Isaac Ghebregziabher

Generation of tunable, 100–800 MeV quasi-monoenergetic electron beams from a laser-wakefield accelerator in the blowout regime

In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30–80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100–800 MeV. These beams have angular divergence of 1–4 mrad, and 5%–25% energy spread, with a resulting brightness 1011 electrons mm−2 MeV−1 mrad−2. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.

Spectral bandwidth reduction of Thomson scattered light by pulse chirping

Based on single particle tracking in the framework of classical Thomson scattering with incoherent superposition, we developed a relativistic, three-dimensional numerical model that calculates and quantifies the characteristics of emitted radiation when a relativistic electron beam interacts with an intense laser pulse. This model has been benchmarked against analytical expressions, based on the plane wave approximation to the laser field, derived by Esarey et al. [Phys. Rev. E 48, 3003 (1993)]. For laser pulses of sufficient duration, we find that the scattered radiation spectrum is broadened due to interferences arising from the pulsed nature of the laser. We find that by appropriately chirping the scattering laser pulse, spectral broadening can be minimized, and the peak on-axis brightness of the emitted radiation is increased by a factor of approximately 5.

Adaptive-feedback spectral-phase control for interactions with transform-limited ultrashort high-power laser pulses

Fourier-transform-limited light pulses were obtained at the laser-plasma interaction point of a 100-TW peak-power laser in vacuum. The spectral-phase distortion induced by the dispersion mismatching between the stretcher, compressor, and dispersive materials was fully compensated for by means of an adaptive closed-loop. The coherent temporal contrast on the sub-picosecond time scale was two orders of magnitude higher than that without adaptive control. This novel phase control capability enabled the experimental study of the dependence of laser wakefield acceleration on the spectral phase of intense laser light.

Repetitive petawatt-class laser with near-diffraction-limited focal spot and transform-limited pulse duration

A repetitive petawatt-class Ti:sapphire laser system operating with high spatial and temporal beam quality is demonstrated. Maximum pulse energy of 30 J is obtained via five multi-pass amplification stages. Closed-loop feedback control systems in the temporal and spatial domains are used to yield Fourier-transform-limited pulse duration (33.7 fs), and diffraction-limited focal spot sizes (with several different tight focusing optics). The laser parameters have been fully characterized at high-power, and are monitored in real-time, to ensure that they meet the experimental requirements for laser-wakefield electron acceleration and x-ray generation.

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.

Selective activation with all-laser-driven Thomson γ-rays

A bright, narrow band MeV γ-ray source-ray source based on Thomson scattering using a laser-driven electron accelerator has been developed. We discuss the application of this source for selective activation in regions of high particle (neutron or gamma) production, with minimal absorption in intervening materials.

Optical control of electron trapping and acceleration in plasma channels: Application to tunable, pulsed sources of multi-color thomson gamma-rays

Summary form only given. Reducing the size of a GeV-scale laser-plasma accelerator to a few millimeters requires maintaining an accelerating gradient as high as 10 GV/cm. This, in turn, dictates acceleration in the blowout regime in high-density plasmas (n0 ~1019 cm-3). With current high-power laser technology, these highly dispersive plasmas are poorly suited as accelerating media. They transform the driving pulse into a relativistic optical shock long before electron dephasing, causing the plasma wake bucket (electron density bubble) to constantly expand and trap background electrons, degrading the beam quality [1, 2]. We show that this can be overcome using a high-bandwidth driver, with up to 400 nm initial bandwidth [2-4]. Introducing a large negative chirp (to compensate for the nonlinear frequency red-shift) and propagating the pulse in a plasma channel (to suppress diffraction of its leading edge) delays pulse self-steepening through electron dephasing and extends the dephasing length. As a result, continuous injection is suppressed, and electron energy is boosted to a GeV level [2, 4]. In addition, periodic self-injection in the channel may produce a sequence of background-free, quasi-monoenergetic bunches with a femtosecond-duration, controllable time delay and energy difference. The number of spectral components, their charge, energy, and energy separation can be controlled by varying the channel radius and length, whereas accumulation of the noise (viz. continuously injected charge) is prevented by the proper dispersion control of the driver via the negative chirp [4]. This level of control is hard to achieve with conventional accelerator techniques. Using the newly-developed relativistic 3D nonlinear Thomson scattering code [5], it is demonstrated that these clean, polychromatic beams can drive high-brightness, tunable, multi-color γ-ray sources.

Background-free, quasi-monoenergetic electron beams from a self-injected laser wakefield accelerator

Stable 200–400-MeV quasi-monoenergetic electron bunches (ΔE/E<10%), ∼ 10-pC charge, and no dark-current are produced when a self-injected laser plasma accelerator is optimized. PIC simulations demonstrate these beams are produced near the threshold for self-injection.