Abstract
Recent experiments with 100 terawatt-class, sub-50 femtosecond laser pulses show that electrons self-injected into a laser-driven electron density bubble can be accelerated above 0.5 gigaelectronvolt energy in a sub-centimetre-length rarefied plasma. To reach this energy range, electrons must ultimately outrun the bubble and exit the accelerating phase; this, however, does not ensure high beam quality. Wake excitation increases the laser pulse bandwidth by red-shifting its head, keeping the tail unshifted. Anomalous group velocity dispersion of radiation in plasma slows down the red-shifted head, compressing the pulse into a few-cycle-long piston of relativistic intensity. Pulse transformation into a piston causes continuous expansion of the bubble, trapping copious numbers of unwanted electrons (dark current) and producing a poorly collimated, polychromatic energy tail, completely dominating the electron spectrum at the dephasing limit. The process of piston formation can be mitigated by using a broad-bandwidth (corresponding to a few-cycle transform-limited duration), negatively chirped pulse. Initial blue-shift of the pulse leading edge compensates for the nonlinear frequency red-shift and delays the piston formation, thus significantly suppressing the dark current, making the leading quasi-monoenergetic bunch the dominant feature of the electron spectrum near dephasing. This method of dark current control may be feasible for future experiments with ultrahigh-bandwidth, multi-joule laser pulses.
Highlights
The electron rest mass, n0 is the background electron density and e is the electron charge
Contrary to the earlier interpretation [78], the contribution of beam loading to the bubble expansion is not dominant, and we associate the origin of dark current with a nonlinear optical effect—laser pulse self-compression
A time-varying electron density bubble created by the radiation pressure of a tightly focused laser pulse guides the pulse through a uniform, rarefied plasma, traps ambient plasma electrons and accelerates them to GeV-level energies
Summary
We examine various scenarios of electron self-injection and acceleration until dephasing and relate them to nonlinear dynamics of the laser pulse. The plasma response to the timeaveraged ponderomotive force is calculated assuming that all plasma electrons (macroparticles) and ions (treated as a non-relativistic fluid) eventually fall behind the laser pulse and the bubble This approach, termed the quasi-static approximation [89], greatly speeds up the simulation, enabling serial runs and parameter scans using small workstations. To capture the laser pulse interaction with non-quasi-static background electrons (and to model self-injection into non-stationary quasi-static wake fields), a group of quiescent test electrons is placed before the laser pulse at each time step. CALDERCirc having fully self-consistent macroparticle dynamics yields the complete electron phase space, and allows the determination of the injected charge and beam emittance
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