Experimental tests of parallel impurity transport theory in Tokamak plasmas

Abstract

In realistic reactor scenarios, high temIperature plasmas will be composed of not only the fusion reactants and products, but also inpurities introduced purposefully or uninitentionally from plasma facing materials. In tokamaks it is often assumed, sometimes erroneously, that surfaces of constant main ion pressure are also surfaces of constant impurity pressure. Although the same underlying physics determinie impurity miomentum balance along closed magnetic field lines, the increased mass and charge of high-Z impurities weights terms differently. Their large mass enhances inertial effects like the centrifugal force from toroidal rotation, and( can lead to accumulation of heavy impurities on the outboard side of a flux surface. Their high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the electrostatic potential. In Alcator C-Mod, 2D (R,Z) measurements of photon emission from high-Z impurities reveal significant variations of impurity density on a flux surface. Poloidal variations, normalized to the flux surface average, I 2/(n ) , have been measured up to ~ 1/3, and separate cases of impurities accumulating on the inhoard and outboard side of a flux surface are observed, depending on local plasmas conditions. Experiments demonstrate that these asymmetries are due to a combination of inertia., poloidal electric fields and ion-impurity friction, and measurements are compared to existing neoclassical parallel impurity transport theory. This is the first time centrifugal force has been observed to cause a substantial asynnnetry in a plasma with no external monmentumn input and where the flow is entirely self-generated. Magnetically trapped fast ions, sustained by ion cyclotron waves, create a poloidally varying electrostatic potential which causes high-Z impurities to accunmulate on the inboard side. Existing theory is extended to include this effect by incorporating a species that has an anisotropic pressure tensor. Experimental measurements in plasmas where the minority resonance layer is scanned show good qualitative and quantitative agreeinent with this extended theory. The sensitivity of 51/(nz) to fast-ions demstrates the opportunity for the impurity asymmetry to be used as a novel diagnpostic tool and calls into question prior work on in/out asymmetries in neutral beam heated plasinas. Measured up/down asymmetries in the banana regime are unable to be explained by ion/nipurity friction in the trace limit, nZZ 2 /n < 1. The sensitivity of such asymeinetries to imain ion poloidal rotation, which depends on impurity density when nZ 2 /n ~ 1, suggests explaining up/down asymmetries requires a more complex model which couples the parallel force balance of main ions and multiple impurity species. Additionally, the conniection between impurity poloidal flow and poloidal density variation is highlighted. Since they are the result of the same parallel transport equation, predictions for both inced to match measurements in order to validate friction-based asynmnetry theory.