Observation of beta-limits in highly elongated tokamak plasmas
According to the Troyon scaling law, the beta-limit is proportional to the plasma current and inversely proportional to the horizontal minor radius and the toroidal magnetic field, where C(T), the Troyon factor, is typically between 2.5 and 4.0, depending on the pressure and current profiles. Idea magnetohydrodynamic (MHD) theory predicts that the Troyon factor is significantly reduced at high elongation or near the plasma current limit. Experimentally, the degradation of the Troyon factor at high elongation has not yet been clearly demonstrated up to now, mainly because very few tokamaks are quipped with feedback systems allowing operation beyond kappa=2. The Tokamak à Configuration Variable (TCV) tokamak can operate routinely up to kappa=2.5. In recent experiments, beta-limits have been observed with Ohmic heating at high elongation, high current and low values of the safety factor. The beta-limit is measured by varying the plasma density, and it manifests itself by the appearance of MHD mode activity, followed by a disruption. These disruptions are not related to the commonly observed density-limit disruptions since they occur at densities below 30 % of the Greenwald limit. At the maximum elongation, kappa=2.5, and for a normalized current, the beta-limit is between 2 and 3 %. This observation is consistent with ideal MHD stability calculations, using reconstructed experimental equilibria.
Bibliographic Reference: Article: 1998 International Congress on Plasma Physics, 25th EPS Conference on Controlled Fusion and Plasma Physics, Prague (CZ), June 29 � July 3, 1998
Record Number: 199811276 / Last updated on: 1998-10-27
Original language: en
Available languages: en