Fluctuations and transport in the TCV scrape-off layer

Abstract

Fluctuations and particle transport in the scrape-off layer of TCV plasmas have been investigated by probe measurements and direct comparison with two-dimensional interchange turbulence simulations at the outer midplane. The experiments demonstrate that with increasing line-averaged core plasma density, the radial particle density profile scale length becomes broader. The particle and radial flux density statistics in the far scrape-off layer exhibit a high degree of statistical similarity with respect to changes in the line-averaged density. The plasma flux onto the main chamber wall at the outer midplane scales linearly with the local particle density, suggesting that the particle flux here can be parameterized in terms of an effective convection velocity. Experimental probe measurements also provide evidence for significant parallel flows in the scrape-off layer caused by ballooning in the transport of particles and heat into the scrape-off layer. The magnitude of this flow estimated from pressure fluctuation statistics is found to compare favourably with the measured flow offset derived by averaging data obtained from flow profiles observed in matched forward and reversed field discharges. An interchange turbulence simulation has been performed for a single, relatively high density case, where comparison between code and experiment has been possible. Good agreement is found for almost all aspects of the experimental measurements, indicating that plasma fluctuations and transport in TCV scrape-off layer plasmas are dominated by radial motion of filamentary structures.

Stochastic Modeling of Intermittent Scrape-Off Layer Plasma Fluctuations

“…In particular, the amplitude distribution of the plasma fluctuations are found to be well described by a Gamma distribution across a broad range of plasma parameters and for all radial positions in the SOL [12]. Excellent agreement was found when comparing analysis of these data with turbulence simulations based on interchange motions [9][10][11].In this Letter, a stochastic model for intermittent fluctuations in the plasma SOL is presented …”

“…Probe measurements on Tokamakà Configuration Variable (TCV) have demonstrated a remarkable degree of universality of the plasma fluctuations in the far SOL region, which is dominated by radial motion of filament structures and a relative fluctuation level of order unity [10][11][12][13]. In particular, the amplitude distribution of the plasma fluctuations are found to be well described by a Gamma distribution across a broad range of plasma parameters and for all radial positions in the SOL [12].…”

“…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]. Measurements on a number of tokamak experiments have demonstrated that as the empirical discharge density is approached, the radial SOL particle density profile becomes broader and plasma-wall interactions increase [11][12][13][14][15].Probe measurements on Tokamakà Configuration Variable (TCV) have demonstrated a remarkable degree of universality of the plasma fluctuations in the far SOL region, which is dominated by radial motion of filament structures and a relative fluctuation level of order unity [10][11][12][13]. In particular, the amplitude distribution of the plasma fluctuations are found to be well described by a Gamma distribution across a broad range of plasma parameters and for all radial positions in the SOL [12].…”

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

“…40,50,53,62,74,81,88 The plasma profiles due to the turbulent (blob) convection have also been compared with experiments. 46,52,93,100,102 A brief discussion of the comparison of turbulence simulations and experiments is given in Sec. VII D.…”

Ideal ballooning modes in the tokamak scrape-off layer

“…As a matter of fact, the key characteristics of the scrape-off layer turbulent phase space appear to be captured by the electromagnetic fluid drift turbulence theory (see, for example, Refs. 8,[15][16][17][18][19][20][21].…”