R. Clary

A new high performance field reversed configuration operating regime in the C-2 devicea)

Large field reversed configurations (FRCs) are produced in the C-2 device by combining dynamic formation and merging processes. The good confinement of these FRCs must be further improved to achieve sustainment with neutral beam (NB) injection and pellet fuelling. A plasma gun is installed at one end of the C-2 device to attempt electric field control of the FRC edge layer. The gun inward radial electric field counters the usual FRC spin-up and mitigates the n = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The combined effects of the plasma gun and of neutral beam injection lead to the high performance FRC operating regime, with FRC lifetimes up to 3 ms and with FRC confinement times improved by factors 2 to 4.

Experimental study on the velocity limits of magnetized rotating plasmas

An experimental study on the physical limits of the rotation velocity of magnetized plasmas is presented. Experiments are performed in the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)], a mirror magnetic field plasma rotating azimuthally. The externally applied parameters that control the plasma characteristics—applied voltage, external magnetic field, and fill pressure—are scanned across the entire available range of values. It is found that the plasma rotation velocity does not exceed the Alfven velocity, in agreement with the equilibrium requirements of magnetically confined plasmas. Measured rotation velocities are also lower than the critical ionization velocity in hydrogen, but a strict limit was not observable within MCX parametric capabilities.

Sub-Alfvénic velocity limits in magnetohydrodynamic rotating plasmas

Magnetized plasmas in shaped fields rely on large, supersonic rotation to effect centrifugal confinement of plasma along magnetic field lines. The results of experiments on the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] to document velocity limits are reported. Previous results have shown a limit at the Alfven speed, consistent with equilibrium limits from ideal magnetohydrodynamic theory. Another speed limit, previously reported as possibly related to a critical ionization phenomenon and depending only on the ion species and the shape of the confining magnetic field, is investigated here for a broad range of the applied parameters. We show that this speed limit manifests at sub-Alfvenic levels and that, as externally applied torques on the plasma are increased, the extra momentum input shows up as enhanced plasma density or lower momentum confinement time, accompanied by an increase in the neutral radiation level. Several key parameters are scanned, including the mirror ratio, the length between insulators, and the species mass. We show that this velocity limit is consistent with the species-dependent critical ionization velocity postulated by Alfven.

Cross-field plasma injection into mirror geometry

The Maryland Centrifugal Experiment (MCX) and HyperV Technologies Corp. are collaborating on a series of experiments to test the use of a plasma gun to inject mass and momentum into a magnetic-confinement device. HyperV has designed, built and installed a prototype coaxial gun to drive rotation in MCX. The gun has been designed to avoid the blow-by instability via a combination of electrode shaping and a tailored plasma armature. Preliminary measurements at HyperV indicate the gun generates plasma jets with a mass of 160 µg, velocities up to 90 km s−1 and plasma density in the high 1014 cm−3. This paper emphasizes characteristics of the plasma gun and penetration of the plasma jet through the MCX magnetic field. Plans for future injection experiments are briefly discussed.

Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anoma...

New high rotation mode in magnetized rotating plasmas

A new high-rotation (HR) mode of operation is reported for magnetized rotating plasmas. Plasma is produced by an externally applied radial electric field in the Maryland centrifugal experiment, a mirror magnetic field plasma rotating azimuthally. The HR mode is achieved by optimizing over the discharge current. Previously reported stationary or steadily declining plasma voltages are now replaced by time-increasing voltages in the HR mode, with maximum voltages significantly higher than those previously recorded. Sonic Mach numbers are close to the theoretical values required for centrifugal confinement and for suppressing large scale magnetohydrodynamic (MHD) instabilities. Plasma momentum confinement time is much longer than the MHD instability time scale and longer than in ordinary mode operation. The mode is sustained over several confinement times.

COMBINED FRC AND MIRROR PLASMA STUDIES IN THE C-2 DEVICE

Abstract 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.

A High Performance Field-Reversed Configuration Regime in the C-2 Device

A high temperature, stable, long-lived field-reversed configuration (FRC) plasma state has been produced in the C-2 device by dynamically colliding and merging two oppositely directed compact toroids, with combining effects of biasing edge plasma near the FRC separatrix from an end-plasma-gun with magnetic-mirror-plugs and of neutral-beam (NB) injection. The plasma-gun creates an inward radial electric field which mitigates the n = 2 rotational instability. The gun also produces E×B velocity shear in the FRC edge layer, which may explain observations of improved transport properties. The FRCs are nearly axisymmetric which enables fast ion confinement, and increasing NB power input clearly extends the FRC lifetime. The combined effects of the plasma-gun with mirror-plugs and of NB injection yield a new High Performance FRC regime with confinement times improved by factors 2 to 4 and FRC lifetimes extended from 1 to 3 ms.

Low Dimensional Model for the Fluctuations observed in the Maryland Centrifugal Experiment

The Maryland Centrifugal eXperiment (MCX) is a novel rotating mirror plasma, which is axially confined by the centrifugal force. The interchange modes can be stabilized by the shear in the azimuthal rotation velocity. Both these goals can be accomplished if the rotation speeds are supersonic with a Mach number∼5. So far Mach numbers∼2 have been accomplished. Thus residual interchange fluctuations are observed on sixteen magnetic pick‐up coils placed azimuthally at an axial location mid‐way between the central section and the mirror throat. The fluctuations are analyzed and modeled using a low‐dimensional system of equations for the interchange mode coupled to zonal flows.

Bifurcated equilibria in centrifugally confined plasma

A bifurcation theory and associated computational model are developed to account for abrupt transitions observed recently on the Maryland Centrifugal eXperiment (MCX) [R. F. Ellis et al. Phys. Plasmas 8, 2057 (2001)], a supersonically rotating magnetized plasma that relies on centrifugal forces to prevent thermal expansion of plasma along the magnetic field. The observed transitions are from a well-confined, high-rotation state (HR-mode) to a lower-rotation, lesser-confined state (O-mode). A two-dimensional time-dependent magnetohydrodynamics code is used to simulate the dynamical equilibrium states of the MCX configuration. In addition to the expected viscous drag on the core plasma rotation, a momentum loss term is added that models the friction of plasma on the enhanced level of neutrals expected in the vicinity of the insulators at the throats of the magnetic mirror geometry. At small values of the external rotation drive, the plasma is not well-centrifugally confined and hence experiences the drag fr...