Accordion effect revisited: Generation of comb-like electron beams in plasma channels

Abstract

International audiencePropagating a short, relativistically intense laser pulse in a plasma channel makes it possible to generate comb-like electron beams – sequences of synchronized, low phase-space volume bunches with controllable energy difference. The tail of the pulse, confined in the accelerator cavity (electron density “bubble”), transversely flaps, as the pulse head steadily self-guides. The resulting oscillations of the cavity size cause periodic injection of electrons from ambient plasma, creating an energy comb with the number of components, their energy, and energy separation dependent on the channel radius and pulse length. Accumulation of noise (continuously injected charge) can be prevented using a negatively chirped drive pulse with a bandwidth close to a one-half of the carrier wavelength. These comb-like beams can drive tunable, multi-color γ-ray sources

Femtosecond pulse trains of polychromatic inverse Compton γ-rays from designer electron beams produced by laser-plasma acceleration in plasma channels

“…The simulation with a 30 fs-length, transformlimited pulse (τ L = 30 fs, κ = 0), is referred to as the reference case [24]. To suppress diffraction of the pulse leading edge and to controllably obtain comb-like electron beams, we introduce a parabolic leaky channel [5]. To avoid pulse self-steepening in the course of propagation in a plasma, we introduce a frequency bandwidth equivalent to a 5 fs transform-limited duration, and temporally advance the higher frequencies.…”

“…The results presented below correspond to a 30 fs NCP (κ = 2.4323) propagating in a uniform plasma and in a channel matched to the self-guided spot size and to a 20 fs NCP (κ = 1.968) propagating in a channel matched to the spot size of the incident pulse. Details of electron beam shaping process and statistics of the resulting "designer" beams are reported elsewhere [5].…”

“…In particular, it is possible to generate trains of fs-length bunches with a low phase space volume and controlled separation in energy [2][3][4][5]. These comb-like beams may find a unique application as drivers of all-optical γ-ray sources [6][7][8][9][10][11] based on the inverse Compton scattering (ICS) mechanism [12][13][14].…”

“…The radiation pressure of the pulse creates an electron density cavity ("bubble") that evolves in lock-step with the optical driver, making it possible to trap ambient plasma electrons, eliminating the need for an external photocathode [23]. Propagating the pulse in an external plasma channel may, under the right circumstances, destabilize the bubble, bringing about oscillations of its size (the accordion effect) and periodic self-injection, shaping the electron beam into a synchronized sequence of QM bunches with a 100 pC-scale charge [2][3][4][5]. Unwanted continuous self-injection (a.k.a.…”

“…Unwanted continuous self-injection (a.k.a. dark current) and accumulation of a massive low-energy tail can be suppressed by using a negatively chirped drive pulse (NCP) with a bandwidth close to a one-half of the carrier wavelength [4,5,24]. Changing the NCP length (as permitted by the large bandwidth) and the channel radius enables control over the number of components in the energy comb and, hence, in the spectrum of polychromatic ICS γ-rays.…”