3D structure of density fluctuations in the T-10 tokamak and new approach for current profile estimation

Abstract

Previous investigations of small-scale density fluctuation by means of correlation reflectometry in T-10 tokamak revealed the existence of several density fluctuation types and strong radial and poloidal variation of their amplitudes and correlation properties. This paper is focused on the new measurements of the 3D spatial distributions of the amplitudes, the radial correlation lengths and the long range correlations along the field lines for the different turbulence types. The properties of the density fluctuations were systematically studied with the improved reflectometers, data analyzing and acquisition hardware. The density fluctuations were measured by heterodyne correlation reflectometry using ordinary mode. New T-10 antenna set have horn antennae arrays at four places distributed toroidally and poloidally over tokamak torus. The experiments confirmed previously found strong poloidal amplitude asymmetry of the broad band and the quasi-coherent oscillations and the uniform poloidal distribution of stochastic low frequency fluctuations. The presence of those turbulence types was also proved by the measurements of perturbation properties using heavy ion beam probe diagnostic. The radial correlation measurements were performed at four poloidal angles to understand the poloidal dependence of the radial correlation length for the different fluctuation types. The significant decrease of the radial correlation lengths towards the high magnetic field side was observed for quasi-coherent and stochastic low frequency turbulence types. The long range correlations along the field lines were measured by the reflectometers in two cross-section separated by 1/4 of the torus. The reflectometers have the same frequency thus provide reflection from the same magnetic surface. Reflection radii are chosen by the frequency variation of the launched wave from shot to shot in a series of reproducible discharges. The measurements were carried out at the low and the high magnetic field side with two currents and simultaneous reverse of the direction of the toroidal magnetic field and the plasma current. Resonance radii were also calculated using 3D tracing of the magnetic field line and demonstrate good agreement with experiments. These results allow to propose the new approach for the current profile measurements in tokamaks.

New Approach to Cross-Correlation Reflectometry Diagnostics of Nonlocality of Plasma Turbulence

“…Taking into account also the assumption about the perturbativity of the finite free path effect used in deriving Equation (45), the integral in Equation (59) can be calculated:…”

“…Subsequently, the same results were obtained on other tokamaks using the same diagnostics-reflectometry (TEXTOR, ToreSupra, KSTAR, HL-2A, and J-TEXT tokamaks). In addition, the spectrum with quasi-coherent components was also observed with the help of another diagnosticheavy ion beam injection (see, e.g., review [44]) on the same T-10 tokamak (the results of these diagnostics are compared in [45]). If the observed symmetric peaks (relative to the frequency of diagnostic probing radiation) in the scattering spectrum on density fluctuations are the result of the presence of a distinguished velocity of their motion relative to the plasma, then we have an analogue of the fine structure of Mandelstam-Brillouin scattering.…”

“…Figure 5. Spectrum of radiation scattered by traveling and almost standing density fluctuations: experimental spectrum (Figure 3a in [50]) (black curve) and the result of the best fitting of this spectrum (orange curve), calculated by Equations (49) and(45).…”

Wigner-function-based solution schemes for electromagnetic wave beams in fluctuating media

“…This can be explained by the small two-point correlation length in the latter case which invalidates the diffusive approximation, see also discussion in Sect. 3.…”

“…In a turbulent fusion plasma, the correlation time C of fluctuations is typically in the order of tens of microseconds [3], whereas the beam is turned on for a time of hundreds of milliseconds or larger [4], and both time scales are orders of magnitude larger than the time of propagation of the beam (estimated by the typical size of the machine divided by speed of light). Heuristically, such time-scale separation can be invoked in order to justify a statistical approach to the description of the effects of random fluctuations on the wave field: Instead of physical time-dependent fluctuations of plasma parameters f(t, x), we consider time-independent random fields f ( , x) where varies in an abstract probability space with probability density ℙ .…”