Eric H. Esarey

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.

Resonantly driven laser‐plasma electron accelerators

A method for generating large‐amplitude nonlinear plasma waves, which utilizes an optimized train of independently adjustable, intense laser pulses, is analyzed in 1‐D both theoretically and numerically (using both Maxwell‐fluid and particle‐in‐cell codes). Optimal pulse widths and interpulse spacings are computed for pulses with either square or finite‐risetime sine shapes. A resonant region of the plasma wave phase space is found where the plasma wave is driven by the laser most efficiently. The width of this region, and thus the optimal finite‐risetime laser pulse width, was found to decrease with increasing plasma density and plasma wave amplitude, while the nonlinear plasma wavelength, and thus the optimal interpulse spacing, was found to increase. Also investigated are the resonance sensitivities to variations in the laser and plasma parameters. Non‐linear Landau damping of the wave by trapped background electrons is found to be important. Resonant excitation by this method is shown to more advantag...

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.

GeV electron beams from a centimeter-scale laser-driven plasma accelerator

Results are presented on the generation of quasi- monoenergetic electron beams with energy up to lGeV using a 40 TW laser and a 3.3 cm-long hydrogen-filled capillary discharge waveguide [1,2]. Electron beams were not observed without a plasma channel, indicating that self- focusing alone could not be relied upon for effective guiding of the laser pulse. Results are presented of the electron beam spectra, and the dependence of the reliability of producing electron beams as a function of laser and plasma parameters.

Performance of capillary discharge guided laser plasma wakefield accelerator

A GeV-class laser-driven plasma-based wakefield accelerator has been realized at the Lawrence Berkeley National Laboratory (LBNL). The device consists of the 40 TW high repetition rate Ti:sapphire LOASIS laser system at LBNL and a gas-filled capillary discharge waveguide developed at Oxford University. The operation of the capillary discharge guided laser plasma wakefield accelerator with a capillary of 225 mum diameter and 33 mm in length was analyzed in detail. The input intensity dependence suggests that excessive self-injection causes increased beam loading leading to broadband lower energy electron beam generation. The trigger versus laser arrival timing dependence suggests that the plasma channel parameters can be tuned to reduce beam divergence.

Optimization of the electron beam properties from intense laser pulses interacting with structured gas jets

Laser plasma acceleration has been intensely investigated for its ability to produce energetic, ultrashort electron bunches in a compact distance. A high intensity laser pulse propagating through a plasma expels the electrons from the optical axis via the ponderomotive force, leaving behind a column of ions and driving a density wake. The accelerating electric fields present in the wake can reach several orders of magnitude greater than those found in radio-frequency cavities, allowing for compact systems much smaller than those using conventional accelerators. This compact source can provide electrons for various applications including stages for a high energy collider or for production of x-ray pulses from coherent undulator radiation. However, these applications require tunable, stable and high-quality electron beams. We report on a study of controlled injection along a shock-induced density downramp of laser-plasma- accelerated electrons through precision tailoring of the density profile produced from a mm-scale gas jet. Using BELLA Center’s TREX Ti:Sapphire laser, the effects of the plasma density profile and the tilt of the shock front on the beam spatial profile, steering, and energy were investigated experimentally. To explain these rela- tionships, we propose simple models which agree well with experimental results. Using this technique, electron beam quality was tailored, allowing for the production of high-quality electron beams with percent-level energy spreads over a range of energies.

Narrow bandwidth Thomson photon source and diagnostic development using laser-plasma accelerators

Compact, high-quality photon sources at MeV energies are being developed based on Laser-Plasma Accelerators (LPAs), and these sources at the same time provide precision diagnostics of beam evolution to support LPA development. We review design of experiments and laser capabilities to realize a photon source, integrating LPA acceleration for compactness, control of scattering to increase photon flux, and electron deceleration to mitigate beam dump size. These experiments are developing a compact photon source system with the potential to enable new monoenergetic photon applications currently restricted by source size, including nuclear nonproliferation. Diagnostic use of the energy-angle spectra of Thomson scattered photons is presented to support development of LPAs to meet the needs of advanced high yield/low-energy-spread photon sources and future high energy physics colliders.

Two-color laser high-harmonic generation in cavitated plasma wakefields

A method is proposed for producing coherent x-rays via high-harmonic generation using a laser interacting with highly-stripped ions in cavitated plasma wakefields. Two laser pulses of different colors are employed: a long-wavelength pulse for cavitation and a short-wavelength pulse for harmonic generation. This method enables efficient laser harmonic generation in the sub-nm wavelength regime.

Design of a Compact ion Beam Transport System for the BELLA Ion Accelerator

The Berkeley Lab Laser Accelerator (BELLA) Center hosts a Ti:sapphire CPA laser providing laser pulses at petawatt-level peak power with a repetition rate of 1 Hz. High irradiances of 1022 W/cm2 can be achieved with a short focal length beamline when the laser is focused to a spot of w0<5 uf06dm. Under this condition, theoretical and particle-in-cell (PIC) simulations have shown that protons and helium ions at energies up to several hundred MeV/u can be expected from the interaction between BELLA laser pulses and different targets. Ion beams of high energies, low energy spread and with high controllability and stability have numerous potential applications. A preliminary ion optics design is presented to collect, transport, and focus the ions generated from the laserdriven ion accelerator.