Control of internal transport barriers on Alcator C-Mod

Abstract

Recent studies of internal transport and double transport barrier regimes in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas 1, 1511] have explored the limits for forming, maintaining, and controlling these plasmas. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions and the plasma has no internal particle or momentum sources. The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz). When enhanced D α high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the Hmode is sustained for ~2 energy confinement times. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. They are characterized by strong central peaking of the electron density, and reduction of the core particle and energy transport. Control of impurity influx and heating of the core plasma in the presence of the ITB have been achieved with the addition of central ICRF power in both Ohmic H-mode and ICRF induced ITBs. The radial location of the particle transport is dependent on the toroidal magnetic field but not on the location of the ICRF resonance. A narrow region of decreased electron thermal transport, as determined by sawtooth heat pulse analysis, is found in these plasmas as well. Transport analysis indicates that reduction of the particle diffusivity in the barrier region allows the neoclassical pinch to drive the density and impurity accumulation in the plasma center. Examination of the gyrokinetic stability at the trigger time for the ITB suggests that the density and temperature profiles are inherently stable to ion temperature gradient (ITG) and trapped electron (TEM) modes in the core inside of the ITB location.

Studies of turbulence and transport in Alcator C-Mod H-mode plasmas with phase contrast imaging and comparisons with GYRO

“…17 This added on-axis ICRF heating arrests the density rise, as shown in Fig. 1(c), where the line-integrated density ceases to increase after the on-axis heating is added.…”

20 years of research on the Alcator C-Mod tokamak

“…However, by adding a small level of onaxis heating, the peaking and impurity accumulation can be controlled. 253,255,256,259,260 The mechanism seems to be through the stimulation of TEM instabilities driven by the steep density gradient. 256 The growth rate of transport due to density gradient driven turbulence in this regime increases strongly with T e , which responds to the heating.…”

“…(4) The principal tool for creating ITBs in C-Mod is off-axis ICRF heating. 202,250,251,[253][254][255] These barriers form in sawtoothing H-mode discharges (that is, with monotonic q profiles and q min < 1) without beam-driven rotation, without a core particle source and with T i ¼ T e . This regime can be made stationary by application of modest on-axis heating.…”

“…Thermal diffusivity can drop to ion-neoclassical levels inside a fully developed barrier. 255 The barrier foot location is itself a function of the safety factor, with a location at r/a ¼ 0.5 at q 95 ¼ 3 moving in to r/a ¼ 0.25 at q 95 ¼ 7. These C-Mod experiments provided the first evidence that intrinsic equilibrium toroidal rotation can generate sufficient E Â B shearing to influence the formation of an internal transport barrier.…”

Benchmarking Nonlinear Turbulence Simulations on Alcator C-Mod

“…RF-heated C-Mod H-mode experiments [3,4], which exhibit an ITB, have been studied with the massively parallel code GS2 [5] towards validation of gyrokinetic microturbulence models. New, linear, gyrokinetic calculations are reported and discussed in connection with transport analysis near the ITB trigger time of shot #1001220016 (Fig.…”