E. G. Sherwood

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.

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.

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.

Theoretical And Experimental Investigation Of Reditrons

We have carried out theoretical and experimental study to optimize the efficiency of microwave production of the reditron. In the optimal configuration, we have achieved the production of 3.3 GW of microwave radiation at 10.0% efficiency with a very narrow spectrum centered at 2.15 GHz. This is roughly a factor of 3.5 increase in efficiency and about 3 in bandwidth narrowing over conventional vircators. In additon, we found that the use of cavities can achieved bandwidth narrowing, stability of frequency during repeated operation, improvement of mode selectivity.