Scaling of edge-plasma turbulence in the Caltech tokamak

Abstract

Edge-plasma turbulence was investigated over a wide range of plasma and field parameters in the Caltech research tokamak. Fluctuation levels and spectra were measured using Langmuir probes in the region r/a = 0.75–1.0. Under almost all conditions the edge plasma was turbulently unstable, with a broadband fluctuation spectrum in the drift-wave range of frequencies f = 10–1000 kHz. A stable state was observed only in the very cold, low-current discharges formed at unusually high neutral filling pressure. Otherwise, the relative fluctuation level as monitored by the ion saturation current J+ was very high, in the range J̃+/J̄+ ≅ 0.2–0.8, while the fluctuation power spectra were roughly invariant in shape. The relative fluctuation level was always highest near the wall and decreased monotonically toward the plasma centre. The same edge turbulence was observed with or without an outer limiter present. The relative fluctuation level was observed to be independent of the local collisionality over a range of nearly 100. The radial variation of J̃+/J̄+ can be related to the radial density scale length Ln by J̃+/J̄+ ≅ (3–5)ρs/Ln.