Study of edge turbulence in dimensionally similar laboratory plasmas

Abstract

Comparative studies between a toroidal low-temperature plasma and drift-Alfvén-wave simulations were carried out in order to investigate the microscopic structure of turbulence. The dimensionless plasma parameters in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)] are similar to those in the edge of a fusion plasma. At the same time the fluctuations can be fully diagnosed by probe arrays. Fluctuation spectra are analyzed by wavelet techniques indicating a large amount of intermittency in both numerical and experimental data. Since in both cases no critical gradient is present, the intermittency is not due to a state in self-organized criticality (SOC). The spectral density P(ω,k) of the turbulence was measured with a 64-tip Langmuir probe array. A broad spectrum indicates fully developed turbulence. The wave-number spectrum of the density fluctuations decays with a power law with an exponent of −3. The experiments confirm predictions from the turbulence code. The cross-phase between potential and density fluctuations is close to zero on all scales and the spectra shift to smaller wave numbers when the drift scale ρs is increased by changing the ion mass from hydrogen to helium and argon. The ρs scaling is confirmed by correlation measurements within the tips of the poloidal array and an 8×8 probe matrix. The results point to the drift-wave mechanism being responsible for the drive of the turbulence in the low-β plasma of TJ-K.