R. K. Linford

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 ...

Particle transport in field-reversed configurations

Particle transport in field‐reversed configurations is investigated using a one‐dimensional, nondecaying, magnetic field structure. The radial profiles are constrained to satisfy an average β condition from two‐dimensional equilibrium and a boundary condition at the separatrix to model the balance between closed and open‐field‐line transport. When applied to the FRX‐B experimental data and to the projected performance of the FRX‐C device, this model suggests that the particle confinement times obtained with anomalous lower‐hybrid‐drift transport are in good agreement with the available numerical and experimental data. Larger values of confinement times can be achieved by increasing the ratio of the separatrix radius to the conducting wall radius. Even larger increases in lifetimes might be obtained by improving the open‐field‐line confinement.

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.

Adiabatic compression of elongated field‐reversed configurations

The adiabatic compression of an elongated field‐reversed configuration (FRC) is computed by using a one‐dimensional approximation. The one‐dimensional results are checked against a two‐dimensional equilibrium code. For ratios of FRC separatrix length to separatrix radius greater than about ten, the one‐dimensional results are accurate within 10%. To this accuracy, the adiabatic compression of FRC’s can be described by simple analytic formulas.

The Ohmic heating of a spheromak to 100 eV

The first spheromaks with Thomson‐scattering‐measured electron temperatures of over 100 eV are described. The spheromak is generated by a magnetized coaxial plasma source in a background gas of 30 mTorr of H2, and it is stably confined in an oblate 80 cm diam copper mesh flux conserver. The open mesh design allows rapid impurity transport out of the spheromak. The peak temperature, measured using multipoint Thomson scattering, is observed to rise from approximately 25 eV to over 100 eV in about 0.2 msec due to Ohmic heating from the decaying magnetic fields. Density (∼5×1013 cm−3) and magnetic fields (approximately 2 kG) are measured using interferometry and magnetic probes.

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.

SPHEROMAK FORMATION AND OPERATION WITH BACKGROUND FILLING GAS AND A SOLID FLUX CONSERVER IN CTX

Spheromaks with lifetimes of 1 ms are produced in the CTX experiment. This paper describes the diagnostics and measurements on plasmas which, for CTX-produced plasmas, are the hottest and longest-lived discharges using a solid copper flux conserver. These spheromaks are formed using a static hydrogen background gas filling the entire vacuum system before the discharge. The density rapidly decays in 150–300 μs from an initial value of (1–3) × 1014 cm−3 to a steady-state plateau with a value of (1–4) × 1013cm−3,determine d by the pressure of the gas fill. A multi-point Thomson scattering system measures the radial profiles of electron temperature and density. Peak temperatures of over 40 eV are observed, and the average temperature increases in time by Ohmic heating from 15 eV to over 30 eV. Equilibrium models for the magnetic field structure are used to calculate values of peak local beta (8–13%), volume-averaged beta (3–8%), and engineering beta (10–25%). The operation with a filling gas results in a reduction of the impurity radiation power as measured by spectroscopy. Improved vacuum practices, discharge cleaning and the use of the static gas fill have resulted in discharges in which the radiation power loss is not dominating the energy balance late in time. Particle loss and the associated ionization and heating of the neutral particles required to maintain the density plateau appear to be the major energy loss processes in the spheromak.

The Los Alamos spheromak programme

Experiments and theory at Los Alamos have contributed to advances and increased understanding of spheromak physics. Application of the relaxation principle and the concept of helicity injection has led to new, improved formation methods and to the ability to sustain spheromaks for long times against resistive decay. Use of oblate flux conservers has provided gross stability of the spheromak, even in the presence of bias magnetic fields. Magnetic diagnostics have seen oscillations caused by rotating non-resonant internal kink modes. The stability thresholds of these modes agree with the measured equilibrium of the spheromak, confirming that those equilibria depart significantly from the minimum-energy state. Reduction of impurities and use of background filling gas have created resistively decaying spheromaks with non-radiation-dominated confinement.

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.