G. R. Plateau

X-ray Emission from Electron Betatron Motion in a Laser-Plasma Accelerator

Single‐shot x‐ray spectra from electron bunches produced by a laser‐plasma wakefield accelerator (LPA) [1, 2] were measured using a photon‐counting single‐shot pixelated Silicon‐based detector [3], providing for the first time single‐shot direct spectra without assumptions required by filter based techniques. In addition, the electron bunch source size was measured by imaging a wire target, demonstrating few micron source size and stability. X‐rays are generated when trapped electrons oscillate in the focusing field of the wake trailing the driver laser pulse [4, 5, 6, 7, 8]. In addition to improving understanding of bunch emittance and wake structure, this provides a broadband, synchronized femtosecond source of keV x‐rays. Electron bunch spectra and divergence were measured simultaneously and preliminary analysis shows correlation between x‐ray and electron spectra. Bremsstrahlung background was managed using shielding and magnetic diversion.

Colliding Laser Pulses for Laser‐Plasma Accelerator Injection Control

Decoupling injection from acceleration is a key challenge to achieve compact, reliable, tunable laser‐plasma accelerators (LPA) [1, 2]. In colliding pulse injection the beat between multiple laser pulses can be used to control energy, energy spread, and emittance of the electron beam by injecting electrons in momentum and phase into the accelerating phase of the wake trailing the driver laser pulse [3, 4, 5, 6, 7]. At LBNL, using automated control of spatiotemporal overlap of laser pulses, two‐pulse experiments showed stable operation and reproducibility over hours of operation. Arrival time of the colliding beam was scanned, and the measured timing window and density of optimal operation agree with simulations [8]. The accelerator length was mapped by scanning the collision point.

High energy, low energy spread electron bunches produced via colliding pulse injection

Electron beams of high energy and narrow, controllable energy spread using significantly reduced laser power are demonstrated by combining control of Laser-Plasma Accelerator (LPA) structure and injection. A high-energy accelerating structure was formed by controlling the phase front of the drive laser in order to obtain collimated propagation over the length of the plasma. This produced electron energies nearly double those previously achieved using comparable lasers. Injection into the accelerator was controlled by using the beat between “colliding” laser pulses to kick electrons at a controlled location into a plasma wave that was operated below the threshold for self injection. This resulted in the production of bunches with controllable energy. Stability of charge, pointing, and energy were demonstrated. With the injection location fixed by the colliding pulses, beam energy up to 200u2005MeV was obtained using 10 TW drive laser pulses, controlled by plasma density and by target location with respect to t...

Low-emittance electron bunches from a laser-plasma accelerator measured using single-shot X-ray spectroscopy

X-ray spectroscopy is used to obtain single-shot information on electron beam emittance in a low-energy-spread 0.5 GeV-class laser-plasma accelerator. Measurements of betatron radiation from 2 to 20 keV used a CCD and single-photon counting techniques. By matching x-ray spectra to betatron radiation models, the electron bunch radius inside the plasma is estimated to be ~0.1 μm. Combining this with simultaneous electron spectra, normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in-cell simulations. Correlations of the bunch radius with electron beam parameters are presented.

Controlling electron injection in laser plasma accelerators using multiple pulses

Use of counter-propagating pulses to control electron injection in laser-plasma accelerators promises to be an important ingredient in the development of stable devices. We discuss the colliding pulse scheme and associated diagnostics.

Optical Sideband Generation: a Longitudinal Electron Beam Diagnostic Beyond the Laser Bandwidth Resolution Limit

Electro‐optic sampling (EOS) is widely used as a technique to measure THz‐domain electric field pulses such as the self‐fields of femtosecond electron beams. We present an EOS‐based approach for single‐shot spectral measurement that excels in simplicity (compatible with fiber integration) and bandwidth coverage (overcomes the laser bandwidth limitation), allowing few‐fs electron beams or single‐cycle THz pulses to be characterized with conventional picosecond probes. It is shown that the EOS‐induced optical sidebands on the narrow‐bandwidth optical probe are spectrally‐shifted replicas of the THz pulse. An experimental demonstration on a 0–3 THz source is presented.

Development of High Gradient Laser Wakefield Accelerators Towards Nuclear Detection Applications at LBNL

Compact high-energy linacs are important to applications including monochromatic gamma sources for nuclear material security applications. Recent laser wakefield accelerator experiments at LBNL demonstrated narrow energy spread beams, now with energies of up to 1 GeV in 3 cm using a plasma channel at low density. This demonstrates the production of GeV beams from devices much smaller than conventional linacs, and confirms the anticipated scaling of laser driven accelerators to GeV energies. Stable performance at 0.5 GeV was demonstrated. Experiments and simulations are in progress to control injection of particles into the wake and hence to improve beam quality and stability. Using plasma density gradients to control injection, stable beams at 1 MeV over days of operation, and with an order of magnitude lower absolute momentum spread than previously observed, have been demonstrated. New experiments are post-accelerating the beams from controlled injection experiments to increase beam quality and stability. Thomson scattering from such beams is being developed to provide collimated multi-MeV monoenergetic gamma sources for security applications from compact devices. Such sources can reduce dose to target and increase accuracy for applications including photofission and nuclear resonance fluorescence.

Ultrafast Diagnostics for Electron Beams from Laser Plasma Accelerators

We present an overview of diagnostic techniques for measuring key parameters of electron bunches from Laser Plasma Accelerators (LPAs). The diagnostics presented here were chosen because they highlight the unique advantages (e.g., diverse forms of electromagnetic emission) and difficulties (e.g., shot-to-shot variability) associated with LPAs. Non destructiveness and high resolution (in space and time and energy) are key attributes that enable the formation of a comprehensive suite of simultaneous diagnostics which are necessary for the full characterization of the ultrashort, but highly-variable electron bunches from LPAs.