 Award ID(s):
 2108970
 NSFPAR ID:
 10417497
 Date Published:
 Journal Name:
 Physics of Plasmas
 Volume:
 30
 Issue:
 5
 ISSN:
 1070664X
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Abstract Due to the highly nonlinear nature of the beamloading, it is currently not possible to analytically determine the beam parameters needed in a twobunch plasma wakefield accelerator for maintaining a low energy spread. Therefore in this paper, by using the Broyden–Fletcher–Goldfarb–Shanno algorithm for the parameter scanning with the code QuickPIC and the polynomial regression together with k fold crossvalidation method, we obtain two fitting formulas for calculating the parameters of triGaussian electron beams when minimizing the energy spread based on the beamloading effect in a nonlinear plasma wakefield accelerator. One formula allows the optimization of the normalized charge per unit length of a trailing beam to achieve the minimal energy spread, i.e. the optimal beamloading. The other one directly gives the transformer ratio when the trailing beam achieves the optimal beamloading. A simple scaling law for charges of drive beams and trailing beams is obtained from the fitting formula, which indicates that the optimal beamloading is always achieved for a given charge ratio of the two beams when the length and separation of two beams and the plasma density are fixed. The formulas can also help obtain the optimal plasma densities for the maximum accelerated charge and the maximum acceleration efficiency under the optimal beamloading respectively. These two fitting formulas will significantly enhance the efficiency for designing and optimizing a twobunch plasma wakefield acceleration stage.more » « less

Pilat, Fulvia ; Fischer, Wolfram ; Saethre, Robert ; Anisimov, Petr ; Andrian, Ivan (Ed.)At the Facility for Advanced Accelerator Experimental Tests (FACETII) accelerator, a pair of 10 GeV highcurrent electron beams is used to investigate Plasma Wakefield Acceleration (PWFA) in plasmas of different lengths. While PWFA has achieved astonishingly high accelerating gradients of tens of GeV/m, matching the electron beam into the plasma wake is necessary to achieve a beam quality required for precise tuning of future high energy linear accelerators. The purpose of this study was to explore how starttoend simulations could be used to optimize two important measures of beam quality, namely maximizing energy gain and minimizing transverse emittance growth in a 2 cm long plasma. These two beam parameters were investigated with an indepth model of the FACETII accelerator using numerical optimization. The results presented in the paper demonstrate the importance of utilizing beamtransport simulations in tandem with particleincell simulations and provide insight into optimizing these two important beam parameters without the need to devote significant accelerator physics time tuning the FACETII accelerator.more » « less

Plasma based acceleration (PBA) is being considered for a next generation linear collider (LC). In typical AsmPBALC designs, the extreme beam parameters are expected to trigger background ion motion, which can lead to longitudinally varying nonlinear focusing forces and result in emittance growth of the beam. While various schemes have been proposed to mitigate this at low beam energies, a solution to minimize the emittance growth in the later high energy stages of a multistage electron acceleration arm is yet to be found. In this paper, we propose to use an adiabatic plasma density ramp as a matching section that is able to match the witness electron beam to the lowdensity plasma entrance, where the beam initially has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma density upramp (downramp), it is adiabatically focused (defocused) and its distribution maintains an equilibrium distribution throughout the entire process even when severe ion collapse has occurred. Simulation results from QPAD show that within a single acceleration stage, this concept can limit the projected emittance growth to only ∼2% for a 25 GeV, 100 nm normalized emittance witness beam and ∼20% for a 100 GeV, 100 nm normalized emittance witness beam.more » « less

Plasmabased acceleration (PBA) is being considered for a next generation linear collider (LC). In some PBALC designs for the electron arm, the extreme beam parameters are expected to trigger background ion motion within the witness beam, which can lead to longitudinally varying nonlinear focusing forces and result in an unacceptable emittance growth of the beam. To mitigate this, we propose to use quasiadiabatic plasma density ramps as matching sections at the entrance and exit of each stage. We match the witness electron beam to the low density plasma entrance, where the beam initially has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma density upramp, it is quasiadiabatically focused, and its distribution maintains a nonGaussian equilibrium distribution in each longitudinal slice throughout the process, even when severe ion collapse has occurred. This only causes small amounts of slice emittance growth. The phase mixing between slices with different betatron frequencies leads to additional projected emittance growth within the acceleration stage. A density downramp at the exit of an acceleration section can eliminate much of the slice and projected emittance growth as the beam and ion motion adiabatically defocuses and decreases, respectively. Simulation results from QuickPIC with Azimuthal Decomposition show that within a single acceleration stage with a 25 GeV energy gain, this concept can limit the projected emittance growth to only ∼2% for a 25 GeV, 100 nm emittance witness beam and ∼20% for a 100 GeV, 100 nm normalized emittance witness beam. The tradeoff between the adiabaticity of the plasma density ramp and the initial ion motion at the entrance for a given length of the plasma density ramp is also discussed.

null (Ed.)Abstract Metrescale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nanoCoulomb electron bunches. To reach useful average currents, however, the enormous energy density that the driver deposits into the wake must be removed efficiently between shots. Yet mechanisms by which wakes dissipate their energy into surrounding plasma remain poorly understood. Here, we report picosecondtimeresolved, grazingangle optical shadowgraphic measurements and largescale particleincell simulations of ion channels emerging from broken wakes that electron bunches from the SLAC linac generate in tenuous lithium plasma. Measurements show the channel boundary expands radially at 1 million metrespersecond for over a nanosecond. Simulations show that ions and electrons that the original wake propels outward, carrying 90 percent of its energy, drive this expansion by impactionizing surrounding neutral lithium. The results provide a basis for understanding global thermodynamics of multiGeV plasma accelerators, which underlie their viability for applications demanding high average beam current.more » « less