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XP-826: Investigation of “X-point limiter” plasmas M. Bell, R. Maingi, K-C. Lee

XP-826: Investigation of “X-point limiter” plasmas M. Bell, R. Maingi, K-C. Lee. Define LCFS by a limiter contact close to and just inside the X-point An alternative to conventional poloidal divertor H-mode has been obtained reliably without an X-point on the boundary

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XP-826: Investigation of “X-point limiter” plasmas M. Bell, R. Maingi, K-C. Lee

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  1. XP-826: Investigation of “X-point limiter” plasmas M. Bell, R. Maingi, K-C. Lee • Define LCFS by a limiter contact close to and just inside the X-point • An alternative to conventional poloidal divertor • H-mode has been obtained reliably without an X-point on the boundary • Flux expansion near X-point and tangential contact spread heat load • XP-826 tried to make an X-point limiter on lower outer divertor plate • SOL contact at location of Liquid Lithium Divertor plates • Approach • Start with 0.9MA, 65kA TF LSN plasma with 2-source NBI • Set PF2L/Ip = 4kA/MA, PF2U/Ip = 2.5kA/MA; PF1AL/Ip, PF1AU/Ip to zero • Adjust vertical position down and, depending on the variation in li, adjust PF2L/Ip to bring the X-point close to lower outer divertor plate • Attempt feedback control of the X-point location with rtEFIT • Repeat shots to condition contact point then assess H-mode access • Experiment could also provide X-point height scan (proposed by K-C. Lee)

  2. Ip 0.9MA; ITF -65kA; PNBI 4.0MW XP≠826 -1.4 Center column XP=826 ZX-point (m) -1.6 Inner divertor Outer divertor 0.2 0.4 0.6 0.8 RX-point (m) Discharges Produced with X-Point Close to Outboard Divertor Plate • ~4 hours total runtime on 3 days (4/25, 5/2, 6/16) • Essentially as “filler” on latter 2 days • 12 productive shots plus initial baseline shot 128756, 0.3s

  3. Stability Appeared to be Reduced • Higher TF needed to maintain q95 > 7 • H-mode transition occurred while plasma limited on inner wall • Confinement appears similar to pre-lithium shots in standard LSN shape • Reduced lower triangularity 0.3–0.4 expected to reduce N limit • Not able to assess conditioning effect with small number of shots • Shots after lithium experiments collapsed at even lower N

  4. Experiment was Limited Success • X-point limiter configuration was achieved transiently • Provides a proof-of-principle for experiments with LLD • Appears to be compatible with H-mode • Need to optimize control strategy to improve reproducibility • Using dRsep control may not be best method • X-point control has not been used routinely on NSTX • Increasing cross-sectional area (taller, fatter plasma) would relax need for high TF (65kA) and modest Ip (0.9MA) to maintain q95 above practical limit • Too few shots were run to assess possible effects of conditioning a new contact area on the PFCs

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