Correlation of magnetic fluctuations and edge transport in the Doublet III tokamak

Abstract

A clear correlation between incoherent magnetic fluctuations and transport in the outer region of the plasma was observed in a set of Doublet III beam-heated discharges which experienced repetitive transitions between a poor-confinement phase and an improved-confinement phase. The fluctuations and associated transport are consistent with a model based on an assembly of saturated microtearing modes.PACS numbers: 52.55. Fa, 52.25.Fi, Observations of improved energy confinement ("//mode" discharges) in tokamaks 1 " 3 suggest that edge confinement plays a critical role in the determination of global energy confinement, i.e., longer edge confinement leads to longer core confinement. Plausible theoretical models have been advanced to explain this relationship, 4 ' 5 and acceptance of this viewpoint shifts the problem of confining a fusion plasma to one of understanding and improving the confinement at the plasma edge.To this end, this Letter reports observations from the Doublet III tokamak wherein the level of magnetic turbulence is directly correlated with (and thus may be responsible for) edge transport. The turbulence is low frequency (5-50 kHz) and broadband, 6 and increases with P p , as previously observed on the ISX-B tokamak. 7 The correlation here is established more directly than in previous experiments 6 " 10 because the observed incoherent magnetic-fluctuation amplitude abruptly changes simultaneously with transitions into and out of the H mode. A model based on microtearing modes will be shown to be consistent with the observed turbulence with regard to the spatial amplitude dependence, frequencies, magnitude of heat transport, and the existence of a temperature threshold to the H mode as observed on the ASDEX tokamak. * Magnetic fluctuations are detected by two arrays (poloidal and toroidal) of Mirnov coils that sense the poloidal magnetic field. They are located just inside the Doublet III vacuum vessel [see Fig. 1(a)] which acts as a perfectly conducting shell in the frequency range of interest (f>2 kHz). Each coil is inside a stainless-steel shield that excludes electrostatic pickup while reducing electromagnetic signals by only 10% at 25 kHz. The highest measurable frequency, 50 kHz, is determined by the maximum data-acquisition rate.In Doublet III (B T <2A T, I p < 1 MA, R = 1A4 m, a =0.4 m), as with other tokamaks, it is found that as neutral-beam heating power is increased in discharges defined by a material limiter, the global energyconfinement time ?E decreases (L mode). 1 ' 11 If, however, the plasma is well diverted, the confinement deterioration is more modest, and XE is typically a factor of 2 higher than limiter discharges at high beam-heating levels. In the Doublet III poloidal divertor configuration, neutral-particle sources are localized in the divertor region, whereas a limited discharge exhibits larger sources, mainly near the limiter. Marginally diverted discharges are intermediate, with recycling at both limiter and divertor regions, and are observed to undergo repetitive transitions...

Confinement physics of H-mode discharges in DIII-D

“…1, at the L to H transition, the magnetic fluctuations in the 10 to 30 kHz range decrease abruptly. This change in the frequency spectrum is seen most clearly on the rnagnetic probes located where the inner and outer separatrix field lines intersect the wall of the vacuum vessel, possibly due to line-tying effects (Ohyabu et al, 1988). It is also seen on magnetic probes located on the centerpost of the vacuum vessel near the vessel midplane.…”

Microtearing modes in tokamak discharges

“…It has been shown that for some tokamak discharges, MTMs are unstable primarily in the plasma edge region, 11,12 regulating heat transport, and, possibly, pedestal evolution between edge-localized modes. 9 Moreover, it has been found that in plasmas where b e (ratio of the electron pressure to the magnetic pressure) and collisionality are sufficiently high, MTMs can become the dominant instability contributing to electron transport in the plasma core.…”

Microtearing Instabilities and Electron Transport in the NSTX Spherical Tokamak

“…One well-known instability is the microtearing mode driven by electron temperature gradient [4]. A nonlinear theory for the saturation level of the instability was developed [5], but it was found later that these modes should be stable in conventional tokamaks [6] except near the plasma edge where electron temperature is low [7,8]. Plasma parameters in current spherical tokamak (ST) experiments are quite different from conventional tokamaks, and microtearing modes can be the most unstable mode in National Spherical Torus Experiment (NSTX) [9] and MegaAmpere Spherical Tokamak (MAST) [10] over a certain range of parameters.…”