Schematic of the experimental setup. The high-power laser pulse is focused using a 1-m focal length, dielectric coated, off-axis paraboloid at the front edge of a supersonic helium gas jet. The electron beam passes through a magnetic spectrometer consisting of a uniform rectangular magnetic field and a drift free region and then impinges on a LANEX screen. Optical emission from the LANEX is imaged onto the CCD1 to measure the beam angular divergence and energy spectrum, along the non-dispersive and dispersive axes respectively. The propagation of the optical pulse through the medium is monitored by imaging the Thomson-scattered light using CCD2. Inset: image of the laser focal spot in vacuum (full-power shot) and its vertical (blue/dark gray) and horizontal (red/light gray) lineouts.
Electron beams obtained from a laser-plasma accelerator in the low-power, high-density regime using underdense Helium target. (a) 20 TW, . (b) 20 TW, . In the former case, the electron beam is quasi-Maxwellian with no quasi-monoenergetic features. A monoenergetic feature (pointed at with an arrow) is observed in the lower density case (b); such features were observed in 10%–20% of shots with significant pointing and energy fluctuation. Raising the laser power beyond 30 TW does not help produce monoenergetic beams at these high densities.
Quasi-monoenergetic electron beams obtained with higher-power, lower-density plasma. (a) 30 TW, . (b) 40 TW, . Reduction of plasma density leads to increase of the beam energy, and decrease in the energy spread and divergence.
Images of spectrally dispersed electron beams as a function of laserpower and plasma density for two different acceleration lengths (a) P = 34 TW, ; (b) P = 42 TW, ; and (c)P = 58 TW, . Images (a) and (b) are obtained with a 3 mm jet. Image (c) is obtained with a 4 mm jet and a higher-resolution spectrometer. and denote the divergence angle in the horizontal and vertical direction.
Structure of the plasma wake at different stages of laser evolution (calder-circ simulation). Axis at the bottom of each panel shows the distance from the plasma border in microns; axis at the top of the panel shows the “co-moving” variable, z–it ct (also in microns; z = ct is the trajectory of the pulse maximum in vacuum). Density cuts are taken in the plane of laser polarization. The pulse focal plane is situated at a distance z = −1 mm from the plasma border. Plasma extends from z = 0 to 3 mm. (a) 1 mm inside the plasma, the pulse is not yet focused, and the wake is still weakly nonlinear and unbroken; (b) 1.5 mm (or roughly ) inside the plasma, the pulse eventually focuses and produces electron cavitation (the bubble); (c) the bubble starts expanding, initiating electron self-injection; (d) near the end of density plateau, the bubble stabilizes, self-injection terminates, and the quasi-monoenergetic beam forms.
Quasi-monoenergetic spectrum of electrons reaching the detector. Parameters of laboratory experiment correspond to 34 TW on target and the plasma density was . For these conditions, the measured electron energy, MeV (panel (a)), is in excellent agreement with the electron energy computed in the simulation of Fig. 5, MeV. In the simulation, electron beam exiting the plasma was propagated through a detector identical to that used in experiment using the GPT code. Resulting computed spectrum (axial lineout of the detector image) is presented in the panel (b).
Self-channeling of laser pulse through the plasma monitored by imaging the Thomson scattered light at 800 nm from the target. (a) 80 TW, . (b) 90 TW, . Based on an estimated Rayleigh range of 720 μm, (a) corresponds to propagation of the laser pulse over 7 Rayleigh ranges, while (b) corresponds to 14 Rayleigh ranges.
Spectrum of electrons obtained with 5 mm slit jet. The laser pulse was at 90 TW and the focus was located inside plasma, at 1.5 mm distance from the edge of the nozzle. On average, the divergence of presented electron beams is ∼10 mrad. The beam charge is rather high, 50–100 pC, but the energy spread and shot-to-shot fluctuations are significant as well. The mean energy is MeV, and the uncertainty in energy is primarily from the fluctuation in the beam pointing.
Spectrum of electron beam obtained with 80 TW laser power and 5 mm slit nozzle for (a) (b) . The laser is focused 1.5 mm inside the jet. 550 MeV electron beams result when the focus is moved to the front edge of the nozzle and the plasma density is reduced to .
(a) Spectrum of electron beam obtained with 90 TW laser power and 10 mm slit nozzle for plasma density and laser focused at the front of the nozzle. (b) Spectrum of electron beam obtained from CALDER-circ simulations with the same experimental geometry, plasma density , and 90 TW laser power on target. The measured energy is lower than the computed one on account of a non-ideal focus which results in a lowering of the effective laser power on target.
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