Field-reversed Configuration Plasma Research Articles

Advanced physics calculations using a multi-fluid plasma model

The multi-fluid plasma model is derived from moments of the Boltzmann equation and typically has two fluids representing electron and ion species. Large mass differences between electrons and ions introduce disparate temporal and spatial scales and require a numerical algorithm with sufficient accuracy to capture the multiple scales. Source terms of the multi-fluid plasma model couple the fluids to themselves (interspecies interactions) and to the electromagnetic fields. The numerical algorithm must treat the inherent stiffness introduced by the multiple physical effects of the model and tightly couple the source terms of the governing equations. A discontinuous Galerkin method is implemented for the spatial representation. Time integration is investigated using explicit, implicit, semi-implicit methods. Semi-implicit treatment is accomplished using a physics-based splitting. The algorithm is applied to study drift turbulence in field reversed configuration plasmas to illustrate the physical accuracy of the model. The algorithm is also applied to plasma sheath formation which demonstrates Langmuir wave propagation.

Combined FRC and Mirror Plasma Studies in the C-2 Device

The Field Reversed Configuration (FRC) is a high-beta Compact Toroid that includes closed and open field line regions of poloidal magnetic field. Improving the transport properties of both regions is important for the overall FRC confinement and may be attempted in the C-2 device. The goal of this experiment is to explore FRC sustainment by combining heating and current drive from neutral beam injection and particle fueling from a pellet injector. Additions to the C-2 device may include magnetic mirror plugs, plasma guns, and electrically-biased limiters. These additions would permit us to explore combined FRC and mirror physics, with emphasis on improving the FRC confinement.

Neutral Beam Dump Utilizing Cathodic ARC Titanium Evaporation

Unlike tokamaks, where the neutral beam shine through is rarely an issue, open magnetic systems with neutral beam injection oftentimes suffer from incomplete beam capture, which necessitates the handling of the shine through power load and beam particle recycling. The cathodic arc gettering, which provides high evaporation rate coupled with a fast time response, is a powerful and versatile technique for depositing clean getter films in vacuum. A compact neutral beam dump utilizing the titanium arc gettering was developed for a field-reversed configuration plasma sustained by 1 MW, 20–40 keV neutral hydrogen beams. The beam dump is capable of handling large, pulsed gas loads, has a high sorption capacity, and is robust and reliable. The beam recycling coefficient, measured under the beam particle flux density of 5 × 1017 H/(cm2s) sustained for 3–10 ms, is ~ 0.7. The use of the beam dump allows to reduce the recycling of the shine through neutral beam by factor of 3–5, as well as to improve the vacuum conditions in the machine.

Two-chord interferometry using 3.39 μm He–Ne laser on a flux-coil-generated FRC

A two-chord λ(IR)∼3.39 μm He-Ne laser interferometer system was developed for a flux-coil-generated field-reversed configuration to estimate the electron density and the total temperature of the field-reversed configuration (FRC) plasma. This two-chord heterodyne interferometer system consists of a single ∼2 mW infrared He-Ne laser, a visible (λ(vis)∼632.8 nm) He-Ne laser for the alignment, a 40 MHz acousto-optic modulator, photodetectors, and quadrature phase detectors. Initial measurement was performed and the measured average electron densities were 2-10×10(19) m(-3) at two different radial positions in the midplane. A time shift in density was observed as the FRC expands radially. The time evolution of the line-averaged density agrees with the density estimated from the in situ internal magnetic probes, based on a rigid-rotor profile model.

Doppler spectroscopy and D-alpha emission diagnostics for the C-2 FRC plasma

Two Doppler spectroscopy diagnostics with complementary capabilities are developed to measure the ion temperatures and velocities of FRC plasmas in the C-2 device. First, the multichord ion doppler diagnostic can simultaneously measure 15 chords of the plasma using an image intensified camera. Second, a single-chord fast-response ion Doppler diagnostic provides much higher faster time response by using a 16-channel photo-multiplier tube array. To study the neutral density of deuterium under different wall and plasma conditions, a highly sensitive eight-channel D-alpha diagnostic has been developed and calibrated for absolute radiance measurements. These spectroscopic diagnostics capabilities, combined with other plasma diagnostics, are helping to understand and improve the field reversed configuration plasmas in the C-2 device.

Separatrix shape of field-reversed configuration

A method of determining three parameters of a separatrix shape on a field-reversed configuration plasma is described. The three parameters are center radius a, length b (half-distance between X-points), and shape index n when the separatrix radius rs is approximated by the radial position derived from the relationship r2/a2+|z|n/bn=1. To demonstrate the details of the method, magnetic fields in a vacuum region around the separatrix are numerically calculated for the assumed separatrix shapes in the ranges 4≤b/a≤10 and 1.2≤n≤4. Excluded flux radii rΔϕ(z) along the plasma are obtained using the calculated magnetic fields. First, it is confirmed that rΔϕ(0) at the axial midplane can be replaced by a with good precision. Second, a trial function including rΔϕ(z) and parameter n′ is defined. The parameters b and n are determined using the trial function that becomes b without any axial dependence when rΔϕ(z)≈rs and n′≈n. Errors in b and n arising from edge-layer plasmas are quantitatively estimated by introducing its width in the trial function. Finally, it is demonstrated, using experimental data, that the separatrix parameters are easily determined by the present method.

Detection of electric field around field-reversed configuration plasma

Electric-field probes consisting of copper plates are developed to measure electric fields in a vacuum region around a plasma. The probes detect oscillating electric fields with a maximum strength of approximately 100 V/m through a discharge. Reproducible signals from the probes are obtained with an unstable phase dominated by a rotational instability. It is found that the azimuthal structure of the electric field can be explained by the sum of an n=2 mode charge distribution and a convex-surface electron distribution on the deformed separatrix at the unstable phase. The former distribution agrees with that anticipated from the diamagnetic drift motions of plasma when the rotational instability occurs. The latter distribution suggests that an electron-rich plasma covers the separatrix.

Toroidal Spin-Up and Velocity Shear of a Field-Reversed Configuration Plasma

A spatial distribution of toroidal flow in a field-reversed configuration plasma generated by the field-reversed theta-pinch method has been measured by an ion Doppler spectroscopy (IDS) system with a line-spectrum of impurity carbon (CV: 227.2 nm) and a Mach probe. With the IDS system, the axial profile of toroidal flow was observed. The observed results show different time evolutions of toroidal spin-up at around the midplane and end regions of the FRC. The radial profile of toroidal flow has also been observed on the weakly ionized plasma outside the separatrix with the Mach probe method. These results indicate propagation of toroidal momentum to the scrape-off plasma outward from the center.

Toroidal Flow Velocity Profile of Impurity Ions in Field-Reversed Configuration Plasma

Particle orbits of impurity ions are calculated in order to estimate their toroidal flow velocity. If the impurity ion flow differs from the ion flow, Doppler shift measurement of impurity spectra requires a suitable modification of experimental results. At the field null, the carbon ion flow coincides with the deuterium ion flow due to the friction force between them. At the separatrix, however, impurity and plasma ions each have their own flow velocities. The diamagnetic drift, which depends on the charge and mass of each fluid species, is found to dominate the toroidal flow velocity.

Flux Supply of a Field-Reversed Configuration by NBI Heating

It is shown that the magnetic flux of a field-reversed configuration plasma can be supplied by neutral beam injection heating. Although the beam ion current leads a flux decay due to interaction between fast ions and electrons that carries the current dominantly, the thermal force affects sustainment of the flux. The azimuthal electric field is the only source of flux supply in the newly developed model.

Fusion Particles and Neutral Beams in FRC Reactors

We study the interaction of fusion-born particles and neutral beams (NBs) with field-reversed configuration (FRC) plasmas. The power deposited and the current generated are calculated for FRC reactors operating with the D-T and D-3He fusion reactions. In the beam studies we specify the beam energy and current, the injection point, and the impact parameter and include an ionization package to determine the position and velocity of the beam particles when they become ionized. In the case of fusion-born particles, we consider a large number of isotropic particle sources distributed inside the FRC. The plasma equilibria are obtained by solving the Grad-Shafranov equation with a pressure that contains linear and quadratic terms in the flux function. A Monte Carlo code that includes particle drag and diffusion is then employed to follow the exact trajectories of the fusion or beam particles and calculate the resulting current and deposited power. The effect of a rotating magnetic field and a toroidal field on the current and deposited power is also studied. In D-T reactors the current generated by the alpha particles is small, but the deposited power fraction is large, and NBs can produce significant currents with reasonable input powers. In D-3He reactors the fusion protons can produce large currents, but the deposited power fraction and the NB current drive efficiencies are low. A small toroidal field, compatible with high β FRCs, reduces the deposited power fraction and the current.

Electron density measurements of a field-reversed configuration plasma using a novel compact ultrastable second-harmonic interferometer

A compact high-sensitivity second-harmonic interferometer for line-integrated electron density measurements on a large plasma machine is presented. The device is based on a fiber coupled near-infrared continuous-wave Nd:YAG laser and is remotely controlled. The performances of the instrument are tested on the Irvine field-reversed configuration machine, and a sensitivity of few 10(14) cm(-2) in measuring line integrated electron density is demonstrated with a time resolution of a few microseconds. The interferometer is self calibrated, has an impressive stability, and it does not require any further alignment after proper installation. These features make this device a real turn-key system suitable for electron density measurement in large plasma machines.

Particle and Energy Recovery Process of a High-Beta Compact Toroid Translated Along an Asymmetric Mirror Field

A field-reversed configuration (FRC) plasma is a high-beta elongated compact toroid only with a poloidal field. Since the plasma torus does not link with confinement coil and vacuum vessel, the FRC can be translated from the formation region to confinement region along the external guide field. In this work, the effects of particle, flux and energy supply to the FRC due to the interaction with background neutral particles has been investigated. Translating an FRC plasma through a neutral gas background is equivalent to the injection of neutral beam end-on into the FRC. The experiments were performed on NUCTE-III and NUCTE-III/T. Due to the effects of back ground particle injection during translation process, the loss rate of particle, flux and energy were reduced.

Self-Generated Toroidal Flow in a High-Beta Compact Toroid with Mirror Configuration

In the field-reversed configuration (FRC) plasma, rotational instability with toroidal mode number n = 2 is known as a sole destructive MHD instability. Though an ideal FRC is unstable for higher n interchange instabilities, those have never been observed in past experiments. As a possible stabilized mechanism, self-generated toroidal rotation and its shear have been investigated experimentally.

Field Reversed Configuration Translation and the Magnetized Target Fusion Collaboration

After considerable design and construction, we describe the status of a physics exploration of magnetized target fusion (MTF) that will be carried out with the first flux conserving compression of a high pressure field-reversed configuration (FRC). The upgraded Los Alamos (LANL) high density FRC experiment FRXL has demonstrated that an appropriate FRC plasma target can be created and translated on a time scale fast enough to be useful for MTF. Compression to kilovolt temperature is expected to form a Mbar pressure, high energy density laboratory plasma (HEDLP). Integrated hardware on the new Field Reversed Compression and Heating Experiment (FRCHX) at the Air Force Research Laboratory Shiva Star facility, has formed initial FRC’s and will radially compress them within a cylindrically symmetric aluminum “liner”. FRXL has shown that time scales for FRC translation to the target region are significantly shorter than the typical FRC lifetime. The hardware, diagnostics, and design rationales are presented. Pre-compression plasma formation and trapping experimental data from FRXL and FRCHX are shown.

The TCS Upgrade: Design, Construction, Conditioning, and Enhanced RMF FRC Performance

The original Translation, Confinement, Sustainment (TCS) experiment was upgraded [TCS Upgrade (TCSU)] to provide an ultrahigh vacuum (UHV) environment with modern discharge cleaning and wall-coating technologies. This has allowed rotating magnetic field formed field reversed configuration plasma temperatures to increase from the TCS radiation-dominated tens of electron volts to >200 eV (Te + Ti), and FRC magnetic fields to double. The improvements are directly attributable to reduced impurity levels and reduced plasma recycling losses. Some of the technologies utilized to achieve these results included replacing O-rings with wire and conflat seals; developing high-temperature, differentially pumped, elastomeric seals for bonding extremely large quartz tubes (needed for rapid field penetration) to the stainless steel vacuum chambers; and using heater blankets for vacuum baking. Extensive testing using electron microprobe and various spectroscopic techniques was performed to establish appropriate UHV cleaning and handling methods.

Effects of internal structure on equilibrium of field-reversed configuration plasma sustained by rotating magnetic field

The effects of an internal structure on the equilibrium of a field-reversed configuration (FRC) plasma sustained by rotating magnetic field is investigated by using detailed electrostatic probe measurements in the FRC Injection Experiment apparatus [S. Okada, et al., Nucl. Fusion. 45, 1094 (2005)]. An internal structure installed axially on the geometrical axis, which simulates Ohmic transformer or external toroidal field coils on the FRC device, brings about substantial changes in plasma density profile. The internal structure generates steep density-gradients not only on the inner side but on the outer side of the torus. The radial electric field is observed to sustain the ion thermal pressure-gradient in the FRC without the internal structure; however, the radial electric field is not sufficient to sustain the increased ion thermal pressure-gradient in the FRC with the internal structure. Spontaneously driven azimuthal ion flow will be accountable for the imbalance of the radial pressure which is modified by the internal structure.

Effects of background neutral particles on a field-reversed configuration plasma in the translation process

A field-reversed configuration (FRC) plasma was translated into a weakly ionized plasma and the effects of heating and particle buildup of the FRC plasma due to the background neutral particles and plasma injection in the translation process were investigated. Improvement of the particle and poloidal flux confinements and delay of onset of n=2 rotational instability were observed in the translation process. It was found that the internal structure of the plasma pressure (plasma temperature and density) at the separatrix and field null was deformed by the particle injection. FRC plasma translation through the background particles was equivalent to an end-on particle beam injection to the FRC plasma. Particles and energy were supplied during the translation. The results obtained for the phenomena of particle supply and plasma heating were also supported by the results of two-dimensional particle simulation. The effects of background particle injection appear to be a promising process for the regeneration of translation kinetic energy to plasma internal energy.

Analysis of magnetic probe signals including effect of cylindrical conducting wall for field-reversed configuration experiment

A confinement field is disturbed by magnetohydrodynamic (MHD) motions of a field-reversed configuration (FRC) plasma in a cylindrical conductor. The effect of the conductor should be included to obtain a spatial structure of the disturbed field with a good precision. For this purpose, a toroidal current in the plasma and an eddy current on a conducting wall are replaced by magnetic dipole and image magnetic dipole moments, respectively. Typical spatial structures of the disturbed field are calculated by using the dipole moments for such MHD motions as radial shift, internal tilt, external tilt, and n=2 mode deformation. Then, analytic formulas for estimating the shift distance, tilt angle, and deformation rate of the MHD motions from magnetic probe signals are derived. It is estimated from the calculations by using the dipole moments that the analytic formulas include an approximately 40% error. Two kinds of experiment are carried out to investigate the reliability of the calculations. First, a magnetic field produced by a circular current is measured in an aluminum pipe to confirm the replacement of the eddy current with the image magnetic dipole moments. The measured fields coincide well with the calculated values including the image magnetic dipole moments. Second, magnetic probe signals measured from the FRC plasma are substituted into the analytic formulas to obtain shift distance and deformation rate. The experimental results are compared to the MHD motions measured by using a radiation from the plasma. If the error included in the analytic formulas and the difference between the magnetic and optical structures in the plasma are considered, the results of the radiation measurement support well those of the magnetic analysis.

Power balance in a high-density field reversed configuration plasma

A global power balance analysis has been performed for the Field Reversed Experiment with Liner high density (>5×1022m−3) field reversed configuration (FRC) plasma. The analysis was based on a zero-dimensional power balance model [D. J. Rey and M. Tuszewski, Phys. Fluids 27, 1514 (1984)]. The key findings are as follows. First, the percentage of radiative losses relative to total loss is an order of magnitude lower than previous lower density FRC experiments. Second, Ohmic heating was found to correlate with the poloidal flux trapping at FRC formation, suggesting that poloidal flux dissipation is primarily responsible for plasma heating. Third, high density FRCs analyzed in this work reinforce the low-density adiabatic scaling, which shows that particle confinement time and flux confinement time are approximately equal.