Search

The influence of low-mode-driven asymmetry on yield degradation in shaped-pulse indirect-drive implosions has been investigated at the 100 kJ laser facility. In this work, P2- and P4-driven asymmetries were tuned by varying hohlraum gas-fill density and capsule diameter. The measured neutron yield varied from 2.1 × 109 to 7.6 × 109 and the yield measured by a 1D simulation (YOC1D) was increased from 3% to 16%. Meanwhile, considering the temporal P2- and P4-driven asymmetries, the yields measured by 2D simulations (YOC2D) were from 26% to 81% (the YOC2D of the majority of the shots was higher than 50%). Furthermore, both the ion temperature and neutron bang-time showed good agreement between the measurements and the 2D simulations. The simulations demonstrated that the temporal P2- and P4-driven asymmetries can decrease the efficiency of PdV work and increase the energy loss due to electron thermal conduction. In addition, the internal energy of the deuterium (DD) fuel clearly decreased and the neutron yield was degraded by low-mode asymmetry. In a future work, we will enlarge the cylindrical hohlraum diameter or use an I-hohlraum to improve the low-mode-driven symmetry, along with measurement of the hot-spot shape.

The re-emitted images of the frame camera indicated that the high-Z (Bi) capsule deviated about 29 μm from the center of the hohlraum in experiments at the Shenguang-II (SG-II) laser facility; however, investigations on this issue have seldom been performed. The influence of three dimensional offsets of a capsule on its radiation asymmetry in inertial confinement fusion (ICF) will be analyzed in this paper. Simulations demonstrate that the axial offset of 100 μm of a capsule from the center of the hohlraum brings an additional 3.5% radiation drive asymmetry and 6.5% P1 asymmetry (Legendre odd model) on the capsule in the SG-II laser facility, and the offset must be within 25 μm if the P1 asymmetry is restricted to below 2%.

By analyzing the monochromatic X-ray backlighting system with spherically bent crystal, the expressions for the spatial resolution and the spectral resolution of this system are deduced, and the relationship between these key performance parameters and the design parameters of the system is analyzed. Analysis results of the imaging system with different imaging modalities are obtained, which coincide with the ray tracing results. A demo test of the monochromatic imaging system was operated on the X-ray source, and the results are consonant with the simulation and the calculation. This diagnosis could have a broad prospect of application in the measurements on Shenguang laser facility.

Gold disk targets were irradiated using focusing and beam smoothing methods on Xingguang (XG-II) laser facilities with 350 nm wavelength, 0.6 ns pulse width and 20–80 Joules energies. Laser absorption, light scattering and X-ray conversion were experimentally investigated. The experimental results showed that laser absorption and scattered light were about 90% and 10%, respectively, under focusing irradiation, but the laser absorption increased 5%–10% and the scattered light about 1% under the condition of beam smoothing. Compared with the case of focusing irradiation, the laser absorption was effectively improved and the scattered light remarkably dropped under uniform irradiation; then due to the decrease in laser intensity, X-ray conversion increased. This is highly advantageous to the inertial confinement fusion. However, X-ray conversion mechanism basically did not change and X-ray conversion efficiency under beam smoothing and focusing irradiation was basically the same.

We have carried out the hohlraum experiments about radiation temperature scaling on the Shenguang-II (SG-II) laser facility with eight laser beams of 0.35 μm, pulse duration of about 1.0 ns and total energy of 2000 J. The reradiated x-ray flux through the laser entrance hole was measured using a soft x-ray spectrometer. The measured peak radiation temperature was 170 eV for the standard hohlraum and 150 eV for the 1.5-scaled one. We have derived the radiation temperature scaling law, in which the laser hohlraum coupling efficiency is included. With an appropriate coupling efficiency, the coincidences between experimental and scaling hohlraum radiation temperatures are rather good.

Inertial confinement fusion utilizes sufficient laser beams to directly illuminate a spherical capsule, or convert the laser into thermal X-rays inside a high Z hohlraum to drive capsule implosion. The direct drive implosion is one of ways toward central ignition and similar to the indirect drive implosion, but has higher laser energy coupling efficiency and the potential for higher-gain implosion than indirect drive, and needs stringent laser condition. In order to develop and execute the direct drive experiment on the laser facility, which is configured initially for indirect drive, the polar direct drive has been proposed and validated on the Omega laser facility and the National Ignition Facility. The polar direct drive repoints some of the beams toward the polar and equator of the target, thus increasing the drive energy on the polar and equator of capsule and achieving the most uniform irradiation. The present article focuses on the laser irradiation uniformity of the target in polar direct drive on ShenGuangIII (SGIII) facility. Firstly, the laser beam configuration of the SGIII, the characteristics of laser spots, the laser beam repointing strategy and the principle of optimization are introduced. The 48 laser beams are distributed over four cones per hemisphere and the beam centerlines are repointed in polar direct drive. The continuous phase plates (CPPs) of the SGIII are designed to have unique shape to make the laser beam with a 250 m-radius circular section at the laser entrance hole in indirect drive, and thus the laser beams have ellipse cross sections with fixed major axis and different minor axes in different cones. Then, the irradiation uniformity of 540 m target is optimized by the three-dimensional (3D) view factor method on the assumption that the laser intensity distribution is super-Gaussian with three and five orders, and the energy deposition distributions are expressed as cos2 and cos . The irradiation nonuniformity of less than 5% on the polar direct drive capsule of 540 m in diameter is achieved. The pressure distribution of the hot spot at the neutron bang time with the optimized parameter is also simulated by Multi2D, and the results of 2D hydrodynamics simulation indicate that the hot spot under the assumption of cos distribution is more symmetric. Finally, the effects on irradiation uniformity of the beam-to-beam power imbalance, the repointing error and the target pointing error are estimated by the Monte Carlo method. According to the simulation results, the laser root mean square nonuniformity on the target will not become worse observably when the maximal beam-to-beam power imbalance is limited to a value of 5%, and the repointing error and the target pointing error are better than 7 m.

The x ray energy loss out of laser-heated hohlraum through laser entrance holes (LEH) is discussed in detail according to a simple theoretical model and is compared with the hohlraum experimental data measured at Shenguang II laser facility. The radiation loss is considered to be composed of two parts, that is, direct contribution from laser spots and re-emitted part from the x ray-heated hohlraum inner wall, and the former accounts for about 20% of the total loss for the Shenguang II hohlraums. Owing to the non-equilibrium characteristics of laser target coupling the direct contribution part is non-equilibrium in spectrum.

The plastic DD filled capsule implosion experiment is performed on Shenguang III prototype laser facility. One-dimensional hydrodynamic numerical simulations show that the implosion compression ratio can be controlled by changing the capsule ablator thickness. In experiments, two types of capsules are studied and most of important implosion parameters are collected, such as neutron yield, X-ray bang-time, trajectory, and shape of hot core. The comparison between post-simulations and experimental results is performed. In our experiments, the neutron yield is 6.8×107 and YOC1D reaches 34% for low compression ratio implosion; the neutron yield is 6.3×106 and YOC1D is only 2.3% for middle compression ratio implosion. Meantime, the shape of hot core obtains an extra higher Legendre partial (P2 is 18% and P4 is 5%). On another side, the trajectory and bang-time are compared with simulations well.

Since ignition target design with layered deuterium and triterium ice had been proposed several decades ago, much effort was devoted to fabricate and implode cryogenic targets. Until recently, direct-drive cryogenic target implosion experiment was carried out on SGIII prototype laser facility. The target consisted of a plastic capsule supported by fill tube. Cryogenic helium gas was used to cool the capsule to a few degrees below the deuterium triple point. The resulting deuterium ice layer was characterized by optical shadowgraph and smoothed by applied temperature gradient. Eight laser beams with total energy of 7 kJ were used to directly drive the implosion. On the path of laser light to the capsule, there were 500 nm sealing film and helium gas of mm length. X-ray pinhole images were analyzed to confirm that the sealing film, and helium gas had little effect on aiming accuracy but caused some loss of laser energy especially when condensation on the sealing film was observed.

Traditional implosion backlight imaging experiment has disadvantages of nonuniform X-ray source, low contrast ablator interface and high requirement for diagnostic device tuning precision. A novel design of multi-point X-ray source combined with phase contrast imaging developed and optimized based on experimental research performed on Shenguang-II facility is presented. The novel design can obtain high-quality experimental result with uniform X-ray source, clear interface between ablator and inner DD gas and large image view. At the same time, the new design using diagnostic silt instead of diagnostic hole improves tuning precision. The experimental result proposes that novel design of laser driven plasma point x-ray source combined with phase contrast imaging has advantage of area X-ray source combined with absorb imaging and can be widely used in inertial confinement fusion and high energy density physics.

Direct drive implosion experiments were conducted on SGIII prototype laser facility. From the time resolvedx-rayimages, the bright ring and the central bright spot are observed. The radial velocity of the convergent bright ring indicates the shell velocity, and the times when the central bright spot is first seen and becomes most intensive indicate the times of shock convergence and later stagnation, respectively. Radiation hydrodynamic simulations were carried out by changing laser energy deposition factors. When the simulated results are brought close to the measured ones, it is found that the energy coupling efficiency is around 70%. The implosion symmetry is indicated by the core x-ray emission pattern which is pancake when viewing from the equator, and splits into several bright spots when viewing close to the pole. A simple model is developed to understand this asymmetry. It is speculated that the observed implosion asymmetry can be attributed to the laser arrangement which is originally designed for indirect drive experiments. Further improvements of energy coupling efficiency and implosion symmetry in future experiments can be achieved by optimizing target design and laser arrangement.

Measurement of radiation field near a implosion pellet M band X-ray is a important area in ICF research. On the SG Ⅱ laser facility, M band X-ray from Au plasma was measured in half-hohlraum radiation from the end. The experimental set-up and typical result were given. And weve also used the view-factor program to simulate this experiment. The results indicate that angular distribution of M band X-ray from the end of half-hohlraum doesnt assume linear relationship with the cosine of eradiating angle. The results are useful for the research of implosion experiment and target design.

A new kind of double-disk gold targets were irradiated by 0.35μm intense laser and-the properties of impvoved X-ray spectra were investigated at “Xingguang”laser facilities.-Collision velocities between the-primary and secondary disks' plasmas have been measured.This double-disk target's design has avoided the influence of the spraying plasma from primary disk to a certain extent, and increased the “clearance” for ablated secondary disk. Two soft X-ray spectrometers were used to measure X-ray radiation from the primary and secondary disks, respectively. An X-ray streaked camera was used to obtain time-spactial images emitted from laser-plasma of double-disks targets. Experimental results were-analyzed physically.

The method of filter difference which is used to measure X－ray spectrum radiate d from laser－produced plasma has been studied in this paper.The basic principle of the method of filter difference is introduced and the calculated and measure d results of filter difference pair are given.The absolute soft X－ray spectra f rom laser plasma have been measured.Finally the errors of measured spectrum inte nsity are analysed and estimated.

X-ray spectra, angular distributions of X-ray fluxes and evolutions of X-ray spe ctra from the front and the rear of gold foil target with different thickness ha ve been measured on “Xingguang” laser facility, in which the gold foil targets were irradiated by smoothing 1.053μm laser beam . The processes of laser and r adiation ablation to gold foil have been studied. Quantitative X-ray spectra fro m the front and the rear of gold foil under several conditions such as different foil thickness, different laser power densities and X-ray emission angles have been obtained. At the same time, radiative heat wave has also been observed fro m the evolutions of X-ray spectra emitted from the rear of gold foils of differe nt thickness.

X-ray reemission from gold disk targets,which were heated by intense soft X-ray radiation((2—6)×1012W·cm-2),had been observed with two time-related soft X-ray spectrometers-The spectra of X-ray reemission were “softer” than those of irradiating X-ray-Temporal evolution of the X-ray reemission was analyzed in terms of a time-modified self-similar solution for the ablative heat wave

Calibration of the energy response for the soft X-ray detection elements using in the ICF diagnostic are reported.The Beijing synchrotron Radiation Facility (3B1 beam line,photon energy～250—1000eV,radiation flux ～1012photon/s·mm2·mr2·0.1% band-width,beam current ～20—80mA) and target chamber of the reflectometer are used.The source intensity is monitored by AXUV-100 photodiode.The calibration of the energy response for the XRD,filter light film,Ⅱa type diamond photoconductive detector,transmission grating and soft X-ray film are given and analysed.