Trapped ion turbulence in tokamaks
Ion turbulence may contribute to ion anomalous heat transport, especially when the ion temperature gradient is large. The thresholds for such a turbulence have been determined by using the code TORRID, which calculates marginal thresholds and associated linear eigenmodes. This kinetic code takes account of circulating and trapped particle resonances and calculates modes within the frame of the ballooning approximation. It appears that the most dangerous instabilities are driven by trapped ions, with a corresponding threshold given by (1+T(i)/T(e)) L(Ti)/R = 0.2/0.3. It seems that most experimental ion temperature profiles do not exceed this threshold. The latter result corresponds to the zeroth order of the ballooning formalisation, i.e. an infinite series of identical Fourier modes localised near their corresponding resonant surfaces. A study of the next order allows the calculation of an envelope and it is shown that its radial scale is a geometric mean of a gradient scale (the smallest of density, temperature and plasma velocity gradient scales) and a thermal banana width. A simple mixing length estimate of the ion thermal diffusivity gives therefore a large value of the ion heat flux, once the L(Ti)/R threshold is exceeded. However, a quasilinear study shows that such a turbulence can induce a differential rotation of the plasma, which is then able to stabilise the largest scale instabilities. The resulting turbulence should, therefore, exhibit smaller scales than otherwise predicted, although the heat ion transport remains large.
Bibliographic Reference: Paper presented: Sherwood International Fusion Theory Conference, Seattle (US), April 22-24, 1991
Availability: Available from (1) as Paper EN 36053 ORA
Record Number: 199110859 / Last updated on: 1994-12-02
Original language: en
Available languages: en