Observation of Ion-Cyclotron-Frequency Mode-Conversion Flow Drive in Tokamak Plasmas

Strong toroidal flow ($V_{\varphi }$) and poloidal flow ($V_{\theta }$) have been observed in D-${}^3\mathrm{He}$ plasmas with ion cyclotron range of frequencies (ICRF) mode-conversion (MC) heating on the Alcator C-Mod tokamak. The toroidal flow scales with the rf power $P_{\mathrm{rf}}$ (up to $30\mathrm{km}/\mathrm{s}$ per MW), and is significantly larger than that in ICRF minority heated plasmas at the same rf power or stored energy. The central $V_{\varphi }$ responds to $P_{\mathrm{rf}}$ faster than the outer regions, and the $V_{\varphi }(r)$ profile is broadly peaked for $r/a\le 0.5$. Localized ($0.3\le r/a\le 0.5$) $V_{\theta }$ appears when $P_{\mathrm{rf}}\ge 1.5\mathrm{MW}$ and increases with power (up to $0.7\mathrm{km}/\mathrm{s}$ per MW). The experimental evidence together with numerical wave modeling suggests a local flow drive source due to the interaction between the MC ion cyclotron wave and ${}^3\mathrm{He}$ ions.

Figure 1

(a) Change of plasma central rotation $V_{\varphi }(r=0)$ after the application of rf power at 0.75 sec in an MC plasma (red solid lines) and an MH plasma (blue dashed lines). Plasma stored energy $W_p$, $T_e$, $⟨n_e⟩$, and $P_{\mathrm{rf}}$ are shown in (b),(c), and (d).

Figure 2

$V_{\varphi }$ traces at different $r/a$. The traces have been shifted to have $⟨V_{\varphi }⟩\simeq 0$ for $t<0.8\sec$.

Figure 3

(a) Chord integrated $V_{\varphi }$ profile (systematic error are about $\pm 10\mathrm{km}/\mathrm{s}$), and (b) $T_i$ (${\mathrm{Ar}}^{16+}$) profile of the MC and MH plasmas in Fig. 1 at two time points.

Figure 4

Poloidal rotation velocity in (a) MC plasma and (b) MH plasma. $V_{\theta }$ before the rf application is subtracted. Uncertainties are approximately $\pm 0.5\mathrm{km}/\mathrm{s}$.

Figure 5

(a) PCI turbulence power spectra in frequency and time, (b) Poloidal rotation vs time, and (c) $P_{\mathrm{rf}}$ vs time.

Figure 6

(a) Direct electron heating power deposition profile from ECE; (b) Power deposition to electrons and ${}^3\mathrm{He}$ ions from TORIC simulation.

Figure 7

2D power deposition from TORIC simulation: (a) Power to ${}^3\mathrm{He}$, (b) Power to electrons. The MC surface and ${}^3\mathrm{He}$ IC resonance are also labeled.