Revision 1

Accepted · first revision · record

Title: Fundamental trade-off between efficiency and beam-breakup instability
Plain: To be efficient, the accelerated bunch must extract much of the wake's energy, but that also makes the bunch wobble sideways and break up. How efficient a stable plasma accelerator can be is not known.
Precise: Strong beam loading raises wake-to-witness energy-transfer efficiency but also strengthens the transverse wakefield that drives the hosing (beam-breakup) instability. Determine the maximum efficiency compatible with a given tolerable emittance growth and driver-witness misalignment, including damping by energy spread, ion motion and plasma-density tailoring. An answer is an upper bound on efficiency versus tolerance, validated by simulation.
Settled by: A derived bound confirmed by 3D particle-in-cell simulations and by measured instability growth in a high-efficiency stage.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: partially-resolved
Status note: Lebedev, Burov and Nagaitsev (2017, https://doi.org/10.1103/PhysRevAccelBeams.20.121301) derived an efficiency-instability relation; 2025 simulations (arXiv:2501.10602) show it is only a lower limit on instability strength, and a bound including all damping mechanisms is lacking.
Posed since: 2017