a Spectroscopic Analysis of Density-Controlled Impurity Behavior in the Diii-D Tokamak during H-Mode
With simultaneous spectral, temporal and chordal resolution, the STRS spectrometer was built to increase both the quality and quantity of spectroscopic impurity information available from tokamak discharges. STRS is capable of tracing the motion of several impurity species through the plasma during a single discharge. By reducing noise from shot irreproducibility, observing unsynchronized events, and reducing the time required to map out impurity behavior, impurity transport and radiation can be examined with much more detailed information than previously available. The astigmatic properties of a grazing incidence spectrometer disperse spectra and provides angular resolution. A 75 mm chevron microchannel plate image intensifier is used as a two dimensional detector to amplify the 50-370 A signals which are then recorded by three CCD cameras controlled by a dedicated microcomputer. The construction, testing and calibration is described. STRS was installed at the DIII-D tokamak, and its data is compared to simulations to analyze impurity behavior. An unusual impurity behavior was discovered at DIII-D during H-mode with giant edge localized modes (ELMs): depending on the plasma current, impurities either accumulate in the plasma center, or are driven out from it. They also modulate at the ELM frequency. This behavior was found to depend on the electron density profile which oscillates between centrally peaked and hollow. Naturally occurring ELM phenomena produce electron density oscillations which cause impurity cycling and allow transport studies of intrinsic impurities. Particle flux is modeled with a constant anomalous diffusion coefficient and a convection coefficient which depends on electron density gradients. MIST impurity transport code simulations use time dependent electron density profiles, oscillating with giant ELMs, to show impurity concentrations follow the density peak as theoretically predicted. This reproduced the observations of intrinsic nickel spectra, and indicates that impurity transport in DIII-D h-mode is dominated by ion density gradients. Central diffusion, at 1.0 times 10^4cm^2 /sec, is found to be anomalous in these H-mode plasmas, approximately two orders of magnitude above neoclassical. Convective flux is found to be large, with the anomalous diffusion reducing the degree of impurity peaking. These results are interpreted as evidence of neoclassical impurity convective fluxes and indicate impurity accumulation in tokamaks will occur unless electron density is flat or confinement is low.
- Pub Date:
- Physics: Fluid and Plasma