Alcator C-Mod

Kenneth Marr

The Role of Momentum Transport Near the Plasma Edge

Updated on July 7, 2006

We are studying momentum transport near the plasma edge on Alcator C-Mod. Momentum transport has been studied primarily on devices where torques are exerted on the plasma through neutral beam injection (used primarily for heating). Alcator C-Mod, with ICRF heating, has the advantage of being a device with no external torque thus simplifying momentum transport study. Specifically, we will probe momentum transport during transitions from L- to H-mode in order to identify the momentum source that drives the spontaneous rotation development at these transitions. Another method that can be used for studying the source and transport of momentum is to follow the changes in the velocity profile as momentum propagates inward during a rapid change in magnetic topology (e.g. from lower-single-null to upper-single-null). Additionally, the study of the velocity profile will lead to a better understanding of the effect of flow shear upon various plasma phenomena including turbulence, transport (energy and particle), and the H-mode power threshold dependence on magnetic topologies.

We will study the momentum profile, and therefore the transport of momentum, via velocity measurements using active charge-exchange recombination spectroscopy (CXRS). The emission line of interest is the n = 7–6 line (494.4nm) from boron charge-exchange with neutral deuterium. This line is desirable because it emits in the visible spectrum and boron is the most prevalent impurity in the plasma, thus making it the brightest of all CXRS lines other than hydrogen/deuterium (which cannot be used due to the strong background signal). There are two methods for injecting neutrals which is the pre-requisite for active CXRS. The first is the gas puff imaging (GPI) system. The second is the Diagnostic Neutral Beam (DNB).

This project would also work closely with the measurements being made by NeSox, HIREX Jr., and the scanning probes to gain a full velocity profile. Using this combined profile we will explore the velocity shear from the SOL to the pedestal region to the core for both the high field and low field sides of the plasma.

In addition to measurements of the momentum profile and its evolution as well as exploring of the effects of velocity shear, this diagnostic provides a measure of the temperature of the emitting boron ion. If we assume that the fully-stripped boron has equilibrated with the background plasma ions before charge-exchanging (an assumption which appears to break down at least several millimeters outside the separatrix where the lifetime of fully stripped boron atoms is very short and they cannot be considered in equilibrium with a cooler background) we can obtain an ion temperature profile in the pedestal region. By helping build the complete ion temperature profile we can play a part in understanding the processes wherein energy is transported down the ion temperature gradient and exchanged between the ions and electrons.