Self-focusing of UV radiation in 1 mm scale plasma in a deep ablative crater produced by 100 ns, 1 GW KrF laser pulse in the context of ICF

Abstract

Experiments at the GARPUN KrF laser facility and 2D simulations using the NUTCY code were performed to study the irradiation of metal and polymethyl methacrylate (PMMA) targets by 100 ns UV pulses at intensities up to 5 × 1012 W cm−2. In both targets, a deep crater of length 1 mm was produced owing to the 2D geometry of the supersonic propagation of the ablation front in condensed matter that was pushed sideways by a conical shock wave. Small-scale filamentation of the laser beam caused by thermal self-focusing of radiation in the crater-confined plasma was evidenced by the presence of a microcrater relief on the bottom of the main crater. In translucent PMMA, with a penetration depth for UV light of several hundred micrometers, a long narrow channel of length 1 mm and diameter 30 μm was observed emerging from the crater vertex. Similar channels with a length-to-diameter aspect ratio of ∼1000 were produced by a repeated-pulse KrF laser in PMMA and fused silica glass at an intensity of ∼109 W cm−2. This channel formation is attributed to the effects of radiation self-focusing in the plasma and Kerr self-focusing in a partially transparent target material after shallow-angle reflection by the crater wall. Experimental modeling of the initial stage of inertial confinement fusion-scale direct-drive KrF laser interaction with subcritical coronal plasmas from spherical and cone-type targets using crater-confined plasmas seems to be feasible with increased laser intensity above 1014 W cm−2.

Deep Penetration of UV Radiation into PMMA and Electron Acceleration in Long Plasma Channels Produced by 100 ns KrF Laser Pulses

“…The GARPUN KrF laser operated at a wavelength λ = 0.248 µm with an unstable telescopic resonator and a magnification parameter M = 6. A narrow-band (∆ υ ~0.1 cm −1 ) radiation of discharge pumped master oscillator Lambda Physik EMG 150 TMSC (200 mJ and 20 ns) was injected into the cavity and provided a narrow-band output of up to 100 J in 100 ns trapezoidal pulses with a rise front of 20 ns (for details see [27]). A slightly convergent output laser beam with an initial cross-section of 14 × 18 cm 2 was directed into the vacuum chamber and focused by a spherical mirror with F = 400 mm in a spot of a 150 µm diameter (at the 0.1 level of the maximum), which contained 75% of the whole energy.…”

“…2D hydrodynamic interaction regime is clearly seen for the PMMA target irradiation image in Figure 3 obtained with a PS-1/S1 streak camera. When a 100 ns laser pulse with a peak intensity of 10 12 W/cm 2 and a rise time of ~20 ns (see [27]) exposed a PMMA target, the maximum speed of plasma expansion into a residual gas of 100-150 km/s was achieved at the pulse plateau. By the end of the laser pulse, the propagation velocity of the plasma glow slowed down, which is obviously due to a rapid decrease in the density and temperature in the plasma corona as it expanded away from the target.…”

“…We believe that the crown-like pattern ending the 1 mm capillary channel in PMMA might be a multitude of deceleration tracks of an electron beam. Fast electrons could be generated as a result of a self-focusing of incident radiation in the deep ablative crater [27,28] and the development of laser plasma instabilities (LPI) favored by a long interaction length of a narrow-band laser radiation with underdense plasma [10,13]. Electrons could then be captured in the adjacent narrow capillary filled with plasma (Figure 5a).…”

“…A high-aspect-ratio capillary was drilled in a PMMA sample by a discharge-pumped master oscillator EMG 150 TMSC operating with 1−2 mJ energy in 20 ns pulses at a 10 Hz repetition rate. Thus, PMMA targets with channels of 3 mm length and 30−40 µm diameter were produced (for details see [27]). Note an important distinction of these channels with a smooth wall from the above-discussed self-produced corrugated capillaries.…”

“…The microscope images were obtained with sample illumination by an unpolarized incoherent light source and in a projection scheme in the polarized light of copper vapor laser [20][21][22]. The formation of the capillary channel is explained by the pre-focusing of incident radiation nearby the crater top due to specular reflection by the plasma density gradient near the conical surface and the self-focusing of radiation transmitted through plasma in the PMMA [27].…”