Plasma Wakefield Experiments at Cornell's Energy Recovery Linac
Mike Downer
Department of Physics, University of Texas at Austin
Tabletop plasma accelerators driven by intense ultrashort laser pulses have captured much attention recently by producing nearly mono-energetic electron beams approaching 1 GeV energy.1 Plasma accelerators driven by charged-particle bunches, however, are more likely to impact the future of high-energy physics because of their potential to double the energy of a parent accelerator by inserting a small, low cost “plasma afterburner” before the interaction point.2 A recent series of experiments at SLAC demonstrated the plasma afterburner principle using nC bunches ranging in duration from 20 ps to <100 fs, with normalized transverse emittance ~ 250 mm-mrad.2 The shortest bunches are most important, because they resonantly drive the densest plasmas (ne ≥ 1017 cm-3), and thus produce the highest accelerating gradient (Ez > 100 GeV/m). However, the requirements of stable propagation and low emittance growth demand that these drive bunches simultaneously focus to a transverse dimension σr < 10 μm less than a plasma wavelength and maintain bunch density greater than the plasma density (nb >ne). The high transverse emittance of the SLAC beam limited access to this so-called “blowout” regime at high plasma densities. The comparatively low transverse emittance of Cornell’s ERL for bunch durations down to 20-50 fs, on the other hand, is ideally suited to explore the “blowout” regime in dense plasmas. Because of the highly nonlinear beam-plasma interaction in this regime, the physics of plasma wave generation is non-trivial. To date particle-in-cell simulations have been the only available tool for its direct study. Measurements of the plasma accelerating structure with micron space- and fs-time resolution are needed. The high repetition rate (MHz vs. 10 Hz for the SLAC experiments) and long operating lifetime of Cornell’s ERL provide unprecedented opportunities for exploring basic physics, as well as technological development, of dense plasma afterburner accelerators. A dedicated facility could spur the development of an important new capability for experimental high-energy physics.
As an initial basic physics study, I will propose to take holographic “snapshots” of wakefield accelerating structures generated in dense plasmas by the Cornell ERL beam. Because of their microscopic size and luminal velocity, critical features of these structures that determine energy, energy spread, collimation and charge of the plasma-accelerated electrons have eluded direct single-shot observation, inhibiting progress in producing high quality beams and in correlating beam properties with wake structure. Recently my group demonstrated single-shot visualization of laser-generated wakefield accelerator structures for the first time, using Frequency Domain Holography (FDH), a technique designed to image structures propagating near the speed of light.3 Our holographic “snapshots” captured evolution of multiple wake periods, detected structure variations as laser-plasma parameters changed, and resolved wavefront curvature, features never previously observed. FDH is equally applicable to beam-driven plasma wakefields, requiring only probe laser pulses synchronized with the ultrafast photocathode laser of Cornell’s ERL. We can reconstruct wake morphology in real time, providing experimental feedback and optimization. I anticipate that FDH measurements will provide unprecedented insight into the physics of dense afterburner accelerators, and a first step toward developing a fully-optimized plasma afterburner accelerator.
References:
1. Faure et al., Nature 431, 541 and companion articles (2004)
2. Joshi, Scientific American 294, 40 and references therein (2006)
3. Matlis et al., submitted to Nature Physics (2006); Le Blanc et al., Optics Letters 25, 764 (2000)
