Density limit studies in the TCA tokamak
Plasma disruption is the most dangerous instability in a tokamak. Different gas programming is one of the ways to change the density disruption limit. In quasi-static density operations in TCA, the plasma is limited to an average density of (4 to 5) x 1.0 E13/cm3. At this limit, and just before the disruption, an MHD mode with m = 2, n = 1 develops and increases in amplitude. Higher n components (n = 2 and n = 3) appear closer to the disruption and are accompanied by a decrease in the mode frequency. Impure plasmas are more likely to disrupt, while in clean plasmas the density is often limited by mode activity without disruption. One of the ways to avoid this quasi-static density disruption, and achieve densities more than a factor of two higher, is by injecting a sudden influx of neutral gas into the plasma chamber ("hard puffing"). In the transition period, the pressure and density profiles become very broad, increasing the plasma stability. The mode frequency increases while the plasma toroidal velocity (measured by the Doppler shift of CV and CIII lines) decreases. The increased mode frequency during hard puffing is accompanied by the appearance of modes with several close frequencies. These modes do not lead to a disruption but degrade the plasma energy confinement. Near the higher density limit, the mode frequency decreases from the higher value achieved during the transition period. A disruption then follows in a similar fashion to a "quasi-static" disruption, except that a beating of several frequencies (oscillating islands) is sometimes visible before the disruption.
Bibliographic Reference: Paper presented: 18th European Conference on Controlled Fusion and Plasma Physics, Berlin (DE), June 3-7, 1991
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Record Number: 199110440 / Last updated on: 1994-12-02
Original language: en
Available languages: en