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Title: Lithium granule ablation and penetration during ELM pacing experiments at DIII-D

Abstract

At DIII-D, lithium granules were radially injected into the plasma at the outer midplane to trigger and pace edge localized modes (ELMs). Granules ranging in size from 300 to 1000 microns were horizontally launched into H-mode discharges with velocities near 100 m/s, and granule to granule injection frequencies less than 500 Hz. While the smaller granules were only successful in triggering ELMs approximately 20% of the time, the larger granules regularly demonstrated ELM triggering efficiencies of greater than 80%. A fast visible camera looking along the axis of injection observed the ablation of the lithium granules. We used the duration of ablation as a benchmark for a neutral gas shielding calculation, and approximated the ablation rate and mass deposition location for the various size granules, using measured edge plasma profiles as inputs. In conclusion, this calculation suggests that the low triggering efficiency of the smaller granules is due to the inability of these granules to traverse the steep edge pressure gradient region and reach the top of the pedestal prior to full ablation.

Authors:
ORCiD logo [1]; ORCiD logo [1];  [1];  [1];  [1];  [1];  [2];  [2];  [1];  [2]
  1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  2. General Atomics, San Diego, CA (United States)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Contributing Org.:
Princeton Plasma Physics Laboratory, Princeton NJ 08543, USA
OSTI Identifier:
1254685
Alternate Identifier(s):
OSTI ID: 1335172; OSTI ID: 1371726
Report Number(s):
PPPL-5219
Journal ID: ISSN 0920-3796; PII: S0920379616303325; TRN: US1601725
Grant/Contract Number:
AC02-09CH11466; FC02-04ER54698
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Fusion Engineering and Design
Additional Journal Information:
Journal Volume: 112; Journal ID: ISSN 0920-3796
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Edge-localized Mode (ELM); injection; pellets; lithium; particle dynamics

Citation Formats

Lunsford, R., Bortolon, A., Roquemore, A. L., Mansfield, D. K., Nagy, A., Maingi, R., Parks, P. B., Jackson, G., Gilson, E., and Chrobak, C. P. Lithium granule ablation and penetration during ELM pacing experiments at DIII-D. United States: N. p., 2016. Web. doi:10.1016/j.fusengdes.2016.04.041.
Lunsford, R., Bortolon, A., Roquemore, A. L., Mansfield, D. K., Nagy, A., Maingi, R., Parks, P. B., Jackson, G., Gilson, E., & Chrobak, C. P. Lithium granule ablation and penetration during ELM pacing experiments at DIII-D. United States. doi:10.1016/j.fusengdes.2016.04.041.
Lunsford, R., Bortolon, A., Roquemore, A. L., Mansfield, D. K., Nagy, A., Maingi, R., Parks, P. B., Jackson, G., Gilson, E., and Chrobak, C. P. Wed . "Lithium granule ablation and penetration during ELM pacing experiments at DIII-D". United States. doi:10.1016/j.fusengdes.2016.04.041. https://www.osti.gov/servlets/purl/1254685.
@article{osti_1254685,
title = {Lithium granule ablation and penetration during ELM pacing experiments at DIII-D},
author = {Lunsford, R. and Bortolon, A. and Roquemore, A. L. and Mansfield, D. K. and Nagy, A. and Maingi, R. and Parks, P. B. and Jackson, G. and Gilson, E. and Chrobak, C. P.},
abstractNote = {At DIII-D, lithium granules were radially injected into the plasma at the outer midplane to trigger and pace edge localized modes (ELMs). Granules ranging in size from 300 to 1000 microns were horizontally launched into H-mode discharges with velocities near 100 m/s, and granule to granule injection frequencies less than 500 Hz. While the smaller granules were only successful in triggering ELMs approximately 20% of the time, the larger granules regularly demonstrated ELM triggering efficiencies of greater than 80%. A fast visible camera looking along the axis of injection observed the ablation of the lithium granules. We used the duration of ablation as a benchmark for a neutral gas shielding calculation, and approximated the ablation rate and mass deposition location for the various size granules, using measured edge plasma profiles as inputs. In conclusion, this calculation suggests that the low triggering efficiency of the smaller granules is due to the inability of these granules to traverse the steep edge pressure gradient region and reach the top of the pedestal prior to full ablation.},
doi = {10.1016/j.fusengdes.2016.04.041},
journal = {Fusion Engineering and Design},
number = ,
volume = 112,
place = {United States},
year = {Wed May 25 00:00:00 EDT 2016},
month = {Wed May 25 00:00:00 EDT 2016}
}

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  • At DIII-D, lithium granules were radially injected into the plasma at the outer midplane to trigger and pace edge localized modes (ELMs). Granules ranging in size from 300 – 1000 microns were horizontally launched into H-mode discharges with velocities near 100 m/sec, and granule to granule injection frequencies less than 500 Hz. While the smaller granules were only successful in triggering ELMs approximately 20% of the time, the larger granules regularly demonstrated ELM triggering efficiencies of greater than 80%. A fast visible camera looking along the axis of injection observed the ablation of the lithium granules. The duration of ablationmore » was used as a benchmark for a neutral gas shielding calculation, and approximated the ablation rate and mass deposition location for the various size granules, using measured edge plasma profiles as inputs. This calculation suggests that the low triggering efficiency of the smaller granules is due to the inability of these granules to traverse the steep edge pressure gradient region and reach the top of the pedestal prior to full ablation« less
  • Experiments have been conducted on DIII-D investigating high repetition rate injection of non-fuel pellets as a tool for pacing Edge Localized Modes (ELMs) and mitigating their transient divertor heat loads. Effective ELM pacing was obtained with injection of Li granules in different H-mode scenarios, at frequencies 3–5 times larger than the natural ELM frequency, with subsequent reduction of strike-point heat flux. However, in scenarios with high pedestal density (~6 × 10 19 m –3), the magnitude of granule triggered ELMs shows a broad distribution, in terms of stored energy loss and peak heat flux, challenging the effectiveness of ELM mitigation.more » Furthermore, transient heat-flux deposition correlated with granule injections was observed far from the strike-points. As a result, field line tracing suggest this phenomenon to be consistent with particle loss into the mid-plane far scrape-off layer, at toroidal location of the granule injection.« less
  • Experiments have been conducted on DIII-D investigating high repetition rate injection of non-fuel pellets as a tool for pacing Edge Localized Modes (ELMs) and mitigating their transient divertor heat loads. Effective ELM pacing was obtained with injection of Li granules in different H-mode scenarios, at frequencies 3-5 times larger than the natural ELM frequency, with subsequent reduction of strike-point heat flux [Bortolon et al, Nucl. Fus., 56, 056008, 2016]. However, in scenarios with high pedestal density (~6×10 19 m -3), the magnitude of granule triggered ELMs shows a broad distribution, in terms of stored energy loss and peak heat flux,more » challenging the effectiveness of ELM mitigation. Furthermore, transient heat-flux deposition correlated with granule injections was observed far from the strike-points. Field line tracing suggest this phenomenon to be consistent with particle loss into the mid-plane far scrape-off layer, at toroidal location of the granule injection.« less
  • Direct measurements of the pedestal recovery during an edge-localized mode cycle provide evidence that quasi-coherent fluctuations (QCFs) play a role in the inter-ELM pedestal dynamics. Using fast Thomson scattering measurements, the pedestal density and temperature evolutions are probed on sub-millisecond time scales to show a fast recovery of the density gradient compared to the temperature gradient. The temperature gradient appears to provide a drive for the onset of quasi-coherent fluctuations (as measured with the magnetic probe and the density diagnostics) localized in the pedestal. The amplitude evolution of these QCFs tracks the temperature gradient evolution including its saturation. Such correlationmore » suggests that these QCFs play a key role in limiting the pedestal temperature gradient. The saturation of the QCFs coincides with the pressure gradient reaching the kinetic-ballooning mode (KBM) critical gradient as predicted by EPED1. Furthermore, linear microinstability analysis using GS2 indicates that the steep gradient is near the KBM threshold. Furthermore, the modeling and the observations together suggest that QCFs are consistent with dominant KBMs (although microtearing cannot be excluded as subdominant).« less