Measurements of Impurity Transport Due to Drift-Wave Turbulence in a Toroidal Plasma

The first direct measurements of an impurity particle flux driven by drift-wave turbulence in a toroidal magnetized plasma are reported. The correlation between the impurity density and radial velocity fluctuations is measured using ion Doppler spectroscopy. The small, very fast radial velocity fluctuation is resolved with the aid of a new linearized spectrum correlation analysis method that rejects uncorrelated noise as the sample size increases. The measured ${\mathrm{C}}^{2+}$ turbulent impurity flux in the edge of the plasma is directed inward and is consistent with impurity density measurements. This is also the first direct evidence for fluctuation-induced transport due to trapped-electron-mode turbulence in reversed field pinch plasmas.

Figure 1

The viewing geometry for the ${\mathrm{C}}^{2+}$ transport measurements. A duo spectrometer [30] accepts two spatial channels whose optical ray paths, shown in pink and green, can be adjusted in relative orientation. For the transport measurement, the two views are radial within 5°, and the near-field foci separation is aligned to the pitch of the magnetic field.

Figure 2

(a) Spectrogram of normalized ${\mathrm{C}}^{2+}$ 464.7 nm emission intensity fluctuations. (b) Spectrogram of normalized chord-integrated electron density intensity fluctuations at impact parameter $R-R_0=43\mathrm{cm}$ ($r/a=0.86$). (c) Tangential component of normalized magnetic fluctuations at $r=a$.

Figure 3

Profiles for $n_e$, $n_{{\mathrm{C}}^{2+}}$, and ${\mathrm{C}}^{2+}$ 464.7 nm emission. Note that, the emission is strongest where the electron and ${\mathrm{C}}^{2+}$ density gradients are in opposite directions.

Figure 4

(a) and (b): spectral channel groupings. (c) and (d): cross spectra. (e) and (f): coherence. The dashed lines in (e) and (f) are a statistical significance level. ${\lambda }_j$ is the wavelength of spectral channel $j$ with respect to 464.7 nm.

Figure 5

Spectral density of the impurity particle flux (red triangles) and the circulating flow (blue circles) due to drift wave turbulence.