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.
Introduction Laser driven inertial confinement fusion (ICF) uses lasers with intensities I ∼ 10 14 -10 15 W/cm 2 to compress spherical capsules of ∼ 1 milligram of deuterium-tritium fuel to densities ≳ 100 g/cm 3 and temperatures ∼ 10 keV over timescales of ∼ 1 -10 ns [1]. This is accomplished by laser-driven ablation of a spherical-shell, often composed of glass [2], plastic [3], or high-density carbon [4,5]. Two approaches have been used historically to accomplish this: indirect-drive [6], where lasers are incident on the interior of a radiationcavity termed a 'hohlraum', and direct-drive [7], where lasers are incident on the capsule surface. This letter concerns a variation of the direct-drive approach where the capsule is immersed in an external magnetic field. The goal of magnetization of the implosion is to reduce electron thermal-conduction losses during the implosion, thereby achieving higher temperatures and greater thermonuclear fusion performance and/or reducing the required laser energy to achieve large energy gain.
A major concern for the viability of direct-drive fusion at ignition energy scales, E laser ≳ 1 MJ, is the generation of suprathermal electrons, or hot electrons, via laser-plasma instability (LPI), that prematurely heat the fuel. Electron preheat reduces the compressibility of the fuel and can therefore be detrimental to ICF implosions as small changes in the density lead to large changes in the fusion yield, E fusion ∝ ρ 2 [7][8][9]. Magnetization of direct-drive implosions has hitherto been conjectured to reduce the amount of preheat via mitigation of LPI that generates hot electrons [10,11]. However, we observe that the hot electron preheat is systematically increased by a factor 1.5 ± 0.1 in magnetized direct-drive fusion experiments. This result presents a concern for the long-term viability of magnetized directdrive implosions for high-yield/high-gain applications, unless laser plasma instabilities in the corona can be avoided, e.g., via the use of broadband lasers [12]. The remainder of this letter is dedicated to the presentation of these data and the explanation of the origin of the enhanced preheat.
Experiments Magnetized direct-drive implosions were carried out at the OMEGA laser using glass capsules with 2.6 ± 0.2 µm thick shells and outer diameters of 850 ± 10 µm. The capsules were filled with a mixture of deuterium (P D 2 ∼ 14.7 atm) and helium-3 (P He 3 ∼ 3 atm). The implosions were driven using 60 laser beams pointed towards target chamber center to produce a spherically uniform irradiation pattern. The laser intensities were ∼ 1.0 -1.2 × 10 15 W/cm 2 achieved utilizing 27 kJ delivered to the capsule in a 1 ns squareshaped pulse. Implosions were performed in which a pair of magneto-inertial fusion electrical discharge system (MIFEDS) [13] coils were utilized to provide a ∼ 10 T axial magnetic field; additional implosions were performed without the magnetic field for reference -note that in the unmagnetized implosions, the coils used to provide the magnetic field were not inserted into the target chamber and potential line-of-sight obstructions due to the coils in magnetized experiments are considered and ruled out.
In each experiment, the flux of hard x-rays hν ≳ 60 keV, and hν ≳ 80 keV, associated with a hot electron population producing bremsstrahlung radiation upon interaction with the ion-species in the capsule shell, were measured using the hard x-ray detector (HXRD) [14].
The energy spectra of charged particles produced by D 3 He and DD reactions were measured using a standard suite of charged-particle spectroscopy diagnostics on OMEGA including wedge range filters (WRFs) and the charged-particle spectrometers (CPS1 and CPS2) [15].
These diagnostics provide detailed information on the time and volume integrated plasma conditions in the capsule core as well as information about electric fields/potentials that may be established during the laser drive due to the production/ejection of hot electrons from the coronal plasma. Such fields lead to systematic up-shifts in the energy spectrum of charged fusion products as they gain energy equivalent to the potential difference between the capsule and the detector [16]. Often, measured charged particle median energies in similar implosions are observed to lie 100’s of keV above the birth energy of the particle due to the capsule potential.
Representative data from the hard x-ray detector (HXRD) are illustrated in Fig. 2.
The signal magnitudes are shown to systematically increase when the 10 T magnetic field is applied. The ratio of the observed signal accumulated in the magnetized experiments compared to the unmagnetized experiments between 0 -1.5 ns is R 60 kev ∼ 1.52 ± 0.1 for the ≳ 60 keV channel and is R 80 kev ∼ 1.55 ± 0.12 for the ≳ 80 keV channel.
Examination of the proton spectra shown in Fig. 3 reveals that the up-shift in energy is substantially smaller for magnetized implosions compared to unmagnetized
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