Scientific Achievement
Researchers from the BELLA Center in the Accelerator Technology & Applied Physics (ATAP) Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have used numerical simulations and experimental measurements to determine that the energy and charge of electron beams can be increased by longitudinally tapering the density profile in a 10-GeV class laser plasma accelerator (LPA). Tapering refers to increasing the plasma density as the laser pulse propagates deeper into the channel, a longstanding theoretical concept. The researchers demonstrated a practical approach toward making an experimentally realizable tapered plasma density profile using a supersonic gas jet.
This research represents one of the first steps towards implementing tapering by demonstrating that an arbitrary, slowly varying gas density profile can be created through a combination of adjusting the tilt and the throat width of an elongated gas jet. It also included Particle-In-Cell (PIC) simulations, which show a significant increase in electron beam energy and charge when a linearly increasing density profile is implemented.
Significance and Impact
In an LPA, a drive laser excites a speed-of-light trailing plasma wakefield, after which background electrons are injected and accelerated to high energies. These injected electrons must remain in phase with respect to the plasma wakefield to stay within an accelerating and focusing region of the electric field and to maximize their energy. The electrons will naturally drift out of phase with the laser, slowly catching up with the drive laser as the plasma slows them down, thereby limiting their final energy. However, by modifying the longitudinal plasma density profile, it is possible to keep the electrons in phase with the laser, thereby increasing the charge and energy of the electron beam.
This work represents a critical step for the LPA community toward high-power, tapered plasma channel LPA experiments, advancing the development of compact accelerators for use in various applications, including free-electron lasers, high-energy particle colliders, microelectronics, and nuclear detection.
Research Details
This research was motivated by two primary goals:
- While it has been previously shown that a tapered density profile can enhance the charge and energy of an LPA, can tapering be applied with a gas jet that is tens of centimeters long, and how can we characterize these gas jets?
- Can we simulate the effects that tapering would have on 10-GeV scale low-power amplifiers and determine the most suitable tapering profile for LPAs?
Density lineouts from simulations and experimental data of the elliptical nozzle shape for two different pressures are shown in (a). The experimental and simulated gas density profiles at 1.48 MPa are shown side by side in (c) and (d), for the elliptical and straight nozzle shapes, respectively. Since shot-to-shot fluctuations in the laser were the dominant source of noise, the lower length of the 2 cm jet results in it having increased noise compared to the 5 cm jet. A lineout at x = 0 is shown for both shapes in (b), with experimental measurements in solid lines and simulation results in dashed lines.
To address these goals, the team employed a Shack-Hartmann wavefront sensor to measure the gas density profile of a 30 cm-long gas jet, which showed very good agreement with OpenFOAM—a computational fluid dynamics code—simulations. The team found that both the throat width of the gas jet and the angle of the gas jet could be used to tailor the longitudinal gas density profile.
The researchers then studied what tapering profiles would be suitable for improving the efficiency of 10 GeV-class LPAs using the PIC code INF&RNO. To establish a reasonable set of input parameters for the simulations, the team used parameters from the recent 9.2 GeV result. The researchers employed a technique known as ionization injection, in which a dopant gas is used to inject electrons that are then accelerated. The dopant region was varied based on the density profile to optimize the properties of the electron beam. The simulations demonstrated that a 2.5-fold linear increase in density could increase the electron energy from 9 GeV to 12 GeV and the beam charge by a factor of approximately 10. These density profiles were experimentally demonstrated to be realizable by tilting a 30-cm-long gas jet.
Future work includes taper optimization and the experimental realization of the increased laser-to-electron beam efficiency predicted by their simulations.
Contact: Anthony Gonsalves
Researchers: Raymond Li, Alex Picksley, Carlo Benedetti, Francesco Filippi, Joshua Stackhouse, Liona Fan-Chiang, Hai-En Tsai, Kei Nakamura, Carl B. Schroeder, Jeroen van Tilborg, Eric Esarey, Cameron G. R. Geddes, Anthony J. Gonsalves
Funding: This work was supported by the Defense Advanced Research Projects Agency and the U.S. Department of Energy, Office of Science, Office of High Energy Physics, and used the computational facilities at the National Energy Research Scientific Computing Center (NERSC).
Publication: R. Li; A. Picksley; C. Benedetti; F. Filippi; J. Stackhouse; L. Fan-Chiang; H. E. Tsai; K. Nakamura; C. B. Schroeder; J. van Tilborg; E. Esarey; C. G. R. Geddes; and A. J. Gonsalves. “Longitudinal tapering in gas jets for increased efficiency of 10-GeV class laser plasma accelerators,” Rev. Sci. Instrum. 96, 043306 (2025). https://doi.org/10.1063/5.0250698
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