High frequency pacing of edge localized modes by injection of lithium granules in DIII-D H-mode discharges

Abstract

A newly installed Lithium Granule Injector (LGI) was used to pace edge localized modes (ELM) in DIII-D. ELM pacing efficiency was studied injecting lithium granules of nominal diameter 0.3–0.9 mm, speed of 50–120 m s−1 and average injection rates up to 100 Hz for 0.9 mm granules and up to 700 Hz for 0.3 mm granules. The efficiency of ELM triggering was found to depend strongly on size of the injected granules, with triggering efficiency close to 100% obtained with 0.9 mm diameter granules, lower with smaller sizes, and weakly depending on granule velocity. Robust ELM pacing was demonstrated in ITER-like plasmas for the entire shot length, at ELM frequencies 3–5 times larger than the ‘natural’ ELM frequency observed in reference discharges. Within the range of ELM frequencies obtained, the peak ELM heat flux at the outer strike point was reduced with increasing pacing frequency. The peak heat flux reduction at the inner strike point appears to saturate at high pacing frequency. Lithium was found in the plasma core, with a concurrent reduction of metallic impurities and carbon. Overall, high frequency ELM pacing using the lithium granule injection appears to be compatible with both H-mode energy confinement and attractive H-mode pedestal characteristics, but further assessment is needed to determine whether the projected heat flux reduction required for ITER can be met.

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

“…This pacing has lead to the reduction of the observed peak heat flux and a clamping of the high-Z metal impurities [5]. The granule injection has been observed by a radially viewing ablation camera, which has noted an ablation behavior consistent with previous pellet injection observations [16][17][18][19].…”

“…This deposition profile indicates why the granule triggering efficiency is primarily dependent upon the size of the granule and is largely insensitive to variations in the granule injection velocity over the ranges accessible to the LGI [5]. As is shown in Fig.…”

“…The lithium granule pacing experiments described herein were performed at the DIII-D National Fusion Facility [5]. The reference discharge into which the granules were injected were deuterium plasmas in a lower single null divertor configuration.…”

“…At DIII-D, lithium granules ranging in size from 300 to 900 m in diameter were injected into various discharges with low natural ELM frequencies to test this principle [5]. A vibrating piezoelectric disk gravitationally fed a rotating impeller which drove granules into the edge plasma at velocities ranging from 50 to 150 m/s.…”

Injected mass deposition thresholds for lithium granule instigated triggering of edge localized modes on EAST

“…Consider that a single 600 micron granule contains approximately 5.24x10 18 lithium atoms, a substantial fraction of which are deposited deep within the discharge, and is successful in providing prompt ELM triggering with an efficiency greater than 95%. In comparison, the injection of a dozen 300 micron granules (as shown in Figure 13), most of which ablate at the pedestal foot, provide 7.86x10 18 lithium atoms and is only able to generate an ELM event, on average 20% of the time.…”