A neutron time-of-flight system for inertial confinement fusion.

A neutron time-of-flight (nTOF) system has been implemented at the largest laser facility in China. The nTOF system is used to measure neutron spectra in inertial confinement fusion experiments. The nTOF system consists of 11 fast plastic scintillation detectors. The detectors employed three designs to measure neutron yield, ion temperature, and neutron bang time. The nTOF system is capable of measuring the primary neutron yield from 107 to 1013, secondary DT neutron yield from 106 to 108, and ion temperature and neutron bang time yields from 108 to 1013. The accuracies of the nTOF system are about 10% for neutron yield and ion temperature measurements and better than 60 ps for neutron bang time measurements. The nTOF system has become one of the most important diagnostics for implosions, and it is used for more than 200 shots per year.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182 eV with drive laser energies of 71 kJ to 84 kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175 eV at 71 kJ. For the flat-top laser pulse of 3 ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Ion temperature measurements of indirect-drive implosions with the neutron time-of-flight detector on SG-III laser facility.

The accuracy of the determination of the burn-averaged ion temperature of inertial confinement fusion implosions depends on the unfold process, including deconvolution and convolution methods, and the function, i.e., the detector response, used to fit the signals measured by neutron time-of-flight (nToF) detectors. The function given by Murphy et al. [Rev. Sci. Instrum. 68(1), 610-613 (1997)] has been widely used in Nova, Omega, and NIF. There are two components, i.e., fast and slow, and the contribution of scattered neutrons has not been dedicatedly considered. In this work, a new function, based on Murphy's function has been employed to unfold nToF signals. The contribution of scattered neutrons is easily included by the convolution of a Gaussian response function and an exponential decay. The ion temperature is measured by nToF with the new function. Good agreement with the ion temperature determined by the deconvolution method has been achieved.

Experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target.

Octahedral spherical hohlraums with a single laser ring at an injection angle of 55^{∘} are attractive concepts for laser indirect drive due to the potential for achieving the x-ray drive symmetry required for high convergence implosions. Laser-plasma instabilities, however, are a concern given the long laser propagation path in such hohlraums. Significant stimulated Raman scattering has been observed in cylindrical hohlraums with similar laser propagation paths during the ignition campaign on the National Ignition Facility (NIF). In this Rapid Communication, experiments demonstrating low levels of laser-driven plasma instability (LPI) in spherical hohlraums with a laser injection angle of 55^{∘} are reported and compared to that observed with cylindrical hohlraums with injection angles of 28.5^{∘} and 55^{∘}, similar to that of the NIF. Significant LPI is observed with the laser injection of 28.5^{∘} in the cylindrical hohlraum where the propagation path is similar to the 55^{∘} injection angle for the spherical hohlraum. The experiments are performed on the SGIII laser facility with a total 0.35-µm incident energy of 93 kJ in a 3 nsec pulse. These experiments demonstrate the role of hohlraum geometry in LPI and demonstrate the need for systematic experiments for choosing the optimal configuration for ignition studies with indirect drive inertial confinement fusion.

First Investigation on the Radiation Field of the Spherical Hohlraum.

The first spherical hohlraum energetics experiment is accomplished on the SGIII-prototype laser facility. In the experiment, the radiation temperature is measured by using an array of flat-response x-ray detectors (FXRDs) through a laser entrance hole at four different angles. The radiation temperature and M-band fraction inside the hohlraum are determined by the shock wave technique. The experimental observations indicate that the radiation temperatures measured by the FXRDs depend on the observation angles and are related to the view field. According to the experimental results, the conversion efficiency of the vacuum spherical hohlraum is in the range from 60% to 80%. Although this conversion efficiency is less than the conversion efficiency of the near vacuum hohlraum on the National Ignition Facility, it is consistent with that of the cylindrical hohlraums used on the NOVA and the SGIII-prototype at the same energy scale.

Note: neutron bang time diagnostic system on Shenguang-III prototype.

A neutron bang time (NBT) diagnostic system has been implemented on Shenguang-III prototype. The bang time diagnostic system is based on a sensitive fusion neutron detector, which consists of a plastic scintillator and a micro-channel plate photomultiplier tube (PMT). An optical fiber bundle is used to couple the scintillator and the PMT. The bang time system is able to measure bang time above a neutron yield of 10(7). Bang times and start time of laser were related by probing x-ray pulses produced by 200 ps laser irradiating golden targets. Timing accuracy of the NBT is better than 60 ps.

Soft x-ray low-pass filter with a square-pore microchannel plate.

A type of low-pass filter devices for soft x rays is investigated by using a microchannel plate (MCP) of small channels with square cross section. The measured transmission spectra on the Beijing Synchrotron Radiation Facility showed that the MCP has excellent bandpass effects below 1.5 keV by grazing incidence and internal multireflections. Combined with filters, the MCP energy bandwidth can be narrowed to 100 eV. In contrast to bandpass made of planar mirrors, the MCP has a much smaller size and better bandpass effects, and can be easily extended to high energy ranges. For low-resolution spectrometer applications of soft x rays, this method allows the monochromator to be replaced by a simple MCP filter and therefore significantly reduces alignment complexity in experiments.

Determination of the hohlraum M-band fraction by a shock-wave technique on the SGIII-prototype laser facility.

The proposal of simultaneously determining the hohlraum peak radiation temperature T(R) and M-band fraction f(M) by shock velocity measurement technique [Y. S. Li et al. Phys. Plasmas 18, 022701 (2011)] is demonstrated for the first time in recent experiments conducted on SGIII-prototype laser facility. In the experiments, T(R) and f(M) are determined by using the observed shock velocities in Al and Ti. For the Au hohlraum used in the experiments, T(R) is about 160 eV and f(M) is around 4.3% under a 1 ns laser pulse of 2 kJ. The results from this method are complementary to those from the broadband x-ray spectrometer, and the technique can be further used to determine T(R) and f(M) inside an ignition hohlraum.

Research of areal density diagnostic technique on Shenguang III laser prototype facility.

In the test run of Shenguang III laser prototype facility, under the low density condition, the yield ratio method was used to measure the fuel areal density. Considering the uncertainty of the neutron yield, detectors with different efficiencies were used. The clear secondary-neutron signals in inertial confinement fusion experiments were obtained for the first time in China, and the values of the areal density were deduced.