Fluctuations in floating potential in the scrape-off layer and plasma edge were analyzed using a conditional statistical analysis technique. The floating potential fluctuations had a nearly Gaussian probability density function with the largest deviation from a Gaussian at the shear layer. The conditional averaging technique followed the statistical evolution of selected conditions in the floating potential signal. The decay rate of a conditional feature in time or space showed a small systematic variation with the amplitude of condition chosen. Either long-lived coherent structures are not present in statistically significant numbers, or the fluctuations are dominated by a large number of coherent structures with a nearly Gaussian distribution of fluctuation amplitudes, or conditional analysis using the amplitude of the floating potential as a condition is not a sensitive technique for identifying coherent structures.
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Electrostatic turbulence has been investigated in the edge region of the Advanced Toroidal Facility (ATF). A reversal in the poloidal phase velocity of the fluctuations has been observed (velocity shear) which determines a characteristic plasma radius. The location of this shear layer depends on the magnetic configuration, the limiter radius and the plasma conditions. Using the shear position as a reference point, the density fluctuation levels in ATF (currentless stellarator) are very similar to those previously reported in TEXT (ohmically heated tokamak), suggesting that the plasma current is not an important drive for the edge turbulence. The drives for the turbulence appear to be different inside and outside the shear location (ashear), with T̃e/Te < ñ/n in the scrape-off layer (r/ashear > 1) and possibly larger T̃e/Te in the plasma edge edge (r/ashear < 1). There is a spatial decorrelation in the fluctuations at the shear location; this suggests that the poloidal shear flow has an important influence on the edge turbulence. The poloidal correlation length depends on local plasma parameters (e.g. velocity and temperature). When neutral beam injection is added, the high frequency components of ̃n increase.
Electron thermal pressure fluctuations measured in the edge plasma of the Texas Experimental Tokamak Upgrade are a fundamental component of plasma turbulence on both sides of the velocity shear layer. The ratio of specific heats, estimated from fluctuations in electron temperature and electron number density measured simultaneously at the same electrode, indicates that observed fluctuations are adiabatic. The observations are made by means of a novel Langmuir probe technique, the time domain triple-probe method, which concurrently measures multiple plasma properties at each of two electrodes with the temporal and the spatial resolution required to estimate thermodynamic properties in a turbulent plasma.
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