Siqun Zhang

Effects of shock waves on spatial distribution of proton beams in ultrashort laser-foil interactions

The characteristics of proton beam generated in the interaction of an ultrashort laser pulse with a large prepulse with solid foils are experimentally investigated. It is found that the proton beam emitted from the rear surface is not well collimated, and a “ring-like” structure with some “burst-like” angular modulation is presented in the spatial distribution. The divergence of the proton beam reduces significantly when the laser intensity is decreased. The “burst-like” modulation gradually fades out for the thicker target. It is believed that the large divergence angle and the modulated ring structure are caused by the shock wave induced by the large laser prepulse. A one-dimensional hydrodynamic code, MED103, is used to simulate the behavior of the shock wave produced by the prepulse. The simulation indicates that the rear surface of the foil target is significantly modified by the shock wave, consequently resulting in the experimental observations.

Preliminary experimental results of tungsten wire-array Z-pinches on primary test stand

The Primary Test Stand (PTS) developed at the China Academy of Engineering Physics is a 20 TW pulsed power driver, which can deliver a ∼10 MA, 70 ns rise-time (10%–90%) current to a short-circuit load and has important applications in Z-pinch driven inertial confinement fusion and high energy density physics. Preliminary results of tungsten wire-array Z-pinch experiments on PTS are presented. The load geometries investigated include 15-mm-tall cylindrical single and nested arrays with diameter ranging from 13 mm to 30 mm, consisting of 132–300 tungsten wires with 5–10 μm in diameter. Multiple diagnostics were fielded to characterize the x-ray radiation from wire-array Z pinches. The x-ray peak power (∼50 TW) and total radiated energy (∼500 kJ) were obtained from a single 20-mm-diam array with 80-ns stagnation time. The highest x-ray peak power up to 80 TW with 2.4 ns FWHM was achieved by using a nested array with 20-mm outer diameter, and the total x-ray energy from the nested array is comparable to that of single array. Implosion velocity estimated from the time-resolved image measurement exceeds 30 cm/μs. The detailed experimental results and other findings are presented and discussed.

Investigation of spectra unfolded for a filtered x-ray diode array with improved smoothness

An unfolding algorithm using parabolic B-splines to smoothly reconstruct the soft x-ray spectra from the measurements of a filtered x-ray diode array is proposed. This array has been fabricated for the study of the soft x ray emitted by Z-pinch plasma. Unfolding results show that for the simulated noise-free blackbody spectra with temperature ranging from 20 to 250 eV, both the spectra and the total power are accurately recovered. Typical experimental waveforms along with the unfolded spectra and total power of x rays are presented. Possible defects due to the adoption of parabolic B-splines instead of conventionally used histograms are discussed.

Diagnosing x-ray power and energy of tungsten wire array z-pinch with a flat spectral response x-ray diode

Fast z-pinch is a very efficient way of converting electromagnetic energy to radiation. With an 8-10 MA current on primary test stand facility, about 1 MJ electromagnetic energy is delivered to vacuum chamber, which heats z-pinch plasma to radiate soft x-ray. To develop a pulsed high power x-ray source, we studied the applicability of diagnosing x-ray power from tungsten wire array z-pinch with a flat spectral response x-ray diode (FSR-XRD). The detector was originally developed to diagnose radiation of a hohlraum in SG-III prototype laser facility. It utilized a gold cathode XRD and a specially configured compound gold filter to yield a nearly flat spectral response in photon energy range of 0.1-4 keV. In practice, it was critical to avoid surface contamination of gold cathode. It is illustrated that an exposure of an XRD to multiple shots caused a significant change of response. Thus, in diagnosing x-ray power and energy, we used each XRD in only one shot after calibration. In a shot serial, output of FSR-XRD was compared with output of a nickel bolometer. In these shots, the outputs agreed with each other within their uncertainties which were about 12% for FSR-XRD and about 15% for bolometer. Moreover, the ratios between the FSR-XRD and the bolometer among different shots were explored. In 8 shots, the standard deviation of the ratio was 6%. It is comparable to XRD response change of 7%.

The magnetically driven plasma jet produces a pressure of 33 GPa on PTS

We report on experiments in which a magnetically driven plasma jet was used to hit a 500 μm thick planar aluminum target. The plasma jet was produced by using a 50 μm thick aluminum radial foil, which was subjected to 4 MA, 90 ns rising time current on the primary test stand pulsed power facility. The subsequent magnetic bubbles propagate with radial velocity reaching 200 km/s and an axial velocity of 230 km/s. After the plasma knocks onto the target, a shock forms in the target. When the shock gets to the backside of the target, we measure the velocity of the moving surface using dual laser heterodyne velocimetry. By using the Hugoniot relations, we know that the plasma jet produced a pressure of 33 GPa. According to the measured pressure and the velocity of the plasma jet, the density of the jet can be also roughly estimated.

Design and primary results of x-ray heating aluminum foil experiments at PTS facility

Summary form only given. The strong X-ray radiations produced by mega-ampere-current-level fast Z-pinch plasmas offer valuable and indispensible opportunities to create hot and dense radiation-heated plasmas1, serving the laboratory investigations of radiation sciences in the fields of opacity measurements, photon-ionized plasmas and so on. Recently, experiments of x-ray heating aluminum foil has been designed and is being carried out at the Primary Test Stand (PTS) facility2, a 8-10 MA, 70 ns pulsed power driver being able to generate x-ray bursts of 80 TW peak power and 3-5 ns FWHM3. In this paper, we present the simulation results of the aluminum foils heated by x-rays using a 1D radiation hydrodynamic code, the design of the sample foils with CH coatings on each side and the diagnostic results of x-ray power and energy in different spectral regions, transmission spectra through the foils and so on. By analyzing the measurements, the states and properties of the radiation-heated aluminum plasmas are estimated and discussed.

Diagnostics forz-pinch on PTS facility

Summary form only given. An extensive diagnostics with spatial and temporal resolution for Z-pinch was developed on the PTS facility at Key Labora tory of Pulsed Power, CAEP. PTS facility can generate a curr ent rising to 8-10 MA in 90 ns, which was used to drive wire array Z pinches. Diagnostics including soft X-ray power meter, DANTE, transmission grating spectrometer, time-resolved X-ray pinhole camera, optical/X-ray streak cameras, X-ray crystal spectrometer, laser differential interferometer and some experimental results are described in this paper.

The Plasma Column Evolution in Gas-Puff Z-Pinches on the Yang Accelerator

The plasma column evolution in gas‐puff z‐pinch was investigated on the Yang accelerator. The pinch process was significantly influenced by the initial gas distribution. Uniformity of the gas‐puff distribution resulted in the asymmetry of the initially load current through the main channels of the plasma layer. The zipper velocity and implosion speed under “trumpet” distribution was given, it was observed the pinch speed and the temperature of the plasma near cathode increased when the rise time of the load current was shortened.