Electron-Bernstein-Wave Current Drive in an Overdense Plasma at the Wendelstein 7-AS Stellarator

Electron-Bernstein-wave (EBW) current drive in an overdense plasma was demonstrated at the Wendelstein 7-AS stellarator for the first time. The EBWs were generated by O-X-B mode conversion. The relatively high current drive efficiency was consistent with theoretical predictions. The experiments provided first investigations of EBW phase space interaction for wave refractive indices much larger than unity.

Figure 1

Variation of the on-axis magnetic field versus the toroidal angle for different ratios of coil currents. The ECRH antenna was located at $27°$ toroidal angle.

Figure 2

Ray trajectories (black lines) in the equatorial plane for different mirror ratios $I_s/I_m$. $\Phi$ is the toroidal angle; $x$ and $y$ are the spatial coordinates. The dashed line represents the plasma center; the grey lines are the cuts through the flux surfaces. The thick grey parts of the ray traces indicate the region where 99% of the power is lost by cyclotron absorption.

Figure 3

Evolution of $N_{\parallel }$ for different ray trajectories according to Fig. 2.

Figure 4

Change of loop voltage during co- and counter-EBCD (0.4–0.65 s). Since the magnetic configuration was different for the two cases, the signals refer to different scales.

Figure 5

Calculated loop voltages (grey) for EBW heating only (top trace) and with additional countercurrent drive (lower trace) in comparison with the measured loop voltage (black curve). The consistency with the measured loop voltage yields a dimensionless CD efficiency $\zeta =0.43$. The resulting EBCD current (bottom trace) is then a function of $T_e$, $n_e$, and $P_{\mathrm{H}\mathrm{F}}$. The latter was modulated with 20% amplitude.

Figure 6

Top: power signals for NBI and ECRH. Bottom: plasma current during EBCD.

Figure 7

Contours of the electron distribution function in a logarithmic scale as a function of the parallel velocity $V_{\mathrm{p}\mathrm{a}\mathrm{r}}$ and of the perpendicular velocity $V_{\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{p}}$. Both are normalized to the thermal velocity. The grey structure is the quasilinear diffusion operator for EBWs with $N_{\parallel }=1.0$. The calculation was limited to $|V|<8V_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}}$.