From concept to reality-A review to the primary test stand and its preliminary application in high energy density physics

Pulsed power technology,whereas the electrical energy stored in a relative long period is released in much shorter timescale,is an efficient method to create high energy density physics(HEDP)conditions in laboratory.Around the beginning of this century,China Academy of Engineering Physics(CAEP)began to build some experimental facilities for HEDP investigations,among which the Primary Test Stand(PTS),a multi-module pulsed power facility with a nominal current of 10 MA and a current rising time~90 ns,is an important achievement on the roadmap of the electro-magnetically driven inertial confinement fusion(ICF)researches.PTS is the first pulsed power facility beyond 10 TW in China.Therefore,all the technologies have to be demonstrated,and all the engineering issues have to be overcome.In this article,the research outline,key technologies and the preliminary HEDP experiments are reviewed.Prospects on HEDP research on PTS and pulsed power development for the next step are also discussed.

Conceptual design of a 15-TW pulsed-power accelerator for high-energy-densityephysics experiments

We have developed a conceptual design of a 15-TW pulsed-power accelerator based on the linear-transformer-driver(LTD)architecture described by Stygar[W.A.Stygar et al.,Phys.Rev.ST Accel.Beams 18,110401(2015)].The driver will allow multiple,high-energy-density experiments per day in a university environment and,at the same time,will enable both fundamental and integrated experiments that are scalable to larger facilities.In this design,many individual energy storage units(bricks),each composed of two capacitors and one switch,directly drive the target load without additional pulse compression.Ten LTD modules in parallel drive the load.Each module consists of 16 LTD cavities connected in series,where each cavity is powered by 22 bricks connected in parallel.This design stores up to 2.75 MJ and delivers up to 15 TW in 100 ns to the constant-impedance,water-insulated radial transmission lines.The transmission lines in turn deliver a peak current as high as 12.5 MA to the physics load.To maximize its experimental value and flexibility,the accelerator is coupled to a modern,multibeam laser facility(four beams with up to 5 kJ in 10 ns and one beam with up to 2.6 kJ in 100 ps or less)that can provide auxiliary heating of the physics load.The lasers also enable advanced diagnostic techniques such as X-ray Thomson scattering and multiframe and three-dimensional radiography.The coupled accelerator-laser facility will be the first of its kind and be capable of conducting unprecedented high-energy-densityephysics experiments.

High energy density physics with intense ion beams

We review the development of High Energy Density Physics(HEDP)with intense heavy ion beams as a tool to induce extreme states of matter.The development of this field connects intimately to the advances in accelerator physics and technology.We will cover the generation of intense heavy ion beams starting from the ion source and follow the acceleration process and transport to the target.Intensity limitations and potential solutions to overcome these limitations are discussed.This is exemplified by citing examples from existing machines at the Gesellschaft fur Schwerionenforschung(GSI-Darmstadt),the Institute of Theoretical and Experimental Physics in Moscow(ITEP-Moscow),and the Institute of Modern Physics(IMP-Lanzhou).Facilities under construction like the FAIR facility in Darmstadt and the High Intensity Accelerator Facility(HIAF),proposed for China will be included.Developments elsewhere are covered where it seems appropriate along with a report of recent results and achievements.

Molar volume of eutectic solvents as a function of molar composition and temperature

The conventional Rackett model for predicting liquid molar volume has been modified to cater for the effect of molar composition of the Deep Eutectic Solvents(DES). The experimental molar volume data for a group of commonly used DES has been used for optimizing the improved model. The data involved different molar compositions of each DES. The validation of the new model was performed on another set of DESs. The average relative deviation of the model on the training and validation datasets was approximately 0.1% while the Rackett model gave a relative deviation of more than 1.6%. The modified model deals with variations in DES molar composition and temperature in a more consistent way than the original Rackett model which exhibits monotonic performance degradation as temperature moves away from reference conditions. Having the composition of the DES as a model variable enhances the practical utilization of the predicting model in diverse design and process simulation applications.

Turbulent hydrodynamics experiments in high energy density plasmas： scientific case and preliminary results of the TurboHEDP project

The physics of compressible turbulence in high energy density(HED) plasmas is an unchartered experimental area.Simulations of compressible and radiative flows relevant for astrophysics rely mainly on subscale parameters. Therefore,we plan to perform turbulent hydrodynamics experiments in HED plasmas(TurboHEDP) in order to improve our understanding of such important phenomena for interest in both communities: laser plasma physics and astrophysics. We will focus on the physics of supernovae remnants which are complex structures subject to fluid instabilities such as the Rayleigh–Taylor and Kelvin–Helmholtz instabilities. The advent of megajoule laser facilities, like the National Ignition Facility and the Laser Megajoule, creates novel opportunities in laboratory astrophysics, as it provides unique platforms to study turbulent mixing flows in HED plasmas. Indeed, the physics requires accelerating targets over larger distances and longer time periods than previously achieved. In a preparatory phase, scaling from experiments at lower laser energies is used to guarantee the performance of future MJ experiments. This subscale experiments allow us to develop experimental skills and numerical tools in this new field of research, and are stepping stones to achieve our objectives on larger laser facilities. We review first in this paper recent advances in high energy density experiments devoted to laboratory astrophysics. Then we describe the necessary steps forward to commission an experimental platform devoted to turbulent hydrodynamics on a megajoule laser facility. Recent novel experimental results acquired on LULI2000, as well as supporting radiative hydrodynamics simulations, are presented. Together with the development of LiF detectors as transformative X-ray diagnostics, these preliminary results are promising on the way to achieve micrometric spatial resolution in turbulent HED physics experiments in the near future.

Competition among the two-plasmon decay of backscattered light,filamentation of the electron-plasma wave and side stimulated Raman scattering

Competition among the two-plasmon decay(TPD)of backscattered light of stimulated Raman scattering(SRS),filamentation of the electron-plasma wave(EPW)and forward side SRS is investigated by two-dimensional particlein-cell simulations.Our previous work[K.Q.Pan et al.,Nucl.Fusion 58,096035(2018)]showed that in a plasma with the density near 1/10 of the critical density,the backscattered light would excite the TPD,which results in suppression of the backward SRS.However,this work further shows that when the laser intensity is so high(>10^(16)W/cm^(2))that the backward SRS cannot be totally suppressed,filamentation of the EPW and forward side SRS will be excited.Then the TPD of the backscattered light only occurs in the early stage and is suppressed in the latter stage.Electron distribution functions further show that trapped-particle-modulation instability should be responsible for filamentation of the EPW.This research can promote the understanding of hot-electron generation and SRS saturation in inertial confinement fusion experiments.

EMP control and characterization on high-power laser systems

Giant electromagnetic pulses(EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers(e.g. the Extreme Light Infrastructure).We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3 D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP,providing an opportunity for comparison with existing charge separation models.

Particle-in-cell simulations of laser–plasma interactions at solid densities and relativistic intensities: the role of atomic processes

Direct numerical simulation of intense laser-solid interactions is still of great challenges, because of the many coupled atomic and plasma processes, such as ionization dynamics, collision among charged particles and collective electromagnetic fields, to name just a few. Here, we develop a new particle-in-cell （PIC） simulation code, which enables us to calculate laser-solid interactions in a more realistic way. This code is able to cover almost ＇all＇ the coupled physical processes. As an application of the new code, the generation and transport of energetic electrons in front of and within the solid target when irradiated by intense laser beams are studied. For the considered case, in which laser intensity is 1020 W. cm-2 and pre-plasma scale length in front of the solid is 10 Izm, several quantitative conclusions are drawn： （i） the collisional damping （although it is very weak） can significantly affect the energetic electrons generation in front of the target, （ii） the Bremsstrahlung radiation will be enhanced by 2-3 times when the solid is dramatically heated and ionized, （iii） the ＇cut-off＇ electron energy is lowered by an amount of 25% when both collision damping and Bremsstrahlung radiations are included, and （iv） the resistive electromagnetic fields due to Ohmic heating play nonignorable roles and must be taken into account in such interactions.

Targets for high repetition rate laser facilities:needs,challenges and perspectives

A number of laser facilities coming online all over the world promise the capability of high-power laser experiments with shot repetition rates between 1 and 10 Hz. Target availability and technical issues related to the interaction environment could become a bottleneck for the exploitation of such facilities. In this paper, we report on target needs for three different classes of experiments: dynamic compression physics, electron transport and isochoric heating, and laser-driven particle and radiation sources. We also review some of the most challenging issues in target fabrication and high repetition rate operation. Finally, we discuss current target supply strategies and future perspectives to establish a sustainable target provision infrastructure for advanced laser facilities.

Implementation of a Faraday rotation diagnostic at the OMEGA laser facility

Magnetic field measurements in turbulent plasmas are often difficult to perform. Here we show that for kG magnetic fields, a time-resolved Faraday rotation measurement can be made at the OMEGA laser facility. This diagnostic has been implemented using the Thomson scattering probe beam and the resultant path-integrated magnetic field has been compared with that of proton radiography. Accurate measurement of magnetic fields is essential for satisfying the scientific goals of many current laser–plasma experiments.

First radiative shock experiments on the SG-II laser

We report on the design and first results from experiments looking at the formation of radiative shocks on the ShenguangII(SG-II)laser at the Shanghai Institute of Optics and Fine Mechanics in China.Laser-heating of a two-layer CH/CH–Br foil drives a∼40 km/s shock inside a gas cell filled with argon at an initial pressure of 1 bar.The use of gas-cell targets with large(several millimetres)lateral and axial extent allows the shock to propagate freely without any wall interactions,and permits a large field of view to image single and colliding counter-propagating shocks with time-resolved,pointprojection X-ray backlighting(∼20µm source size,4.3 keV photon energy).Single shocks were imaged up to 100 ns after the onset of the laser drive,allowing to probe the growth of spatial nonuniformities in the shock apex.These results are compared with experiments looking at counter-propagating shocks,showing a symmetric drive that leads to a collision and stagnation from∼40 ns onward.We present a preliminary comparison with numerical simulations with the radiation hydrodynamics code ARWEN,which provides expected plasma parameters for the design of future experiments in this facility.

Magnetic reconnection driven by intense lasers

Laser-driven magnetic reconnection(LDMR) occurring with self-generated B fields has been experimentally and theoretically studied extensively, where strong B fields of more than megagauss are spontaneously generated in highpower laser–plasma interactions, which are located on the target surface and produced by non-parallel temperature and density gradients of expanding plasmas. For properties of the short-lived and strong B fields in laser plasmas, LDMR opened up a new territory in a parameter regime that has never been exploited before. Here we review the recent results of LDMR taking place in both high and low plasma beta environments. We aim to understand the basic physics processes of magnetic reconnection, such as particle accelerations, scale of the diffusion region, and guide field effects. Some applications of experimental results are also given especially for space and solar plasmas.

Measurement and analysis of K-shell lines of silicon ions in laser plasmas

We present laboratory measurement and theoretical analysis of silicon K-shell lines in plasmas produced by Shenguang II laser facility, and discuss the application of line ratios to diagnose the electron density and temperature of laser plasmas.Two types of shots were carried out to interpret silicon plasma spectra under two conditions, and the spectra from 6.6 ?A to6.85 ?A were measured. The radiative-collisional code based on the flexible atomic code(RCF) is used to identify the lines, and it also well simulates the experimental spectra. Satellite lines, which are populated by dielectron capture and large radiative decay rate, influence the spectrum profile significantly. Because of the blending of lines, the traditional G value and R value are not applicable in diagnosing electron temperature and density of plasma. We take the contribution of satellite lines into the calculation of line ratios of He-α lines, and discuss their relations with the electron temperature and density.

Performance of an elliptical crystal spectrometer for SGⅡX-ray opacity experiments

A new crystal spectrometer for application in X-ray opacity experiments is proposed. The conditions necessary to yield broad spectral coverage with a resolution >500, strong rejection of hard X-ray backgrounds and negligible source broadening for extended sources are formulated. In addition, the design, response modeling and reporting of an elliptical crystal spectrometer in conjunction with a linear detector are presented. The measured results demonstrate the performance of the new crystal spectrometer with a broad energy coverage range, high spectral resolution, and high luminosity(good collection efficiency). This spectrometer can be used in combination with point-projection backlighting techniques as utilized in X-ray opacity experiments. Specifically, the X-ray source, transmission and self-emission spectra of the sample can be measured simultaneously in a single shot, which can reduce the experimental uncertainties from shot-to-shot fluctuations. The new crystal spectrometer has been used in the X-ray opacity experiment to precisely measure the aluminum K-absorption edge shift in the energy range around 1.560 keV in strongly compressed matter. It is demonstrated that the spectrometer can be used to realize measurements of new and unpredictable physical interactions of interest, as well as basic and applied high-energy-density science.

New scheme to trigger fusion in a compact magnetic fusion device by combining muon catalysis and alpha heating effects

The application of laser pulses with psec or shorter duration enables nonthermal efficient ultrahigh acceleration of plasma blocks with homogeneous high ion energies exceeding ion current densities of 10^(12) A cm^(-2). The effects of ultrahigh acceleration of plasma blocks with high energy proton beams are proposed for muon production in a compact magnetic fusion device. The proposed new scheme consists of an ignition fusion spark by muon catalyzed fusion(μCF) in a small mirror-like configuration where low temperature D–T plasma is trapped for a duration of 1 μs. This initial fusion spark produces sufficient alpha heating in order to initiate the fusion process in the main device. The use of a multi-fluid global particle and energy balance code allows us to follow the temporal evolution of the reaction rate of the fusion process in the device. Recent progress on the ICAN and IZEST projects for high efficient high power and high repetition rate laser systems allows development of the proposed device for clean energy production. With the proposed approaches,experiments on fusion nuclear reactions and μCF process can be performed in magnetized plasmas in existing kJ/PW laser facilities as the GEKKO-LFEX, the PETAL and the ORION or in the near future laser facilities as the ELI-NP Romanian pillar.

Magnetic field annihilation and charged particle acceleration in ultra-relativistic laser plasmas

Magnetic reconnection driven by laser plasma interactions attracts great interests in the recent decades. Motivated by the rapid development of the laser technology, the ultra strong magnetic field generated by the laser-plasma accelerated electrons provides unique environment to investigate the relativistic magnetic field annihilation and reconnection. It opens a new way for understanding relativistic regimes of fast magnetic field dissipation particularly in space plasmas,where the large scale magnetic field energy is converted to the energy of the nonthermal charged particles. Here we review the recent results in relativistic magnetic reconnection based on the laser and collisionless plasma interactions.The basic mechanism and the theoretical model are discussed. Several proposed experimental setups for relativistic reconnection research are presented.

Analytical modelling of the expansion of a solid obstacle interacting with a radiative shock

In this paper, we present a model characterizing the interaction of a radiative shock(RS) with a solid material, as described in a recent paper(Koenig et al., Phys. Plasmas, 24, 082707(2017)), the new model is then related to recent experiments performed on the GEKKO XII laser facility. The RS generated in a xenon gas cell propagates towards a solid obstacle that is ablated by radiation coming from the shock front and the radiative precursor, mimicking processes occurring in astrophysical phenomena. The model presented here calculates the dynamics of the obstacle expansion,which depends on several parameters, notably the geometry and the temperature of the shock. All parameters required for the model have been obtained from experiments. Good agreement between experimental data and the model is found when spherical geometry is taken into account. As a consequence, this model is a useful and easy tool to infer parameters from experimental data(such as the shock temperature), and also to design future experiments.

A platform for high-repetition-rate laser experiments on the Large Plasma Device

We present a new experimental platform for studying laboratory astrophysics that combines a high-intensity, highrepetition-rate laser with the Large Plasma Device at the University of California, Los Angeles. To demonstrate the utility of this platform, we show the first results of volumetric, highly repeatable magnetic field and electrostatic potential measurements, along with derived quantities of electric field, charge density and current density, of the interaction between a super-Alfv′enic laser-produced plasma and an ambient, magnetized plasma.