Strong Scattering of High Power Millimeter Waves in Tokamak Plasmas with Tearing Modes

In tokamak plasmas with a tearing mode, strong scattering of high power millimeter waves, as used for heating and noninductive current drive, is shown to occur. This new wave scattering phenomenon is shown to be related to the passage of the $O$ point of a magnetic island through the high power heating beam. The density determines the detailed phasing of the scattered radiation relative to the $O$-point passage. The scattering power depends strongly nonlinearly on the heating beam power.

Figure 1

Schematic of the experimental setup. The gyrotron and both the inline-ECE and CTS diagnostics are viewing the plasma (solid circle) from the low-field side. The gray mirror in the ECRH or inline-ECE beam line indicates the dielectric mirror separating ECE (and backscattered) radiation from the ECRH beam [11]. The gray oval at the intersection of ECRH and CTS beams indicates the CTS scattering region. Backscattered ECRH along the inline-ECE view can come from the entire ECRH beam path in the plasma up to the resonance where the ECRH is absorbed.

Figure 2

TEXTOR discharge 108 103. The CTS toroidal viewing angle is scanned during a 400 kW ECRH pulse. Overlap around $R=2\mathrm{m}$ between ECRH (injected with a toroidal angle of 4° with respect to the inward major radial direction) and CTS occurs near $t=3\mathrm{s}$. From top to bottom the frames show the evolution of the plasma current (with ECRH power in an inset), central line averaged electron density, a standard ECE measurement at 141 GHz originating from close to the $q=2$ surface on the high-field side (the insert indicates DED currents), and a CTS signal at 138.58 GHz. The oscillations in the 141 GHz ECE signal reveal the $m=2$, $n=1$ magnetic island triggered by the DED.

Figure 3

CTS data from discharge 108 103 during beam overlap. The top frame shows the currents in the two sets of DED coils (red for coils 1–4 and blue for coils 5–8). Vertical lines indicate the times that the $O$ point of the island crosses the ECRH beam on the low-field side.

Figure 4

The CTS spectrum in the presence of a rotating $m=2$, $n=1$ mode in discharge 107 128. Estimated times of the passage of the island $O$ point through the ECRH beam on the low-field side are indicated by vertical lines.

Figure 5

Inline spectrum of scattered radiation during a density scan in discharge 108 115. A 38 ms period between scattering pulses is explained by aliasing between the 1 kHz acquisition rate of the inline spectrometer and the 974 Hz rotation frequency of the magnetic island which is locked to the DED.

Figure 6

Inline spectrum of scattered radiation from $t=2.8$ to 3.4 s in discharge 108 115. The bottom frame shows the currents in the DED coil sets (red for coils 1–4 and blue for coils 5–8) subsampled at the times of the recording of the inline spectra. Vertical lines indicate the passage of the $O$ point through the ECRH beam on the low-field side.

Figure 7

Evolution of CTS scattering intensity for different ECRH powers. From top to bottom: 200 (discharge 108 111), 400 (108 114), and 600 kW (108 115). The data are shown using a 10 ms sliding time average. CTS channels with frequencies 138.18, 138.34, 138.50, and 138.66 GHz are shown. Identical density scans from 1.5 to $3.5\times {10}^{19}{\mathrm{m}}^{-3}$ are performed during the 2 s ECRH pulse (see also Fig. 5).