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


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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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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

    • 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)


Discharges produced with x point close to outboard divertor plate

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


Stability appeared to be reduced

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


Experiment was limited success

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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