Hohlraum Targets Research Articles

Intense Electromagnetic Pulses Generated From kJ-Laser Interacting With Hohlraum Targets

Irradiating hohlraum targets with high-power laser pulses is a promising approach to achieve inertial confinement fusion (ICF). Unfortunately, such interaction is accompanied by intense electromagnetic pulses (EMPs). The EMPs can interfere with various physical diagnostic signals and even cause a malfunction of some crucial equipment. In this study, EMPs generated due to kJ-laser coupling with hohlraum targets are recorded using B-dot probes inside the target chamber. The configuration of the hohlraum has a more remarkable effect on EMP amplitudes compared to laser energy. 0.6 mg/cc of He gas filling in the hohlraum and the presence of the capsule inside the hohlraum both dramatically reduce EMPs. Moreover, EMP frequencies corresponding to various hohlraums are basically below 1.75 GHz. Soft X-ray (0.1–4 keV) radiant flux is used to reveal the characteristics of EMPs. The conclusions are expected to enable us to better understand the relationship of EMPs with target configuration and laser energy.

Transient magnetic field diffusion considerations relevant to magnetically assisted indirect drive inertial confinement fusion

Application of a magnetic field to an indirect drive inertial confinement fusion target requires diffusion of the field through the high-Z and electrically conducting Hohlraum. The onset of the external field generates eddy currents in the Hohlraum wall that result in (1) a reduction of the peak field at the capsule, (2) heating of the Hohlraum wall through Ohmic dissipation, and (3) wall movement due to the inward force from the eddy current interacting with the field. Heating of the wall causes an increase in blackbody radiation which can preheat the capsule and frozen deuterium–tritium fuel, while wall motion leads to potential misalignment of the lasers at the Hohlraum wall. Limiting these detrimental effects sets requirements on the tolerable magnitude of each effect. We present a nonlinear model for B-field diffusion through an infinitely long thin-walled cylinder with a temperature dependent resistivity, to show that a 15 μm thick wall of pure gold fails to meet these requirements. A new Hohlraum material made from an alloy of Au and Ta has a measured resistivity of ≥60 times that of Au and is shown with the nonlinear model to meet the requirements for magnetization. We compare the nonlinear model to simulations of the actual Hohlraum target using a finite element code which includes temperature-dependent Hohlraum resistivity.

Saturation of Raman scattering in nonlinear regime

Saturation of Raman scattering of laser pulses in plasma, which is expected for hohlraum targets in ICF, is investigated. Trapping and Langmuir decay instability are two major candidates for saturation of Raman scattering in nonlinear regime. Coupled equations of plasma and scattered waves are derived and solved analytically in nonlinear regime where the effect of Landau damping and nonlinear frequency shift is considered. The solution leads to derivation of scattered waves growth and their reflectivity. Plots of reflectivity of scattered waves under various laser beam and plasma conditions are presented. The effects of plasma temperature and its density, as well as, laser frequency, its intensity and pulse duration on reflectivity are discussed.

Recent research progress of laser plasma interactions in Shenguang laser facilities

We report experimental research on laser plasma interaction (LPI) conducted in Shenguang laser facilities during the past ten years. The research generally consists of three phases: (1) developing platforms for LPI research in mm-scale plasma with limited drive energy, where both gasbag and gas-filled hohlraum targets are tested; (2) studying the effects of beam-smoothing techniques, such as continuous phase plate and polarization smoothing, on the suppression of LPI; and (3) exploring the factors affecting LPI in integrated implosion experiments, which include the laser intensity, gas-fill pressure, size of the laser-entrance hole, and interplay between different beam cones. Results obtained in each phase will be presented and discussed in detail.

Enhanced energy coupling for indirectly driven inertial confinement fusion

Recent experiments in the study of inertial confinement fusion (ICF) at the National Ignition Facility (NIF) in the United States have reached the so-called alpha-heating regime1–3, in which the self-heating by fusion products becomes dominant, with neutron yields now exceeding 1 × 1016 (ref. 4) However, there are still challenges on the path towards ignition, such as minimization of the drive asymmetry, suppression of laser-plasma instabilities, and mitigation of fabrication features5. In addition, in the current cylindrical-hohlraum indirect drive schemes for ICF, a strong limitation is the inefficient (≤10%) absorption of the laser-produced hohlraum X-rays by the capsule as set by relative capsule-to-hohlraum surface areas. Here we report an experiment demonstrating ~30% energy coupling to an aluminium capsule in a rugby-shaped6, gold hohlraum. This high coupling efficiency can substantially increase the tolerance to residual imperfections and improve the prospects for ignition, both in mainline single-shell hot-spot designs and potential double-shell targets. High coupling efficiency between laser-induced hohlraum X-rays and targets is essential for reaching long-sought regimes for viable inertial confinement fusion. Experiments with a rugby hohlraum shape and an improved capsule now allow demonstration of more than 30%.

Laser pulse shape design for laser-indirect-driven quasi-isentropic compression experiments

Laser pulse shape design is a key work in the design of indirect-laser-driven experiments, especially for long pulse laser driven quasi-isentropic compression experiments. A method for designing such a laser pulse shape is given here. What’s more, application experiments were performed, and the results of a typical shot are presented. At last of this article, the details of the application of the method are discussed, such as the equation parameter choice, radiation ablation pressure expression, and approximations in the method. The application shows that the method can provide reliable descriptions of the energy distribution in a hohlraum target; thus, it can be used in the design of long-pulse laser driven quasi-isentropic compression experiments and even other indirect-laser-driven experiments.

Target alignment in the Shen-Guang II Upgrade laser facility

The Shen-Guang II Upgrade (SG-II-U) laser facility consists of eight high-power nanosecond laser beams and one short-pulse picosecond petawatt laser. It is designed for the study of inertial confinement fusion (ICF), especially for conducting fast ignition (FI) research in China and other basic science experiments. To perform FI successfully with hohlraum targets containing a golden cone, the long-pulse beam and cylindrical hohlraum as well as the short-pulse beam and cone target alignment must satisfy tight specifications (30 and $20~\unicode[STIX]{x03BC}\text{m}$ rms for each case). To explore new ICF ignition targets with six laser entrance holes (LEHs), a rotation sensor was adapted to meet the requirements of a three-dimensional target and correct beam alignment. In this paper, the strategy for aligning the nanosecond beam based on target alignment sensor (TAS) is introduced and improved to meet requirements of the picosecond lasers and the new six LEHs hohlraum targets in the SG-II-U facility. The expected performance of the alignment system is presented, and the alignment error is also discussed.

Design and Fabrication of Opacity Targets for the National Ignition Facility

Accurate models for opacity of partially ionized atoms are important for modeling and understanding stellar interiors and other high-energy-density phenomena such as inertial confinement fusion. Lawrence Livermore National Laboratory is leading a multilaboratory effort to conduct experiments on the National Ignition Facility (NIF) to try to reproduce recent opacity tests at the Sandia National Laboratory Z-facility. Since 2015, the NIF effort has evolved several hohlraum designs that consist of multiple pieces joined together. The target also has three components attached to the main stalk over a long distance with high tolerances that have resulted in several design iterations. The target has made use of rapid prototyped features to attach a capsule and collimator under the hohlraum while avoiding interference with the beams. This paper discusses the evolution of the hohlraum and overall target design and the challenges involved with fabricating and assembling these targets.

Ultrafast probing of magnetic field growth inside a laser-driven solenoid

We report on the detection of the time-dependent B-field amplitude and topology in a laser-driven solenoid. The B-field inferred from both proton deflectometry and Faraday rotation ramps up linearly in time reaching 210 ± 35 T at the end of a 0.75-ns laser drive with 1 TW at 351 nm. A lumped-element circuit model agrees well with the linear rise and suggests that the blow-off plasma screens the field between the plates leading to an increased plate capacitance that converts the laser-generated hot-electron current into a voltage source that drives current through the solenoid. ALE3D modeling shows that target disassembly and current diffusion may limit the B-field increase for longer laser drive. Scaling of these experimental results to a National Ignition Facility (NIF) hohlraum target size (∼0.2cm^{3}) indicates that it is possible to achieve several tens of Tesla.

Analysis of Raman scattering of self-focused Gaussian laser beam in plasma without WKB approximation

The self-focusing and propagation of intense Gaussian laser beams in plasmas are investigated, and the explicit form of intensity of self-focused laser beams is derived without the use of WKB approximation. Propagation of self-focused laser beams in plasmas is strongly affected by Raman scattering and Brillouin scattering that are expected for hohlraum targets in inertial confinement fusion. The intensity of Raman and Brillouin scattered waves is derived in paraxial approximation where the effect of plasma temperature and Landau damping is considered through the kinetic theory of plasmas. The effect of plasma temperature and its density, as well as laser wavelength and its intensity, on self-focusing and spatial growth of scattered waves is considered.

A novel experimental setup for energy loss and charge state measurements in dense moderately coupled plasma using laser-heated hohlraum targets

We report on a new experimental setup for ion energy loss measurements in dense moderately coupled plasma which has recently been developed and tested at GSI Darmstadt. A partially ionized, moderately coupled carbon plasma (ne ≤ 0.8• 1022 cm-3, Te = 15 eV, z = 2.5, Γ = 0.5) is generated by volumetrical heating of two thin carbon foils with soft X-rays. This plasma is then probed by a bunched heavy ion beam. For that purpose, a special double gold hohlraum target of sub-millimeter size has been developed which efficiently converts intense laser light into thermal radiation and guarantees a gold-free interaction path for the ion beam traversing the carbon plasma. This setup allows to do precise energy loss measurements in non-ideal plasma at the level of 10 percent solid-state density.

Optical field distribution on a hohlraum wall during indirect laser-driven inertial confinement fusion

A model to calculate the optical field distribution of quadruplet beams on a hohlraum target wall is presented. This model combines geometrical ray tracing, coordinate transformation, and Fresnel diffraction integral to capture the quadruplet beams propagating in four different directions and the typically non-planar geometry of the hohlraum wall. The results demonstrate that the optical field distribution arises mainly from individual beam diffraction, and the interference with other beams in the quadruplet hardly devotes to the distribution. A movie is also produced to interpret the spatio and temporal evolution of the optical field on a cylindrical hohlraum wall.

强激光辐照平面靶与柱腔靶产生电磁脉冲对比研究

强激光辐照平面靶与柱腔靶产生电磁脉冲对比研究

Laser parametric instability experiments of a 3ω, 15 kJ, 6-ns laser pulse in gas-filled hohlraums at the Ligne d'Intégration Laser facility

Experimental investigation of stimulated Raman (SRS) and Brillouin (SBS) scattering have been obtained at the Ligne-d'Intégration-Laser facility (LIL, CEA-Cesta, France). The parametric instabilities (LPI) are driven by firing four laser beamlets (one quad) into millimeter size, gas-filled hohlraum targets. A quad delivers energy on target of 15 kJ at 3ω in a 6-ns shaped laser pulse. The quad is focused by means of 3ω gratings and is optically smoothed with a kinoform phase plate and with smoothing by spectral dispersion-like 2 GHz and/or 14 GHz laser bandwidth. Open- and closed-geometry hohlraums have been used, all being filled with 1-atm, neo-pentane (C5H12) gas. For SRS and SBS studies, the light backscattered into the focusing optics is analyzed with spectral and time resolutions. Near-backscattered light at 3ω and transmitted light at 3ω are also monitored in the open geometry case. Depending on the target geometry (plasma length and hydrodynamic evolution of the plasma), it is shown that, at maximum laser intensity about 9 × 1014 W/cm2, Raman reflectivity noticeably increases up to 30% in 4-mm long plasmas while SBS stays below 10%. Consequently, laser transmission through long plasmas drops to about 10% of incident energy. Adding 14 GHz bandwidth to the laser always reduces LPI reflectivities, although this reduction is not dramatic.

Use of external magnetic fields in hohlraum plasmas to improve laser-coupling

Efficient coupling of laser energy into hohlraum targets is important for indirect drive ignition. Laser-plasma instabilities can reduce coupling, reduce symmetry, and cause preheat. We consider the effects of an external magnetic field on laser-energy coupling in hohlraum targets. Experiments were performed at the Omega Laser Facility using low-Z gas-filled hohlraum targets which were placed in a magnetic coil with Bz ≤ 7.5-T. We found that an external field Bz = 7.5-T aligned along the hohlraum axis results in up to a 50% increase in plasma temperature as measured by Thomson scattering. The experiments were modeled using the 2-D magnetohydrodynamics package in HYDRA and were found to be in good agreement.

Beam guiding system geometric arrangement in the target area of high-power laser drivers

With the increasing number of laser beams, the main difficulty in arranging beam guiding systems (BGSs) involves determining the corresponding relationships between the output and input ports to realize the identified light path length of all beams. Given the basic constraints of geometric arrangement, a BGS model is established, and a base-line algorithm is proposed to address the difficulty mentioned above. Boundary conditions of target area and target chamber are discussed to increase the number of laser beams, and a maximum value exists for a specific target area. Finally, the compatibility of a cylindrical hohlraum target chamber with a spherical hohlraum is analyzed, and a moveable final optics assembly is proposed to execute the switch between the two different targets.

A novel double hohlraum target to create a moderately coupled plasma for ion stopping experiments

We present a new double hohlraum target for the creation of a moderately coupled (0.1<Γ<1) carbon plasma for energy loss and charge state measurements of projectile ions interacting with this plasma. A spherical cavity of 600μm in diameter is heated with a 150-J laser pulse (λL=527nm) within 1.2ns to produce a quasi-Planckian X-ray source with a radiation temperature of Tr≈100eV. These X-rays are then used to heat volumetrically two thin carbon foils in a secondary cylindrical hohlraum to a dense plasma state. An axi-symmetric plasma column with a free-electron density of up to 8×1021cm-3, a temperature of T≈10eV, and an average ionization degree of Z≈3 is generated. This plasma stays in a dense and an almost uniform state for about 5ns. Ultimately, such targets are supposed to be used in experiments where a heavy ion beam is launched through the sample plasma, and the ion energy losses as well as the charge distributions are to be measured. The present paper is in a certain sense a symbiotic one, where the theoretical analysis and the experimental results are combined to investigate the basic properties and the prospects of this type of plasma targets.

A Full Aperture Backscattering Light Diagnostic System Installed on the Shenguang-III Prototype Laser Facility

A full aperture backscattering light diagnostic system (FABLDS) implemented on the Shen Guang-III Prototype Laser Facility is described in the paper. FABLDS measures both stimulated brillouin scattering (SBS) and stimulated Raman scattering (SRS) with a series of optical detectors. Energy sensors record the integrated energy, and streak cameras coupled with spectrometers measure the temporal spectrum of the backscattering light. This paper provides an overview of the FABLDS and detailed descriptions of the optical path. Special components, including off-axis parabolic mirror, spatial filter and optical light filters, are incorporated along the beam path for purifying the scattering light. Several hohlraum targets were employed, including C5H12 gas-filled targets and empty targets in the experiments. Results presented in the paper indicate that the fraction of backscatter light has been obviously shrinked when the laser is smoothed by continuous phase plates (CPP).

Progress toward ignition at the National Ignition Facility

Progress toward ignition at the National Ignition Facility (NIF) has been focused on furthering the understanding of implosion performance. Implosion performance depends on the capsule fuel shape, on higher mode asymmetries that may cause hydrodynamic instabilities to quench ignition, on time-dependent asymmetries introduced by the hohlraum target, and on ablator performance. Significant findings in each of these four areas is reported. These investigations have led to improved in-flight capsule shape, a demonstration that a capsule robust to mix can generate high levels of neutrons (7.7 × 1014), hohlraum modifications that should ultimately provide improved beam propagation and better laser coupling, and fielding of capsules with high-density carbon (HDC) ablators. A capsule just fielded with a HDC ablator and filled with DT gas generated a preliminary record level of neutrons at 1.6 × 1015, or 5 kJ of energy. Future plans include further improvements to fuel shape and hohlraum performance, fielding robust capsules at higher laser power and energy, and tuning the HDC capsule. A capsule with a nanocrystalline diamond (HDC) ablator on a DT ice layer will be fielded at NIF later this year.

Optimization of the parameters of plasma liners with zero-dimensional models

The efficiency of conversion of the energy stored in the capacitor bank of a high-current pulse generator into the kinetic energy of an imploding plasma liner is analyzed. The analysis is performed by using a model consisting of LC circuit equations and equations of motion of a cylindrical shell. It is shown that efficient energy conversion can be attained only with a low-inductance generator. The mode of an "ideal" load is considered where the load current at the final stage of implosion is close to zero. The advantages of this mode are, first, high efficiency of energy conversion (80%) and, second, improved stability of the shell implosion. In addition, for inertial confinement fusion realized by the scheme of a Z pinch dynamic hohlraum, not one but several fusion targets can be placed in the cavity on the pinch axis due to the large length of the liner.