Enhanced Hot Electron Preheat Observed in Magnetized Laser Direct-Drive Implosions

TL;DR

The study reveals that applying a $10$ T axial magnetic field to direct-drive ICF experiments increases hot-electron preheat by about a factor of $1.5$, contrary to expectations that magnetization would suppress preheat. A quasi-steady radial field confines hot electrons in a mirror-mode and enables rapid pitch-angle scattering into the capsule, while the total hot-electron production remains largely unchanged. A two-temperature hot-electron distribution with $T_{\rm hot}\approx 90$ keV and $T_{\rm cold}\approx 3$ keV yields a preheat ratio $R$ in the range $1.5$–$1.6$, consistent with measurements; PIC simulations reproduce a comparable enhancement ($R_{\rm sim}\approx 2.1$) and highlight the role of transport over generation. These findings stress the need to mitigate laser-plasma instabilities (e.g., via broadband lasers) and to improve modeling of external magnetic-field effects on hot-electron transport in magnetized direct-drive implosions to maintain fusion gain and implosion efficiency.

Abstract

Hard x-ray emission, associated with hot electron preheat, in direct-drive implosions was observed to be enhanced by a factor of $1.5\pm0.1$ by application of a $10$ T magnetic field. The applied magnetic field reaches a quasi steady-state aligned with the ablation flow prior to the onset of laser-plasma instabilities in the corona. Hot electrons that would otherwise escape the corona and lead to capsule charging in unmagnetized implosions are confined in a mirror-mode of the magnetic field in magnetized implosions. These hot electrons are shown to subsequently pitch-angle scatter from the mirror onto the capsule, thereby leading to the observed hard x-ray generation in magnetized implosions. Consequently, the energy of charged-fusion products, associated with the capsule charging, are observed to decrease when the implosion is magnetized. These results intensify the need to mitigate laser-plasma instabilities -- particularly for magnetized implosions -- to maximize fusion gain and implosion efficiency.

Figures (9)

• Figure 1: Experimental diagrams from a) unmagnetized and b) magnetized implosions. MIFEDS coils are not drawn to scale. Lines of sight from the diagnostics indicated are unobstructed by the coils.
• Figure 2: Representative hard x-ray detector (HXRD) measurements for magnetized and unmagnetized implosions at OMEGA. Shaded regions are drawn to indicate the standard error in the measurement. These signals are associated with hot electrons generated by laser plasma interactions colliding with the shell and producing bremsstrahlung. The average $\omega/2$ signal from TPD scattered light in unmagnetized implosions is denoted by the dashed black curve and scaled to peak at unity; The average $\omega/2$ signal from magnetized implosions is denoted by the solid black curve, drawn on the same scale as the unmagnetized signal. The time of minimum volume is demarcated with the vertical, dotted, gray line.
• Figure 3: D$^3$He and DD proton spectra relative to the proton birth energy measured from magnetized and unmagnetized experiments. In all cases, the median energy is seen to have shifted above the birth energy due to the capsule becoming positively charged during the laser drive. However, in magnetized implosions the capsule is evidently less charged.
• Figure 4: Numerical solution to ideal MHD model for the magnetic field evolution at $800$ ps for $u_{\rm{abl}} = 3 \times 10^8$ cm/s and $r_{\rm{shell}} = 5\times10^{-2}$ cm. The field is normal to the quarter-critical surface several $100$ picoseconds prior to the peak of TPD in our experiments. All field lines (white) are directed towards the top of the image.
• Figure 5: Diagrams illustrating the hot electron trajectories in various implosions. In all cases, hot electrons with trajectories inside the x-ray cone will collide with the capsule and generate preheat. In unmagnetized implosions, hot electrons with trajectories outside the x-ray cone will contribute to capsule charging. Conversely, in magnetized implosions, due to confinement by the magnetic field and pitch-angle scattering, electrons with trajectories in the mirror-region will strike the capsule surface and contribute to preheat.