Fast Heating of Imploded Core with Counterbeam Configuration

A tailored-pulse-imploded core with a diameter of $70\mu \mathrm{m}$ is flashed by counterirradiating 110 fs, 7 TW laser pulses. Photon emission ($>40\mathrm{eV}$) from the core exceeds the emission from the imploded core by 6 times, even though the heating pulse energies are only one seventh of the implosion energy. The coupling efficiency from the heating laser to the core using counterirradiation is 14% from the enhancement of photon emission. Neutrons are also produced by counterpropagating fast deuterons accelerated by the photon pressure of the heating pulses. A collisional two-dimensional particle-in-cell simulation reveals that the collisionless two counterpropagating fast-electron currents induce mega-Gauss magnetic filaments in the center of the core due to the Weibel instability. The counterpropagating fast-electron currents are absolutely unstable and independent of the core density and resistivity. Fast electrons with energy below a few MeV are trapped by these filaments in the core region, inducing an additional coupling. This might lead to the observed bright photon emissions.

Figure 1

(a) Schematic of the experiment showing the $f/2.58$ laser beam, spherical shell target, $2\omega$ probe [18, 19], and diagnostics layout. A box (right top) represents the laser pulse trace taken by a biplanar phototube. (b) Implosion diagram. Radius of fluid elements versus time. Closed circles are photon-emission trajectories in the experiments. The dashed line is the fitting slope used to evaluate the implosion velocity of $1.2\pm 0.2\times 10^7\mathrm{cm}/\mathrm{s}$. (c) Laser power versus time for the counterpulse implosion and fast heating. (d) star-1d results. Spatial imploding density profiles before (blue curve) and after (red curve) stagnation.

Figure 2

X-ray streak images of counterirradiation (a) without heating pulses and (b) with heating pulses taken in the 30 ns streak range. (c) X-ray streak image with a 2 ns streak range for a region marked by broken lines in (b) for a different laser shot. (d) Spatial photon emission profiles for (c) before the heating pulse irradiation (at 36.1 ns, blue curve) and at the time of heating pulse irradiation (at 36.2 ns, red curve). Dashed curves represent the fits to Gaussian profiles. (e) X-ray streak images of one-side irradiation taken in the 30 ns streak range. (f) Spatial photon emission profiles for (b) counterirradiation (red curve) and (e) one-side (blue curve) irradiation at the time of the heating pulse arrival (36.2 ns).

Figure 3

(a) Time-of-flight spectrum for the heating pulse shot obtained from the on-axis gated scintillator with a single shot. (b) Time-of-flight spectrum of neutron signals for an averaging of nine shots in series. Yellow regions represent the beam fusion neutron energy ranges induced from deuterons with energies in the 0.2–1.0 MeV range. The dashed line indicates 2.45 MeV, the thermal neutron energy.

Figure 4

picls2d results: (a) Estimated bremsstrahlung power (normalized) by calculating $Z^2{n}_e{n}_i{T}_e^{1/2}$ for the one-beam case at 800 fs (about 400 fs after the laser interaction). (b) Same plot as (a) but for the counterpropagating beam case. (c) Magnetic fields at 500 fs for the one-beam case in the core region between $Y=20$ and $60\mu \mathrm{m}$, marked by broken lines in (a). (d) Same plot as (c) for the counterpropagating beam case. (e) Electron energy distribution in the core integrated in the region between $X=55$ and $65\mu \mathrm{m}$ at 500 fs. Solid (red), dotted (blue), and dashed (black) lines indicate the data for the cases of counterpropagating beams, one beam, and one beam with 2 times the intensity and the energy, respectively.