B.M. Hegelich

Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments.

A double-foil target for improving beam quality in laser ion acceleration with thin foilsa)

A double-foil target is proposed for laser ion acceleration with thin targets to take advantage of high efficiency of such targets while avoiding beam degradation in late stage of acceleration. Laser heating of electrons co-moving with the ion beam is stopped by the second foil. It is found that the second foil can also modify and substantially improve the spectral and spatial properties of the ion beam and reduce the temperature of the co-moving electrons, leading to better preservation of the beam quality. Details of the dynamics are studied with particle-in-cell simulations.

Pulse shape measurements using single shot-frequency resolved optical gating for high energy (80 J) short pulse (600 fs) laser.

Relevant to laser based electron/ion accelerations, a single shot second harmonic generation frequency resolved optical gating (FROG) system has been developed to characterize laser pulses (80 J, ∼600 fs) incident on and transmitted through nanofoil targets, employing relay imaging, spatial filter, and partially coated glass substrates to reduce spatial nonuniformity and B-integral. The device can be completely aligned without using a pulsed laser source. Variations of incident pulse shape were measured from durations of 613 fs (nearly symmetric shape) to 571 fs (asymmetric shape with pre- or postpulse). The FROG measurements are consistent with independent spectral and autocorrelation measurements.

Fast ignition driven by quasi-monoenergetic ions: Optimal ion type and reduction of ignition energies with an ion beam array

Fast ignition of inertial fusion targets driven by quasi-monoenergetic ion beams is investigated by means of numerical simulations. Light and intermediate ions such as lithium, carbon, aluminum and vanadium have been considered. Simulations show that the minimum ignition energies of an ideal configuration of compressed Deuterium-Tritium are almost independent on the ion atomic number. However, they are obtained for increasing ion energies, which scale, approximately, as Z2, where Z is the ion atomic number. Assuming that the ion beam can be focused into 10 ?m spots, a new irradiation scheme is proposed to reduce the ignition energies. The combination of intermediate Z ions, such as 5.5 GeV vanadium, and the new irradiation scheme allows a reduction of the number of ions required for ignition by, roughly, three orders of magnitude when compared with the standard proton fast ignition scheme.

Gas-filled hohlraum experiments at the National Ignition Facility

Experiments done at the National Ignition Facility laser [J. A. Paisner, E. M. Campbell, and W. Hogan, Fusion Technol. 26, 755 (1994)] using gas-filled hohlraums demonstrate a key ignition design feature, i.e., using plasma pressure from a gas fill to tamp the hohlraum-wall expansion for the duration of the laser pulse. Moreover, our understanding of hohlraum energetics and the ability to predict the hohlraum soft-x-ray drive has been validated in ignition-relevant conditions. Finally, the laser reflectivity from stimulated Raman scattering in the fill plasma, a key threat to hohlraum performance, is shown to be suppressed by choosing a design with a sufficiently high ratio of electron temperature to density.

Physics: A tabletop neutron source

The molecular clock not only controls the rhythmic expression of genes with physiological roles, but also regulates the creation of ribosomes — molecular machines that translate messenger RNA into protein. Frédéric Gachon at the University of Lausanne in Switzerland and his colleagues found that mRNAs that encode components of the translation machinery — including some involved in making ribosomes — are rhythmically expressed in the livers of mice. Production of these RNAs peaks in the nocturnal animals shortly before nightfall, when the energy needed for protein synthesis is most likely to be available.

Recent progress on ion-driven fast ignition

We report on the encouraging progress from research on fusion fast ignition (FI) initiated by laser-driven ion beams. Compared to electrons, FI based on a beam of quasi-mono-energetic ions (including protons and especially heavier ions such as C) has the advantage of a more localized energy deposition, which minimizes the required total beam energy. High-current, laser-driven ion beams are very promising for this purpose, and because of their ultra-low transverse emittance, these beams may be focused to the required dimension, ∼ tens of microns. Because they are created in ps timescales, these beams can deliver the power required to ignite the compressed D-T fuel, ∼ 10 kJ / 50 ps. Our recent integrated calculations of ion-based FI include high fusion gain targets and a proof of principle experiment, which indicate the concept is feasible. The beam requirements derived from high-gain calculations using realistic DT fuel implosion are presented. The scientific issues and technical issues in the generation of the required laser-driven ion beams, and recent progress in their realization, are summarized.

PW performance ion acceleration from the LANL 200TW Trident laser facility

The high contrast front-end for the 200 TW Trident laser has shown in recent experiments the importance roles that Amplified Spontaneous Emission (ASE) and prepulse contrast play in laser-ion acceleration. Ion energies above 58 MeV with efficiencies of greater than 5% into ions above 4 MeV have been observed even at modest intensities, on par with the Nova Petawatt results² at half the intensity¹, and a fifth of the energy and power, at an intrinsic laser ASE contrast of ≫ 10⁻⁷. Scalings for, laser energy, intensity, and target thickness are presented and compared to other empirical scalings and theories, including results of the new ultra-high contrast ≫10⁻¹⁰ Optical Parametric Chirped-Pulse Amplification (OPCPA) front-end system³.