Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments

Abstract

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 μm), electron temperature (∼3 to 5 keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

One-dimensional hydrodynamic simulations of low convergence ratio direct-drive inertial confinement fusion implosions

“…These include stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), filamentation and two-plasmon decay (TPD) [45]. Such instabilities result in the reduced coupling of drive energy and the generation of hot electrons, which contribute to preheating of the fuel and reduced yield [46]. This work aims to minimize these instabilities by limiting the peak laser power in order to keep Iλ 2 , the product of intensity and the square of wavelength, below 1014 false(W cm−2false) μm2, a threshold value above which significant onset of these instabilities is observed [47].…”

Direct-drive laser fusion: status, plans and future

“…With the above caveats, the majority of LDD experiments on the NIF to date have focused on laser–plasma coupling with a specific focus on instabilities such as the two-plasmon-decay instability (2 ω pe ) and SRS that produce energetic electrons that can preheat and reduce the compressibility of the imploding DT shell [25]. Another process of concern to all laser-driven approaches that is being addressed on the NIF is CBET [17,18].…”

“…Figure 11 summarizes hot-electron experiments executed on the NIF that are discussed in detail in the references [25,28]. Not surprising, the scattered-light spectrum (figure 11 a ) from the large coronas from irradiated planar targets (table 1) where CBET is negligible, shows that SRS occurring in plasma densities less than nc/4 is the dominant source of the approximately 40–50 keV hot electrons [16,29].…”

“…In the smaller plasmas on OMEGA, the two-plasmon-decay instability is dominant [30,31]. Hot-electron levels deduced from hard X-ray emission (figure 11 b ) can be as large as several per cent of the laser energy if the laser intensity is greater than 5 × 10 14 W cm −2 at nc/4 [25]. These results show the importance of conducting experiments at the MJ scale even with the poor-quality NIF beams.…”

“…The data are taken with 4.5 nsec pulses and with CH targets (one with a thin Si overcoat). The inner and outer beams refer to the different NIF beam cones [25]. (Online version in colour.)…”