Session PO04: ICF: Alternative Approaches

Characterization of Supersonic Spherically Imploding Plasma Liners on PLX

The Plasma Liner Experiment (PLX) at Los Alamos National Laboratory is a mid-size experimental facility that has been built to explore the idea of using a spherically imploding plasma liner, formed via the merging of discrete plasma jets, as a transformative driver for magneto-inertial fusion. [1] We present results from PLX with fully spherical plasma liner implosions, formed from an array of 36 plasma jets. Prior investigations on PLX have studied the merging of smaller numbers of plasma jets, and indicated the significance of inter-jet streaming and interpenetration between merging jets, and the possible impact of density perturbations due to shock waves between the jets. We will present experimental characterization of the plasma liner convergence, symmetry, and stagnation morphology, informing the potential for integrated plasma compression experiments to be driven by the facility.

[1] Hsu, Scott C., et al. ”Spherically imploding plasma liners as a standoff driver for magnetoinertial fusion.” IEEE Transactions on Plasma Science 40.5 (2012): 1287-1298.

We acknowledge the contributions of the many scientists and personnel in the broader team and plasma physics community who have contributed to the PLX program.   

Magnetized Plasma Guns for the BETHE PLX PJMIF Project

In plasma jet driven magneto-inertial fusion (PJMIF), an array of discrete supersonic plasma jets is used to form a spherically imploding plasma liner, which then compresses a magnetized plasma target to fusion conditions[1]. We are currently pursuing the formation of a central high-beta magnetized plasma target via the collision of an array of magnetized jets. Magnetized coaxial plasma guns have been developed by adding a bias field coil to the previously developed contoured gap plasma liner gun. We have achieved (exceeded) our goal of magnetized hydrogen plasma jets with 3 x 10¹⁴ cm^-3 muzzle density, temperature above 5 eV, 100 km/s, and with an embedded field of 1 kG. This was achieved partly by reducing the pfn to only 400μF as suggested by Mach2 modeling and also by fine tuning operating parameters. We are currently assessing methods to minimize plasma impurities. We provide an overview of experimental results, and plans for a 2-jet colliding experiment. [1] Hsu et al., IEEE Trans. Plasma Sci. 40, 1287 (2012).   

Magnetized Plasma Gun Development for the PLX PJMIF Project

We present engineering and technical details of magnetized plasma gun development for the Plasma Liner Experiment (PLX) project [1]. Intended to form a magnetized hydrogen plasma target for plasma-jet-driven magneto-inertial fusion (PJMIF), the magnetized guns are an extension of HyperJet Fusion's most recently developed coaxial plasma guns (HJ1), which form the plasma liner for target compression. Each gun incorporates a high power, pulsed magnet coil designed to provide linked magnetic flux between gun electrodes in the plasma formation region such that the resultant hydrogen plasma jet exits the gun with an embedded magnetic field. A 25 kJ capacitor bank with SCR switching and crowbar diode was used to energize a 30-turn magnet coil for initial testing and validation, which provided up to 3.1 kG of peak field within the gun at maximum magnet charge voltage, resulting in up to 4 mWb of flux linking electrodes in the breech. With this magnet system, we have exceeded our goal of magnetized hydrogen plasma jets with ∽3 x 10¹⁴ cm^-3 muzzle density, ∽5 eV electron temperature, ∽100 km/s velocity and an embedded field of ∽1 kG. [1] Hsu et al., IEEE Trans. Plasma Sci. 40 (2012).    

OSIRIS Particle-in-Cell Simulations of Colliding Plasma Jets for Plasma Liner Experiment

OSIRIS Particle-in-Cell simulations modeling target formation in Plasma Liner Experiment [PLX, Hsu et al. IEEE Transaction on Plasma Science 40, 1287 (2012)] will be presented. The results confirm that the interaction between colliding plasma jets are in a regime of plasma beta β>1 and Hall parameters  χ_i,e>>1. The jets stop each other with only limited interpenetration mainly by increased magnetic pressures, rather than Coulomb collisions. This result supports further modeling using radiation magnetohydrodynamic simulation codes to obtain ab initio large scale simulations of the PLX target formation.    

Updates on the X-Pinch Platform and Faraday Rotation Imaging Diagnostic on the MAIZE Pulsed Power Facility

X-pinches, formed by driving intense current through the crossing of 2 or more wires, provide an excellent platform for the study of “micro-pinches” due to their propensity to generate a single micro-pinch at a predetermined location in space (i.e., where the wires cross) [1,2]. Ideally, micro-pinches are areas of run-away compression to very small radii (~1 µm) leading to pressures on the order of ~1 Gbar for currents on the order of ~0.1 MA. However, the fraction of the total current that is driven through the dense micro-pinch plasma at small radii versus that being shunted through the surrounding coronal plasma at larger radii is not well known. To allow for the study of micro-pinches and their current distribution on the 1-MA MAIZE facility, a Faraday rotation imaging diagnostic (1064 nm) [3], as well as corresponding modular X-pinch load hardware, was developed. Presented is the status of these developments including preliminary experimental results characterizing X-pinches on the MAIZE LTD.

 

[1] S.A. Pikuz et al., Plasma Phys. Rep., 41, 291 (2015);

[2] S.A. Pikuz et al., Plasma Phys. Rep., 41, 445 (2015);

[3] G.F. Swadling et al., Rev. Sci. Instrum., 85, 11E502 (2014);   

Understanding electrode plasma formation on wires and thin foils via vacuum ultraviolet spectrsocopy of desorbed surface contaminants

Power flow studies on the 30-MA, 100-ns Z facility at Sandia National Labs (SNL) have shown that plasmas in the facility's magnetically insulated transmission lines (MITLs) can result in a loss of current delivered to the load. During the current pulse, thermal energy deposition into electrodes (ohmic heating, charged particle bombardment, etc.) causes neutral surface contaminants layers (water, hydrogen, hydrocarbons, etc.) to desorb, ionize, and form plasmas in the anode-cathode (AK) gap. Shrinking typical electrode thicknesses (~1 cm) down to that of thin foils (5−200 μm) produces observable amounts of plasma on smaller pulsed power drivers (≤1 MA). We suspect that as the electrode material bulk thickness decreases relative to the skin depth of the current pulse (50−100 μm for a 100−500-ns pulse in aluminum), the thermal energy delivered to the neutral surface contaminant layers increases, and thus more surface contaminants desorb from the current carrying surface. In this talk, we review our efforts to develop a thin-foil-based platform to study electrode plasma formation on smaller-scale facilities (≤ 1 MA) and present results from a vacuum ultraviolet (VUV) spectroscopy system developed to measure the hydrogen Lyman-α line (121.6 nm) from wires and foils with varying thicknesses (5−200 μm). We use the VUV measurements to compare hydrogen inventories to those predicted to be released via thermal processes in the electrode. The VUV range (100−200 nm) was chosen due to the expectation of low levels of background continuum emission relative to the visible range; this expectation is supported by preliminary simulations with PrismSPECT.   

Current Distribution in Liner-on-target Gas-Puff Implosions on the CESZAR Linear Transformer Driver

Gas-puff Z-pinches serve as powerful x-ray and neutron sources and have been studied in a number of configurations for their potential use in nuclear fusion. Understanding the Z-pinch process necessitates diagnosis of the evolution and behavior of the currents and magnetic fields which drives the implosion. Liner-on-target gas-puff (an annular jet surrounding a central column jet) z-pinches experiments have been performed on the CESZAR linear transformer-driver (500 kA and 160 ns current rise time) at UC San Diego using a Zeeman-based polarization spectroscopy technique to measure the azimuthal magnetic field both in the target and liner regions. Values of B_θ were measured up to 4 T in the target regions and up to 8 T in the liner region. In addition, a fast-framing extreme ultra-violet camera provided information about pinch dynamics. Experimental measurements of the azimuthal magnetic field and implosion dynamics are compared with the magnetohydrodynamic simulations carried out in HYDRA.   

Magneto-Rayleigh-Taylor instabiliity mitigation in multi-species gas-puff Z-pinches

The gas-puff Z-pinch is a well-known, efficient source of X-rays and/or neutrons, in which an axially applied current interacts with the azimuthal self-magnetic field to drive a radial implosion. It is highly susceptible to the magneto-Rayleigh-Taylor instability (MRTI), which can disrupt the pinch if unmitigated.  Axial pre-magnetization can mitigate MRTI growth but tends to reduce yield [1] as the initial field, B_z0, is increased. Here, we present 2-D magnetohydrodynamic simulations [2] of Ne-liner, deuterium-target gas-puff loads driven by an 850 kA, 160 ns driver which show that the tradeoff between stability and yield can be reduced by adding a second liner. A B_z0 of 0.7 T is required to stabilize the single-liner implosion with initial radius 2.5 cm, but thermonuclear neutron yield decreases from 1.2x10⁹ with B_z0 = 0 T to 1.0x10⁷ with B_z0 = 0.7 T. When a second liner is added with radius 1.25 cm, the required B_z0 is reduced to 0.3 T. Consequently, the penalty to yield is significantly reduced: from 1.2x10⁹ with B_z0 = 0 T to 9.8x10⁸ with B_z0 = 0.3 T. We show this concept scales favorably with current to 10 MA, at which level thermonuclear yields of ~10¹³ are predicted.

[1] F. Conti et al., Phys. Plasmas 27, 012702 (2020).

[2] J. Narkis et al., Phys. Rev. E (under review).   

Characterization of hot electrons in experiments relevant to the Shock Ignition approach to Inertial Confinement Fusion

Shock Ignition (SI) is an alternative approach to direct-drive Inertial Confinement Fusion based on the separation of the compression and the ignition phases.  The high laser intensity required in the ignition phase exceeds the thresholds for the generation of laser-plasma instabilities (LPI), generating large amount of supra-thermal electrons. These electrons could preheat the hotspot, with detrimental effects for the SI scheme, or assist in generating a strong shock.

In this context, we present the results coming from an experiment conducted on the OMEGA-EP laser facility aiming at characterizing this hot electron source. In the experiment, an high intensity UV interaction beam (1-ns UV I ~10¹⁶ W/cm²) was focused on a multi-layer planar target (175 μm CH / 20 μm Cu, 500 μm diameter), generating a strong shock and copious amounts of hot electrons. The hot-electron source was characterized in terms of maxwellian temperature T_h and laser to hot-electron energy conversion efficiency η using different spectrometers, exploiting the radiation emitted by the passage of the electrons in the target.

The post-processing of the spectrometer data relies on using Monte Carlo codes in which the propagation of the electron beam in the target and the detector responses are simulated.   

A Dual Laser-Beam Configuration Compatible with Both Symmetric Direct Drive and Spherical Hohlraums

This work continues to investigate the feasibility of laser-beam configurations that would make possible both spherical direct drive and spherical indirect drive on the same ignition-scale laser system, exploring, in particular, the port arrangement proposed in Ref. [1]. This geometry permits the use of octahedral hohlraums (with six laser entrance holes based on the faces of a cube), proposed by Lan et al.,[2] which offer excellent uniformity without the need for different pulse shapes in different beams. The trade-off between uniformity and radiation temperature is examined in designs with different case-to-capsule ratios. For direct drive, the beams require a small amount of repointing (much less than is typical for direct-drive designs on the National Ignition Facility). The sensitivity of uniformity to the laser pointing and spatial profile is investigated. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.

NIF Characterization of Hot Electrons Applied to Shock Ignition, using 3D Monte-Carlo Tracking within 2D ALE Simulations

When simulated hydrodyamically, shock ignition (SI) achieves the highest fusion energy gain at the scale of current inertial confinement fusion (ICF) facilities [1]. This gain is critical if ICF is to be used for energy generation. The National Ignition Facility (NIF) is the world's largest laser facility, with 1.8MJ of laser energy. The NIF has not yet achieved ignition and, although not designed for spherical direct drive, simulations indicate that polar direct drive (PDD) [2] could be used to ignite an implosion [3]. Scaled down SI experiments have shown that hot electrons generated from laser plasma instabilities (LPI) can degrade implosion performance. Hot electrons are a significant unknown that cannot be captured by hydrodynamics alone.

In this talk, the effects of hot electrons on a cryogenic SI implosion will be analysed using a 3D Monte-Carlo hot electron scattering and energy deposition model integrated into a 2D radiation hydrodynamics ALE code (Odin). The talk will focus on the key areas of hot electron preheat, shock support and shock timing. The hot electron source is based on a NIF experiment which used PDD illumination of a solid target to achieve SI-relevant ablation-plasma and intensity conditions, in addition to characterizing the hot electron population.

[1] Betti, R., et al. Physical review letters 98.15 (2007): 155001.

[2] Skupsky, S., et al. Physics of Plasmas 11.5 (2004): 2763-2770.

[3] Anderson, K. S., et al. Physics of Plasmas 20.5 (2013): 056312.   

A Shock-Augmented approach to Laser Inertial Fusion

Shock ignition¹ enables high gain at low implosion velocity, easing the ablative Rayleigh-Taylor instability which degrades conventional direct drive. With this method, driving a strong shock requires high laser power and intensity, resulting in inefficiencies in the drive and the generation of hot electrons that can preheat the fuel. A new ‘shock-augmented ignition’ pulse-shape² is described which, by launching a strong shock, enables the shock-ignition of thermonuclear fuel, but with substantially reduced laser power and intensity requirements. The reduced intensity requirement with respect to shock-ignition limits laser-plasma instabilities, such as Stimulated Raman and Brillouin Scatter, reducing the risk of hot-electron pre-heat and restoring the laser coupling advantages of conventional direct drive. Simulations indicate that, due to the reduced power requirements, high gain (~100) ignition of large-scale direct drive implosions is possible within the power and energy limits of existing facilities such as the National Ignition Facility. Moreover, this concept extends to indirect drive implosions, which exhibit substantial yield increases at reduced implosion velocity. Shock-augmented ignition expands the viable ignition design-space of laser inertial fusion.   

Experimental study of intense proton beam transport through plastic foam using 2-D hybrid-PIC simulations with a unique proton source model

Laser-accelerated proton beams have proven useful in many ways, including proton radiography and fast heating of solids to warm dense regimes. For applications requiring high current density, such as Fast Ignition Inertial Confinement Fusion, transport behaviors of the proton beam have not been well-studied. We report on an experimental study, done on the Omega EP laser, of intense (~10⁹ A/cm²) proton beam transport through plastic foam targets (380 mg/cm³) of length 0.55 mm or 1.0 mm. Copper foils were placed on the foams’ rear faces, and a spherical crystal imager tuned to Cu-Kα photon energies revealed proton- and electron-induced emission images, showing minimal beam break-up at both depths. 2-D hybrid fluid-PIC simulations implementing a unique proton-energy-dependent beam divergence showed that the foam could be heated to temperatures of several keV, and good agreement was found between the experimental and simulated Cu-Kα emission profiles.   

Dynamic smoothing of fuel-target implosion non-uniformity by phase control in heavy ion inertial fusion

  The dynamic smoothing method is applied to direct-driven fuel target implosion in heavy ion inertial fusion. A dynamic mitigation method was proposed to stabilize plasma instabilities and to smooth plasma non-uniformities based on a phase control [1]. We found that the wobbling motion of heavy ion beam (HIB) axis induces a phase-controlled HIBs energy deposition, and it realizes the phase-controlled implosion acceleration non-uniformity, so that the HIBs illumination non-uniformity is well smoothed [2, 3]. HIB accelerators provide a well-established capability to oscillate HIB axis with a high frequency. In inertial confinement fusion, a fuel implosion uniformity is essentially required to compress the DT fuel and to release the fusion energy. The non-uniformity of the implosion acceleration should be less than a few % [4]. The results in this paper also demonstrate that the wobbling HIBs would provide an improvement in the fusion energy output gain. [1] S. Kawata, Phys. of Plasmas, 19, 024503 (2012). [2] R. Sato, et al., Scientific Reports 9, 6659 (2019). [3] S. Kawata, Advances in Phys. X, 6, 1873860 (2021). [4] S. Kawata and K. Niu, J. Phys. Soc. Jpn., 53, 3416 (1984).