Observation of Self-Generated Flows in Tokamak Plasmas with Lower-Hybrid-Driven Current

In Alcator C-Mod discharges lower hybrid waves have been shown to induce a countercurrent change in toroidal rotation of up to $60\mathrm{km}/\mathrm{s}$ in the central region of the plasma ($r/a\sim <0.4$). This modification of the toroidal rotation profile develops on a time scale comparable to the current redistribution time ($\sim 100\mathrm{ms}$) but longer than the energy and momentum confinement times ($\sim 20\mathrm{ms}$). A comparison of the co- and countercurrent injected waves indicates that current drive (as opposed to heating) is responsible for the rotation profile modifications. Furthermore, the changes in central rotation velocity induced by lower hybrid current drive (LHCD) are well correlated with changes in normalized internal inductance. The application of LHCD has been shown to generate sheared rotation profiles and a negative increment in the radial electric field profile consistent with a fast electron pinch.

Figure 1

Time histories of (a) lower hybrid power, (b) loop voltage, (c) line averaged density, (d) central argon toroidal rotation velocity and (e) the normalized internal inductance ${\ell }_i$ for discharges with LHCD (solid lines) and without LHCD (dashed lines). Both discharges had the same magnetic field (5.4 T) and plasma current (800 kA).

Figure 2

Trajectories in the $-\Delta V_{\varphi }$ vs $-\Delta {\ell }_i$ plane for discharges with LHCD at varying $n_{\parallel }$. Each diamond represents the average value over a 20 ms time period. The LH power in all four discharges was $\sim 400\mathrm{kW}$.

Figure 3

LH induced changes in central toroidal rotation and normalized internal inductance for a wide variety of plasmas (Each point represents a single discharge). The trajectories from Fig. 2 (dashed lines) are included for comparison.

Figure 4

Profiles of (a) electron density, (b) impurity toroidal velocity, (c) electron and impurity temperature and (d) inferred radial electric field before and during an the application of 600 kW of LHCD ($n_{\parallel }=2.3$).