Ohmic Flux Research Articles

Ohmic flux consumption during initial operation of the NSTX spherical torus

Spherical tokamaks, because of their slender central columns, have very limited volt second capability relative to that of standard aspect ratio tokamaks of similar plasma cross-section. Recent experiments on NSTX have begun to quantify and optimize the ohmic current drive efficiency in an MA class ST device. Sustainable ramp rates in excess of 5 MA/s during the current rise phase have been achieved on NSTX, while faster ramps generate significant MHD activity. Discharges with Ip exceeding 1 MA have been achieved in NSTX with nominal parameters: aspect ratio A = 1.3-1.4, elongation κ = 2-2.2, triangularity δ = 0.4, internal inductance li = 0.6, Ejima coefficient CE = 0.35. Flux consumption efficiency results, performance improvements associated with first boronization and comparisons with neoclassical resistivityare described.

Simple microwave preionization source for ohmic plasmas

A simple economical 2.45 GHz microwave system has been developed and utilized for preionization on the Korea Advanced Institute of Science and Technology (KAIST)-TOKAMAK. The magnetron microwave source was obtained from a widely used, household microwave oven. Since ac operation of the magnetron is not suitable for tokamak application, the magnetron cathode bias circuit was modified to obtain continuous and stable operation of the magnetron for several hundred milliseconds. Application of the developed microwave system to KAIST-TOKAMAK resulted in a reduction of ohmic flux consumption.

Anomalous dependence of the dissipation upon magnetic field angle in layered high Tc superconductors

We have investigated the dependence of the electrical dissipation upon the magnetic field angle in narrow single-crystal bismuth strontium calcium copper oxide (BSCCO) whiskers of the 2:2:1:2 phase. In the ohmic flux flow regime, which occurs at lower currents and for temperatures close to T co, we find unusual angular dependences. As the magnetic field angle is varied in the bc -plane close to H| b we observe a single minimum in the resistance, two minima with a central maximum, three minima and two maxima, or a single minimum with shoulders, depending upon the temperature. Scaling shows that these features depend only upon the magnitude of the magnetic field component parallel to the bic-axis. We propose the following explanation. At very low fields the dissipation is determined by thermally excited pancake vortices. As an applied bic-axis field is increased vortices enter in the very mobile vortex gas phase and the dissipation rapidly increases. As the field is further increased the vortices enter the solid phase and the dissipation falls due to reduced mobility. At far higher fields the dissipation increases with increasing vortex numbers.

AC magnetic susceptibility measurements near the superconducting transition temperature of YBCO sintered samples

The temperature dependence of the ac susceptibility of sintered YBCO pellets is experimentally analysed in detail few Kelvin below the transition temperature. The quite complex dependence of χ ′ and χ ″ on the frequency and the amplitude of the ac magnetic field cannot be explained by an ohmic Flux Flow or Critical State models, so that intermediate regimes or different non linear phenomena are to be considered.

Neutral particle transport in a tokamak toroidal belt pump limiter

The Advanced Limiter Test‐II (ALT‐II) is an experimental toroidal belt pump limiter in the TEXTOR tokamak. This configuration is designed to study power and particle removal capabilities of the toroidal limiter during the long pulse (4 s), high power (7 MW) tokamak discharges. Probe measurements of plasma parameters in the limiter shadow are used to calibrate the incident ion flux. Neutral gas pressure and particle throughput to the pumps are measured with Bayard–Alpert gauges. In the ohmic phase, the plasma density in the limiter scoops is 3 to 8×1011 cm−3, electron temperatures are 8 to 12 eV, and the neutral pressure can reach 0.5 mTorr. Numerical calculations show that the background plasma influences atomic and molecular gas transport in the scoops, and the estimated particle removal rates agree with experimental measurements under ohmic (0.1–0.2 Torr l/s) and high flux conditions with neutral beam injection (NBI) (>1.0 Torr l/s).

Magnetic Buoyancy with Viscosity and Ohmic Dissipation and Flux Tube Formation

In the framework of the magnetohydrodynamic equations with dissipative terms in the form of turbulent viscosity (Vt) the linear stage of the instability of a subphotospheric field with respect to magnetic buoyancy is considered. For an exponential in z, isothermal plane-parallel atmosphere with a constant Alfvenic velocity the perturbations of the form are described by differential equation with constant coefficients. The qualitative dependence of the growth rate of instability on transverse wave number is determined (see Figures 1 and 2) and characteristic scales of the magnetic tubes are evaluated at the linear stage of instability (Kuznetsov, 1987, 1988).

Magnetic helicity injection for configurations with field errors

The relative magnetic helicity and its rate of injection are computed for several examples of configurations representing magnetic fusion experiments, specifically reversed field pinches (RFP’s) with field errors. Field errors, here defined to be normal components of magnetic field at the wall, are believed to have a significant effect on the loop voltage, and therefore the volt-second consumption, in RFP’s. The perturbations producing these errors can arise from imbalance of the vertical field, leakage of Ohmic transformer flux, or toroidal field ripple. In the examples given, the relative helicity is the difference between two terms, which are called, respectively, the plasma linkage and the vacuum linkage. The latter quantity, which is zero for the simplified configurations studied previously, represents the linkage between vacuum fluxes occurring within the region of interest. On the other hand, the plasma linkage is the part of the relative helicity directly related to the instantaneous state of the plasma. It is found that the rate of change of the plasma linkage in the presence of poloidal field errors contains a new term proportional to the time rate of change of toroidal flux.

Observational constraints on the generation process of the Earth's magnetic field -- correction

where the integrals are taken over the Earth's core. F. J . Lowes has pointed out that (1) also holds when the integral on the right-hand side is taken over V + V' , the whole of space, and includes the external magnetic field. This form of the inequality furnishes better bounds on the heat flux, which are derived below. Paper 1 contains some algebraic errors which are corrected. The Ohmic heat flux was bounded in 1 by minimizing the integral: