PLASMA BEHAVIOUR AT RATIONAL ROTATIONAL TRANSFORM IN WENDELSTEIN VII-A
Plasma confinement in W VII-A is characterized by enhanced losses if the rotational transform at the plasma edge approaches a low order rational number t(a) is the sum of the external transform t-o and the transform of the plasma current. Three mechanisms responsible for these enhanced losses can be identified. 1) Islands in the vacuum field. Perturbations in the helix produce islands around t = 1/2, 1/3,... 2) Island formation due to finite beta effects. 3) Convective cells excited by external asymmetric sources. (1) strongly modify the pattern of the Pfirsch-Schlueter currents which generate a magnetic field comparable with the external perturbation field. The rotational transform is changed by the plasma currents. (Result obtained from the Johnson-Anania-Weimer code and Kibetalinger-Wobig code). Island formation and ergodization of field lines introduce difficulties into the calculation of stellarator equilibria. A friction term varies asv in the momentum balance removes the singularities of the plasma currents at rational values of t. The final set of equations is a quasilinear elliptic system for the unknown quantities OE, P, T (electric potential, plasma pressure, plasma temperature). The model allows to study the effect of asymmetric external sources. In a simplified model (cylindrical plasma) the equation for OE and T have been solved assuming asymmetric momentum sources. In case of closed magnetic field lines analogy to Benard convection exists.
Bibliographic Reference: 5TH INTERNATIONAL WORKSHOP ON STELLARATORS, SCHLOSS RINGBERG, BAVARIA (GERMANY), SEPT. 24-28, 1984 VOL. I, PP. 363-374, EUR 9618 EN (1985) FS, VOL. I AND VOL. II, 787 P., BFR 3500 (BOTH VOLUMES), EUROFFICE, LUXEMBOURG, POB 1003
Record Number: 1989124120000 / Last updated on: 1987-01-01
Available languages: en