In this community white paper, we describe an approach to achieving fusion which employs a hybrid of elements from the traditional magnetic and inertial fusion concepts, called magneto-inertial fusion (MIF). The status of MIF research in North America at multiple institutions is summarized including recent progress, research opportunities, and future plans.

Summary form only given. The objective of the Field-Reversed Configuration Heating Experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high energy density plasmas (HEDPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion (MIF), laboratory astrophysical research, and intense radiation sources, among others. To create the HEDP, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum shell (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically-driven implosion of the shell. A requirement is that once the FRC is stopped within the shell, the trapped flux inside the FRC must persist while the compression process is completed. With the present shell dimensions and drive bank parameters, the total time required for implosion is ~25 microseconds. Lifetime measurements of recent FRCHX FRCs indicate trapped lifetimes now approaching ~14 microseconds, and with recent experimental modifications the liner compression can be initiated considerably earlier before formation is completed in order to close that gap further. A discussion of FRC lifetime-limiting mechanisms will be presented along with a description of FRCHX and recent changes that have been made to it. Results from recent experiments aimed at lengthening FRC lifetime will also be presented.

Magneto-inertial fusion (MIF) approaches take advantage of an embedded magnetic field to improve plasma energy confinement by reducing thermal conduction relative to conventional inertial confinement fusion (ICF). MIF reduces required precision in the implosion and the convergence ratio. Since 2008 (Wurden et al 2008 IAEA 2008 Fusion Energy Conf. (Geneva, Switzerland, 13-18 October) IC/P4-13 LA-UR-08-0796) and since our prior refereed publication on this topic (Degnan et al 2008 IEEE Trans. Plasma Sci. 36 80), AFRL and LANL have developed further one version of MIF. We have (1) reliably formed, translated, and captured field reversed configurations (FRCs) in magnetic mirrors inside metal shells or liners in preparation for subsequent compression by liner implosion; (2) imploded a liner with interior magnetic mirror field, obtaining evidence for compression of a 1.36 T field to 540 T; (3) performed a full system experiment of FRC formation, translation, capture, and imploding liner compression operation; (4) identified by comparison of 2D-MHD simulation and experiments factors limiting the closed-field lifetime of FRCs to about half that required for good liner compression of FRCs to multi-keV, 10(19) ion cm(-3), high energy density plasma (HEDP) conditions; and (5) designed and prepared hardware to increase that closed-field FRC lifetime to the required amount. Those lifetime experiments are now underway, with the goal of at least doubling closed-field FRC lifetimes and performing FRC implosions to HEDP conditions this year. These experiments have obtained imaging evidence of FRC rotation, and of initial rotation control measures slowing and stopping such rotation. Important improvements in fidelity of simulation to experiment have been achieved, enabling improved guidance and understanding of experiment design and performance.

Summary form only given. In MTF liner compression experiments, MG magnetic fields heat up the inner liner surface during compression, possibly leading to gas or plasma formation and mixing of wall material with the fuel. To investigate the conditions leading to plasma formation from an inner metal liner surface, experiments have been performed on the 1-MA Zebra generator, by passing a fast-rising (1.1×1013 A/s rise rate) current through `barbell'-shaped aluminum (Al 6061) and copper (Cu 101) rods with diameters between 0.5 mm and 2 mm. The barbell shape avoids direct line-of-sight between arcs at contacts and the heated surface under investigation. Plasma formation is observed when the surface magnetic field approaches 2.2 MG for Al.1, 2, 3 The experiment also fulfills a need for detailed experimental data to benchmark radiation-MHD and plasma spectroscopy modeling. The metal plasma is well characterized by UV (266 nm) and visible (532 nm) 2-frame laser shadowgraphy, multi-frame optical imaging, filtered visible4 and EUV photometric measurements, and timeresolved visible and EUV spectroscopy. The magnetic field threshold for plasma formation, the expansion speed, the plasma temperature, and the emissions in visible and EUV bands have been compared with the results of a variety of numerical simulations, both Lagrangian and Eulerian, using several different sets of EOS, resistivity, and opacity tables.5,6 For the first time, a spectroscopic quality radiation transport line-of-sight integration for this Al plasma has been performed. Radiation-MHD modeling results from the MHRDR5 simulation is used as input for PrismSPECT spectral modeling. The line-of-sight integration takes into account the emission, absorption, and transmission of each plasma layer and finally is convoluted with the resolution of the EUV spectrometer. The resultant simulated spectrum compares well with the experimental EUV spectra.

Summary form only given. Megagauss magnetic fields on metallic surfaces form plasmas that have higher temperatures than would be expected based upon simple diffusion models, as has been demonstrated by a series of aluminum “thick-wire,” i.e., rod, experiments that have been conducted on the University of Nevada, Reno (UNR) Zebra generator (2 TW, 1 MA, 100 ns). One physical situation of current interest where such fields and plasmas may be encountered is Magnetized Target Fusion (MTF), where magnetically driven liners compress magnetized plasmas to fusion temperatures, e.g., the Russian MAGO program, the MagLIF program recently initiated at the Sandia National Laboratories and the FRCHX experiments being conducted at the Air Force Research Laboratory. Computations by Garanin and the UNR team have predicted and/or “matched” many of the previously reported Zebra experimental observations, including an observed magnetic field threshold for surface plasma formation. In this paper we report Eulerian computations that show improved agreement with the observations. It is quite common to use computations that “match the experiment” to interpret the experimental results, and we report an interpretation based upon the Eulerian results. However, we have also compared the Eulerian computations with the Lagrangian calculations of Garanin and Lagrangian calculations performed with the computer code Raven. The comparisons between the codes give rise to serious concerns about the differences between Eulerian and Lagrangian simulations and about the differences between van der Waals and Maxwell-construct equations-of-state.

We discuss various verification and validation issues in computational modeling of a series of aluminum "thick-wire," i.e., rod, experiments that have been conducted on the University of Nevada, Reno (UNR) Zebra generator (2 TW, 1 MA, 100 ns). These conceptually simple experiments involve multi-megagauss surface magnetic fields and have proven to be exceptionally rich in physical phenomena. The experiments also present severe computational challenges. We show that with a proper choice of equation-of-state and resistivity models Eulerian simulations can reproduce many of the observations to a reasonable degree. We also show that Eulerian and Lagrangian computations agree only with serious caveats, leading to the question of whether or not the computations are satisfactorily verified.

Summary form only given. Understanding the physical processes that can lead to the formation of plasma on the surface of metals subjected to megagauss magnetic fields and magnetic pressures of 0.1 Mbar and more is vital for both basic science and a wide variety of applications. “Thick” wire, i.e., rod, experiments on the University of Nevada, Reno (UNR) Zebra generator (2 TW, 1 MA, 100 ns) have provided an extensive data base on aluminum surface plasma formation. “Cold start” magnetohydrodynamic (MHD) computer models, one using a Lagrangian technique with an equation-of-state (EOS) that has VanderWaals loops and the second using an Eulerian technique with a Maxwell-construct EOS, have satisfactorily predicted many of the observations and trends in the observations as experimental parameters are varied. UNR Eulerian modeling has computationally predicted a magnetic field threshold for plasma formation and has led to a conclusion that the plasma formation in the Zebra experiments is predominantly a thermal process driven by Ohmic heating, although the modeling demonstrated significant dependence on the choice of equation-of-state (EOS) and resistivity models. In this paper, we examine the sensitivity of the computational results to various computational aspects such as physical model (e.g., with or without thermal conduction), computational approach (Eulerian or Lagrangian), computational grid size, time-step control, vacuum treatment, EOS (Maxwell construct or VanderWaals loops), and other computational issues. We also discuss the insight into experimental behavior that can be learned from the computations.

Untangling the roles of different plasma formation mechanisms in high-current electrical discharges requires controlling each mechanism and isolating its effects. Recent experiments use this method to find that thermal formation of plasma occurs from 6061-alloy Al conductors by ohmic heating when the surface field reaches a magnetic field threshold of 2.2 MG (independently of ∂B/∂t ranging from 30 to 80 MG/μs) [T. J. Awe et al., Phys. Rev. Lett. 104, 035001 (2010)]. The experiments pulse 1.0 MA peak current on the surface of thick (∼1.0 mm diameter) aluminum rods in 100 ns. Strong electric fields are necessarily introduced, and precursor plasma may form (e.g., from arcing electrical contacts). To ensure plasma formation is predominantly thermal, a systematic study of multiple combinations of high-current electrodes and central-rod profiles was completed. Data from multiple diagnostics show that the time and location of plasma formation strongly correlate with the hardware combination used. An extensive dataset (greater than 100 shots) enabled numerous hypotheses to be tested regarding plasma formation mechanisms. For hardware with compressed, large-diameter current joints, plasma formation is a predominantly thermal process driven by ohmic heating.

Plasma formation from thick Al rods ohmically heated during the diffusion of pulsed multimegagauss magnetic field is examined experimentally. Rods in the Z-pinch configuration are driven to 1.0-MA peak current. The evolution of the resultant surface plasma is complex yet highly reproducible. Surface plasmas first form at discrete hot spots. As the pulsed current grows, plasma filaments form, first along the current, and then transverse to it. As the plasma temperature rises, emissions become increasingly uniform until instabilities grow and modulate the surface.

Numerical simulations of experiments in which plasma is formed on an aluminum surface by megagauss magnetic fields provide the first computational demonstration of a magnetic-field threshold that must be reached for aluminum plasma to begin to form. The computed times of plasma initiation agree reasonably well with the observations across the full range of rod diameters, leading to the conclusion that plasma formation is a thermal process. Computationally, plasma forms first in low-density material that is resistive enough to expand across the magnetic field and yet conductive enough that Ohmic heating exceeds expansion cooling.

The first measurement of the threshold for thermal ionization of the surface of thick metal by pulsed magnetic field (B) is reported. Thick aluminum-with depth greater than the magnetic skin layer-was pulsed with partial derivative B/partial derivative t from 30-80 MG/mu s. Novel loads avoided nonthermal plasma (from electron avalanche, or energetic particles or photons from arcs). Thermal plasma forms from 6061-alloy aluminum when the surface magnetic field reaches 2.2 MG, in qualitative agreement with numerical simulation results by Garanin et al. [J. Appl. Mech. Tech. Phys. 46, 153 (2005)].

Summary form only given. Recent aluminum rod experiments driven by 1-MA Zebra generator at University of Nevada, Reno (UNR) have provided a benchmark for magnetohydrodynamic (MHD) modeling. The innovative 'hourglass' and 'barbell' load geometries used in the experiments made it possible to distinguish between plasma formation due to Ohmic heating, which can be studied numerically utilizing MHD codes, and plasma formation due to high electric fields, by introducing a large-diameter contact with the electrodes. This prevents the explosive electron emission (EEE) at the contacts which triggers initial plasma formation in the conventional rod explosion experiments.The UNR megagauss rod experiments were modeled by employing the state-of-the-art radiation-magnetohydrodynamic code MHRDR. Numerical simulations were performed for a wide range of rods, varying from 100 to 580 microns in radius. A "cold start" initiation was employed in order to create initial parameters close to the experimental conditions. Material properties of aluminum, crucial for such simulations, were modeled employing a set of well tested SESAME format equations-of-state (EOS), ionization, and thermal and electrical conductivity tables. The cold start initiation also allowed observation of the numerical phase transitions of the aluminum rod, from solid to liquid to vapor and finally to low density plasma as it is ohmically heated by the megaampere driving current. Numerical results indicate that plasma forms at the surface of the expanding low density aluminum vapor, when and where the magnetic field is about 2.7 MG. This result is in agreement with a previous simulation by Garanin3 et al., as well as with data from the UNR rod experiments.