Rapid Pedestal Pressure Increase in High Triangularity, Double-Null QH-mode Discharges

Transcription

1 Rapid Pedestal Pressure Increase in High Triangularity, Double-Null QH-mode Discharges by K.H. Burrell with X. Chen, A.M. Garofalo, G.R. McKee, C.M. Muscatello, T.H. Osborne, T.L. Rhodes, P.B. Snyder, W.M. Solomon, and Z. Yan Presented at the 2015 US/EU Transport Task Force Workshop Salem, MA April 28 May 1, K.H. Burrell/TTF/April 2015

2 New Discovery: Rapid Transition to Improved Pedestal Pressure Pedestal pressure rapidly increases about 60% in high triangularity, double null QH-mode plasmas during NBI torque ramp down Edge pressure pedestal height and width show stepwise increase as rotation drops Transition is associated with Increased width of inner side of edge Er well Increased density and broadband MHD fluctuations Cessation of coherent EHO in most cases Edge plasma can operate below peeling stability boundary even with higher pedestal pressure Rotation characteristics of coherent EHO and broadband MHD suggest they are different modes 2 K.H. Burrell/TTF/April 2015

3 Coherent EHO and Broadband MHD Both Provide Density Control in QH-mode But Have Different Characteristics Coherent EHO and broadband MHD exist in different but somewhat overlapping density and rotation regimes Coherent EHO exists at the lower end of the density regime and fades away as plasma approaches the density limit Coherent EHO usually fades out as rotation decreases Broadband MHD is more prevalent in highly shaped discharges at higher triangularity, higher density and lower rotation 3 K.H. Burrell/TTF/April 2015

4 Dedicated Experiment Run to Investigate Effect of Shear in ω E = E r /RB θ Plasma conditions: Plasma current: 1.0 MA (forward I p ), 1.1 MA (reverse I p ) Toroidal field: T Density scan: 2 to 6 x m -3 Shape: high triangularity, balanced double null Outer edge of plasma swept over 4 cm range to improve edge diagnostic resolution Sweep carried out every 200 ms starting at 1500 ms into the shot NBI torque ramped from counter to co-i p and back again to alter rotation and ω E 4 K.H. Burrell/TTF/April 2015

5 QH-mode Shot Goes through Zero Rotation without Locked Mode 5 K.H. Burrell/TTF/April 2015

6 Periodic Edge Sweep Coupled with New CER Edge Chords Provides High Spatial Resolution ω E Profiles Complete ω E profile every 200 ms 6 K.H. Burrell/TTF/April 2015

7 Shear in ω E Characterized Using Width Parameters from Functional Fit Functional form inspired by mtanh fit to edge pedestal properties [Groebner et al., Nuclear Fusion (2001)] Form used:! E! = A IN (1+! IN x IN )exp(x IN )! exp(!x IN ) exp(x IN )+ exp(!x IN )!A OUT exp(x OUT )! (1+! OUT x OUT )exp(!x OUT ) exp(x OUT )+ exp(!x OUT ) + B x IN = (R IN! R) / w IN!!!! x OUT = (R OUT! R) / w OUT! IN,!! OUT!!Asymptotic!Slope!Parameters R IN,!R OUT!!Location!(Symmetry!Point)!Parmeters w IN,!w OUT!!Half "Width!Parameters B!!!Offset!Parameter 7 K.H. Burrell/TTF/April 2015

8 Coherent EHO Usually Disappears at Small Rotation Only Broadband MHD Remains β N ~ , q 95 = K.H. Burrell/TTF/April 2015

9 Increased edge density fluctuations accompany enhanced broadband MHD activity Magnetic probe, B1 ρ~0.95 ρ~0.93 Pedestal width ρ~0.92 Phase data from DBS system consistent with the broadband activity being located in the pedestal gradient region, >0.91. ρ~ K.H. Burrell/TTF/April 2015

10 Intermediate-k ñ increases in upper-pedestal region during increased pedestal pressure height and width DBS ñ spectra Density profile ρ~ ñ_rms, (a.u.) Before elevated peped 2584 ms During elevated peped 3578 ms Shown are RMS levels of Doppler shifted intermediate-k ñ k s ~ K.H. Burrell/TTF/April 2015 rho

11 Width of Inner Side of ω E Profile Increases Significantly when Pedestal Height and Width Step Up 11 K.H. Burrell/TTF/April 2015

12 Plasmas in Balanced Double Null Shape Operate Below Peeling Boundary at Low Rotation What process maintains operating point below peeling boundary in shot without ELMs? 12 K.H. Burrell/TTF/April 2015

13 Phase with Low Rotation and Broadband MHD Operates Below Peeling Boundary Phase with High Rotation and Coherent EHO Operates on Boundary Edge Current [(j max + j sep )/ 2 <j>] Edge Current [(j max + j sep )/ 2 <j>] Shot ms Coherent EHO High Rotation Edge Current [(j max + j sep )/ 2 <j>] Edge Current [(j max + j sep )/ 2 <j>] Shot ms Broadband MHD Only Low Rotation Shot ms Broadband MHD Only Low Rotation Shot ms Coherent EHO High Rotation Normalized Pressure Gradient (α) Normalized Pressure Gradient (α) 13 K.H. Burrell/TTF/April 2015

14 Phase with Low Rotation and Broadband MHD Operates Below Peeling Boundary Phase with High Rotation and Coherent EHO Operates on Boundary Edge Current [(j max + j sep )/ 2 <j>] Shot ms Coherent EHO High Rotation Edge Current [(j max + j sep )/ 2 <j>] Shot ms Broadband MHD Only Low Rotation Edge Current [(j max + j sep )/ 2 <j>] Shot ms Coherent EHO High Rotation Edge Current [(j max + j sep )/ 2 <j>] Shot ms Broadband MHD Only Low Rotation Normalized Pressure Gradient (α) Normalized Pressure Gradient (α) 14 K.H. Burrell/TTF/April 2015

15 Pedestal Pressure and Pressure Width Are Larger in Low Rotation Portion of Discharges At low rotation (3423 ms): P tot PED = 4 kpa and w P = 1.7 cm At high rotation (1963 ms): P tot PED = 3 kpa and w P = 1.2 cm 15 K.H. Burrell/TTF/April 2015

16 Cessation of Coherent EHO is Not Key Part of Pedestal Pressure Increase In most cases (~10), several quantities change together Pedestal pressure height and width increase Inner width of edge Er well increases Frequency range of broadband MHD increases Coherent EHO ceases In two cases, coherent EHO continues into phase of higher pedestal pressure These two cases demonstrate that cessation of coherent EHO is not a key part of the pedestal pressure increase 16 K.H. Burrell/TTF/April 2015

17 Pedestal Pressure Increase Occurs with Similar Midplane Plasma Separatrix Radius Small radius means larger distance to wall 17 K.H. Burrell/TTF/April 2015

18 Rotation characteristics of coherent EHO and broadband MHD suggest they are different modes Broadband MHD rotates toroidally in co-ip direction independent of plasma rotation Coherent EHO rotates toroidally in direction of plasma rotation independent of plasma current direction Poloidal phase velocities of coherent EHO and broadband MHD have opposite signs when NBI is in counter-i p direction 18 K.H. Burrell/TTF/April 2015

19 Coherent EHO Rotates Toroidally in Same Direction as Plasma Rotation Forward (CCW) I p, counter-i p NBI, coherent EHO n<0, counter-i p EHO rotation Forward (CCW) I p, co-i p NBI, coherent EHO n>0, co-i p EHO rotation 19 K.H. Burrell/TTF/April 2015

20 Microwave imaging reflectometer measures localized density fluctuations in 2D N.C. Luhmann Jr., C.M. Muscatello, C.W. Domier, X. Ren, A. Spear, B. Tobias Quasi-optical, active, microwave imaging for spatially localized (in r,θ,φ) measurements X-mode operation Tunable over GHz 12 (poloidal) x 4 (radial) x 1 (toroidal) Poloidal resolution ~ 3 cm Radial resolution depends on n e & B but typically < 5 mm in pedestal *C.M. Muscatello, et al, Rev. Sci. Instrum. 85, 11D702 (2014) **A. Spear, et al, Rev. Sci. Instrum. 85, 11D834 (2014) 20 K.H. Burrell/TTF/April 2015

21 Microwave Imaging Reflectometer Shows Poloidal Phase Velocity of Coherent EHO and Broadband MHD Can Have Opposite Signs NBI is in counter- I p direction 21 K.H. Burrell/TTF/April 2015

22 Beam Emission Spectroscopy Shows Poloidal Phase Shift Has Opposite Signs for Coherent EHO and Broadband MHD Indicating Opposite Phase Velocities BES and MIR systems both show opposite phase velocity 22 K.H. Burrell/TTF/April 2015

23 Stability and Transport Are Both Involved in Pedestal Improvement Operation below peeling stability boundary suggests enhanced transport in steep gradient region of edge pedestal is allowing transport limited solution In spite of increased local transport, shots exhibit H-mode global confinement (H 98y2 = 1.3) Transport limited operation favored by Excellent peeling-ballooning stability in highly shaped, diverted plasmas Reduction of input power needed to maintain nearly constant global β as confinement improves at low rotation Importance of PB stability is consistent with rapid pedestal pressure increase occurring only in balance double null plasmas Decreased E x B shear may allow increased transport due to broadband MHD and density fluctuations in steep gradient region of edge pedestal Pedestal height and width change together with inner width of edge Er well To further investigate possible role of E x B shear, we want to Identify turbulence mode in plasma edge Decide whether local E x B shear is important or whether mode is so broad that some average over edge region is needed Find theory to predict how big E x B shear needs to be to stabilize mode and compare to measurements 23 K.H. Burrell/TTF/April 2015

24 Summary: Rapid Transition to Improved Pedestal Pressure Pedestal pressure rapidly increases about 60% in high triangularity, double null QH-mode plasmas during NBI torque ramp down Have one case where density ramp up produced same effect Edge pressure pedestal height and width show stepwise increase as rotation drops Energy confinement improves even though transport in steep gradient region of edge pedestal appears to increase Transition is associated with Increased width of inner side of edge Er well Increased density and broadband MHD fluctuations Cessation of coherent EHO in most cases Edge plasma can operate below peeling stability boundary even with higher pedestal pressure Rotation characteristics of coherent EHO and broadband MHD suggest they are different modes Results are potentially quite significant for future burning plasmas Operation without ELMs at low rotation is essential Increased pedestal pressure leads to improved fusion performance 24 K.H. Burrell/TTF/April 2015