Shock X-ray emission image measurement in Z-pinch dynamic hohlraum

Abstract

Owing to high efficiency for delivering thermal radiation from Z-pinch plasma to an inertial fusion capsule, Z-pinch dynamic hohlraum (ZPDH) is a promising indirect-drive inertial confinement fusion (ICF) approach. ZPDH is created by accelerating an annular tungsten Z-pinch plasma radially inward to an internal low density convertor. The collision launches a radiating shock traveling inward. Radiations emitted from the shock, after being trapped and thermalized by the optically thick tungsten plasma, drive the internal fusion capsule to implode. In our previous experiments, shock propagating process has never been imaged or even never been formed, due to low drive current (about 1.3 MA). In this paper, the ZPDH has a load of single tungsten wire array embedded in a cylindrical 16 mg/cm3 C15H20O6 foam, and the tungsten wire array is explored using JuLong-1 facility (also named PTS facility) driven by current with a peak value of 7-8 MA and rising time of 60-70 ns (from 10% to 90%). Several results are presented for improving the understanding of the physics of the shock propagating and hohlraum forming. For the high optical depth in tungsten plasmas around the foam, radially directly diagnosing hohlraum radiation distribution along axis is impossible. The most convenient way to diagnose the radiation symmetry and the shock evolution is to take the end-on X-ray images. The time-resolved X-ray images of annular radiating shock evolution, which are performed with a 10-frame time-gated X-ray pinhole camera located at 0 with respect to the Z-pinch axis, are obtained for the first time in China. By analyzing the radial X-ray emission power waveform and intensity distribution of end-on radiation image, the process of wire array plasma impacting on the foam convertor and properties of dynamic hohlraum radiation are discussed. The shock emission structures are found to be circular, similar to the results predicted theoretically. The shock velocity which seems to be constant in the whole process of inward propagating is linearly fitted to be (14.21.7) cm/s. The annular width of shock emission is 0.8-0.9 mm, which is inferred from the full width at half maximum of radial lineout of end-on X-ray image at time t=-11.9 ns and the blurring effect of shock velocity. The radiation symmetry is assessed by statistic property of mean intensity of 36 sectors of end-on X-ray image evenly divided by 10. The standard deviation of azimuthal shock emission intensity is 10% while that of hohlraum region prior to shock impact is 4.2%. The azimuthal symmetry improvement from shock emission to hohlraum radiation is a piece of exciting news for ZPDH driven ICF.