John Slough

Flux and particle life-time measurements in field-reversed configurations

Flux and particle life-times of field-reversed configurations (FRC) have been measured in the TRX-1 field-reversed theta pinch. These measurements have been correlated with detailed numerical transport calculations based on classical and lower-hybrid-drift (LHD) resistivity. The data appear to imply spatially uniform resistivity profiles with a magnitude somewhat lower than that given by the wave energy bound of LHD fluctuations. Using a programmed formation technique it has been possible to form FRCs with large amounts of poloidal flux, and thus large separatrix radii. The particle confinement time has been found to scale approximately linearly with the amount of poloidal flux, and 150 μs particle confinement times have been achieved in 3 X 1015 cm−3 density FRCs with separatrix radii of only 7 cm. The linear scaling with poloidal flux is highly encouraging for scaling to higher-field, larger-size devices.

Suppression of the n=2 rotational instability in field‐reversed configurations

Compact toroid plasmas formed in field‐reversed theta pinches are generally destroyed after 30–50 μsec by a rotating n=2 instability. In the reported experiment, instability is controlled, and the plasma destruction is avoided in the TRX‐1 theta pinch through the application of octopole magnetic fields. The decay times for loss of poloidal flux and particles are unaffected by the octopole fields. These decay times are about 100 μsec based on inferences from interferometry and excluded flux measurements. The weak, rotating elliptical disturbance (controlled n=2 mode) also made possible a novel determination of the density profile near the separatrix using single‐chord interferometry. The local density gradient scale length in this region is found to be about one ion gyrodiameter.

High Power Helicon Thruster

The High Power Helicon (HPH) plasma thruster under development at MSNW and the University of Washington is an emerging electrode-less in-space thruster that is potentially capable of high thrust level (1-2 Newtons) at moderate power levels of 20 to 50 kW. Unlike previous lower power (2 to 5 kW) helicon thruster schemes, which have been shown to produce moderate temperature of 5 to 10 eV thermal plasmas, the HPH axial and radial plasma characteristics show that the plasma is created in the helicon coil and is then accelerated in the axial direction downstream away from the HPH. The bulk acceleration of the plasma is believed to be due to a directional coupling of the plasma electrons with the helicon wave field, which in turn transfers energy to the ions via an ambipolar electric field. Downstream energy distribution functions obtained with an ion energy retarding field analyzer show a highly non-thermal or beamlike supersonic ion flow away from the HPH thruster. The system is very versatile and is capable of operation at variable input power levels in either pulsed or DC modes. Additionally, the HPH system has been shown to operate utilizing different propellants with hydrogen, nitrogen, argon and xenon having been tested to date. Baseline Isp levels for argon, nitrogen and hydrogen are 1500, 3000 and 5000 respectfully, giving some variability in Isp and thrust by the choice of propellants or propellant mixtures. Current work focuses on the optimization of the system and increasing output plasma power levels.

Flux generation and sustainment of a field reversed configuration with rotating magnetic field current drive

A new experimental device has been constructed to study the flux build-up and sustainment of a field reversed configuration (FRC) with a rotating magnetic field (RMF). Even though complete penetration was expected from RMF theory, the RMF field was observed to penetrate only a few centimeters inside the FRC separatrix. Despite the limited penetration, significantly larger toroidal currents (40 kA) were driven than in previous experiments (∼2 kA) with the same RMF field. The high currents and lack of deep penetration allowed the axial field to be the dominant field throughout the FRC. The radially inward pondermotive force arising from axial screening currents at the FRC edge had a significant influence on energy and particle confinement, reducing convective losses to the limit of observability. With only ohmic heating, the measured low ion temperatures (2 eV) left the ions unmagnetized while the electrons (∼40 eV) were well magnetized. No destructive instability was observed for the RMF driven FRC despite...

The TCS Rotating Magnetic Field FRC Current-Drive Experiment

Field-reversed configurations (FRCs) have extremely attractive reactor attributes because of their singly connected geometry. They have been created in theta-pinch devices, but being compact toroids and lacking a center hole, their toroidal current cannot be sustained by transformer action as in other toroidal configurations. A new device, the Translation, Confinement, and Sustainment (TCS) facility has been constructed to use rotating magnetic fields (RMFs) to build up and sustain the flux of hot FRCs formed by the normal theta-pinch method. RMF formation and sustainment of similar, but cold, pure poloidal field configurations have been demonstrated in devices called rotamaks, and RMF formation, but not sustainment, has been achieved in a smaller FRC facility called the Star Thrust Experiment (STX). Initial formation and sustainment have now been achieved in TCS, albeit still with cold (Te ~ 50 eV) plasmas. Both the formation and final steady-state conditions are found to agree with newly developed analytic and numerical models for RMF flux buildup and sustainment inside a standard cylindrical flux conserver. The required plasma conditions (mainly resistivity but also density) can now be determined for the planned hot FRC, RMF flux buildup experiments and for eventual reactor conditions.

Transport, energy balance, and stability of a large field‐reversed configuration

Experiments have been conducted on the Large s Experiment (LSX) [Phys. Rev. Lett. 69, 2212 (1992)] field‐reversed theta pinch, where plasmas confined in a field‐reversed configuration (FRC) have exhibited record energy, particle, and configuration lifetimes. By careful control of the formation process, it was possible to form symmetric, quiescent FRCs with s values (the number of ion gyroradii from the field null to the separatrix of the FRC) as large as 5. LSX particle confinement showed a strong scaling with s. The inferred particle diffusivity, Ds, at large s approached ∼2 m2/s, which, along with previous experimental results, indicate a favorable Ds∼s−1/2 scaling. At large s, both electron and ion cross‐field thermal conduction losses become negligible compared to convective losses, with the inferred χ⊥e∼4 m2/s, which was near classical values. Data from several diagnostics employed on the LSX device were analyzed to seek correlation between distortions in the plasma shape and the confinement properti...

A model for inferring transport rates from observed confinement times in field‐reversed configurations

A one‐dimensional transport model is developed to simulate the confinement of plasma and magnetic flux in a field‐reversed configuration. Given the resistivity, the confinement times can be calculated. Approximate expressions are found which yield the magnitude and gross profile of the resistivity if the confinement times are known. These results are applied to experimental data from experiments, primarily TRX‐1, to uncover trends in the transport properties. Several important conclusions emerge. The transport depends profoundly, and inexplicably, on the plasma formation mode. The inferred transport differs in several ways from the predictions of local lower‐hybrid‐drift turbulence theory. Finally, the gross resistivity exhibits an unusual trend with xs (separatrix radius rs divided by the conducting wall radius rc ), and is peaked near the magnetic axis for certain predictable conditions.

Formation of Field-Reversed Configurations Using Scalable, Low-Voltage Technology

Field-reversed configurations are compact toroids confined solely by poloidal fields. Recent experiments and numerical calculations have demonstrated that they can be formed in field-reversed theta pinches on time scales longer than the radial Alfven time. This considerably eases the technological requirements for large devices, and permits reasonable formation schemes to be developed for future experiments. Scaling laws are developed for both flux trapping and heating effectiveness as a function of the formation time scale and poloidal flux level.

Creation of a high-temperature plasma through merging and compression of supersonic field reversed configuration plasmoids

A new device, the Inductive Plasma Accelerator, was employed to simultaneously form and accelerate two oppositely directed field reversed configurations (FRCs) where the relative velocity (600 km s−1) of the plasmoids was much larger than their internal thermal motion. Upon collision all of the FRC directional energy was observed to be rapidly thermalized concurrent with complete magnetic reconnection of the two FRCs. Upon merging, the resulting FRC was compressed to kilovolt ion temperatures exhibiting a configuration lifetime better than predicted by past scaling of in situ formed FRCs. With the improved FRC confinement scaling, a pulsed plasma device based on this approach capable of achieving fusion gain is examined. For an FRC with a poloidal flux 20 mWb or greater, the fusion energy yield per pulse exceeds the plasma energy for compression fields of 10 T or more. The scaling is insensitive to the compression chamber radial scale, providing for the possibility of a very compact fusion neutron source.

Plasma characteristics of a high power helicon discharge

A new high power helicon (HPH) plasma system has been designed to provide input powers of several tens of kilowatts to produce a large area (0.5?m2) of uniform high-density, of at least 5 ? 1017?m?3, plasma downstream from the helicon coil. Axial and radial plasma characteristics show that the plasma is to a lesser extent created in and near the helicon coil and then is accelerated into the axial and equatorial regions. The bulk acceleration of the plasma is believed to be due to a coupling of the bulk of the electrons to the helicon field, which in turn transfers energy to the ions via ambipolar diffusion. The plasma beta is near unity a few centimetres away from the HPH system and Bdot measurements show ?B perturbations in the order of the vacuum magnetic field magnitude. In the equatorial region, a magnetic separatrix is seen to develop roughly at the mid-point between the helicon and chamber wall. The magnetic perturbation develops on the time scale of the plasma flow speed and upon the plasma reaching the chamber wall decays to the vacuum magnetic field configuration within 200??s.