Study of microturbulence in the TEXTOR Tokamak using CO₂laser scattering

Abstract

CO, laser collective scattering has been used on TEXTOR to measure electron density fluctuation amplitude ff2 ( , f , k ) in the parameter range 50 kHz < .f < 3.5 MHz and 3 cm-' c k < 130 cm-' using both forward and far-forward scattering techniques. Measurements are presented for ohmically heated plasmas and for discharges with ICRF auxiliary heating. Frequency spectra have maxima in the range 100 kHz < , / , , , < 400 kHz and decay asf-O with 1.5 < < 3.5 depending on plasma conditions. The frequency of maximum fluctuation amplitude and the spectral width in frequency at fixed k scales linearly with electron temperature. Wave number spectra decaying as k-" are observed, with CI evolving during the plasma discharge and being approximately equal to 4 during the steady-state phase of the discharge. The wave number component of maximum amplitude obtained from far forward scattering measurements lies in the interval 7 cm-l < k,,, < 10 cm-'. A density scaling experiment shows little dependence of thc normalized turbulence level associated with one observation wave number on the gross energy confinement time, but indicates a strong effect of the electron temperature on (A),/n, for a fixed value of T ~. Strong correlations between sawtooth oscillations and turbulence levels were measured using small-k scattering. Also, changes in the fluctuation level during ICRF heating were correlated to changes in the electron density profile, giving (A'),, -iL,j-5, where L, is the density scale length for 0.45 < r n < 0.65. No reproducible variation of fluctuation level with ICRF heating power could be established.

Transport through dissipative trapped electron mode and toroidal ion temperature gradient mode in TEXTOR

“…(3) the conditions that both Y T1 , and Y DTE (Y = growth rate) must be small i are consequently probably the restrictions which determine the profiles corresponding to each experimental condition and, to a large extent, profile consistency ; (4) we suggest finally that the deviation from neo-Alcator scaling, the density limit, and the phenomenon of plasma detachment are interrelated effects which arise at high densities when the constraint on the electron heat flux becomes crucial . 1 .…”

“…In section 2 we recall our theories of nonlinear saturation and turbulent transport for the DTE instability ; we describe our numerical code and present the results obtained for a density scan of ohmic discharges and a power scan (ICRH) at constant density, both at qa ti 2 . 4 .…”

“…A compensation result . 4 . Thus, in a broad sense, the DTE mode can also be said to be marginally unstable .…”

“…fig . 10) are compared to the calculated values on figure4 . A striking observation is that the code reproduces extremely well the neo-Alcator scaling (tiE "' N e ) ; that this would be the case was far from obvious, considering the complexity of eq .…”

CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements

“…[5][6][7][8][9][10][11] Since the frequency of 10.6 m radiation is much greater than the plasma frequency in fusion machines, absorption and refraction of the beam are negligible which makes the CO 2 laser a valuable tool for probing the entire plasma volume. Traditional collective scattering using the CO 2 laser provides measurements of wave numbers in a range of order 1-100 cm Ϫ1 .…”