B.L. Wright

Axial dynamics in field-reversed theta pinches. II: Stability

Detailed stability studies are made with new diagnostics in the FRX‐C/LSM field‐reversed theta pinch [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. II, p. 517]. These studies seek the origin of a degradation of the confinement properties of field‐reversed configurations (FRC’s) that appears associated with strong axial dynamics during plasma formation. Several instabilities are observed, including rotational modes, interchanges, and tilt instabilities. Only the latter are strongly correlated with FRC confinement. Tilt instabilities are observed for FRC’s with larger average number of ion gyroradii (s∼3–5) and smaller separatrix elongations (e∼3–4). Coincidently, strong axial dynamics occurs for cases with larger s and smaller e values, through increases in either reversed bias field or fill pressure. These data provide some understanding of FRC stability. In agreement with finite Larmor radius theory, there is a regime of gross stability for the very kinetic and elongate...

Energy confinement studies in spheromaks with mesh flux conservers

The paper presents experiments and analysis of energy confinement on the CTX spheromak. Compared to previous published results from 0.4 m radius flux conservers, in a 0.67 m radius mesh flux conserver (with the current density kept constant), the magnetic field increases while the plasma density is kept the same. However, the electron temperature does not rise, and hence βvol drops. The plasma resistivity remains constant (the resistance drops as the size increases), and the energy confinement time stays the same. Plasma energy content results from spheromaks during sustainment by helicity injection are also presented and show confinement equivalent to that during the decay phase. Increased magnetic field in the same size experiment produces very little improvement in electron temperature and a decrease in confinement time. The resistive decay time is found to be empirically independent of the core electron temperature. It is, instead, proportional to the strength of the magnetic field at constant plasma density, while the ratio of magnetic field to decay time depends on plasma density, consistently with ionization breakdown at the edge of the spheromak dominating helicity dissipation. The possible causes of this observed confinement are examined separately in detailed quantitative and qualitative studies. Absolutely calibrated multichord bolometry shows that impurity radiation is not the cause of the low electron temperatures. The particle confinement time has increased with size, but does not show an increase with increasing magnetic field. At the lower βvol of the larger experiment, the particle replacement power cannot explain the unaccounted energy losses. Any important role of pressure driven modes in the CTX energy balance is shown to be inconsistent with the available CTX data. The possibility that rotating coherent current driven kink modes can seriously degrade energy confinement is evaluated and discounted owing to the lack of improvement when the modes are absent. The role of anomalous ion heating is examined, and the available data are presented. Finally, a hypothesis explaining these results is presented, suggestions for future work are made, and the results are summarized.

High-power magnetic-compression heating of field-reversed configurations

Magnetic compression heating experiments at the 1 GW level on field‐reversed configuration (FRC) compact toroid plasmas are reported. FRC’s formed in a tapered theta‐pinch coil have been translated into a single‐turn compression coil, where the external magnetic field is slowly raised up to seven times its initial value. Significant electron and ion heating consistent with the expected B4/5 adiabatic scaling is observed, despite significant particle diffusion, which is enhanced during compression. The n=2 rotational instability is enhanced during compression, but has been controlled to an extent by the application of an external quadrupole field. The particle and flux confinement times, τN and τφ, remain approximately equal and decrease roughly with the square of the plasma radius R during compression, implying a constant nonclassical field‐null resistivity. The observed τN and τφ magnitudes and scalings are compared with classical and anomalous transport theories, and existing empirical models. Particle ...

Axial dynamics in field‐reversed theta pinches. I: Formation

Bias field scans are performed at various fill pressures in the FRX‐C [Fusion Technol. 9, 13 (1986)] and FRX‐C/LSM [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. II, p. 517] field‐reversed theta pinches. These data show a systematic degradation of the confinement properties of field‐reversed configurations whenever strong axial implosions occur during plasma formation. This limitation prevents access to the desired regime of large‐size and long‐lived field‐reversed configurations. The cause of the confinement degradation must be due to some formation or gross stability problem. Here many studies are reported that attempt to correlate confinement degradation with some formation characteristic. These investigations remain inconclusive and suggest further stability studies presented in a companion paper.

The m=1 helicity source spheromak experiment

In this paper extensive measurements of magnetic equilibrium and source parameters in the m=1 helicity source spheromak experiment are described (previously called the kinked z‐pinch source [Comments Plasma Phys. Control Fusion 9, 161 (1985)]). In the cylindrical entrance region connecting the stabilized z‐pinch helicity source to the spheromak flux conserver, the observed equilibrium configuration is the helical azimuthal m=1 state with no net axial flux. In the flux conserver, the equilibrium is a spheromak (m=0) state with an m=1 distortion. The magnetic equilibria observed are compared to theory. The performance of the source relative to coaxial helicity sources is also examined.

Electron energy confinement in field reversed configuration plasmas

Electron energy transport in field reversed configuration (FRC) equilibria is studied experimentally for a variety of conditions. Up to 37% of the global plasma power loss is attributed to non-convective processes by electrons. The electron temperatures are approximately two times larger than those measured without reversed bias field, which indicates confinement by the poloidal magnetic field in the FRC. The inferred average cross-field thermal diffusivities χ⊥e are anomalous, ranging between 35 and 140 times classical, and are consistent with transport from turbulent magnetic fluctuations of 1 to 2%.

The n =1 rotational instability in field-reversed configurations

Data from an array of external probes provide the first clear evidence for the n=1 rotational instability in field‐reversed configurations. The time evolution and the axial structure of this instability are clarified.

Zero-dimensional energy balance modelling of the CTX spheromak experiment

A zero-dimensional time-dependent energy balance model is used to explore the energy loss mechanisms of the CTX spheromak experiment at Los Alamos National Laboratory. A coupled set of model equations representing electron, ion, neutral, and impurity particle balance, electron and ion temperature, and magnetic field decay, are solved from initial values and the results compared to the time behaviour of experimentally measured average densities, temperature, and magnetic field. The energy balance model considers all the major atomic physics processes, especially the effects of radiation from a non-equilibrium distribution of impurity charge states evolving in time. The model includes the effects of a strong neutral-particle source which replaces by ionization the plasma being lost by a short particle confinement time. The neutral source is required in experiments to prevent the sudden termination of the discharge associated with low densities. A major new conclusion is that all the data from resistively decaying spheromaks can be effectively modelled, when the flux loss effects of a resistive flux conserver are also included, using plasma resistivity increased by a factor of 3.2 ? 0.6 over the Spitzer-H?rm value evaluated with the volume-average temperature. This factor appears to be constant for all discharges at all times. The analysis has determined that the power density associated with the particle replacement is the most important loss in the warm, non-radiation-dominated CTX spheromaks.

Flux confinement measurements in large field‐reversed configuration equilibria

The poloidal magnetic flux φ in large field‐reversed configuration plasmas is examined experimentally. A wide range of initial equilibrium conditions, with 1≤φ≤8 mWb, is produced by varying the reverse bias magnetic field strength. The flux confinement time τφ at first improves with bias, albeit with field‐null resistivities an order of magnitude larger than classical. A further increase in bias results in a reduction of τφ, which is inconsistent with either classical or anomalous diffusion theory. The data suggest the importance of nondiffusive processes such as instabilities or formation dynamics.

Field reversed configurations and spheromaks

The status of controlled nuclear fusion research is reviewed for two major compact toroidal confinement concepts: the field reversed configuration (FRC) and the spheromak. The FRC is an inherently high beta concept offering the advantages of a cylindrical geometry, a natural divertor and the possibility of axial movement of the confined plasma during the formation/burn cycle. The essential techniques of FRC formation, translation and magnetic compression heating have been successfully demonstrated, and gross instabilities driven by plasma rotation have been controlled. Emphasis of present FRC research is on formation symmetry, and on the effects of reducing the large gyroradii of the ions on stability and confinement. The spheromak, like the reversed field pinch, embodies a nearly force-free equilibrium whose evolution is governed significantly by the principle that magnetic helicity is conserved during relaxation processes that minimize magnetic energy. Recent spheromak research has shown how improved edge conditions can enhance global confinement by reducing relaxation and turbulence. Attention to this issue has led to major gains in spheromak energy confinement and plasma beta, and to the discovery of new pressure driven effects.