Novel soft x-ray diagnostic techniques for the study of particle transport phenomena in magnetically confined fusion plasmas

The understanding of impurity behavior in a magnetically confined fusion (MCF) plasma is one of the challenges of current fusion research as the exhaust from high power plasmas will ultimately strike and erode the material surfaces of the contain ment device. The aim of this work is to study the properties of particle transport in a spherical tokamak (ST) in a variety of scenarios, including changes in the toroidal magnetic field, plasma current, collisionality, etc. The perturbative impurity transport experiments performed at the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory (PPPL), used a brief neon puff injected from an outboard midplane port as an impurity particle source with a strong spectral signature in the soft X-ray range of 0.1-2.0 keV. In order to study the transport of such impurities, we developed a fast (≤0.1 ms) and compact multi-energy scintillatorbased (optical) soft X-ray (OSXR) system. This instrument consists of three horizontal midplane rows of 16 channels, each viewing an overlapping plasma volume with a different soft X-ray (SXR) beryllium filter; such filtering technique enables the observation of different charge states of neon ions and thus their transport description across the NSTX cross section. The signals from the low energy array filtered by a beryllium 10 mum foil are dominated by emission lines from the He- and H-like (Ne8+, Ne9+) neon charge states located primarily near the plasma boundary (r/a ≳ 0.6) while, the fully stripped Neon (Ne10+) contribution from the inner plasma is imaged by the medium- and high-energy detectors filtered by the thicker Be 100 mum and 300 mum foils. The injected neon impurity was followed by the multi-energy OSXR array; neon ions penetrated the core on a hundred millisecond time scale, and the measurements indicated a low particle diffusivity (∼ 1 m2/s). The usually large peripheral (r/a > 0.8) particle diffusivity in the NSTX high confinement (H-mode) seem to be due to a transport associated with the high q ≈ 6 - 10 value, high ion density (∼ 6 - 7 x 1019 m-3) and low ion temperature (∼ 200 eV). Both the impurity buildup and their subsequent penetration to the core were changed substantially when the plasma current and the toroidal field were increased; nevertheless, the calculated particle diffusivities found are in good agreement with the turbulence-free nature of neoclassical particle transport. Thus, the main result of the research presented in this work suggests that anomalous ion particle transport associated with turbulent long-wavelength electrostatic instabilities is largely suppressed in the NSTX core.; The significance of this result is that an ST operated at high fields does not experience the deleterious effect of the core impurity accumulation. It should also be pointed out that even though the results at low field suggest an anomalous inward pinch, this phenomena appears to be due to either magnetohydrodynamic (MHD) activity or to a partial suppression of electrostatic plasma modes (e.g. ITGs).