Generation and acceleration of electron bunches from a plasma photocathode

Abstract

Plasma waves generated in the wake of intense, relativistic laser 1,2 or particle beams 3,4 can accelerate electron bunches to giga-electronvolt (GeV) energies in centimetre-scale distances. This allows the realization of compact accelerators having emerging applications, ranging from modern light sources such as the free-electron laser (FEL) to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavoltper-metre (GV m -1 ) wakefields can accelerate witness electron bunches that are either externally injected 5,6 or captured from the background plasma 7,8 . Here we demonstrate optically triggered injection 9,10,11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This "plasma photocathode" decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical 11 density down-ramp injection 18,19,20 , is highly tunable and paves the way to generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness 12 . This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh brightness beams.The advent of photoinjectors in state-of-the-art linear accelerators (linacs) has enabled the substantial increases in electron beam quality that have ushered in an era of new scientific capabilities, as exemplified by the introduction of the hard X-ray FEL 13 . These photoinjectors produce electron beams in electric fields of ~100 megavolts-per-metre (MV m -1 ). This injection environment largely determines key beam qualities such as the transverse emittance (phase space area) and beam brightness. The strong accelerating field restricts emittance dilution and pulse lengthening by quickly increasing the relativistic Lorentz factor of the beam, γ = (1-v 2 /c 2 ) -1/2 , where v is the electron velocity and c is the speed of light, thus diminishing these effects

Plasma accelerator-based ultrabright x-ray beams from ultrabright electron beams

“…Recently, an experimental breakthrough has been achieved by the E-210: Trojan Horse collaboration at the Stanford Linear Accelerator (SLAC) Facility for Advanced Accelerator Experimental Tests (FACET), by demonstrating the feasibility of the plasma photocathode method for the first time. 7 This breakthrough, combined with the development of novel conceptual techniques for energy chirp compensation are very encouraging milestones towards laboratory-scale ultra-high brightness electron beam accelerators. The combination of ultra-high energy gain and ultra-high beam quality thus constitute game-changing advances which may allow, for example, driving future light sources with unique features, and other applications such as in high energy physics.…”

“…The combination of suitable experimental environment and the added installation of a synchronized laser system for preionization and plasma photocathode capability, PWFA expertise at FACET, and extensive in-depth theoretical and experimental work by academics and researchers from a multi-national collaboration mainly from Europe (University of Strathclyde, University of Hamburg, DESY, University of Oslo) and the US (RadiaBeam Technologies, UCLA, RadiaSoft, University of Colorado Boulder, University of Texas at Austin, Tech-X) eventually allowed to unlock the plasma photocathode injection scheme experimentally. 7,14,25 This was realized in 90 • geometry between electron driver beam axis and plasma photocathode laser pulse for reasons described below. This achievement marks a major milestone towards producing ultrahigh brightness electron beams in PWFA, has triggered the co-development of many auxiliary techniques e.g.…”

“…This method is called "Plasma Torch" injection 26,27 and is an alloptical version of plasma density downramp injection which we used as a stepping stone towards the plasma photocathode injection method: by delaying the injection laser pulse arrival, and by reducing the laser pulse energy, we successively approached and entered the Trojan Horse mode. 7 Both related laser-triggered injection methods are tuneable and flexible, and constitute experimental firsts. A challenge specifically to the Trojan Horse method is the inherent shot-to-shot temporal jitter between the FACET linac-generated driver electron beam and the plasma photocathode laser pulse.…”

“…Additionally, the limited plasma channel width introduced highly complex wakefield dynamics along the acceleration direction, and even wakefields which became decelerating during passage through the plasma . 7 The inadequate plasma channel width also amplified sensitivity to shot-to-shot jitters of preionization laser intensity, pointing etc., and driver electron beam and injector laser jitters. The small plasma blowout size with the comparably long electron driver beam, and the 90 • geometry with the comparably long plasma photocathode laser pulse Rayleigh length, furthermore increase the emittance as explained e.g.…”

“…14 We developed various methods and techniques which address and allow to overcome the limitations of previous experiments in the future. 7,14,25 The flagship experiment for this at FACET-II is the "E-310: Trojan Horse-II" collaboration, dedicated to explore and realize the full potential of the Trojan Horse scheme, and to realize tunable "designer" electron bunches with nm-rad scale emittances and associated brightness orders of magnitude better than state-of-the-art. This can be obtained by operating at substantially reduced plasma wavelengths and/or by using shorter plasma photocathode laser Rayleigh lengths or methods such as simultaneously space-time focused laser beams 18 or similar techniques to confine the "effective" Rayleigh length of the laser beam, and/or by moving towards smaller angles between driver particle beam and plasma photocathode laser beam in (near-)collinear or (near-)countercollinear geometry.…”