Serge Y. Kalmykov

Optical control of electron phase space in plasma accelerators with incoherently stacked laser pulsesa)

It is demonstrated that synthesizing an ultrahigh-bandwidth, negatively chirped laser pulse by incoherently stacking pulses of different wavelengths makes it possible to optimize the process of electron self-injection in a dense, highly dispersive plasma ( n0∼1019 cm−3). Avoiding transformation of the driving pulse into a relativistic optical shock maintains a quasi-monoenergetic electron spectrum through electron dephasing and boosts electron energy far beyond the limits suggested by existing scaling laws. In addition, evolution of the accelerating bucket in a plasma channel is shown to produce a background-free, tunable train of femtosecond-duration, 35–100 kA, time-synchronized quasi-monoenergetic electron bunches. The combination of the negative chirp and the channel permits acceleration of electrons beyond 1 GeV in a 3 mm plasma with 1.4 J of laser pulse energy, thus offering the opportunity of high-repetition-rate operation at manageable average laser power.

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

Propagating a short, relativistically intense laser pulse in a plasma channel makes it possible to generate comb-like electron beams for advanced radiation sources. The ponderomotive force of the leading edge of the pulse expels all electrons facing the pulse. The bare ions attract the ambient plasma electrons, forming a closed bubble of electron density confining the pulse tail. The cavity of electron density evolves slowly, in lock-step with the optical driver, and readily traps background electrons. The combination of a bubble (a self-consistently maintained, “soft” hollow channel) and a preformed channel forces transverse flapping of the laser pulse tail, causing oscillations in the bubble size. The resulting periodic injection produces a sequence of background-free, quasi-monoenergetic bunches of femtosecond duration. The number of these spectral components, their charge, energy, and energy separation is sensitive to 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. As a result of dispersion compensation, self-steepening of the pulse is reduced, and continuous injection almost completely suppressed. This level of control on a femtosecond time scale is hard to achieve with conventional accelerator techniques. These comb-like beams can drive high-brightness, tunable, multi-color γ-ray sources.

Controlled generation of comb-like electron beams in plasma channels for polychromatic inverse Thomson γ-ray sources

Propagating a relativistically intense, negatively chirped laser pulse (the bandwidth >150 nm) in a plasma channel makes it possible to generate background-free, comb-like electron beams—sequences of synchronized bunches with a low phase-space volume and controlled energy spacing. The tail of the pulse, confined in the accelerator cavity (an electron density ‘bubble’), experiences periodic focusing, while the head, which is the most intense portion of the pulse, steadily self-guides. Oscillations of the cavity size cause periodic injection of electrons from the ambient plasma, creating an electron energy comb with the number of components, their mean energy, and energy spacing dependent on the channel radius and pulse length. These customizable electron beams enable the design of a tunable, all-optical source of pulsed, polychromatic γ-rays using the mechanism of inverse Thomson scattering, with up to ∼10 −5 conversion efficiency from the drive pulse in the electron accelerator to the γ-ray beam. Such a source may radiate ∼10 7 quasi-monochromatic photons per shot into a microsteradian-scale cone. The photon energy is distributed among several distinct bands, each having sub-30% energy spread, with a highest energy of 12.5 MeV.

Multi-Color γ-Rays from Comb-Like Electron Beams Driven by Incoherent Stacks of Laser Pulses

Trains of fs-length, GeV-scale electron bunches with controlled energy spacing and a 5-D brightness up to 1017 A/m2 may be produced in a mm-scale uniform plasma. The main element of the scheme is an incoherent stack of 10-TW-scale laser pulses of different colors, with mismatched focal spots, with the highest-frequency pulse advanced in time. While driving an electron density bubble, this stack remains almost proof against nonlinear red-shift and self-compression. As a consequence, the unwanted continuous injection of background electrons is minimized. Weak focusing of the trailing (lower-frequency) component of the stack enforces expansions and contractions of the bubble, inducing controlled periodic injection. The resulting train of electron bunches maintains exceptional quality while being accelerated beyond the energy limits predicted by accepted scalings. Inverse Thomson scattering from this comb-like beam generates a sequence of quasi-monochromatic, fs-length γ-ray beams, an asset for nuclear forens...

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

Propagating 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

Propagating a short, relativistically intense laser pulse in a plasma channel makes it possible to generate clean comb-like electron beams – sequences of synchronized, low phase-space volume bunches with controllable energy spacing [S. Y. Kalmykov et al., “Accordion Effect Revisited: Generation of Comb-Like Electron Beams in Plasma Channels,” in Advanced Accelerator Concepts: 16th Workshop, AIP Conference Proceedings; this volume]. All-optical control of the electron beam phase space structure via manipulation of the drive pulse phase (negative chirp) and parameters of the channel enables the design of a tunable, all-optical source of polychromatic pulsed γ-rays using the mechanism of inverse Compton scattering.