Laser-plasmas in the relativistic-transparency regime: Science and applications

Abstract

Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at "table-top" scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼10⁴ T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 μm laser pulses irradiating planar foils up to 250 nm thick at 2-8 × 10²⁰ W/cm². These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.

FIG. 1.

(a) Isometric projection of the Trident West target chamber in its baseline configuration. (b) Top view of the experimental configuration used for the γ-ray generation experiments. The “kaleidoscope” object for point-projection high-resolution imaging and the thick objects for near-contact imaging are not all used simultaneously. The inset shows a top-view detail of the laser-target nanofoil and Ta-converter arrangement. The nanofoil-converter registration is done by spacers off the field of view. Magnets and shielding to avoid direct electron and scattered γ-ray detector irradiation are not shown.

FIG. 2.

Measured calibration of IP type SR.

FIG. 3.

Profiles at 2013 fs (610 optical cycles). Laser propagation is from left to right. The initial target location is x = 95 λ_L. Top: Magnetic field profile. The range is ±10 kT. Middle: Electron-density profile, illustrating the plasma jet along the central channel. Bottom: Ion density distribution. The ion energy distribution has not yet narrowed.

FIG. 4.

Channel-averaged profiles of electron and ion density, net charge, and the resulting longitudinal electric field.

FIG. 5.

Self-organization process leading to peaking of the ion-energy distribution.

FIG. 6.

Point projection radiographs of thick objects recorded on collocated image plates (left) and on an a-Si gamma-ray detector (right).

FIG. 7.

Gamma-ray point-projection radiographs of the AWE Kaleidoscope object, taken at DARHT (left) and at Trident (right).

FIG. 8.

Laser-driven ion-beam focusing geometry for fast ignition.

FIG. 9.

Flexible neutron converter made out of Be disks 4 cm in diameter and 0.9 cm long. The photo shows a setup where a radiochromic film is placed in between layers to study deuteron transport within the converter.

FIG. 10.

Neutron spectra for two different cylindrical Be neutron converters.

FIG. 11.

Radiography of thick objects with the laser-driven neutron-source at Trident.

FIG. 12.

Neutron production by deuterium breakup.

FIG. 13.

Optimization of laser parameters for laser-driven ion acceleration.