Observations of impurity toroidal rotation suppression with ITB formation in ICRF and ohmic H mode Alcator C-Mod plasmas

Abstract

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ⩽ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.

Dimensionless size scaling of intrinsic rotation in DIII-D

“…At C-Mod, Rice and colleagues did some of the first experiments on intrinsic rotation in diverted tokamaks [21][22][23]. These experimental measurements were made in the interior in minor radius and an on-axis toroidal velocity was usually reported.…”

“…The measurements of V ϕ were made using X-ray spectra from trace injection of argon, and their experiments over time have shown this to be a good indicator of the bulk ion core velocity in C-Mod [24]. A machine parameter scaling for V ϕ was found where the magnitude of the positive core toroidal velocity increased linearly with W/I p , where W is the total plasma stored energy and I p is the magnitude of the plasma current [21][22][23]. This is referred as "Rice scaling" for intrinsic rotation.…”

Control of internal transport barriers on Alcator C-Mod

“…10,11,12,13 While several different criteria are often used to define an ITB, the most general definition maintains that an ITB exists in the plasma when there is a clear change in the plasma transport at an internal boundary in some quantity such as energy, particles, or momentum 14 . Steady state ITBs (lasting at least 10 energy confinement times) arise during EDA H-mode plasmas when either the H-mode is produced in an Ohmic plasma or when ICRF heating is applied well off axis, typically with the resonance location being at or greater than r/a=0.5 on either the low or high field side of the plasma.…”

“…In addition, a narrow region of decreased electron heat transport as determined from analysis of the heat pulse following a sawtooth crash is observed at or near the location of the break in the density profile 12 . ITBs in Alcator C-Mod are often accompanied by a reduction and sometimes reversal of the central rotation velocity determined by Doppler shifts of the argon impurity line radiation 10,13,15 . It should be noted that most of the discharges discussed here are sawtoothing throughout the ITB presence indicating that the q profile increases monotonically and that the magnetic shear is positive in the ITB region.…”

Overview of recent Alcator C-Mod research

“…There is a clear energy transport barrier, located just outside the half-radius for this 0.21m minor radius plasma. A simple model for the heat-pulse χ profile, with a nearly singular dip at the While these barriers are most reliably triggered using off-axis ICRF heating, they also occur in ohmic EDA-H-modes [5]. In the ohmic cases, the time scale to develop the ITB is longer (typically ~10 energy confinement times), but the subsequent dynamics are very similar to those of the ICRF-triggered barrier discharges.…”