Csaba Toth

Laser guiding for GeV laser–plasma accelerators

Guiding of relativistically intense laser beams in preformed plasma channels is discussed for development of GeV-class laser accelerators. Experiments using a channel guided laser wakefield accelerator at Lawrence Berkeley National Laboratory (LBNL) have demonstrated that near mono-energetic 100 MeV-class electron beams can be produced with a 10 TW laser system. Analysis, aided by particle-in-cell simulations, as well as experiments with various plasma lengths and densities, indicate that tailoring the length of the accelerator, together with loading of the accelerating structure with beam, is the key to production of mono-energetic electron beams. Increasing the energy towards a GeV and beyond will require reducing the plasma density and design criteria are discussed for an optimized accelerator module. The current progress and future directions are summarized through comparison with conventional accelerators, highlighting the unique short-term prospects for intense radiation sources based on laser-driven plasma accelerators.

Demonstration of a plasma mirror based on a laminar flow water film

A plasma mirror based on a laminar water film with low flow speed (0.5–2 cm/s) has been developed and characterized, for use as an ultrahigh intensity optical reflector. The use of flowing water as a target surface automatically results in each laser pulse seeing a new interaction surface and avoids the need for mechanical scanning of the target surface. In addition, the breakdown of water does not produce contaminating debris that can be deleterious to vacuum chamber conditions and optics, such as is the case when using conventional solid targets. The mirror exhibits 70% reflectivity, while maintaining high-quality of the reflected spot.

Contrast Enhancement of the LOASIS CPA Laser and Effects on Electron Beam Performance of LWFA

A nonlinear optical pulse cleaning technique based on cross‐polarized wave (XPW) generation filtering [1] has been implemented to improve laser pulse contrast, and consequently to control pre‐ionization in laser‐plasma accelerator experiments. Three orders of magnitude improvement in pre‐pulse contrast has been achieved, resulting in 4‐fold increase in electron charge and improved stability of both the electron beam energy and THz radiation generated as a secondary process in the gas‐jet‐based LWFA experiments.

Powerful, pulsed, THz radiation from laser accelerated relativistic electron bunches

Coherent THz radiation was produced from relativistic electron bunches of subpicosecond duration. The electron beam was produced by strongly focused (≈ 6 μm), high peak power (up to 10 TW), ultra-short (≥50 fs) laser pulses of a 10 Hz repetition rate Ti:sapphire chirped pulse amplification (CPA) laser system via self-modulated laser wakefield acceleration (SM-LWFA) in a high density (> 1019 cm-3) pulsed gas jet. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation yield in the 0.3 to 19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured total radiated energy in the THz range for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be applied as a useful tool for measuring the properties of laser accelerated bunches at the exit of the plasma accelerator.

Thomson scattering from laser wakefield accelerators

The production of ultrashort (fs) x‐ray pulses through Thomson scattering of high power laser pulses off relativistic electron bunches generated by laser‐plasma accelerators is discussed. These accelerators can typically produce two types of electron bunches: i) a high charge bunch with a broad energy distribution, resulting in x‐ray pulses with high flux and a broadband spectrum, and ii) electron bunches with a narrow energy distribution, resulting in an ultrashort x‐ray pulse with a nearly monochromatic spectrum. Nonlinear Thomson scattering (high laser intensity) will be considered, as well as the effects of the laser pulse profile. Thomson scattering may provide a useful diagnostic for determining the electron bunch properties via the scattering x‐ray spectrum.

Staging Laser Plasma Accelerators for Increased Beam Energy

Staging laser plasma accelerators is an efficient way of mitigating laser pump depletion in laser driven accelerators and necessary for reaching high energies with compact laser systems. The concept of staging includes coupling of additional laser energy and transporting the electron beam from one accelerating module to another. Due to laser damage threshold constraints, in‐coupling laser energy with conventional optics requires distances between the accelerating modules of the order of 10 m, resulting in decreased average accelerating gradient and complicated e‐beam transport. In this paper we use basic scaling laws to show that the total length of future laser plasma accelerators will be determined by staging technology. We also propose using a liquid jet plasma mirror for in‐coupling the laser beam and show that it has the potential to reduce distance between stages to the cm‐scale.

Transition of the BELLA PW laser system towards a collaborative research facility in laser plasma science

The advancement of Laser-Plasma Accelerators (LPA) requires systematic studies with ever increasing precision and reproducibility. A key component of such a research endeavor is a facility that provides reliable, well characterized laser sources, flexible target systems, and comprehensive diagnostics of the laser pulses, the interaction region, and the produced electron beams. The Berkeley Lab Laser Accelerator (BELLA), a PW laser facility, now routinely provides high quality focused laser pulses for high precision experiments. A description of the commissioning process, the layout of the laser systems, the major components of the laser and radiation protection systems, and a summary of early results are given. Further scientific plans and highlights of operational experience that serve as the basis for transition to a collaborative research facility in high-peak power laser-plasma interaction research are reviewed.The advancement of Laser-Plasma Accelerators (LPA) requires systematic studies with ever increasing precision and reproducibility. A key component of such a research endeavor is a facility that provides reliable, well characterized laser sources, flexible target systems, and comprehensive diagnostics of the laser pulses, the interaction region, and the produced electron beams. The Berkeley Lab Laser Accelerator (BELLA), a PW laser facility, now routinely provides high quality focused laser pulses for high precision experiments. A description of the commissioning process, the layout of the laser systems, the major components of the laser and radiation protection systems, and a summary of early results are given. Further scientific plans and highlights of operational experience that serve as the basis for transition to a collaborative research facility in high-peak power laser-plasma interaction research are reviewed.

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.

Stable electron beams with low absolute energy spread from A laser wakefield accelerator with plasma density ramp controlled injection

Laser wakefield accelerators produce accelerating gradients up to hundreds of GeV/m, and recently demonstrated 1-10 MeV energy spread at energies up to 1 GeV using electrons self-trapped from the plasma. Controlled injection and staging may further improve beam quality by circumventing tradeoffs between energy, stability, and energy spread/emittance. We present experiments demonstrating production of a stable electron beam near 1 MeV with hundred-keV level energy spread and central energy stability by using the plasma density profile to control self- injection, and supporting simulations. Simulations indicate that such beams can be post accelerated to high energies, potentially reducing momentum spread in laser accelerators by 100-fold or more.

Innershell femtosecond x-ray lasers pumped by Larmor radiation and characteristics of Larmor radiation

High-repetition-rate, femtosecond x-ray lasers would be useful for dynamical study of ultra-fast phenomena in nature. One of routes to get fs x-ray lasers is to use inner-shell processes in atomic and ionic systems. In this paper, the two inner-shell schemes recently proposed will be reviewed and compared in detail. One of important issues using inner-shell schemes is fast and intense x-ray pumping source. One of good candidate sources for that purpose is Larmor radiation produced by electrons under an intense fs laser field. The relativistic, nonlinear Thomson scattering by an electron of an intense laser field is investigated in computer simulation. Under a laser field with a pulse duration of 20 fs Full Width Half Maximum and an intensity of 1020 W/cm2, the motion of an electron is highly relativistic and generates an ultra-short radiation of 2 attoseconds with photon energies of 100 to 600 eV. An interesting modulated structure of the spectrum is observed and analyzed. A radiation produced by the zigzag motion of an electron under a linearly polarized laser has better characteristics than by a helical motion under a circularly polarized laser pulse in terms of an angular divergence and an energy spectrum.