A fast scanning assembly has been widely used on the TCV tokamak to insert a probe head equipped with an array of single Langmuir probe tips up to the separatrix at the plasma midplane. Using fast voltage sweeping, we obtain IV-characteristics every 8 μs, allowing an estimate of the electron temperature, Te on this timescale. Since this temporal resolution corresponds to typical Te autocorrelation times , it is just fast enough to resolve the temperature of individual turbulent structures (blobs). Since at this voltage sweep frequency (~60 kHz) hysteresis is observed in the IV-characteristics, some effort is required to demonstrate the credibility of the Te derived from the characteristics. Following the methodology proposed in , we use both numerical (5spice code) and lab simulations of the equivalent probe circuit, together with a simplified plasma circuit to study the capacitative coupling both across the plasma sheath and in the probe circuit itself. Comparisons are also made between the results from higher frequency sweeping and the standard values derived from a slower sweep to show that the fast measurement is reliable. Considerable effort has been expended in recent years to compare the statistical character of turbulence in the SOL particle flux on TCV with results from the 2D fluid electrostatic model ESEL . Using results from the fast sweeping, similar comparisons can now be made with the fluctuating Te and will be described in this contribution. We also present basic statistics derived from the Te time series obtained at different radii in the SOL plasma and show, in particular, that the relationship between higher moments of the probability distribution function from both experimental and simulated Te’s may be well described by the Beta probability distribution function, introduced for SOL turbulence in . The fast Te capability also allows the SOL response to Edge Localised Modes (ELMs) to be studied and new results will be presented for the far SOL Te response during Type III ELMs.
Energiteknologier på vej
Main Research Area:
13. EU-US TTF workshop 2008 and 1. EFDA Transport Topical Group meeting