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

Abstract

In Alcator C-Mod discharges lower hybrid waves have been shown to induce a countercurrent change in toroidal rotation of up to 60 km=s in the central region of the plasma (r=a $ <0:4). This modification of the toroidal rotation profile develops on a time scale comparable to the current redistribution time ($100 ms) but longer than the energy and momentum confinement times ($20 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. The beneficial effects of rotation on toroidal plasmas have been well documented. Strong rotation can help stabilize destructive magneto-hydrodynamic instabilities (i.e., resistive wall modes [1,2]) while gradients in rotation can improve confinement by suppressing turbulence [3,4]. In many experiments the rotation profiles associated with improved performance are generated through the use of neutral beam injection. This approach may prove impractical in the large, high density plasmas envisioned for next generation devices such as ITER [5,6]. As a result, there is a need to develop alternative methods for driving plasma rotation. Significant self-generated flows have been observed on a number of tokamaks [7] suggesting that it may be possible to reap the benefits of rotation without the use of neutral beams.Self-generated flows associated with lower hybrid current drive (LHCD) have been observed in both L-mode and H-mode discharges on Alcator C-Mod when lower hybrid waves are launched such as to drive positive current. These changes to the toroidal rotation profile are core localized (r=a $ <0:4) and always in the countercurrent direction. When the waves are launched against the inductive toroidal electric field, very little current is driven and no effect on the rotation profile is observed. This result indicates that it is the LHCD (as opposed to heating) that is responsible for the countercurrent change in toroidal rotation. In discharges with sufficient LHCD, a region of enhanced velocity shear forms concurrently with a negative increment in the radial electric field profile.The results presented in this Letter were obtained on Alcator C-Mod [8], a compact tokamak (major radius R ¼ 0:67 m, typical minor radius ¼ 0:21 m) that operates at the high magnetic fields (B t > 5 T) and high densities (n e $ 10 20 m À3 ) envisaged for burning plasma reactors such as ITER and DEMO [9]. In the experiments described here, the lower hybrid waves were injected by an 88 wave guide launcher capable of delivering up to 1.2 MW of power at 4.6 GHz with an n k range of 1.5-4 in either direction [10]. (Here n k is the refractive index of the injected ...

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.…”

“…During the launch of lower hybrid waves, increasing the wave-driven current while fixing the total current is accompanied by variations in current density profile, which is usually measured by normalized internal inductance. Therefore, our theory has explained the mechanism of the strong correlation between rotation speed and normalized internal inductance observed in experiments [5]. In addition, it is able to explain the recent experiments in which both counter-and co-current rotations are observed during the launch of lower hybrid waves with different initial currents.…”

“…The time history of flow in Fig. 2 (c) and (d) can be fitted by exponential functions with a characteristic time of 65 ms, which is shorter than the typical 150 ms observed in experiments [5]. The reason of this difference might be that we have ignored the time it takes to establish toroidal current in Eq.…”

“…Strong rotation can stabilize magneto-hydrodynamic (MHD) instabilities [1,2] and gradients of plasma rotation help improve confinement by reducing turbulence [3,4]. According to experiments in Alcator C-Mod, significant changes of toroidal rotation speed have been observed after the launch of lower hybird waves for driving current [5][6][7]. The changes of rotation speed are either in counter-current or co-current direction.…”

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

“…LHCD power levels between 0.4 and 1.1 MW have been injected with n in the range of 1.5-4, both co-and counter-current, including pure heating phasing. Shown in Fig.1 internal inductance (consistent with a broadening of the current density profile) and an increment of the central toroidal rotation velocity in the counter-current direction [26]. (Here a negative velocity corresponds to counter-current rotation.)…”