Formation of a Stable Field Reversed Configuration through Merging

The Inductive Plasma Accelerator (IPA) Field Reversed Configuration (FRC) experiment is a plasmoid accelerator/interaction experiment designed to explore the acceleration, reconnection and compression of high beta compact toroids. The IPA experiment is designed to be able to form, accelerate, and merge two FRCs having a mass of 0.1–0.2 mg at velocities ranging from 150 km/s to 250 km/s in a centrally located interaction/compression chamber. The interaction/compression chamber magnetic fields are arranged in a mirror configuration to trap and allow the FRCs to merge. The merged FRCs are then magnetically compressed to high density and temperature. After compression, ion temperatures are predicted to exceed several kV at densities greater than 10²² m⁻³. The experimental device now in operation will be discussed. Initial results of FRC merging will be presented, as well as results from 2D numerical calculations based on the current experiment.     

Experimental study of single-translated field-reversed configuration in KMAX

For collisional merging field-reversed configurations (FRCs), it is desired to have both FRCs tuned to be approximately the same, as well as to optimize each FRC to have high temperature and high translation speed so as to retain most of the equilibrium flux after traveling a distance to the middle plane for merging. The present study reports the experimental study of a single-translated FRC in the KMAX-FRC device with various diagnostics, including a triple probe, a bolometer, several magnetic probe arrays, and a novel 2D internal magnetic probe array. According to the measurements conducted in the present study, a maximum toroidal magnetic field equal to ∼1/3 of the external magnetic field inside the FRC separatrix radius is observed, and the typical parameters of a single-translated FRC near the device’s mid-plane are ne ∼ (2–4)×10¹⁹ m⁻³, T_e ∼ 8 eV, T_i ∼ 5 eV, r_s ∼ 0.2 m, l_s ∼ 0.6 m and ϕ_p(RR) ∼ 0.2 mWb. The 2D magnetic topology measurement revealed, for the first time, the time evolution of the overall internal magnetic fields of a single-translated FRC, and an optimized operation regime is given in the paper.     

Recent Results on Field Reversed Configurations from the Translation, Confinement and Sustainment Experiment

The field-reversed configuration (FRC) offers an attractive alternative approach to magnetically confined fusion because of its extremely high β, simple linear geometry, and natural divertor for helium ash removal. Multi-hundred eV and high density FRCs have been produced using the standard Field Reversed Theta Pinch (RFTP) method, with a confinement scaling that leads to fusion conditions. These FRCs are, however, limited to only tens of mWb fluxes and sub-msec lifetime. Recent progress has been made in building up the flux and sustaining the FRC current using Rotating Magnetic Fields (RMF) in the Translation, Sustainment, and Confinement (TCS) facility at the University of Washington. TCS has demonstrated formation and steadystate sustainment of standard, flux-confined, prolate FRCs. The RMF also provides stability for the n = 2 rotational mode, which is the dominant global instability observed experimentally. Simple calculations show that a strong radially inward force imposed by the RMF increases proportionally to any local outward deformation of the plasma cross section. Evidence of this has been experimentally demonstrated, and the effects of various RMF antenna geometries studied. High temperature FRCs could also be produced in TCS by translating high energy plasmoids formed in the normal theta pinch manner into the confinement chamber containing the RMF antennas. Extremely interesting results were obtained for this translation and capture process. The plasmoids can survive the violent dynamics of supersonic reflections off magnetic mirror structures, producing a stable high-β, near-FRC state with substantial flux conversion from toroidal to poloidal. This is a tribute not only to the robustness of FRCs, but also to the tendency of anFRC to assume a preferred state for a magnetized plasma. The magnetic helicity, as inferred by a simple interpretive model, is approximately preserved, possibly conforming to a high-β relaxation principle.     

High-Beta Steady-State FRC Plasma Sustained by Rotating Magnetic Field with Spatial High-Harmonic Components

Field-reversed configurations (FRCs) driven by rotating magnetic fields (RMFs) with spatial high-harmonic components have been studied in the metal flux conserver of the FRC injection experiment (FIX). The high-harmonic RMF method has some unique features; (1) field lines of the RMF do not penetrate or cross the vessel wall, (2) selective penetration/exclusion of the fundamental/high-harmonic RMF component will result in a generation of effective magnetic pressure near the separatrix, which helps to keep the separatrix away from the vessel wall, (3) strong azimuthal non-uniformity of the RMF will cause the n = 4 deformation of the core FRC plasma, which will eliminate the destructive modes caused by the rotation of the plasma column. The RMF method with high harmonics will provide quasi-steady current drive of high-beta FRC plasmas without destructive n = 2 rotational mode and will be helpful in reducing the particle loss and thermal load when applied to the fusion core plasma.     

Operation of TCSU with Internal Flux-Conserving Rings

The Translation, Confinement, and Sustainment Upgrade (TCSU) device is a facility to form and sustain a field-reversed configuration (FRC) in quasi-steady state using rotating magnetic fields (RMF). Recent campaigns include Ti gettering, the installation of a set of internal flux rings, and RMF frequency scans. The Ti gettering campaign was successful, reduced impurities, and reduced deuterium recycling from the walls allowing density control and hotter FRCs [J.A. Grossnickle et al., Phys. Plasmas 17, 032506 (2010)]. Internal flux rings have been installed to provide a uniform flux surface and minimize plasma-wall contact. Results from the internal flux ring operation and an additional Ti gettering campaign are reported. RMF frequencies of 123 kHz and 170 kHz have been investigated and initial results are reported.     

Recent Advances in the SPIRIT (Self-organized Plasma with Induction, Reconnection, and Injection Techniques) Concept

Since its inception at the 1997 Innovative Confinement Concept meeting, the Self-organized Plasma with Induction, Reconnection, and Injection Techniques (SPIRIT) concept has been continuously advanced both theoretically and experimentally. The main features of this concept are: (1) formation of large-flux Field Reversed Configuration (FRC) plasmas by merging two spheromaks with opposite helicities; (2) flexibility to assess FRC stability by varying the plasma shape and kinetic parameter, by using passive stabilizers, and by injecting energetic ions; (3) sustainment of the FRC for a time significantly longer than the energy confinement time using an ohmic transformer and/or neutral beam injection. Experiments carried out in TS-3/4 and SSX and more recently in Magnetic Reconnection Experiment (MRX) have further verified the effectiveness of this formation scheme for large-flux FRCs. An improved understanding of FRC stability over plasma shape and kinetic parameter has been obtained in MRX. New numerical simulations showed that FRC plasmas can be globally stabilized by injecting energetic ions. Many of these aspects of the SPIRIT concept can be further studied in the current MRX device.     

High Flux FRC Facility for Stability and Confinement Studies

To provide a path for advancing the FRC concept into a more fusion-like regime, the existing TCSU facility will be modified to take advantage of the new FRC formation method of dynamic formation and merging of FRCs. Results from recent experiments have shown that this methodology provides appreciable increases in the key parameters of ion temperature, poloidal flux and FRC lifetime. FRC stability has been found in numerical calculations where a subpopulation of high energy particles is present in sufficient numbers. A critical goal of the high flux FRC facility will be to form FRCs with poloidal fluxes sufficiently large to fully confine high energy ion orbits introduced from neutralized ion beams injected during FRC formation. A key aspect of the experiments will be to validate theoretical models and simulation codes, such as the 3D extended-MHD code NIMROD, in a in a high beta regime with large two-fluid effects, plasma flows, and an energetic minority species.     

Simulations of the Field-Reversed Configuration with the NIMROD Code

The recently formed Plasma Science and Innovation Center (PSI-Center) is refining the NIMROD code to simulate field-reversed configurations (FRCs). The NIMROD code can resolve highly anisotropic heat conduction and viscosity. This, combined with its ability to include two-fluid effects, allows us to capture more detailed physics than previous calculations. Some initial simulations are focused on 2D (n = 0 only) non-linear two-fluid simulations. We present initial validations of a translating FRC and note good conservation of density and magnetic flux. As a validation of the effects of anisotropic thermal conduction, we present a comparison of an FRC with standard thermal transport to one with anisotropic conduction. Two-fluid simulations are shown which produce significant spin-up due to the end-shorting boundary condition. Finally, simulations of the tilt instability are presented, which show that Hall physics significantly retards, but does not eliminate the growth rate.     

Physics Basis and Progress for a Translating FRC for MTF

We describe a physics scaling model used to design the high density field reversed configuration (FRC) at LANL that will translate into a mirror bounded compression region, and undergo Magnetized Target Fusion compression to a high energy density plasma. At Kirtland AFRL the FRC will be compressed inside a flux conserving cylindrical shell. The theta pinch formed FRC will be expelled from inside a conical theta coil. Even though the ideal FRC has zero helicity and toroidal magnetic field, significant non-ideal properties follow from formation within a conical (not cylindrical) theta coil. The FRC stability and lifetime properties may improve. Several experimental features will also allow unique scientific investigations of this high Lundquist number but collisional plasma.     

Quasi-static magnetic compression of field-reversed configuration plasma: amended scalings and limits from two-dimensional MHD equilibrium

In this work, several key scaling laws of the quasi-static magnetic compression of field reversed configuration(FRC) plasma(Spencer et al 1983 Phys. Fluids 26 1564) are amended from a series of two-dimensional FRC MHD equilibriums numerically obtained using the Grad–Shafranov equation solver NIMEQ. Based on the new scaling for the elongation and the magnetic fields at the separatrix and the wall, the empirically stable limits for the compression ratio, the fusion gain, and the neutron yield are ...     

FRC Simulations using the NIMROD Code

The Plasma Science and Innovation Center (PSI-Center) is benchmarking and refining the NIMROD code for simulations of field-reversed configurations (FRCs). The NIMROD code can resolve highly anisotropic heat conduction and viscosity (Sovinec et al., JCP 195:355, 2004). This combined with its ability to include two-fluid effects, allows us to capture more detailed physics than previous calculations. Recent modifications to the radial boundary conditions capture most of the effects of multiple discrete coils found in many FRC experiments. When the tangential electric field on the end boundaries (open field lines) is set to zero and the Hall term is included in the calculation, the open field-line plasma spins up due to end-shorting effects, which in turn couples to the main FRC plasma through shear viscosity. The spin-up rate is found to be sensitive to the open field-line plasma profile. We are also investigating recent observations (Guo et al., Phys. Rev. Lett. 95:175001, 2005] that imply that a small toroidal field could help stabilize the n = 2 rotational instability. We find that a combination of a relatively weak toroidal magnetic field and the inclusion of the Hall term in the calculation can lead to a change in the character of the mode and a dramatic reduction to its growth rate.     

Comparative Observation of Ar, Ar-H2 and Ar-N2 DC Arc Plasma Jets and Their Arc Root Behaviour at Reduced Pressure

Results observed experimentally are presented, about the DC arc plasma jets and their arc-root behaviour generated at reduced gas pressure without or with an applied magnetic field. Pure argon, argon-hydrogen or argon-nitrogen mixture was used as the plasma-forming gas. A specially designed copper mirror was used for a better observation of the arc-root behaviour on the anode surface of the DC non-transferred arc plasma torch. It was found that in the cases without an applied magnetic field, the laminar plasma jets were stable and approximately axisymmetrical. The arc-root attachment on the anode surface was completely diffusive when argon was used as the plasma-forming gas, while the arc-root attachment often became constrictive when hydrogen or nitrogen was added into the argon. As an external magnetic field was applied, the arc root tended to rotate along the anode surface of the non-transferred arc plasma torch.     

Compression, Heating and Fusion of Colliding Plasmoids by a Z-theta Driven Plasma Liner

A potentially promising approach to fusion employs a plasma shell to radially compress two colliding plasmoids. The presence of the magnetic field in the target plasma suppresses the thermal transport to the confining shell, thus lowering the imploding power needed to compress the target to fusion conditions. With the momentum flux being delivered by an imploding plasma shell, many of the difficulties encountered in imploding a solid metal liner are eliminated or minimized. The best plasma for the target in this approach is the FRC. It has demonstrated both high β, and robustness in translation and compression that is demanded for the target plasma. A high density compressed plasmoid is formed by a staged axial and radial compression of two colliding/merging FRCs where the energy that is required for the implosion compression and heating of the magnetized target plasmoid is stored in the kinetic energy of the plasmas used to compress it. An experimental apparatus is being constructed for the demonstration of both the target plasmoid formation as well as the compression of the plasmoid by a plasma liner. It is believed that with the confinement properties and the high β nature of the FRC, combined with the unique approach to be taken, that an nτ_E T _i triple product ∼5 × 10¹⁷ m⁻³ s keV can be achieved.     

Challenge End-Plugged FRC Concept

A fusion concept is proposed in which plug mirror cells are applied at the ends of a field-reversed configuration (FRC) to improve overall confinement by reducing the end-loss rate in the scrape-off layer. As such this combines a closed “toroidal” system with an open “mirror” system. Arguments are presented that the plug cells would produce a dramatic increase in confinement of the FRC itself, as well as being significantly better than a stand-alone mirror system. This approach is based on more-or-less existing technology and does not require pulsed, high-density operation.     

Confinement & Current Drive Measurements for Steady-State FRCs

Detailed measurements in the TCS Rotating Magnetic Field (RMF) driven FRC device display a highly non-uniform resistivity profile, highly peaked near the separatrix where the ratio of electron drift velocity v _de to ion sound speed v _s is large. The RMF parameters determine the plasma density. The plasma temperatures are governed by power balance, and higher temperatures result in higher diamagnetic currents, mostly inside the magnetic field null, and higher magnetic fields, with surprisingly little increase in absorbed power. The results are well modeled by a ‘Chodura’ type resistivity scaling with electron collision frequency scaling as ν_ch∼ω_pi(1− exp[−v _de/v _s]).     

On the safety of conceptual fusion-fission hybrid reactors

A preliminary examination of some potential safety questions for conceptual fusion-fission hybrid reactors is presented in this paper. The study and subsequent analysis was largely based upon one design, a conceptual mirror fusion-fission reactor, operating on the deuterium-tritium plasma fusion fuel cycle and the uranium-plutonium fission fuel cycle. The major potential hazards were found to be: (a) fission products, (b) actinide elements, (c) induced radioactivity, and (d) tritium.As a result of these studies, it appears that highly reliable and even redundant decay heat removal must be provided. Loss of the ability to remove decay heat results in melting of fuel, with ultimate release of fission products and actinides to the containment. In addition, the studies indicate that blankets can be designed which will remain subcritical under extensive changes in both composition and geometry. Magnet safety and the effects of magnetic fields on thermal parameters were also considered.     

The Influence of the Magnetic Field on the Electrical Breakdown Phenomena

A simple phenomenological model and detailed simulation studies of the breakdown phenomena in argon and nitrogen under the simultaneous action of electric and magnetic fields are presented in this paper. Expressions for the breakdown voltage have been derived taking into account variations of both the ionization coefficient and the secondary electron yield in a magnetic field. Calculations were performed by using XOOPIC code, an Object Oriented Particle in Cell code, with both the original and the improved secondary emission model with inclusion of the influence of the magnetic field on the secondary electron production. The simulation results presented here clearly show that the inclusion of the dependence of the secondary electron yield on the magnetic field leads to better agreement with existing experimental results.     

Physics of Formation, Acceleration, Reconnection and Compression of Two Field Reversed Configurations

A new experiment aims to form two field-reversed configurations (FRCs) and accelerate them into an interaction chamber in which they will collide, and undergo an adiabatic compression. Previous results obtained in a similar configuration showed that ion temperatures varied as a function of compression coil voltage, in the keV range. This paper outlines the physics of formation, acceleration, reconnection and compression of two FRCs.     

Comparison of double layer in argon helicon plasma and magnetized DC discharge plasma

We present in this paper the comparison of an electric double layer (DL) in argon helicon plasma and magnetized direct current (DC) discharge plasma. DL in high-density argon helicon plasma of 13.56 MHz RF discharge was investigated experimentally by a floating electrostatic probe and local optical emission spectroscopy (LOES). The DL characteristics at different operating parameters, including RF power (300–1500 W), tube diameter (8–60 mm), and external magnetic field (0–300 G), were measured. For comparison, DL in magnetized plasma channel of a DC discharge under different conditions was also measured experimentally. The results show that in both cases, DL appears in a divergent magnetic field where the magnetic field gradient is the largest and when the plasma density is sufficiently high. DL strength (or potential drop of DL) increases with the magnetic field in two different structures. It is suggested that the electric DL should be a common phenomenon in dense plasma under a gradient external magnetic field. DL in magnetized plasmas can be controlled properly by magnetic field structure and discharge mode (hence the plasma density).