Hybrid Laser and Beam Driven Plasma Wakefield Acceleration and Light Sources

Laser-plasma-acceleration (LWFA) and beam-driven plasma wakefield acceleration (PWFA) are promising novel concepts  which can provide extremely large accelerating gradients up to the TV/m scale, offering the potential to reduce cost and size and to simplify future accelerators, light sources and colliders. While there are a lot of similarities between laser-driven and beam-driven plasma wakefield acceleration schemes, there are also profound differences. For example, while laser pulses propagate in plasma with a group velocity vg < c, leading to dephasing, electron beam drivers with relativistic energies do propagate nearly with the speed of light. Even larger driver beam energy spreads can be tolerated. Also, transversal driver beam expansion is much smaller for typical parameters when compared to the diffraction of strongly focused laser pulses, resulting in extended acceleration distances when using electron beams as particle drivers. On the other hand, laser pulses are ideal to selectively ionize gaseous materials.

Combining the advantages of LWFA and PWFA in hybrid plasma accelerators is a main task of the group. Such hybrid systems were for the first time considered in 2008 [1], and R&D in the field has led to an advanced hybrid scheme known as Trojan Horse Plasma Wakefield Acceleration, aka underdense photocathode PWFA [2-5]. Here, the driver beam (coming either from a conventional accelerator such as FACET, FLASH or CLARA, or from a Laser-Plasma-Accelerator) sets up a plasma wave based on a gaseous low-ionization threshold (LIT) component such as hydrogen. This plasma wave serves as a transient accelerating plasma cavity, which -- in contrast to coventional accelerators, where multiple consecutive, stationary metallic cavities in the lab frame are accelerating the particles -- propagates with the speed of light in the co-moving frame. Next, another central ingredient of the scheme are comparably low-power laser pulses which are used to generate and shape electron beams directly within the LIT-based plasma wave via ionization processes. Apart from synchronization of these laser pulses with the driver beam, the bunch-generation process is completely decoupled from the driver electron beam. These laser pulses, either in collinear geometry [3] or propagating at an arbitrary angle with respect to the driver beam axis [2], are used to release ultracold electrons by ionizing an additional high-ionization-threshold (HIT) component, which has not been ionized before, neither by a LIT pre-ionization laser nor by the electric self-fields of the driver electron beam itself. For example at Ti:Sapph wavelength of 800 nm, focused intensitites of the order of 1014-1015 W/cm2 are sufficient to release ultracold electrons directly inside the plasma wave. This is a major advantage compared to standard LWFA, where the laser pulse typically needs intensities of the order of 1018-1019 W/cm2. Such huge intensities are necessary because of the oscillating laser pulse electromagnetic fields, which need to exert an high enough ponderomotive force to catapult plasma electrons radially off axis in order to excite the plasma wave. This leads to a higher residual transverse momentum (nd therefore, of emittance) of electrons eventually injected in state-of-the-art LWFA schemes. In contrast, in the Trojan Horse scheme, the laser pulse does only have to exceed the ionization threshold in order to set free electrons in the already existing plasma wave. The residual momentum is proportional to the laser intensity a0, which scales linearly with the laser wavelength. Minimized laser intensities and wavelengths therefore can minimize the residual oscillation momentum. The laser contribution to the emittance of generated electrons is proportional to the laser spot size and normalized laser intensity. In contrast to conventional solid photocathodes, in the Trojan Horse scheme the photoionization laser(s) can propagate through the underdense plasma. Another advantage is the rapid acceleration in the GV/m to TV/m scale electric plasma fields, thereby quickly leaving behind the low electron energy region where space charge effects can have detrimental effects on emittance and brightness, and a transient shielding effect thanks to the ions which are generated at the same time. Depending on the trapping potential, the electrons are then trapped and accelerated to highest energies on ultrashort distance. By tuning the release laser and plasma wake parameters, extremely low normalized emittance down to ~10-10 m rad, and at currents of 100's and even 1000's of amps extremely bright electron beams with electron bunch brightness up to ~1020 A m-2 rad-2 may be generated. For the first time, a plasma based scheme therefore promises the generation of electron bunches with qualities orders of magnitude better than even the finest high-current electron accelerators such as FLASH or the LCLS. Such electron beams are natural candidates for light sources such as free-electron lasers, as explored for the first time in [3].  

Next to efforts at various laser-plasma accelerators worldwide, the group leads a flagship experiment at FACET called E210 Trojan Horse Plasma Wakefield Acceleration, having been proposed in 2011. In collaboration with UCLA; RadiaBeam Technologies, the University of Strathclyde and SLAC; we are currently installing a time-of-arrival diagnostic needed in the context of synchronization between the photocathode laser(s) and the electron drive beam. Beam time for the first stage of the E210 experiment is scheduled for early 2014.    

Various variations of the basic underdense photocathode scheme are possible [2], some of which are displayed in the above figure. For example, in an advanced all-optical setup, many laser pulse fractions would be used: one to generate the electron beam driver in the LWFA stage, one to preionize the subsequent Trojan Horse stage, one (or more) to release electron bunches via the underdense photocathode scheme, probe beams to monitor the interaction, and a laser beam for Thomson scattering of the generated high brightness electron witness beam.     

The Hybrids group is part of the Institute of Experimental Physics of the University of Hamburg, is part of LAOLA, the Laboratory for Laser- and Beam-driven Plasma Acceleration, a collaborational research framework between University of Hamburg and DESY, and is part of the Center for Free-Electron Laser Science (CFEL) in Hamburg. It is also part of the Helmholtz Virtual Institute of Plasma Wakefield Acceleration. Furthermore, it operates in close collaboration with the Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) at the University of Strathclyde, which is the flagship project of SUPA, the Scottish Universities Physics Alliance of seven Scotttish universities, with UCLA's Particle Beam Physics Lab (PBPL), and RadiaBeam Technologies.

[1] Monoenergetic energy doubling in a hybrid laser-plasma wakefield accelerator, B. Hidding, T. Königstein, J. Osterholz, S. Karsch, O. Willi, and G. Pretzler, Physical Review Letters 104, 195002, 2010

[2] Method for generating high-energy electron beams with ultra short pulse length, width, divergence and emittance in a hybrid laser-plasma accelerator, B. Hidding et al., filed as German Patent June 2011, AZ 10 2011 104 858.1. Filed as PCT/US patent via UCLA under the title "Method for generating electron beams in a hybrid laser-plasma-accelerator" on June 18, 2012, PCT/US Ser. No. PCT/US12/043002. US/PCT commercialization by RadiaBeam Technologies, Santa Monica, USA.

[3] Ultracold Electron Bunch Generation via Plasma Photocathode Emission and Acceleration in a Beam-driven Plasma Blowout, B. Hidding, G. Pretzler, J.B. Rosenzweig, T. Königstein, D. Schiller, D.L. Bruhwiler, Physical Review Letters 108, 035001, 2012

[4] Beyond injection: Trojan horse underdense photocathode plasma wakefield acceleration, B. Hidding, J. B. Rosenzweig, Y. Xi, B. O'Shea, G. Andonian, D. Schiller, S. Barber, O. Williams, G. Pretzler, T. Königstein, F. Kleeschulte, M. J. Hogan, M. Litos, S. Corde, W. W. White, P. Muggli, D. L. Bruhwiler and K. Lotov, AIP Conf. Proc. 1507, 570 (2012)

[5] Hybrid modeling of relativistic underdense plasma photocathode injectors, Y. Xi, B. Hidding, D. Bruhwiler, G. Pretzler, and J. B. Rosenzweig, PRSTAB 16, 031303 (2013)