Plasma wakefield acceleration experiments on FACET II and the DOE's Strategic Plan for Advanced Accelerator R&D
Professor Chandrashekhar Joshi, UCLA, USA
A dense, ultra-relativistic electron bunch propagating through a uniform plasma can produce a highly nonlinear wake that can be employed for accelerating a second, trailing bunch in a scheme known as the Plasma Wakefield Accelerator (PWFA). Recent work has shown that a PWFA cavity can accelerate a low energy spread electron bunch containing a significant charge at both high gradients and high energy-extraction efficiency - necessary conditions for making future particle accelerators both compact and less expensive. The next important challenge that must be addressed is the high efficiency acceleration of a low emittance and narrow energy spread electron beam while pump depleting the drive beam. Professor Joshi will explain how such an ambitious experiment is well aligned with the U.S. Department of Energy-High Energy Physics Division’s long-range strategic plan for Advanced Accelerator Research and Development. A new facility, FACET II is being constructed at SLAC that will enable such an experiment. Professor Joshi will describe the plans and the progress for the upcoming experimental campaign on FACET II.
Beam quality preservation challenges and strategies
Professor Michael Litos, University of Colorado Boulder, USA
Plasma-based electron accelerators have now demonstrated the ability to reliably provide a large energy gain with a small final energy spread. Beam emittance preservation, however, remains an outstanding challenge for the field. If the beam size is appropriately matched to the plasma, the beam will not suffer chromatic emittance growth as it traverses the plasma source. Unfortunately, the typically small matched beam sizes present a serious problem for external beam injection and beam extraction. This makes it difficult for plasma accelerators to interface with conventional accelerator components, including electromagnetic beam transport systems and magnetic undulators. In this talk, a solution for beam matching into and out of a plasma accelerator is presented, based on the use of tailored plasma density ramps at the entrance and exit of the plasma source. A simple model for beam transport through the plasma will be discussed, and it will be shown that a plasma ramp of a practical length can perfectly match a high-energy electron beam into or out of a plasma source with experimentally realistic beam and plasma parameter tolerances. Further, empirical generalized scaling laws will be presented that give a prescription for the ideal plasma density profile as a function of the desired beam parameters entering or leaving the plasma source.
FLASHForward – plasma wakefield accelerator science for high average power applications
Dr Jens Osterhoff, DESY, Germany
The field of particle acceleration in plasma waves has seen remarkable progress in the last two decades. These days, acceleration gradients of more than 10 GV/m can be readily achieved using either ultra-short intense laser pulses or high-current density particle beams as plasma wakefield drivers. With the demonstration of first GeV electron beams and a trend towards improved reproducibility, beam quality and control over the involved plasma processes, plasma-acceleration techniques are drawing considerable interest in the traditional accelerator community. As a consequence, DESY, Germany's leading accelerator centre, has established a research programme for beam-driven plasma-based novel acceleration techniques with the goal to symbiotically combine conventional and new accelerator concepts for applications. This presentation will give an introduction into these emerging activities, show first theoretical and experimental results and outline the DESY PWFA flagship project, FLASHForward.
FLASHForward is a pioneering beam-driven plasma-wakefield experiment that aims to produce, in a few centimetres of ionized hydrogen, electron beams of energies exceeding 1.5 GeV that are of sufficient quality to demonstrate gain in a free-electron laser. The experimental beamline will allow for milestone studies assessing plasma-internal particle injection regimes, external injection, and controlled beam capturing and release for subsequent applications. The facility provides a unique combination of low-emittance GeV-class electrons from the superconducting MHz repetition rate, high-average power accelerator FLASH synchronized to a 25 TW laser interacting in a windowless, optically accessible, versatile plasma target. Experiments will commence in 2018 and are foreseen to run for the next decade, opening up new avenues in this highly dynamic research field.
On the phase space dynamics of high brightness injection and staging in plasma wakefield acceleration
Professor Wei Lu, Tsinghua University of Beijing, China
In this talk, the current physics understanding of phase space dynamics for different injection methods and matching between stages in Plasma Wakefield Acceleration (PWFA) will be reviewed, with a focus on how to generate and maintain high quality electron beams of ultra-high brightness and low energy spread. Two major injection methods (plasma density modulation and ionization based injection) will be discussed in detail to show their potentials and limitations. Furthermore, phase space manipulation methods on staging and energy chirp reduction to 0.1% level will also be discussed.
Experimental demonstration of electron bunch generation from a plasma photocathode
Dr Aihua Deng, UCLA, USA
Dr Deng will talk about the experiments performed at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory, which is termed 'E210: Trojan Horse Plasma Wakefield Acceleration'. A new technique will be introduced, called the 'plasma photocathode'. It generates high quality electron beams directly within the large fields of a particle beam driven plasma wave by means of an ionizing laser pulse. The talk will demostrate controlled electron bunch generation in a plasma wakefield accelerator with an ionizing laser pulse to liberate electrons from helium gas in a preformed hydrogen plasma. Two injection modes will be introduced in the experiments: all-optical density down ramp injection and the real plasma photocathode injection. Optically-triggered density ramp injection is shown to provide a stepping stone for experimental parameter adjustments that yield beam generation from a plasma photocathode. The 'plasma photocathode' technique opens a path to beam phase space characteristics exceeding those of conventional photocathodes, and thus to combine the ultra-high energy gain of plasma accelerators with ultra-high beam quality. It will open the door to production of 'designer electron beams' with unprecedentedly low emittance, short duration and high brightness based on precision control of injection in extremely high field waves.