Compact source of shaped drive beams for high transformer ratio wakefield accelerators

Compact high-gradient acceleration of ultra-short electron beams is a key element of the strategic plan for the DOE Office of High Energy Physics. High wakefield transformer ratios are essential for efficient beam- driven acceleration of such electron beams in plasma and dielectric structures, and this requires careful shaping of the drive beam current profile. The use of conventional magnetic optics to create a triangular or double-triangular longitudinal profile is subject to problems with nonlinear space charge forces when the charge exceeds 1 nC. Also, the use of conventional magnetic optics to longitudinally compress high-charge electron beams can generate coherent synchrotron radiation, resulting in unacceptable emittance growth and energy spread. We propose to demonstrate through simulations that both of these fundamental difficulties can be solved through use of the underdense plasma photocathode concept aka Trojan horse). The Trojan horse variant of beam-driven plasma acceleration uses low-power Ti-Sapphire laser pulses for controlled ionization injection of electrons into the plasma bubble. Recent Trojan horse simulations have shown 3 nC of injected charge with slice energy spread of 0.1% and good emittance. Use of shaped, overlapping laser pulses will enable the creation of high-charge triangular electron beams, including control of beam loading in the plasma wake to minimize energy spread. These beams will be suitable for driving plasma or dielectric wakefield accelerators with high transformer ratio. The parallel Warp framework will be used. To keep the system compact, we will consider GeV-scale beams from a laser-plasma accelerator as the Trojan horse driver. We will develop nonlinear optimization capabilities to determine parameters of the multiple overlapping laser pulses, including longitudinal profile shaping. We will also study whether the triangular shape can be modified to further increase the average transformer ratio in a standard PWFA system, including the effects of drive beam evolution. In order to benchmark our Warp simulations, we will provide computational support to the Iliad experiment, tentatively approved at SLACs user facility FACET to demonstrate use of the SSTF simultaneous spatial and temporal focusing) technique for tunneling ionization of fs-scale electron bunches. Also, the Warp framework will be further developed for cloud-based simulations of laser-plasma, beam-plasma and beam- dielectric accelerators, suitable for education of students and postdocs, as well as use by industry. The commercialization strategy is to develop a subscription-based website to productize the open source Warp framework. Warp can be used to simulate a wide variety of electrostatic beam and plasma structures with complicated geometries, including a wide range of particle accelerator systems. Warp is also a state- of-the art simulation tool for electromagnetic beam and plasma systems, capable of effectively using large parallel computers when necessary. More generally, Warp includes sophisticated and very general spectral solvers that could be applied to a wide range of partial differential equations for other problem domains. By the end of Phase II, cloud-based Warp simulations of high-gradient plasma and dielectric accelerators will be supported. The range of applications will grow over time during Phase III, to continue growing market share.