Broweret al. reply

Abstract

Brower et al. Reply:The phenomenological model described in the Comment [1] represents a novel interpretation of the long-time precursors to high-density-limit disruptions observed on TEXT [2]. The prediction that D e and V e have the same poloidal resistivity dependence and, consequently, that the ratio D e /V e and the density profile are invariant to changes in rj^, is most interesting. Relating microturbulence to anomalous poloidal resistivity, however, provides no information on the specific instability mechanism responsible for the density fluctuation levels. Our paper offers evidence that an ion-pressuregradient-driven mode may be driving changes in the transport, perhaps driven more unstable by impurity accumulation [2,3] and/or changes in the parameter rj, (experimentally unknown). This is further supported by data from helium-and pellet-fueled discharges where the ion feature in the fluctuation spectra is absent and the disruption limit is exceeded. In particular, for highdensity helium-gas-fueled discharges, the electron particle diffusivity in the core plasma (D e = \A m 2 /s) is significantly reduced when compared to the hydrogen-gasfueled equivalent (D e =2.2 m 2 /s).With respect to impurities, the theory of Haas and Thyagaraja [1] predicts that along with the measured degradation of heat and particle confinement, impurity confinement should also deteriorate. Experimentally, it has previously been shown on TEXT [3], as well as other devices [4][5][6], that in fact the confinement of impurities actually increases prior to disruptions at the high-density limit. The implications of this for the Haas and Thyagaraja model [l] need to be addressed. Although the authors' original interpretation that increased microturbulence, perhaps an n, instability, leads to confinement degradation and eventually a disruption via the Greenwald scenario [7] is unchanged, a potential relationship to anomalous poloidal resistivity represents an intriguing possibility which warrants further investigation.

An empirical scaling law for improved confinement in reversed-field pinch plasmas

“…First, we note that I θ , driven by the externally applied poloidal electric field in PPCD, could modify the current density profile so that the tearing modes may be suppressed, as has been numerically demonstrated [25] as well as experimentally measured [26]. We also note that I θ /I p increases with .…”

Edge shear flows and particle transport near the density limit of the HL-2A tokamak

“…turbulent particle and heat fluxes [2]. We also note that enhanced particle transport has been observed in advance of any change in the MHD activity in both experiments and numerical simulations when n G is approached [15][16][17][18][19]. At fixed pressure, the higher density usually implies reduced temperature and hence increases dissipative and resistive effects.…”

Radial profile and q dependence of electron heat diffusion measured with ECH modulation in RTP

“…In both Refs [6] and [7] it is noted that the sawtooth heat pulse data cannot distinguish between a radial profile and a qa dependence. The evidence presented in this paper shows that most if not all of the qa dependences found earlier are indeed due to the radial x p e r t profile.…”

“…Therefore, in these studies the qa dependence and the radial dependence could not be separated. Typical results are x p e r t 0; q l 2 found in TEXT [6], and xPert c ( p1.4, where p r / a denotes the normalized minor radius, found in JET [7]. In both publications, it is noted that it is undetermined whether these dependences should be interpreted as a radial profile or as a qa dependence.…”