Objective In laser inertial confinement fusion (ICF) research, the suprathermal electrons generated by the high-power laser and plasma interaction (LPI) have always been a research hotspot. Whether using a direct or indirect drive central ignition scheme, suprathermal electrons with energies greater than 50 keV will deposit some of their energy in the fuel, resulting in preheating and a reduction in implosion performance. It is discovered in the shock ignition scheme that suprathermal electrons with energies less than 100 keV generated by the spike pulse can help enhance the intensity of the shock wave and increase the energy gain. The suprathermal electrons are generally considered to be accelerated by the electron plasma waves, which are generated by LPI instability, such as stimulated Raman scattering (SRS), two plasmon decay (TPD), and resonance absorption. However, it is still unknown which instability dominates the generation of hot electrons. As a result, experimental research on suprathermal electrons’ spatial energy spectrum distribution, obtaining the main sources of suprathermal electrons, and exploring ways to control suprathermal electron generation is critical for ICF research.
Methods The experiment was carried out at the ShenGuang II high-power laser facility. The schematic of the experimental setup is shown in Fig.1. The four north laser beams (5th, 6th, 7th, and 8th beams) were used and irradiated on the 10-μm thick CH foil at 45° and P polarization direction. The wavelength of the laser is 351 nm, the pulse is a 1-ns square wave, and the energy of one beam is 250 J. The diameters of laser focal spots with or without continuous phase plate are approximately 250 μm and 120 μm, respectively. One laser beam’s corresponding average power density is 5.1×10 14 and 2.2×10 15 W/cm 2. In the experiment, the plasma density distribution was measured with a Normaski interferometry system, and the probe beam was an 80-ps duration pulse with a wavelength of 527 nm. The suprathermal electrons were measured by two calibrated electron spectrometers (ESM A and ESM B). The spatial distribution of suprathermal electrons emitted from the target’s back was measured by a big size Image Plate, which was covered by a 50-μm thick Al foil. The backward-scattered light produced by LPI instabilities was collected by two optical fibers connected to an Ocean FX light spectrometer.
Results and Discussions The energy spectra of suprathermal electrons measured in the experiment were fitted well with the Maxwellian distribution with the electron temperatures of approximately 30--65 keV (Fig.2). It demonstrates that in all laser conditions used in our experiment, the fitted electron temperatures and intensities obtained in the normal front direction of the targets are greater than those obtained in the normal back direction. Furthermore, the angular distributions of total suprathermal electron energy in the backward direction are obtained, which have a Gaussian distribution with a peak along with the target’s normal (Fig.3). In the obtained scattering light spectra, there is a strong convective SRS signal, a TPD signal with the characteristic of a double peak (Fig.4). By analyzing the intensities of scattering light and suprathermal electron, we find that the hot electrons generated by SRS are more dominant than TPD under the condition of high-temperature large-scale plasma (Fig.6). In addition, the relationship between the kinetic energy of accelerated electrons and the wavelength of scattering light is investigated. It is found that the accelerated electron kinetic energy increases as the scattering light wavelength increases, which is consistent with the experimental observation (Fig.7).
Conclusions In this paper, we measured the energy spectrum and spatial distribution of the suprathermal electrons and the scattering light for the C8H8 foil targets irradiated with three laser conditions. By comparing the scattered light spectra and the hot electron kinetic energy spectra in the range of 20--500 keV, we found that the changes in the SRS scattered light intensity and amount of hot electrons with different laser conditions are synchronous, from which we speculated that SRS is more responsible for the diagnosed hot electrons than the TPD. Furthermore, the calculation confirms the hypothesis that the accelerated electron kinetic energy increases with the wavelength of the scattering light, which is consistent with the experimental observation. This conclusion implies that the development of laser-driven ICF will result in higher temperature and larger-scale plasma conditions than current experiment conditions and an increase in the contribution of SRS to suprathermal electrons compared to TPD. As a result, effective measures to control the growth of SRS, and reduce the preheating effect caused by suprathermal electrons are required.
