Triggering internal disruptions in tokamaks at high-{beta}
- Princeton Plasma Physics Lab., NJ (United States); and others
At high-{beta}, both minor or major tokamak disruptions can arise abruptly without precursor. This fact is difficult to explain based on the picture of crossing some stability limit. Our analysis of TFTR supershots shows that highly peaked pressure profiles in tokamaks may create conditions for coupling between the internal reconnection mode and the ballooning modes. In standard supershot regimes, the q{sub 0} value at the plasma center sits below 1. Although the ideal MHD theory predicts the m = 1 instability in these regimes, the sawtooth oscillations are stabilized by FLR effects. This stabilization is proportional to the pressure gradient at the q = 1 surface. When the pressure gradient approaches the ballooning stability limit near the q = 1 surface, the excitation of ballooning modes suppresses FLR stabilization of the reconnection m = 1 mode. In turn, the growing reconnection mode significantly enhances the local pressure gradient p{prime} = p{prime}{sub 0} (1 + {xi}{prime} cos({theta} - {phi})) at the q = 1 surface where {xi}{prime} {approx} 1 and, thus, amplifies the ballooning modes. This mechanism reveals the positive feedback between the reconnection mode and ballooning modes and explains the abrupt onset of internal disruptions at high-{beta}. Because of the positive feedback, the disruptions can start either with or without the m = 1 or ballooning precursors present as it is observed in experiment. The criterion for sawtooth stabilization has been validated earlier for all of TFTR regimes. The same two-fluid model, now applied to ballooning modes, shows good correlation with experimental data. In particular, it predicts that the central {beta}{sub N}, calculated within {approximately} 1/3 of the minor radius, rather than the global {beta}{sub N} is responsible for the onset of internal disruptions. This explains the fact that depending on the peakedness of the pressure profile, disruptions may occur at unexpectedly low levels of the total {beta}.
- DOE Contract Number:
- AC02-76CH03073
- OSTI ID:
- 489419
- Report Number(s):
- CONF-960354-; TRN: 97:011563
- Resource Relation:
- Conference: International Sherwood fusion theory conference, Philadelphia, PA (United States), 18-20 Mar 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 international Sherwood fusion theory conference; PB: 244 p.
- Country of Publication:
- United States
- Language:
- English
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