Nonlinear gyrokinetic simulations of trapped electron mode (TEM) turbulence, within an internal particle transport barrier, are performed and compared with experimental data. The results provide a mechanism for transport barrier control with on-axis radio frequency heating, as demonstrated in Alcator C-Mod experiments [S. J. Wukitch et al., Phys. Plasmas 9(5) 2149 (2002)]. Off-axis heating produces an internal particle and energy transport barrier after the transition to enhanced Dα high confinement mode. The barrier foot reaches the half-radius, with a peak density 2.5 times the edge density. While the density profile peaks, the temperature profile remains relatively unaffected. The peaking and concomitant impurity accumulation are controlled by applying modest central heating power late in the discharge. Gyrokinetic turbulence simulations of the barrier formation phase, using the GS2 code [W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)] show that toroidal ion temperature gradient driven modes are suppressed inside the barrier foot, but continue to dominate in the outer half-radius. As the density gradient steepens further, trapped electron modes are driven unstable. The onset of TEM turbulence produces an outflow that strongly increases with the density gradient, upon exceeding a new nonlinear critical density gradient, which significantly exceeds the linear critical density gradient. The TEM turbulent outflow ultimately balances the inward Ware pinch, leading to steady state. Moreover, the simulated turbulent particle diffusivity matches that inferred from particle balance using measured density profile data and the calculated Ware pinch. This turbulent diffusivity exhibits a strong unfavorable temperature dependence that allows control with central heating.

The time evolution of toroidal rotation velocity profiles has been measured 2

The Thomson scattering (TS) diagnostics set [Rev. Sci. Instrum. 70, 759 (1999); Rev. Sci. Instrum. 72, 1107 (2001)] on the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] has been upgraded to deliver measurements of electron temperature (T e ) and density (n e ) with improved temporal and spatial resolution. Two 30 Hz, 1.3 J Nd-doped yttrium aluminum garnet (Nd:YAG) lasers share a vertical beam path through the tokamak plasma and are fired in alternating fashion, allowing collection of scattered light with a 60 Hz rep rate. Also, plasma variation occurring on faster time scales (10 −5 < ∼ τ (s) < ∼ 10 −2 ) can be studied by closely spacing the pulses from either laser for operation in 30 Hz burst mode. Eight compact polychromators [Rev. Sci. Instrum. 61, 2858 (1990)] have been added to the system, better than doubling the spatial resolution of core plasma TS measurements and allowing better diagnosis of core n e and T e gradients. Up to 14 fiber bundles view the core plasma, and a linear array of up to 22 single fibers of 1 mm diameter observes the plasma edge. Each fiber holder has been upgraded to incorporate a cam key and follower, so that the holders can be mounted to a cammed plate that automatically orients each holder to the correct viewing angle as its height is adjusted. This allows the fiber bundles to be repositioned easily between tokamak discharges, making it possible to concentrate core measurements in regions of particular interest, such as the vicinity of an internal transport barrier, with spatial resolution of ∼ 1 cm. The newly designed mount can be positioned with sub-millimeter precision, and its alignment with respect to the Nd-YAG beam can be adjusted between discharges using remotely controlled stepper motors. This capacity for correcting misalignment increases the reliability of density measurements. Cross-calibration of TS n e measurements with electron cyclotron emission (ECE) diagnostics is performed using dynamic ramps of both plasma density and magnetic field to force ECE signal cutoffs at known values of n e . This technique, which can be performed on any given day of an experimental campaign, supplements an absolute calibration based on Raman scattering in hydrogen and deuterium gas, which normally can not be done during a campaign.