cold pre-formed hollow plasma channels lack focusing and defocusing fields, and thus in principle enable emittance-preserving acceleration of both electrons and positrons, but are prone to beam-breakup instabilities. Recent simulations [1] and measurements [2] of ion dynamics at time-delays ∆t ~ tens of picoseconds behind a nonlinear PWFA driver, however, revealed that hollow plasma channels of ∆t- tunable radius and “warm” electrons temperature form self-consistently. Because of their finite temperature, plasma electrons accumulate at the center of the thin hollow channel, providing focusing forces for accelerating, while unlocking beam breakup suppressions mechanisms that have been previously inaccessible in hollow channels [1].
Here, I will describe a scheme for observing and diagnosing these warm plasma channels at FACET II using ultrashort laser probe pulses that impinge on the PWFA propagation path at grazing incidence (θ≈20 mrad) and time delay ∆t ~ tens of ps behind the drive bunch. In this configuration, the thin warm channel diffracts probe light outward so that it interferes with the unperturbed outer annulus of the probe profile. This interference pattern encodes the channel’s shape, enabling its reconstruction. I will present PIC simulations that show the self-consistent, robust formation of the channels and numerical Fourier probe propagation routines that show diffraction of a probe pulse from them. Finally, I will show preliminary experimental results obtained at FACET II.
| Working group | WG3 |
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