Time-Resolved Measurements of Suprathermal Ion Transport Induced by Intermittent Plasma Blob Filaments

Suprathermal ion turbulent transport in magnetized plasmas is generally nondiffusive, ranging from subdiffusive to superdiffusive depending on the interplay of the turbulent structures and the suprathermal ion orbits. Here, we present time-resolved measurements of the cross-field suprathermal ion transport in a toroidal magnetized turbulent plasma. Measurements in the superdiffusive regime are characterized by a higher intermittency than in the subdiffusive regime. Using conditional averaging, we show that, when the transport is superdiffusive, suprathermal ions are transported by intermittent field-elongated turbulent structures that are radially propagating.

Figure 1

Experimental setup showing the TORPEX device. The suprathermal ion source and the gridded energy analyzer are shown. The poloidal plane in red indicates the position of the reference probes used for the CAS. A helical magnetic field line is shown in violet. The field lines crossing the suprathermal ion beam between the source and the detector cross the reference probe plane twice. Examples of computed suprathermal ion trajectories are shown in red along with a snapshot of the electric potential fluctuations as simulated by the gbs code [10, 22].

Figure 2

Suprathermal ion current time traces measured with the GEA positioned at the locations indicated by the black crosses in Fig. 3. The suprathermal ion source is turned on (red) and off (blue) periodically. The time trace of 30 eV ions (a) shows a high intermittency level contrary to the 70 eV ions time trace (b). Insets show the PDFs of the suprathermal current fluctuations. The PDF for the 30 eV case is strongly positively skewed, reflecting the intermittency of the signal, whereas the PDF for the 70 eV case is not.

Figure 3

Poloidal profiles of the time-averaged suprathermal ion current density, in $\mathrm{A}/{\mathrm{m}}^2$, for ions at 30 (a) and 70 eV (b) and of the skewness (c) and (d). The profile of the skewness of the 30 eV ions (c) reveals a crown of high skewness around the peak of the time-average profile. This indicates that the broadening of the 30 eV suprathermal ion beam is due to intermittent bursts perturbing the gyromotion of the ions. This pattern is not visible on the skewness profile for 70 eV ions (d). Gray circles show the positions of measurements.

Figure 4

Conditionally averaged dynamics of the suprathermal ion current density [$\mathrm{A}/{\mathrm{m}}^2$], for the 30 eV case (left) and the 70 eV case (right) at three different times. Dashed gray contours represent time-average profiles (Fig. 3) and the green contour represents the averaged blob $I_{\text{sat}}$ profile. The center of mass positions of the beam and of the time-averaged profiles are indicated by blue and gray crosses, respectively. The effect of the blob, as it passes through the suprathermal ion beam, is much more important for 30 eV ions than for 70 eV ions.

Figure 5

Horizontal (top row) and vertical (bottom row) displacement of the suprathermal ion beam due to large blobs (${\tilde{I}}_{\text{sat}}>4\sigma$) for 30 and 70 eV suprathermal ions. On all panels, the $x$ axis represents the time relative to the detection of the blob on the reference probe and the $y$ axis represents the vertical position of the reference probe. The crosses and the circles indicate the positions of the beam center, when it reaches the detector, and of the source, respectively, projected along the helical magnetic field lines on the poloidal plane of the reference probes (see Fig. 1). The measurement error is $\pm 1\mathrm{mm}$. The vertical periodicity of the magnetic field is reflected in the pattern with a field line return distance of $\mathrm{\Delta }\simeq 18.8\mathrm{cm}$.