Carsten Lechte

Study of edge turbulence in dimensionally similar laboratory plasmas

Comparative studies between a toroidal low-temperature plasma and drift-Alfven-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 betw...

ρs scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusiv...

Comparison of Langmuir and emissive probes as diagnostics for turbulence studies in the low-temperature plasma of the torsatron TJ-K

For the investigation of turbulent transport in magnetized plasmas, the cross-phase between density and plasma potential fluctuations is a key parameter. In the majority of the experimental studies, the floating potential of a conventional Langmuir probe is used as an estimate for the plasma potential. It is well known, however, that plasma and floating potential fluctuations can have different phases if temperature fluctuations are present in the plasma. A diagnostic giving a direct estimate of the plasma potential fluctuations is the emissive probe. In this work, potential measurements from both emissive and Langmuir probes are directly compared. The experiments are carried out in the magnetically confined low-temperature plasma of the TJ-K torsatron. Potential, electric field and density fluctuations are measured with a three-tip probe array in the entire plasma volume. Profiles of mean values and fluctuations of the parameters are compared. The deviation of the mean plasma potential from the floating potential is larger than expected from simple probe theory. The measurements of fluctuations, however, which are relevant for turbulence studies, do not show a significant difference. This justifies the use of Langmuir probes for turbulence studies at least in toroidal low-temperature plasmas.

Investigation of intermittency in simulated and experimental turbulence data by wavelet analysis

Turbulent transport in magnetized plasmas has an intermittent nature. Peaked probability density functions and a 1/frequency decay of the power spectra have been interpreted as signs of self-organized criticality generated, similar to a sand pile, by the critical gradients of ion- (ITG) or electron-temperature-gradient (ETG) driven instabilities. In order to investigate the degree of intermittency in toroidally confined plasmas in the absence of critical pressure or temperature gradients, data from the drift-Alfven-wave turbulence code DALF3 [B. Scott, Plasma Phys. Controlled Fusion 39, 1635 (1997)], running with a fixed background pressure gradient, and from a weakly driven low-temperature plasma are analyzed. The intermittency is studied on different temporal scales, which are separated by a wavelet transform. Simulated and experimental data reproduce the results on intermittent transport found in fusion plasmas. It can therefore be expected that in fusion plasmas, too, a substantial fraction of the bur...

Plasma parameter limits of magnetically confined low temperature plasmas from a combined particle and power balance

A global particle and power balance calculation is carried out to assess the accessible plasma parameter range in a toroidal confined low temperature plasma. Power and particle sources and sinks are calculated from the relevant atomic processes. The balance is fulfilled for hyperbolic-like curves in the temperature–density plane. They constitute an upper limit accessible for plasmas at a given heating power, neutral density, and sort of gas. The calculations are compared with results from the TJ-K torsatron [N. Krause, C. Lechte, U. Stroth, S. Niedner, J. Stober, R. Westphal, E. Ascasibar, and J. Alonso, “The Torsatron TJ-K, a toroidal plasma experiment for low-temperature plasma research,” Rev. Sci. Instrum. (in press).