Precision Shock Tuning on the National Ignition Facility

Abstract

Ignition implosions on the National Ignition Facility [J. D. Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a very carefully tailored sequence of four shock waves that must be timed to very high precision to keep the fuel entropy and adiabat low and ρR high. The first series of precision tuning experiments on the National Ignition Facility, which use optical diagnostics to directly measure the strength and timing of all four shocks inside a hohlraum-driven, cryogenic liquid-deuterium-filled capsule interior have now been performed. The results of these experiments are presented demonstrating a significant decrease in adiabat over previously untuned implosions. The impact of the improved shock timing is confirmed in related deuterium-tritium layered capsule implosions, which show the highest fuel compression (ρR~1.0 g/cm(2)) measured to date, exceeding the previous record [V. Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)] by more than a factor of 3. The experiments also clearly reveal an issue with the 4th shock velocity, which is observed to be 20% slower than predictions from numerical simulation.

Observation of strong electromagnetic fields around laser-entrance holes of ignition-scale hohlraums in inertial-confinement fusion experiments at the National Ignition Facility

“…The capsule inside a hohlraum was a warm plastic (CH) one with ∼2 mm diameter and filled with D 3 He gas. Each hohlraum was driven by 192 laser beams forming four single irradiation rings, with total laser energy of ∼1-1.5 MJ in a typical [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] four-shock pulse (lasting ∼14.9-19.9 ns). The individual laser beams had full spatial and temporal smoothing [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

“…Such an exciting scientific breakthrough is being vigorously pursued at the National Ignition Facility (NIF) [5] through the indirect-drive approach, in which the capsule implosion occurs in response to tremendous radiation pressure generated by the thermal x-rays in a high-Z enclosure, i.e. hohlraum, when the enclosure's inner wall is irradiated by high-power lasers [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The lasers would have a temporal pulse shape designed to launch four radially convergent shock waves which would coalesce at the capsule center, creating a self-igniting 'hot spot' which would generate a self-sustaining burn wave that propagates into the main fuel region.…”

“…The National Ignition Campaign (NIC) aims to eventually realize this important goal at the NIF, which has unique capabilities including 192 laser beams and 1.8 MJ of 3ω (0.35 µm) laser energy, enhanced pulse-shaping capabilities and pulse durations and greatly improved irradiation symmetry, etc [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Several ongoing experimental campaigns have been conducted to tune implosion conditions and to address a wide range of critical physics issues relevant to ignition sciences, including laser coupling, implosion dynamics and symmetry, hot spot formation and burn physics, etc.…”

$$\mathrm{ND}^2\mathrm{AV}$$ ND 2 AV : N-dimensional data analysis and visualization analysis for the National Ignition Campaign

“…We will focus here on parameters that affect the time-history of the laser brightness as it is delivered to the target. This tailoring of the laser intensity in time is known as pulse shaping, and it is used to set up a sequence of shock waves that precondition, or stiffen, the DT fuel before accelerating it for implosion [34]. The pulse is shaped such that a sequence of four spherical shocks merges at a carefully chosen radial location in the target.…”

Raman Backscatter as a Remote Laser Power Sensor in High-Energy-Density Plasmas

“…The experiments are performed with the NIF laser [18] using an indirect drive ignition hohlraum target [19][20][21]. This target is a gold cylinder (∼1 cm long and ∼ 0.5 cm diameter), cryogenically cooled to 21.…”