Scientific Achievement
Future linear particle colliders typically propose using high-quality flat beams to achieve the desired collision rate while avoiding beam degradation effects at the interaction point. Researchers from the BELLA Center in the Accelerator Technology & Applied Physics (ATAP) Division at Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with colleagues at Deutsches Elektronen-Synchrotron (DESY) and CERN, have analyzed the acceleration of flat particle beams in plasma-based accelerators for the first time. They have identified a plasma-based source of emittance mixing and potential mitigation strategies.
Significance and Impact
Plasma-based accelerators are promising candidate technologies for future linear colliders due to their ultra-high accelerating fields. These fields can be orders of magnitude larger than conventional metallic-cavity, radio frequency-based accelerators, resulting in a compact accelerator footprint.
To maximize a collider’s discovery potential, the collision rate must increase while minimizing harmful radiation effects during the collision, known as beamstrahlung. A proposed solution to achieve the necessary collider performance is to operate with flat beams at the collision point, utilizing a large beam aspect ratio in both horizontal and vertical directions. Operating the collider with flat beams requires maintaining these beams’ horizontal and vertical emittances during acceleration.
This work explores how the coupling of wakefields in plasma accelerators results in beam emittance mixing between the horizontal and vertical planes and how, in some instances, the characteristics of the plasma wakefield accelerator can be selected to alleviate this effect.
Research Details
In a plasma accelerator, a driver—either an intense electron beam or a laser pulse—propagates through a plasma at relativistic velocities, exciting an electron plasma wave or wakefield. This plasma wakefield then accelerates a trailing charged particle beam. Large accelerating gradients require the excitation of large wakefields, and plasma accelerators often operate in a regime where the driver is intense enough to expel all the electrons, leaving an ion cavity in its wake. A uniform distribution of background ions within the cavity is ideal for preserving emittance, as the horizontal and vertical wakefields are decoupled, which prevents emittance exchange. Various nonlinear effects can perturb the transverse wakefields, leading to coupling and emittance mixing. Such effects arise in collider-relevant beams that require high beam charge and low emittance, generating extreme space-charge fields capable of ionizing the background plasma or causing ion motion. Both of these effects can lead to the formation of nonlinearly coupled wakefields.
Simulation, using HiPACE++, of (a) plasma wakefield (co-moving ion cavity) accelerating a witness beam (b). Inset shows the beam aspect ratio between the horizontal (x) and vertical (y) planes. (c) Evolution of horizontal and vertical beam emittances for two cases (Li plasma and Ar plasma) as drive and witness beams propagate in plasma.
Nonlinear wakefields are sometimes desired, as nonlinearities in the wake due to ion motion can suppress instabilities without degrading beam quality if the witness beam is appropriately matched. However, this work demonstrates that coupled wakefields in plasma accelerators can lead to severe emittance mixing of flat beams when there is resonance between beam particle oscillations in the horizontal and vertical planes. This effect results in a decrease in emittance in the plane where it is largest and increases in the plane where it is smallest, resulting in an overall growth of the geometric average, which impacts the expected event rate in the collider (decreased luminosity). This work shows that, without mitigating this effect, a flat beam in plasma will become round.
The plasma-based source of emittance mixing is analyzed using analytical models and 3D simulations. Configuring the focusing fields in the plasma can enhance energy exchange between the transverse degrees of freedom of the particles. Moreover, the extent of emittance mixing relies on the ratio of resonant particles within the beam. The analysis also suggests possible solutions, such as preventing particle oscillation resonance, which may help minimize emittance mixing, adjusting the shape of the driver to achieve this symmetry breaking, and employing laser and flat beam drivers that can decrease the number of resonant particles and lessen emittance mixing.
The HiPACE++ code, developed at Berkeley Lab and DESY, was used to perform the 3D simulations that describe this effect. This quasi-static particle-in-cell code, designed to model short-pulse beam and laser-plasma interactions with significantly reduced computational cost, enabled these studies of transverse emittance exchange.
The emission exchange mechanism described here will significantly influence any future plasma collider design, and proposed plasma collider designers have already integrated this effect into their plans.
Contact: Carlo Benedetti, Jens Osterhoff, and Carl Schroeder
Researchers: Carlo Benedetti, Jens Osterhoff, Carl Schroeder (Berkeley Lab); Angel Ferran Pousa, Alexander Sinn, and Maxence Thévenet (DESY); and Severin Diederichs (CERN);
Funding: This research was supported by 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) at Berkeley Lab.
Publication: S. Diederichs, C. Benedetti, A. Ferran Pousa, A. Sinn, J. Osterhoff, C. B. Schroeder, and M. Thevenet. “Resonant emittance mixing of flat beams in plasma accelerators,” Phys. Rev. Lett. 133, 2024. https://doi.org/10.1103/PhysRevLett.133.265003
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