Nonlinear convective motion in plasmas

Measurements of cross‐field diffusion were made in the levitated octupole using two independent methods. First, the magnitude and scaling of D⊥ with density, n, and field strength, B, were determined by direct measurements of the particle flux onto an internal ring surface. Second, the shape of the density profile and the time decay of the density at the separatrix were compared with solutions of a diffusion equation model. Both methods indicated that D⊥ was proportional to n−1/2 and independent of B. The plasma was created by gun injection into the purely poloidal magnetic field and the initial parameters were ne∼5 ×109 cm−3, Ti∼20 eV, Te∼5 eV. The observed diffusion scaling can be understood within the framework of vortex diffusion in a high‐dielectric constant plasma if the spectrum of waves is enhanced in energy over thermal levels with an effective temperature, T*, of 104 eV.

Diffusion Coefficient Scaling in the Wisconsin Levitated Octupole.

Internal-conductor devices for low-β toroidal confinement are reviewed. These devices are characterized by high stability against low-frequency oscillations. Experimental observations amply demonstrate this stability property. Plasma confinement approaching the classical limit has been observed in a limited range. It is also shown that the confinement time exceeds the so-called Bohm time by a factor of 200 to 300. The plasma confinement is remarkably sensitive to non-ideal effects such as field error, existence of symmetry-spoiling supports and concentration of impurities.

https://iopscience.iop.org/article/10.1088/0029-5515/13/3/015/pdf

Experiments on plasma confinement in internal-ring devices

Large scale, DC type plasma convection is investigated in the JFT-2M tokamak. In the plasma edge region, E*B type convective flows are observed, which are caused by interactions with both a poloidal limiter and an RF wave injection. Observation of changes in the edge potential during lower hybrid (LH) wave injection indicated that the confinement improvement in the LH phase is due to the reduction of an intrinsic E*B type convection. These results are used in the construction of a model that explains both the L and H modes found in the tokamak NBI experiments. The model consists of both a deterioration mechanism based on the obstacle induced convection and a recovery due to the increase of the electric conductivity along the magnetic field. Agreement between the model and typical experimental results is shown

Convective loss in tokamaks

A historical survey is given of the experimental multipole and tokamak fusion research programme carried out at the University of Wisconsin since 1962. The programme has concentrated on axisymmetric toroidal confinement through the utilization of a series of different multipole devices and of a poloidal divertor tokamak. The gross plasma stability and good confinement properties of the multipoles led to a number of experimental results not easily achievable in other toroidal devices. The paper describes the major results obtained in multipoles with and without Ohmic heating current and toroidal field, and their influence on the development of plasma theory. In the poloidal divertor tokamak, stable discharges have been found at very low values of the safety factor.

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Multipole and tokamak research at the University of Wisconsin

The effects of hoop supports and other obstacles penetrating the plasma containment region of a toroidal octupole were determined. Obstacles produced large scale convective cells which move plasma toward the field boundary at the azimuth of the obstacles and toward the separatrix at other azimuths. Under certain obstacle bias conditions, 3 V floating potential steps were observed across a dummy support along with 100% density steps. The density step showed an exponential dependence on the obstacle bias potential. A model is discussed which predicts the observed properties of the potential and density steps. Electron cooling at the obstacle was also detected.

G. L. Schmidt

Papers

Obstacle Induced Convection in a Toroidal Octupole