Authors: R. Lunsford, A. Bortolon, A.L. Roquemore, D.K. Mansfield, M.A. Jaworski, R. Kaita, R. Maingi, A. Nagy and the NSTX-U team

Abstract:  By  employing   a  neutral  gas  shielding  (NGS)  model  to  characterize  impurity   granule injection, the pedestal atomic deposition for three different species of granule: lithium, boron, and  carbon,   are  determined.     Utilizing   the  duration  of  ablation  events   recorded  on experiments  performed at DIII-D to calibrate the NGS model, we quantify the ablation rate and mass deposition location with respect to the plasma density profile.  The species-specific granule shielding constant is then used to model granule ablation within NSTX-U discharges. Simulations of 300, 500 and 700 micron diameter granules injected at 50 rnlsec are presented for NSTX-U L-mode type plasmas, as well as H-mode discharges with low natural ELM frequency.  Additionally, ablation calculations of 500 micron granules of each species are presented at velocities ranging from 50 - 150 rnlsec. In H-mode discharges these simulations show that the majority of the injected granule is ablated within or just past the edge steep gradient region.   At this radial position, the perturbation to the background plasma generated by the ablating granule can lead to conditions advantageous for the rapid triggering of ELM crashes.

Submitted to: Nuclear Fusion
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Download PPPL-5331 1.1 MB (pdf  19 pp)
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