Measurements of ion temperature fluctuations in the Tokamak Fusion Test Reactor

Abstract

First of a kind measurements of high-frequency ion temperature microturbulence in fusion-grade plasmas have been made in TFTR. The ion temperature fluctuations and carbon density fluctuations were found to have spectra similar to those of the ion density fluctuations across the plasma radius. The ratio of the relative fluctuation levels, (T /T)/(ñ/n), is 2 ± 0.5 from r/a = 0.59 to r/a = 0.99. The fact that this ratio is greater than unity is consistent with the general expectations of ion temperature gradient driven turbulence theory and suggests that ion drift modes dominate trapped electron modes in the turbulent spectrum. The temperature fluctuation spectra were found to exhibit a narrow transition region between distinctive edge and core turbulent modes, as has been seen with ion density fluctuations. The ratio of the relative fluctuation levels is greater than unity across this transition, which suggests that, despite the different modes present, the underlying instability is driven by the ion temperature gradient.

Direct multiscale coupling of a transport code to gyrokinetic turbulence codes

“…The assumptions employed in ordering the terms are as follows: (1) The fluctuations are assumed to be low amplitude compared to macroscopic quantities, such that δf ∼ f . This is in good agreement with core measurements from a number of modern fusion experiments, which find density and temperature fluctuations 1% of the macroscopic densities and temperatures; 29,30,31 (2) The microturbulence is assumed to be spatially anisotropic with macro-scale variations along and micro-scale (i.e. Larmor radius) variations across the equilibrium magnetic field.…”

Linewidth-modulated motional Stark effect measurements of internal field structure in low-field configurations

“…An alternative approach, which we consider here, is to use linewidth modulation spectroscopy and borrow techniques from turbulence measurements. [6][7][8][9] The basic approach is to rotate a linear polarizer at frequency ⍀ to repeatedly vary the observed spectrum from the to components, and use a high-speed, high-throughput multichannel spectrometer to measure the oscillations in the resultant linewidth. This linewidth will modulate at a fundamental frequency of 2 ⍀ and the frequency characteristics of this linewidth are readily measured as the frequency characteristics of the second moment of the spectral distribution…”

“…The actual statistical noise floor is raised by the finite spectrometer resolution which reduces the modulation depth of the linewidth. 8 The photon-noise level in the actual Stark manifold variance is given by the product of the measured linewidth and the logarithmic derivative of the real and measured linewidths…”

“…8 It plays the same role as a reduction in the polarization fraction does in conventional MSE measurements in that it reduces the contrast of the desired measurement. Measurement of the linewidth modulation has the advantage that it is not susceptible to variations in the beam energy since the measurement analysis automatically compensates for changes in the lower order moments of the linewidth ͑i.e., the total intensity and line center or E B ͒.…”

“…8 It consists of an f /4 Echelle grating spectrometer with a 1/2 m focal length. With a resolution of ϳ0.4 Å ͑0.1 mm slits͒ and 2 cm high slits, the optical throughput is ϳ0.001 cm 2 sr.…”