Light impurity transport at an internal transport barrier in Alcator C-Mod

Abstract

Density profiles for a light impurity, boron, are reported for internal transport barrier (ITB) discharges in Alcator C-Mod. During the ITB, the light impurity gradient steepens because the impurity pinch increases relative to diffusion. The ITB-induced impurity profile steepening is at approximately the same major radius as that for the main-ion profile. Neoclassical transport does not describe the light impurity profiles but transport is closer to neoclassical in the ITB region. In previous work on C-Mod, profiles of seeded heavy impurities (introduced by puffing) peaked during the ITB, but a marked difference between transport of heavy and light impurities has been reported for other tokamaks. With the addition of light impurity profiles described here, the ITB on C-Mod is shown to share additional profile traits with the ITB on other tokamaks. This confirms that the macroscopic features of the C-Mod ITB are similar to those on other devices although it leaves open the details of the onset of the ITB.