The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak is to investigate current profile control under plasma conditions relevant to future devices such as ITER and DEMO. Experimental observations of an LHCD "density limit" for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 10 20 m −3 (ω/ω LH ∼3-4), well below the density limit previously observed on other experiments (ω/ω LH ∼ 2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit, and a model including collisional absorption in the SOL, which dramatically improves agreement with experiment above the observed density limit. These results show that strong absorption of LH waves in the SOL is possible on a high density tokamak and the SOL must be included in simulations of LHCD at high density.

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 ...

Recent experiments on EAST have achieved the first long pulse H-mode (61 s) with zero loop voltage and an ITER-like tungsten divertor, and have demonstrated access to broad plasma current profiles by increasing the density in fully-noninductive lower hybrid current-driven discharges. These long pulse discharges reach wall thermal and particle balance, exhibit stationary good confinement (H 98y2 ~ 1.1) with low core electron transport, and are only possible with optimal active cooling of the tungsten armors. In separate experiments, the electron density was systematically varied in order to study its effect on the deposition profile of the external lower hybrid current drive (LHCD), while keeping the plasma in fully-noninductive conditions and with divertor strike points on the tungsten divertor. A broadening of the current profile is found, as indicated by lower values of the internal inductance at higher density. A broad current profile is attractive because, among other reasons, it enables internal transport barriers at large minor radius, leading to improved confinement as shown in companion DIII-D experiments. These experiments strengthen the physics basis for achieving high performance, steady state discharges in future burning plasmas.