Transport phenomena in the edge of Alcator C-Mod plasmas

Abstract

Two aspects of edge turbulence and transport in Alcator C-Mod are explored. The quasi-coherent mode (QCM), an edge fluctuation present in Enhanced Dα H-mode plasmas, is examined with regard to its role in the enhanced particle transport found in these plasmas, its in/out asymmetry, its poloidal wave number and its radial width and location. It is shown to play a dominant role in the perpendicular particle transport. The QCM is not observed at the inboard midplane, indicating that its amplitude there is significantly smaller than on the outboard side. The peak amplitude of the QCM is found just inside the separatrix, with a radial width 5 mm, leading to a non-zero amplitude outside the separatrix and qualitatively consistent with its transport enhancement. Also examined are the characteristics of the intermittent convective transport, associated with the larger scale turbulent structures, also called blobs, and typically occurring in the scrape-off-layer (SOL). These turbulent structures are qualitatively similar in Land H-mode. When their perpendicular extent, occurrence frequencies and magnitudes are compared, it is found that their size is somewhat smaller in ELMfree H-Mode, while their frequency is similar. A clear difference is seen in the magnitude of these turbulent fluctuations in the far SOL, with ELMfree H-mode showing a smaller perturbation there than L-mode. As the Greenwald density limit is approached (n/n GW 0.7), blobs are seen inside the separatrix consistent with the observation that the high cross-field transport region, normally found in the far SOL, penetrates the closed flux surfaces at high n/n GW .

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…5 of Ref. 137). It was concluded that the blob birth zone was associated with the gradients rather than with the transition from open to closed field lines.…”

“…126). Finally, a series of L-mode shots on Alcator C-Mod 137 showed that the blob birth zone moved inwards as the normalized density (n e =n G ) was increased, where n G is the Greenwald density limit. For n e =n G > 0:6 the blob birth zone moved inside the separatrix and both the gradients and fluctuation levels inside the separatrix were observed to increase.…”

“…In this case the blobs are born near the location of the maximum linear growth rate, whether that is inside or outside of the LCFS. 137 The change of magnetic topology from closed to open field lines can either play a role or not in determining the maximum growth rate, depending on the regime. In the interchange regime, the mode extends far along the field line, effectively performing a weighted average over the curvature (of the flux surface, or of the SOL field line).…”

Intermittent fluctuations in the Alcator C-Mod scrape-off layer

“…As the discharge density limit is approached, the far SOL profile becomes broader and the break point moves radially inwards such that the far SOL profile effectively extends all the way to the magnetic separatrix or even inside it. 26 As an illustration of the plasma dynamics in the SOL we present in Fig. 3 the GPI signals at three different radial The PDF of the GPI signals is presented in Fig.…”

Stochastic Modeling of Intermittent Scrape-Off Layer Plasma Fluctuations

“…Under very general conditions, this model predicts a parabolic relation between the skewness and kurtosis moments of the plasma fluctuations. In the case of exponentially distributed burst amplitudes and waiting times, the probability density function for the fluctuation amplitudes is shown to be a Gamma distribution with the scale parameter given by the average burst amplitude and the shape parameter given by the ratio of the burst duration and waiting times.Cross-field transport of particles and heat in the scrape-off layer (SOL) of non-uniformly magnetized plasmas is caused by radial motion of blob-like structures [1][2][3][4][5][6]. This results in singlepoint recordings dominated by large-amplitude bursts, which have an asymmetric wave form with a fast rise and a slow decay, and positively skewed and flattened amplitude probability density functions [7][8][9][10][11][12][13][14].…”