Session ZM11: Mini-Conference: Pulsed Magnetic Fusion Energy III

A comparison of MagLIF: Status and plans to improve performance

The Magnetized Liner Inertial Fusion (MagLIF) concept combines fuel magnetization, laser preheat and compression of a fuel-filled liner with a pulsed power driver to achieve thermonuclear yields [1] [2]. Recent work has focused on improving the performance of experiments by increasing inputs (current drive, applied magnetic field [3] and preheat energy [4]) and improving the implosion velocity and stability with dielectric-coated liners [5]. The combined improvements resulted in yields approximately a factor 2 greater than the highest previously reported. Further improvements may be possible that increase the yield by optimizing the liner height, incorporating anti-mix layers, and utilizing novel materials to improve implosion stability and stagnation confinement. This talk aims to summarize this progress and relate it to the potential for MagLIF to achieve high yields on future facilities.

 

[1]: S. A. Slutz et al., Physics of Plasmas 17, 056303 (2010)

[2]: M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)

[3]: M. R. Gomez et al., Phys. Rev. Lett. 125(15), 155002 (2020)

[4]: A. J. Harvey-Thompson et al., Rev. Sci. Instrum. 94, 053506 (2023)

[5]: D. J. Ampleford et al., Physics of Plasmas 31(2), 022703 (2024)   

Developments in assessing the generalized Lawson criterion in magnetized liner inertial fusion

Magnetized liner inertial fusion (MagLIF) is a magneto-inertial fusion concept fielded at Sandia's Z Pulsed Power Facility. The extreme environments produced in these experiments along with limited diagnostic lines of sight and spatio-temporal resolving power make characterization of the resulting fusion plasma challenging. Recently, a Bayesian data assimilation method was demonstrated to allow inference of plasma conditions and the corresponding generalized Lawson criterion by simultaneously matching a variety of multi-modal and multi-dimensional x-ray and neutron diagnostics. We report on application of this methodology to quantify of the Lawson criterion for several new experiments, discussing corresponding modifications and improvements to the MagLIF experimental platform. We will also address ongoing work to assess and improve model assumptions used in the inference and additional experimental diagnostic approaches being considered.   

Bayesian inferences of radial current losses and symmetry in multi-terawatt magnetically insulated transmission lines via photonic doppler velocimetry

Assessments of the electrical current delivered by 20-30 MA pulsed power facilities has historically been enabled by bdot measurements at large-radius and at a limited number of azimuthal locations [1]. This sparsity, and that bdot data is unreliable for some configurations [2] precludes realizing the full design potential of sophisticated (but under-constrained) particle-in-cell simulations, and slows efforts to optimize magnetically insulated transmission line geometries. Relying on the Z machine at Sandia National Laboratories, we collected photonic doppler velocimetry (PDV) data at 8 radii and each of 3 azimuths within 55 mm. Using these PDV velocimetry traces together with Bayesian inference, we estimate delivered current vs. radius vs. time, and will discuss radial current losses and azimuthal asymmetries for a short-circuit synchronous Z pulse. This data will be compared with PIC modeling of the experiment.

 

[1] H. M. Hess, et al, PoP 25 042702 (2018)

[2] T. M. Hutchinson, et al, JAP, 134, 155901 (2023).   

MITL Electrode Surface Plasma Density Measurements

TW-class pulsed power accelerators like Sandia National Laboratories’ Z machine experience current loss within their inner Magnetically Insulated Transmission Line (MITL) and convolute regions that results from the formation and transport of 10^15 – 10^17 cm^-3 electrode plasmas [1]. However, little experimental data has been collected to characterize these plasmas. Verification experiments presented herein have been conducted on Sandia National Laboratories’ 1 MA Mykonos accelerator [2] using a diagnostically accessible parallel plate transmission line [3] which generates similar electrode current densities and field strengths to that of the Z machine’s inner MITL region [1]. A colinear Second-Harmonic Orthogonally Polarized dispersion interferometer (SHOPi) diagnostic with a sensitivity of ∼ 10^14 cm^−2 [4] temporally resolved the electrode plasma’s refractive index at a variety of probing locations across the transmission line Anode-Cathode (A-K) gap. The results provide constraining data to validate large-scale particle-in-cell simulations of transmission line power flow, plasma formation, subsequent current loss, and the scaling of current delivery.

[1] N. Bennett et al., Physical Review Accelerators and Beams 24, 060401 (2021); ibid. 26, 040401 (2023); K. Tummel et al., Phys. Plasmas, 29, 113102 (2022).

[2] J. Schwarz et al., Journal of High Energy Density Physics, in review (2024).

[3] D. Lamppa et al., IEEE Pulsed Power Conference, Invited Talk, (2023).

[4] N. R. Hines et al., Review of Scientific Instruments 93, 113505 (2022).   

Neutron energy spectrum measurements from MagLIF implosions on Z

The stagnation column generated in Magnetized Liner Inertial Fusion (MagLIF) experiments performed on the Z machine generates neutrons for a duration of ~1-2 ns before rapidly disassembling. Measurements from neutron time of flight (nTOF) detectors, therefore, no longer represent a direct neutron energy spectrum measurement as the signal is a combination of neutron energy and the nuclear reaction history. A CR-39 based Compact Recoil Spectrometer (CRS) was recently commissioned on Z and has measured the time integrated neutron energy spectrum from MagLIF experiments for the first time. It will be shown that simultaneous fits of measured CRS and nTOF data provides a more complete understanding of the neutron energy spectrum from MagLIF implosions.

Sandia National Laboratory is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.   

Measured nuclear burn widths for Magnetized Liner Inertial Fusion (MagLIF) experiments

The Magnetized Liner Inertial Fusion (MagLIF) concept being pursued on the Z machine offers a pathway to generate >1 MJ fusion yields with significant alpha heating on a future machine [1]. Understanding the role of nuclear burn widths can be informative for assessing how MagLIF scales to higher yields. A technique for measuring nuclear burn widths from collinear neutron time-of-flight (nToF) detectors is presented. The widths of measured nToF traces are a function of the nuclear burn width, energy broadening, an instrument response function (IRF), and scattering along the line-of-sight. For each nToF detector, IRFs are measured as a function of the PMT voltage bias and neutron scattering is numerically calculated. At least two collinear nToF detectors at different distances from the neutron source are needed to disambiguate the contributions of nuclear burn width and thermal broadening to the widths of the measured nToF traces. A forward fitting technique [2] designed for simultaneously analyzing multiple nToF traces has been adapted for fitting two collinear nToF detectors on the Z machine. Nuclear burn widths and apparent ion temperatures are calculated from fitted widths of the nToF traces.

 

[1] D.A. Yager-Elorriaga et al. Nucl. Fus. 62 042015 (2022).

[2] J.M. Mitrani et al. Rev. Sci. Instrum. (2024) article in press.

 

Prepared by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Sandia National Laboratory is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.   

Nuclear diagnostic techniques for >10 MJ yields at the National Ignition Facility (NIF)

Ignition and routine MJ yields at the National Ignition Facility (NIF) have opened a new regime for Inertial Confinement Fusion (ICF), and introduced new challenges and opportunities for nuclear measurements. Nuclear diagnostics provide the most direct measure of the state of the fusing D-T fuel, and are crucial to evaluating implosion performance and refining experimental designs to push to higher yields. At the NIF, a large effort is underway to evaluate new and existing nuclear diagnostic techniques for >10 MJ yields. At these yields backgrounds from the neutron signal itself become large, necessitating neutron attenuation, precise background characterization, or other methods to compensate. Burn times drop to significantly less than 100 ps, challenging temporal measurements and requiring ultrafast detection and recording systems. At the same time exciting new diagnostic techniques become accessible, as previously undetectable signals become measurable. This talk will give an overview of nuclear diagnostic techniques at the NIF and considerations for future facilities in this exciting new era of ICF.

*This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-866350.   

Prototype 4-stage 60-GW impedance-matched Marx generator: design, simulations, and performance

We have designed, assembled, and tested a prototype 4-stage 60-GW coaxial impedance-matched Marx generator (IMG) [1-3].  Each stage is powered by two LCR circuits, which are referred to as “bricks.”  Each brick comprises two 100-kV 160-nF capacitors connected in series with a single 200-kV field-distortion gas switch.  The IMG performs as predicted theoretically: simulated and measured load-power time histories agree to within experimental uncertainties.  A system of IMGs is an attractive candidate prime-power source for next-generation pulsed-power accelerators.  We have designed an IMG-powered accelerator that could deliver as much as 90 MA to a physics load.  Such a machine would achieve thermonuclear-fusion yields as high as 1 – 10 GJ, and revolutionize high-energy-density-physics experiments in support of the national-security mission.

[1]  W. A. Stygar, K. R. LeChien, M. G. Mazarakis et al., Impedance-matched Marx generators, Phys. Rev. Accel. Beams 20, 040402 (2017).

[2]  K. R. LeChien, W. A. Stygar, M. G. Anderson et al., Sirius I: prototype of a prime-power source for future 1 - 10 GJ fusion-yield experiments, LLNL Report LLNL-TR-846570 (2023).

[3]  M. G. Anderson, K. R. LeChien, W. A. Stygar et al., Sirius I Prototype: A Prime-Power Source for Future 1 – 10 GJ Fusion-Yield Experiments, 2023 IEEE Pulsed Power Conference (2023).   

Magnetic compression and melt of electrically thick metal driven by lineal current densities characteristic of pulsed-power-driven fusion devices

Approaches to magnetic fusion utilizing advances in electrical pulsed-power technology offer an attractive path toward commercialization due to their high energy conversion efficiency. In Z-pinch and liner-based fusion schemes such as MagLIF, the stability of the imploding conductor is critical to the fusion performance. The electrothermal instability (ETI) is found in many high-energy-density and fusion experiments, dramatically affecting performance by seeding MHD instabilities. Computational modeling of electrically driven conductors is challenging due to uncertainties in the equation-of-state (EOS) and electrical conductivity during the metal-insulator transition. Photonic Doppler velocimetry (PDV) was used to measure the surface motion of extremely smooth mm-diameter pure aluminum rods driven to 860 kA with 70 ns risetime (10-90%) by the Sandia Mykonos generator. For the first time, aluminum rods were measured with sufficient PDV resolution to observe radial compression before expansion. The reflective surface experiences several changes in acceleration during the Mykonos current rise. PDV data was compared with MHD simulations to diagnose the phase-space trajectory of the metal surface, including magnetic compression, the duration of the solid-liquid phase transition, and the subsequent early-time motion until plasma formation, constraining the conditions from which ETI arises. The experimental measurements are being used to benchmark MHD calculations, and thereby inform the choice of EOS and conductivity tables for modeling.   

Dynamics and Instabilities of Low-Density Gas Plasma on Thin Metal Foil Implosions

Magnetized liner inertial fusion (MagLIF) targets [Yager-Elorriaga et al. Nuclear Fusion 2021] are subject to detrimental instabilities such as the magneto-Rayleigh-Taylor instability (MRTI) and current driven magnetohydrodynamics (MHD) instabilities, which can lead to degradation of the assembly and confinement of fusion fuel. Simulation show low-density plasma (LDP) originating from the surrounding electrodes [Bennett et al. PRAB 2023] compressing onto the liner’s outer surface prior to the liner imploding. To investigate the effects of LDP on instability development, an experimental platform is developed on the University of Michigan’s MAIZE facility (~0.5 MA, 150 ns rise time). The platform involves magnetically imploding a low-density (10¹⁷ cm^-3 ), annular gas-puff plasma onto a thin (400-nm thickness) metallic cylindrical foil has been axially pre-magnetized. The dynamics of the implosion and instability growth are captured using a 12-frame optical camera. A suite of laser-based diagnostics (shadowgraphy, schlieren, interferometry) enables the measurement of electron density of the LDP layer. Discussion and results from this study will be presented.   

Spatiotemporally Resolved Measurements of Hard and Soft X-rays at the UCSD CESZAR Z-Pinch Device

A 2D x-ray camera capable of imaging energetic photons exceeding 5keV has been developed, capable of generating 16x16 pixel movies of hard X-ray emissions with an effective interframe time of 18ns. This 2D X-ray camera was deployed at the CESZAR Z-pinch device to create spatiotemporally resolved measurements of X-rays generated during the lifetime of the pinch. The target consisted of a central deuterium gas jet gas with a liner of either argon or krypton to stabilize Magneto-Rayleigh Taylor Instability which destroy the viability of the pinch. Preliminary tests have demonstrated energetic X-ray signals, which were detected in a spatially unresolved manner in 1 out of 24 shots. This report intends to present results from ongoing experiments. The camera is planned to be used as a coarse X-ray energy spectrometer by imaging X-rays through filters constructed by interleaving multiple species of metal. Any potential resulting data will be compared against predicted transmission through the filters given beam-target or thermal X-ray distributions. Further, with access to a second spatial dimension in the movies generated by the diagnostic, the location of any observed X-rays in the image plane will be readily determined. Finally, it is planned to correlate these energy spectra and spatially-resolved movies with time-of-flight neutron detectors to establish the nature and location of X-rays emitted by the pinch, thus giving insight into their underlying generative mechanism.