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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