DIII D Quiescent H-Mode Experiments with Co Plus Counter Neutral Beam Injection

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1 Quiescent H-Mode Experiments with Co Plus Counter Neutral Beam Injection by K.H. Burrell for W.P. West, M.E. Fenstermacher, P. Gohil, P.B. Snyder, T.H. Osborne, W.M. Solomon* Lawrence Livermore National Laboratory, Livermore, California. *Princeton Plasma Physics Laboratory, Princeton, New Jersey. Presented at Forty-Ninth APS Meeting of the Division of Plasma Physics Orlando, Florida November 12 16, 2007

2 Introduction Quiescent H-mode (QH-mode) is in many ways the ideal H-mode Exhibits H-mode confinement level Has ELM-free operation with stationary pedestal density and radiated power Long term goal of QH-mode research is to broaden the parameter range where QH-mode is obtained QH-mode with balanced or co-dominated NBI would be a major achievement In 2006 campaign, adding some co-nbi to counter-injected QH-mode reduced edge rotation and increased pedestal density and pressure ELMs returned as pedestal pressure went up In 2007, we developed double-null plasma shape with improved edge stability, allowing QH-mode operation at lower edge rotation Peeling-ballooning mode stability theory used as guide Changing rotation allows controlled change in edge density and pressure by factor of two while retaining ELM-free state Improved error field compensation aided low rotation operation We now have a technique for actively controlling edge particle transport and edge pressure

3 Quiescent H-Mode Runs ELM-Free for Long Pulses With Constant Pedestal Density and Radiated Power Duration limited by neutral beam pulse length I P (MA) Divertor D α (au) 5 T i PED (kev) T e PED (kev) 3 2 Density (10 19 m 3 ) n e 1 n e PED 15 Power (MW) P NBI P rad Time (ms)

4 Operating Points of Shots at Various Torques Are Consistent with Edge Peeling-Ballooning Stability Theory Stability calculations performed with ELITE code QH-mode plasma with EHO operates near but below peeling stability boundary ELMing shots are closer to peeling boundary Edge Current [(j max +j sep )/2 j ] UNSTABLE STABLE t = 3450 γ/(ω * /2) Contours EHO ELM Normalized Pressure Gradient (α) /KHB/rs

5 Plasma Shape With Improved Edge Stability Developed for 2007 Campaign Edge Current [(jmax+jsep)/2 j ] UNSTABLE STABLE t=3750 γ=ω./2 contours Normalized Pressure Gradient (α)

6 Plasma Shape With Improved Edge Stability Developed for 2007 Campaign Edge Current [(jmax+jsep)/2 j ] UNSTABLE STABLE t=3750 γ=ω./2 contours Normalized Pressure Gradient (α)

7 Plasma Shape With Improved Edge Stability Developed for 2007 Campaign Edge Current [(jmax+jsep)/2 j ] UNSTABLE STABLE t=3750 γ=ω./2 contours Normalized Pressure Gradient (α)

8 Edge Pedestal Width and Height are Significantly Larger in Double-Null QH-Mode Plasma 0 Peak pressure gradient and pedestal width are both higher in double-null QH-mode plasma p ψ ( ) kpa rad Wb J φ R o 2 /R 2 R o /R (MA/m 2 ) Double-Null 30 Single-Null P TOT (kpa) ψ NORM

9 Rotation Control of Density Used to Access Higher Pedestal Pressure Permitted by Improved Shape Input torque is reduced factor of five 3 β N PED P TOT (kpa) PED n e (10 19 m 3 ) Divertor D α VPED φ (km/s) Torque (n-m) P inj (MW) Time (ms)

10 Edge Density Increases as Toroidal Rotation Decreases Global stored energy and energy confinement time increase 35% at lower rotation V φ /R (krad/s) 2 n e (10 19 m 3 ) ρ T e (kev) ρ ρ T i (kev) ρ

11 Change in Co-Counter Beam Balance and Edge Rotation Modifies Edge Harmonic Oscillation Previous work has shown that EHO enhances edge particle transport, facilitating density control using the DIII-D cryopump Density (10 19 m 3 ) Coherent EHO occurs at lower pedestal densities V φ PED (km/s) n = 4 Broadband MHD modes occur at higher pedestal densities Frequency (khz) n = +4 n = +3 Coherent EHO rotates in counter direction but broadband MHD rotates in co direction n = +2 n = +1 n = 2 n = Time (ms)

12 Experiments in 2007 Benefited from Improved Error Field Correction Improved error field correction prevented locked mode formation 2 I P x B r (G) 60 N1RMS 5 N2RMS v φ PED (km/s) TORQUE (nt m) Time (ms)

13 QH-Mode Operates with Balanced Beams and No Edge Rotation for About 500 ms Balanced beam QH-mode terminated after about 500 ms by locked mode Further improvement in error field correction needed 3 β N l i * ESLD079 ESLD199 ESLD N2RMS (G) N1RMS (G) 150 v φ PED (km/s) 5 Beams Co Beams Counter-Beams 10 Torque (nt-m) Time (ms)

14 Conclusion In the 2007 campaign, we developed a double null divertor plasma with much broader range of edge stability than the single null plasmas used previously Pedestal width and height both significantly greater in double null Changing rotation in highly shaped, double null plasma allows controlled change in edge density and pressure by factor of two while retaining ELM-free state Improved error field compensation aided low rotation operation We now have a technique for active control of edge particle transport and edge pressure Further improvements in error field compensation needed to extend QH-mode operation to co-dominated neutral beam injection RWM feedback technique is available to do this