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X-Point Height and H-mode P th (Gyrocenter shift vs. l c )

X-Point Height and H-mode P th (Gyrocenter shift vs. l c ). K.C. Lee , R. Maingi , V.A. Soukhanovskii, R . E . Bell, B.P. LeBlanc, R.J. Maqueda , D.P. Stotler, P.W. Ross, D.A. Gates. NSTX (H-mode ok) NSTX (no H-mode). 117747-0.23s (no shift) 117747-0.23s (shifted) 123644-0.3s.

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X-Point Height and H-mode P th (Gyrocenter shift vs. l c )

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  1. X-Point Height and H-mode Pth(Gyrocenter shift vs. lc) K.C. Lee, R. Maingi, V.A. Soukhanovskii, R.E. Bell, B.P. LeBlanc, R.J. Maqueda, D.P. Stotler, P.W. Ross, D.A. Gates

  2. NSTX (H-mode ok) NSTX (no H-mode) 117747-0.23s (no shift) 117747-0.23s (shifted) 123644-0.3s NSTX shot with downward z-axis shift had H-mode access (but also lower Ip and higher q95) L-H

  3. Background Physics on X-pointand H-mode Pth( Gyrocenter shift and connection length) Er (ExB flow) ∇nn ∇ni (pedestal) lower X-point height high Er & low Pth → decrease ∂x → increase ∇nn (=∂nn/∂x) → or smaller connection length → lower Pth

  4. Neutral Density Measurement on X-Point diverter D-alpha tangential TV T.S. [ ne, Te] (mid pl) emission= f(nn,ne,Te) mid plane : D-beta Er from ERD DIII-D [Colchin, Maingi et al., Nuclear Fusion (2000)]

  5. ► NSTX has capability to measure the neutral density profiles for same study which was done on DIII-D, assuming that the midplane Thomson can be mapped to the X-point region (this is reasonably valid for the low density L-mode phase preceding the L-H transition). ► NSTX has mid plane D-beta camera and edge rotational diagnostics to measure the neutral density and edge rotation profiles just before the L-H transition. ► NSTX will be the first machine to investigate the H-mode power threshold in conjunction with the gyrocenter shift theory by measuring the neutral density gradient at variable X-point height. ► The run plan includes verification of x-point height and effect of connection length by scanning of plasma current.

  6. Experimental Run Plan (14 shots, ½ day) #117747 : Ip=0.6 MA, BT=4.5 kG, with NBI power increase (I MW, 2 MW, 4 MW) 1. Start with low X-point height, Pth measurement, with all diagnostics (4-5 shots) 2. Pth measurement with high X-point (2-3 shots) 3. Increase Ip to 0.8 MA to decrease lc (reduces gas), Pth measurement (2-3 shots) 4. Repeat #1 with 0.8 MA (4-5 shots) Goal : to differentiate effect of Xpt height from lc on H-mode Pth Also important for the study of recycling Influence on H-mode ( ELMs & Lithium)

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