rapid pedestal pressure increase in high triangularity ...3 k.h. burrell/ttf/april 2015...

K.H. Burrell/TTF/April 2015 1 040-15/KHB/rs

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, 2015


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


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


Dedicated Experiment Run to Investigate Effect of Shear in ωE= Er/RBθ

•  Plasma conditions:

–  Plasma current: 1.0 MA (forward Ip), –1.1 MA (reverse Ip)

–  Toroidal field: - 2.05 T

–  Density scan: 2 to 6 x 1019 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-Ip and back again to alter rotation and ωE


QH-mode Shot Goes through Zero Rotation without Locked Mode


Periodic Edge Sweep Coupled with New CER Edge Chords Provides High Spatial Resolution ωE Profiles

•  Complete ωE profile every 200 ms


Shear in ωE 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 ! = AIN(1+! IN xIN )exp(xIN )! exp(!xIN )

exp(xIN )+ exp(!xIN )

!AOUTexp(xOUT )! (1+!OUT xOUT )exp(!xOUT )

exp(xOUT )+ exp(!xOUT )+B

xIN = (RIN ! R) /wIN !!!!xOUT = (ROUT ! R) /wOUT

! IN ,!!OUT !!Asymptotic!Slope!ParametersRIN ,!ROUT !!Location!(Symmetry!Point)!ParmeterswIN ,!wOUT !!Half "Width!ParametersB!!!Offset !Parameter


Coherent EHO Usually Disappears at Small Rotation Only Broadband MHD Remains

•  βN ~ 1.5–1.7, q95 = 5.5


•  Phase data from DBS system consistent with the broadband activity being located in the pedestal gradient region, ρ>0.91.

Increased edge density fluctuations accompany enhanced broadband MHD activity

Magnetic probe, B1

Pedestal width

ρ~0.95

ρ~0.93

ρ~0.92

ρ~0.91


0.6 0.7 0.8 0.9rho

0.00

0.05

0.10

0.15

0.20ñ_RMS, (a.u.)

Before elevated peped 2584 ms

157109

During elevated peped 3578 ms

•  Shown are RMS levels of Doppler shifted intermediate-k ñ –  kθρs~ 0.8–1.2

Intermediate-k ñ increases in upper-pedestal region during increased pedestal pressure height and width

DBS ñ spectra

ρ~0.91 Density profile


Width of Inner Side of ωE Profile Increases Significantly when Pedestal Height and Width Step Up


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?


Phase with Low Rotation and Broadband MHD Operates Below Peeling Boundary Phase with High Rotation and Coherent EHO Operates on Boundary

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

] Ed

ge

Cur

rent

[(j m

ax+

j se

p)/

2 <j>

]

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

] Ed

ge

Cur

rent

[(j m

ax+

j se

p)/

2 <j>

]

Normalized Pressure Gradient (α)

0.3

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Shot 157109 1963 ms Coherent EHO High Rotation0.3

Shot 157109 2863 ms Broadband MHD Only Low Rotation

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Shot 157109 4060 ms Coherent EHO High Rotation

0.3

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Normalized Pressure Gradient (α)


2


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Phase with Low Rotation and Broadband MHD Operates Below Peeling Boundary Phase with High Rotation and Coherent EHO Operates on Boundary

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

]

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

]

Normalized Pressure Gradient (α) Normalized Pressure Gradient (α)

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

]

Edg

e C

urre

nt [(

j ma

x+ j s

ep)/

2 <j>

]

20.3

0.4

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3 4 5 6 7 8


20.3

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20.3

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3 4 5 6 7 8



Pedestal Pressure and Pressure Width Are Larger in Low Rotation Portion of Discharges

•  At low rotation (3423 ms): Ptot

PED = 4 kPa and wP = 1.7 cm

•  At high rotation (1963 ms): Ptot

PED = 3 kPa and wP = 1.2 cm


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


Pedestal Pressure Increase Occurs with Similar Midplane Plasma Separatrix Radius

•  Small radius means larger distance to wall


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-Ip direction


Coherent EHO Rotates Toroidally in Same Direction as Plasma Rotation

•  Forward (CCW) Ip, counter-Ip NBI, coherent EHO n<0, ⇒ counter-Ip EHO rotation

•  Forward (CCW) Ip, co-Ip NBI, coherent EHO n>0, ⇒ co-Ip EHO rotation


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 56 – 74 GHz

•  12 (poloidal) x 4 (radial) x 1 (toroidal)

•  Poloidal resolution ~ 3 cm

•  Radial resolution – depends on ∇ne & ∇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)


Microwave Imaging Reflectometer Shows Poloidal Phase Velocity of Coherent EHO and Broadband MHD Can Have Opposite Signs

•  NBI is in counter-Ip direction


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


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 (H98y2 =

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


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

rapid pedestal pressure increase in high triangularity ...3 k.h. burrell/ttf/april 2015...

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