W. T. Armstrong

Field‐reversed experiments (FRX) on compact toroids

Equilibrium, stability, and confinement properties of compact toroids produced in field‐reversed theta‐pinch experiments (FRX) are reported. Two experimental facilities, FRX‐A and FRX‐B, have been used to study highly elongated compact toroid plasmas confined in a purely poloidal field geometry. Spatial scans and fill pressure scaling of the equilibrium plasma parameters are presented. Plasma conditions range from Te∼150 eV, Ti∼800 eV, nm∼1×1015 cm−3 to Te∼100 eV, Ti∼150 eV, nm∼4×1015 cm−3. Typical confined plasma dimensions are: major radius R∼4 cm, minor radius a∼2 cm, and total length 35–50 cm. The plasma configuration remains in a stable equilibrium for up to 50 μsec followed by the destructive n = 2 rotational instability. The stable period prior to the onset of the rotational mode is up to one hundred times greater than characteristic Alfven transit times of the plasma. This stable period increases and the mode growth rate decreases with increased a/ρi (where ρi is the ion gyroradius). Agreement of ...

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.

Electron temperature measurements in the field-reversed configuration experiment FRX-C

Electron temperature data are presented from Thomson scattering measurements performed on field-reversed configuration plasmas generated in the FRX-C experiment. Two experimental conditions have been investigated, corresponding to initial deuterium fill pressures of 5 and 20 mtorr. At 5 mtorr, Te values of (175±25) eV are observed, which are nearly independent of time and radial position inside the separatrix. An electron power loss of approximately 80 MW is inferred. At 20 mtorr, Te values of 70–120 eV are measured and good agreement with radial pressure balance is obtained. This agreement indicates that radiation from low-Z impurity ions does not dominate the observed energy confinement.

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.

ϑ‐pinch ionization for field‐reversed configuration formation

The behavior of a plasma produced by a ringing ϑ‐pinch discharge in the presence of a 2.3‐kG bias field is examined for the case where the net field passes through zero. Experimental studies, employing internal and external field probes, indicated ∼50% of the initially applied bias flux is excluded by the plasma with this ionization technique. A theoretical model incorporating field diffusion and elastic ion‐neutral collisions is used to describe the observed dynamics of the plasma sheath.

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.

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.