Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

Abstract

Following application of lower hybrid current drive (LHCD) power, the core toroidal rotation in Alcator C-Mod L-and H-mode plasmas is found to increment in the countercurrent direction, in conjunction with a decrease in the plasma internal inductance, l i . Along with the drops in l i and the core rotation velocity, there is peaking of the electron and impurity density profiles, as well as the ion and electron temperature profiles. The mechanism generating the counter-current rotation is unknown, but it is consistent with an inward shift of energetic electron orbits, giving rise to a negative core radial electric field. The peaking in the density, toroidal rotation (in the counter-current direction) and temperature profiles occurs over a time scale similar to the current relaxation time but slow compared to the energy and momentum confinement times. Most of these discharges exhibit sawtooth oscillations throughout, with the inversion radius shifting inward during the LHCD and profile evolution. The magnitudes of the changes in the internal inductance and the central rotation velocity are strongly correlated and found to increase with increasing LHCD power and decreasing electron density. The maximum effect is found with a waveguide phasing of 60 • (a launched parallel index of refraction n ∼1.5), with a significantly smaller magnitude at 120 • (n ∼3.1), and with no effect for negative or heating (180 • ) phasing. These results resemble the current drive efficiency which scales as P LH /n e n 2 . Regardless of the plasma parameters and launched n of the waves, there is a strong correlation between the rotation velocity and l i changes, possibly providing a clue for the underlying mechanism.

On the toroidal plasma rotations induced by lower hybrid waves

“…As displayed in Fig.2 (a) and (c), the rotation speed reaches its peak value at plasma core, this radial profile is consistent with experimental results [5,32]. The time scale of rotation is also important.…”

Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod

“…As shown previously, 14 central rotation becomes more counter-current following the application of LH into H-mode, just as is the case in L-mode plasmas. Figure 5 shows a change in rotation ΔV of approximately -35km/s that lags the initial application of LH by more than 100ms.…”

20 years of research on the Alcator C-Mod tokamak

“…225,226 Rotation "reversals" have also been observed in discharges with the LH current drive (LHCD). For target plasmas with high plasma current, the intrinsic rotation experiences a change in the counter-current direction, 227,228 while for low plasma current targets, the rotation increment is in the co-current direction. 229,230 This reversal of the change in rotation with LHCD has been traced to the current density, through the q profile.…”