K. F. McKenna

Experimental determination of the conservation of magnetic helicity from the balance between source and spheromak

The conjecture that magnetic helicity (linked flux) is conserved in magnetized plasmas for time scales that are short compared to the resistive diffusion time is experimentally tested in the CTX spheromak [Phys. Rev. Lett. 45, 1264 (1980); 51, 39 (1983); Nucl. Fusion 24, 267 (1984)]. Helicity is created electrostatically by current drawn from electrodes. The magnetized plasma then flows into a conducting flux conserver where the energy per helicity of the plasma is minimized and a spheromak is formed on a relaxation time scale of many Alfven times. The magnetic field strength of the equilibrium is subsequently increased and sustained. The amount of helicity created by the magnetized coaxial plasma source, the helicity content of the spheromak equilibrium, and the resistive loss of the helicity are measured to determine the balance of helicity between source and spheromak with a ±16% uncertainty. In CTX the amount of energy that must be rapidly dissipated within the conducting boundary while conserving hel...

Experimental studies of field‐reversed configuration translation

In the FRX‐C/T experiment [Proceedings of the 9th Symposium for Engineering Problems of Fusion Research (IEEE, New York, 1981), p. 1751], field‐reversed configuration (FRC) plasmas have been formed in, and launched from, a field‐reversed theta‐pinch source and subsequently trapped in an adjacent confinement region. No destructive instabilities or enhanced losses of poloidal flux, particles, or thermal energy are observed for FRC total trajectories of up to 16 m. The observed translation dynamics agree with two‐dimensional magnetohydrodynamic (MHD) simulations. When translated into reduced external magnetic fields, FRC’s are observed to accelerate, expand, and cool in partial agreement with adiabatic theory. The plasmas reflect from an external mirror and after each reflection, the axial kinetic energy is reduced by approximately 50%. Because of this reduction, FRC’s are readily trapped without the need of pulsed gate magnet coils.

Review of the Los Alamos FRX-C experiment

The FRX-C device is a large field-reversed theta pinch experiment with linear dimensions twice those of its FRX-A and FRX-B predecessors. It is used to form field-reversed configurations (FRCs), which are high-beta, highly prolate compact toroids. The FRX-C has demonstrated an R/sup 2/ scaling for particle confinement in FRCs, indicating particles are lost by diffusive processes. Particle losses were also observed to dominate the energy balance. When weak quadrupole fields were applied to stabilize the n = 2 rotational mode, FRC lifetimes >300..mu..s were observed. Detailed studies of the FRC equilibrium were performed using multichord and holographic interferometry. Measurements of electron temperature by Thomson scattering showed a flat profile and substantial losses through the electron channel. The loss rate of the internal poloidal flux of the FRC was observed to be anomalous and to scale less strongly with temperature than predicted from classical resistivity.

Flux loss during the equilibrium phase of field-reversed configurations

Field‐reversed configurations are consistently formed at low filling pressures in the FRX‐C device, with decay time of the trapped flux after formation much larger than the stable period. This contrasts with previous experimental observations.

Confinement of translated field-reversed configurations

The confinement properties of translating field‐reversed configurations (FRC) in the FRX‐C/T device [Phys. Fluids 29, ▪ ▪ ▪ ▪ (1986)] are analyzed and compared to previous data without translation and to available theory. Translation dynamics do not appear to appreciably modify the FRC confinement. Some empirical scaling laws with respect to various plasma parameters are extracted from the data. These are qualitatively similar to those obtained in the TRX‐1 device [Phys. Fluids 28, 888 (1985)] without translation and with a different formation method. Translation with a static gas fill offers new opportunities such as improved particle confinement or refueling of the FRC particle inventory.

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.

Helical and straight quadrupole stabilization of the n = 2 rotational instability in translated field-reversed configurations

The n=2 rotational instability has been completely suppressed on translated field‐reversed configurations (FRC’s) in the FRX‐C/T device [Phys. Fluids 29, 852 (1986)] by the application of either helical or straight quadrupole fields. Quadrupole field thresholds approximately equal to 7% and 9% of the external confinement field are required for stabilization with straight and helical coils, respectively. Comparisons with linearized magnetohydrodynamic (MHD) theory and 2 1/2 ‐dimensional hybrid simulations are made. The influence of quadrupoles on translation and plasma confinement is also reported.

Scaling studies in field reversal experiments

The stable period of field‐reversed configurations, defined by the onset of the rotational n = 2 instability, is observed to scale with R2/ ρi over a new, wider range of experimental conditions, where R is the major radius and ρi is the ion gyro‐radius indexed to the external field. The scaling factor is approximately 6.0×10−7 sec cm−1 over a range of R2/ ρi from 18 to ∼100 cm in which 1/ ρi varied from 1 to 5 cm−1 and R varied by approximately 30%. In a complimentary study, the stable period was observed to be independent of Ti over a range of 200–1200 eV when R2/ ρi was held approximately constant. The theoretical correlation of the stable period with the particle containment time, and hence with R2/ ρi, are discussed.

Interpretation of end‐on interferometry in field‐reversed configurations

For field‐reversed configurations formed from a 20 mTorr static gas filling in the FRX‐C device, the density in the end regions is determined by side‐on interferometry. This allows us to understand the radial profiles obtained from end‐on holograms, and provides a more accurate estimate of the FRC particle confinement time than previously obtained.

Field-reversed configuration research at Los Alamos

Exploratory field-reversed-configuration (FRC) experiments, initiated at Los Alamos in the midseventies, demonstrated FRC lifetimes substantially longer than predicted from MHD stability theory. Subsequent experimental and theoretical advances have provided considerable understanding of FRC stability physics, the characteristics of the configuration loss processes, and the particle confinement scaling with size. The critical FRC physics issues, which directly relate to the development of an FRC fusion reactor and need to be addressed in a new generation of experiments, have been clearly identified.