Understanding impurity transport in the scrape-off layer (SOL) of tokamak plasmas is a necessary piece of developing the physics basis for designing next-generation reactors. A system for inferring impurity transport parallel and perpendicular to local magnetic field lines has been developed on Alcator C-Mod using gas-injection "plumes". In this system, impurity gas is injected at a fixed position in the SOL via a reciprocating fast-scanning probe, and the resulting emission is imaged. In this paper visible light emission patterns from C+1 and C+2 ions are presented.

The Plasma Theory and Simulation Group (PTSG) is collaborating with LLNL in order to model the edge region of a tokamak plasma and its interaction with the diverter plate. In the overall framework of the project, MHD will be used to model the bulk plasma. Near the edge, the MHD model will interface with the gyrokinetic code UEDGE developed at LLNL. Since the UEDGE model approximations may not be accurate within a few cyclotron radii of the diverter plate, the UEDGE code will interface with a collisional PIC-hybrid code developed by the PTSG under this project. The PTSG PIC code will include a self-consistent potential with kinetic or fixed hydrogen ions. The sputtering profile of the plate, under development at LLNL, will be used as input to the PIC code in order to correctly model the kinetic behavior of sputtered carbon. These carbon products will interact with hydrogen according to known chemistry cross-sections. While some kinetic electrons may be used to model the fast tail of the distribution function (if necessary), the bulk of the electron population will be modeled as being in thermal equilibrium using the Boltzmann relation, resulting in a significant improvement in code speed. Coulomb collisions may also be considered. The Boltzmann model has been implemented with various features in three of the PTSG codes: XPDP1 and OOPD1 (both 1d-3v), and OOPIC (2d-3v), according to the methodology of Cartwright [1]. When the model is fully implemented, it will include fluid interaction with the boundaries, energy conservation through the temperature term, and take into account collisions with the Boltzmann species. A more rigorous convergence analysis has been developed than is outlined in [1]; boundary effects are included explicitly in a formulation valid in arbitrary coordinate systems. In OOPD1, the Boltzmann model is included in an object-oriented manner as part of a general fluid model framework. The basic Boltzmann solver has been implemented and shown to give self-consistent results. The details and results were described in detail in a talk presented at LLNL (updated slides attached). Currently, the output of the three codes is being compared for a test case of a current-driven DC discharge. Computational speed-up and accuracy will be compared between PIC and the Boltzmann-PIC hybrid. A framework for general binary and three-body collisions is being developed for OOPD1. Given known cross-sections or reaction rates, this will function as a chemistry model for the code. The framework may then be imported into OOPIC.

Impurity transport is a subject of fundamental importance in plasma physics in general and in tokamak physics in particular. The behaviour of the various impurity species and the evolution of their concentration determines, among other things, the fuel dilution and the fusion reaction rate, the plasma radiation pattern and the local energy balance, the plasma effective charge and resistivity and the neutral beam particle and power deposition profile. It is therefore important to develop both a sound experimental base and reliable models to interpret the experimental results and to predict the transport properties of impurities. Time-dependent helium and methane gas puff experiments have been performed on the Mega Ampere Spherical Tokamak (MAST) during a two point plasma current, Ip, scan in L-mode and a confinement scan at constant Ip. For the Ip scan, a dimensionless safety factor, q, scan was attempted by using a constant toroidal magnetic field and by moderating the beam power to match the plasma temperature. The temperature and magnetic field was also kept constant during the confinement scan to probe the effects of the electron density gradient. An evaluation of the He II (n = 4 -> 3) and C VI (n = 8 -> 7) spectral lines, induced by active charge exchange emission and measured using the RGB 2D camera on MAST, indicate that carbon experiences moderately higher rates of diffusion and inward convection than helium in the L-mode high Ip plasma. Lowering Ip in Lmode caused a moderate increase in the helium diffusion and convection coefficients near the plasma edge. Neoclassical simulations were carried out which indicate anomalous rates of helium and carbon diffusion and inward convection in the outer regions of both L-mode plasmas. Similar rates of helium diffusion are found in the H-mode plasma, however these rates are consistent with neoclassical predictions. The anomalous inward pinch found for helium in the L-mode plasmas is also not apparent in H-mode. An outward flux of helium and carbon is found at mid-radius in H-mode, corresponding the region of positive electron density gradient. Linear gyrokinetic simulations of one flux surface in L-mode using the gs2 and gkw codes were performed which show that equilibrium flow shear is sufficient to stabilise ion temperature gradient (ITG) modes, consistent with BES observations, and suggest that collisionless trapped electron modes (TEMs) may dominate the anomalous helium particle transport. A quasilinear estimate of the dimensionless peaking factor associated with TEMs is in good agreement with experiment. Collisionless TEMs are more stable in H-mode because the electron density gradient is flatter. The steepness of this gradient is therefore pivotal in determining the inward neoclassical particle pinch and the particle flux associated with TEM turbulence.

Motivated by a renewed interest in impurity transport in nuclear fusion research, this reference text covers the diagnostics, experimental approach, and results of recent impurity transport studies in tokamak and helical plasmas. It also covers the impurity transport parallel to the magnetic field in the scrape-off layer (SOL) and the impact of magnetic topology on impurity transport. Topics covered include an introduction to impurity transport; the diagnostics system with passive and active spectroscopy; impurity transport across magnetic flux surfaces; the effect of magnetic topology; and the control of impurity transport using electron cyclotron resonance heating and ion cyclotron resonance heating. With few books available on impurity transport in plasmas, this book would appeal to academic researchers and graduate students on the field of plasma physics and nuclear fusion research. Key Features Describes recent developments in diagnostics for impurity transport Covers recent developments of an experimental approach for impurity transport study Provides a comprehensive understanding of impurity transport in different magnetic configurations Covers a new area of impurity transport experiment including the effect of magnetic topology Written by high-profile authors