Abstract
Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb 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 picosecond-time-resolved, grazing-angle optical shadowgraphic measurements and large-scale particle-in-cell 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 metres-per-second 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 impact-ionizing surrounding neutral lithium. The results provide a basis for understanding global thermodynamics of multi-GeV plasma accelerators, which underlie their viability for applications demanding high average beam current.
Highlights
Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb electron bunches
Simulations reveal that the initial wake transfers energy into the surrounding medium via the following sequence of events: the initial wake breaks, expelling fast electrons from the plasma; radial electric fields arise that propel ions outward at tens of keV while escorting electrons; outwardly streaming electrons and ions ionize and excite surrounding neutral lithium, expanding plasma volume several hundred-fold
Unlike small-scale plasma wakefield accelerator experiments, in which parameters of drive e-bunches delivered from a tabletop laser wakefield accelerator could only be estimated[5], here e-bunches from SLAC were characterized with high precision
Summary
Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb electron bunches. The present study aims to understand the fundamental processes by which plasma accelerator structures at ne ~ 1017 cm−3 dissipate their energy into surrounding plasma, and to evaluate their global energy budget over a nanosecond (ns) time scale It builds a thermodynamic foundation on which future engineering solutions of the heat management problem can be based. No experiments at any ne have yet explored how the enormous energy density stored in a nonlinear wake redistributes over nanoseconds among accelerated electrons, undirected hot electrons, freely streaming ions, radiation, electrostatic fields, and ionization of surrounding gas, as well as collective ion motion Understanding this complex process at ne ~ 1017 cm−3 demands experiments with precisely characterized multi-GeV drive bunches (or petawatt laser pulses), probes that track particle and energy flow over millimeters, and simulation of multifarious plasma processes over nanoseconds. Benchmarking simulated plasma expansion against measurements quantifies energy retention in the plasma column and elucidates mechanisms that drive its expansion
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
