High frequency pacing of edge localized modes by injection of lithium granules in DIII-D H-mode discharges
Authors: A. Bortolon, R. Maingi, D.K. Mansfield, A.
Nagy, A.L. Roquemore, L.R. Baylor, N. Commaux, G.L. Jackson, E.P.
Gilson, R. Lunsford, P.B. Parks, C. Chrystal, B.A. Grierson, R.
Groebner, S. Haskey, M.J. Makowski, C.J. Lasnier, R. Nazikian, T.
Osborne, D. Shiraki, M.A. Van Zeeland
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 and
average injection rates up to 100 Hz for 0.9 mm granules and up to
700 Hz for 0.3 mm granules. The eefficiency 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.
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