C. Bellei

Radiation pressure acceleration of thin foils with circularly polarized laser pulses

A new regime is described for radiation pressure acceleration of a thin foil by an intense laser beam of above 1020 W cm−2. Highly monoenergetic proton beams extending to giga-electron-volt energies can be produced with very high efficiency using circularly polarized light. The proton beams have a very small divergence angle (<4°). This new method allows the construction of ultra-compact proton and ion accelerators with ultra-short particle bursts.

Targets for direct-drive fast ignition at total laser energy of 200-400 kJ

Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400kJ (with less than 100kJ for the igniting beams) are discussed by means of a simple integrated gain model. Gain curves are generated and their sensitivity to several parameters is analyzed. A family of scaled target is designed, based on 1D hydrodynamic simulations of the implosion stage and 2D model simulations of ignition and burn. It is found that ignition and propagating burn can be achieved by targets compressed by 100–150kJ, properly shaped laser pulses (with wavelength λc=0.35μm), and ignited by 80–100kJ pulses. This requires adiabat shaped implosions to limit Rayleigh-Taylor instability, at the same time keeping the fuel entropy at a very low level. In addition, the igniting beam should be coupled to the fuel with an efficiency of about 25%, and the hot-electron average penetration depth should be at most 1.2–1.5g∕cm2. According to the present understanding of ultraintense laser-matter interaction, t...

Fast-ignition transport studies: Realistic electron source, integrated particle-in-cell and hydrodynamic modeling, imposed magnetic fields

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell (PIC) code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum and a divergent angle spectrum (average velocity-space polar angle of 52°). Transport simulations with the PIC-based divergence do not ignite for >1 MJ of fast-electron energy, for a modest (70 μm) standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ∼50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility hav...

Fast ignitor target studies for the HiPER project

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3ω (wavelength λ=0.35μm), multibeam, multi-ns pulse of about 250kJ and a 2ω or 3ω pulse of 70–100kJ in about 15ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency ηig of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency ηig exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2ω or 3ω ...

Dynamic Control of Laser-Produced Proton Beams

The emission characteristics of intense laser driven protons are controlled using ultrastrong (of the order of 10(9) V/m) electrostatic fields varying on a few ps time scale. The field structures are achieved by exploiting the high potential of the target (reaching multi-MV during the laser interaction). Suitably shaped targets result in a reduction in the proton beam divergence, and hence an increase in proton flux while preserving the high beam quality. The peak focusing power and its temporal variation are shown to depend on the target characteristics, allowing for the collimation of the inherently highly divergent beam and the design of achromatic electrostatic lenses.

Third harmonic order imaging as a focal spot diagnostic for high intensity laser-solid interactions

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime ( 10(19) Wcm(-2) is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (> 10(20) Wcm(-2)) an important application for the field of high laser contrast interaction science.

Fast ignition: Dependence of the ignition energy on source and target parameters for particle-in-cell-modelled energy and angular distributions of the fast electrons

The energy and angular distributions of the fast electrons predicted by particle-in-cell (PIC) simulations differ from those historically assumed in ignition designs of the fast ignition scheme. Using a particular 3D PIC calculation, we show how the ignition energy varies as a function of source-fuel distance, source size, and density of the pre-compressed fuel. The large divergence of the electron beam implies that the ignition energy scales with density more weakly than the ρ−2 scaling for an idealized beam [S. Atzeni, Phys. Plasmas 6, 3316 (1999)], for any realistic source that is at some distance from the dense deuterium-tritium fuel. Due to the strong dependence of ignition energy with source-fuel distance, the use of magnetic or electric fields seems essential for the purpose of decreasing the ignition energy.

Plasma adiabatic lapse rate.

The plasma analog of an adiabatic lapse rate (or temperature variation with height) in atmospheric physics is obtained. A new source of plasma temperature gradient in a binary ion species mixture is found that is proportional to the concentration gradient ∇α and difference in average ionization states Z(2)-Z(1). Application to inertial-confinement-fusion implosions indicates a potentially strong effect in plastic (CH) ablators that is not modeled with mainline (single-fluid) simulations. An associated plasma thermodiffusion coefficient is derived, and charge-state diffusion in a single-species plasma is also predicted.

Comparative study of betatron radiation from laser-wakefield and direct-laser accelerated bunches of relativistic electrons

The dynamics of relativistic electrons in a laser driven plasma cavity are studied via measurements of their radiation. For ultrashort laser pulses at comparatively low focused laser intensities (3 < a0 < 10), low density and long f-number of 10, electrons are predominantly accelerated in the wakefield leading to quasi-monoenergetic collimated electron beams and well collimated (< 12 mrad) beams of comparatively soft x-rays (1-10 keV) with unprecedented small source size (2-3 μm). For laser pulses with increasing laser intensity (10 < a0 < 30), density and short f-number (< 5), electrons are accelerated directly by the laser, leading to divergent quasimaxwellian electron beams and divergent (50-95°) beams of hard x-rays (20-50 keV) with relatively large source size (> 100 μm). In both cases, the measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. At low laser intensity transverse oscillations are small as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser. A betatron resonance leads to a tenfold increase in transverse oscillation amplitude and electrons enter a highly radiative regime with up to 5% of their energy converted into x-rays.

Self-Guided Wakefield Experiments Driven by Petawatt-Class Ultrashort Laser Pulses

We investigate the extension of self-injecting laser wakefield experiments to the regime that will be accessible with the next generation of petawatt-class ultrashort pulse laser systems. Using nonlinear scalings, current experimental trends, and numerical simulations, we determine the optimal laser and target parameters, i.e., focusing geometry, plasma density, and target length, that are required to increase the electron beam energy (to >1 GeV) without the use of external guiding structures.