Core Plasma Constraints on Divertor Design By A.W. Leonard - PowerPoint PPT Presentation

Core Plasma Constraints on Divertor Design By A.W. Leonard Presented to IAEA-TM on Divertor Concepts Vienna, Austria Sept. 29 – Oct. 2, 2015

JET’s ILW experience illustrates important roles of boundary plasma JET • Operational space to limit impurity accumulation – Divertor conditions to limit W source – Fueling for ELM frequency, W transport • High Confinement – Role of Z eff on pedestal pressure – Pedestal dependence on neutral density • Learning curve to optimize performance – Core performance optimization must be designed into future tokamak divertors C. Challis NF 2015 2 IAEA-TM Div. Concepts, Vienna Sept. 2015

Core Performance a Critical Constraint on Divertor Design • Divertor design must simultaneously accommodate core plasma as well as divertor target constraints – Divertor: q ⊥ ≤ 10 MWm -3 , T e ≤ 5 eV, no transients (ELMs) – Core: High confinement, High β , etc. • What are the Core constraints on divertor operation? – Maintain robust H-mode confinement – High β operation – Low central impurity density to limit radiation and fuel dilution – No/small ELMs • Core constraints must be translated into divertor/SOL design metrics – Separatrix values: Density, temperature, neutral flux, impurity density, turbulence, etc. – These values are not well defined for future devices 3 IAEA-TM Div. Concepts, Vienna Sept. 2015

Core Confinement is a Convolution of Multiple Physics Processes • Confinement degradation can occur through several pathways – Pedestal pressure degradation – Profile peaking/flattening – Rotation – MHD instabilities; NTMs, RWMs, locked modes, etc. • Global confinement is not a good metric for core compatibility divertor solutions in existing devices – The divertor and SOL most directly affects the pedestal through the separatrix – Most other transport processes can be described in terms of pedestal top conditions; Density, Temperature, Rotation, impurities, collisionality, etc. 4 IAEA-TM Div. Concepts, Vienna Sept. 2015

Fusion Performance Relies on Robust H-mode Pedestal ITER Fusion Power • Fusion gain scales strongly with pedestal pressure • Predictive EPED model accurate over range of conditions • EPED requires Div/SOL input – Density: Dependent on separatrix density and fueling Pedestal Pressure – Z eff : Dependent on SOL impurity transport Comparison of EPED Model to 288 Cases on 5 Tokamaks • Operational space for EPED validity 10 2 Measured Pedestal Height (kPa) JET (137) uncertain for DIII-D ELM (109) DIII-D QH (11) – P sep à P LH JT-60U (16) C-Mod (10) AUG (5) – High collisionality 10 1 ITER – High/Low edge recycling 10 0 10 0 10 1 10 2 P. Snyder NF 2015 EPED Predicted Pedestal Height (kPa) 5 IAEA-TM Div. Concepts, Vienna Sept. 2015

Uncertain relationship between Pedestal density and SOL conditions Density and N Dependence of EPED1 for ITER Ref and Hybrid • Optimal pedestal density a 150 combination of several factors EPED1 Pedestal Height (kPa) 125 – Pedestal pressure 100 – Fusion Gain 75 – Current drive efficiency 50 • Div/SOL models predict n sep and Γ n 0 Reference (I p =15MA, N =2) Hybrid (I p =12MA, N =2) 25 – n ped results from ionization source Hybrid (I p =12MA, N =2.6) Hybrid (I p =12MA, N =3.2) and transport 0 0 5 10 15 20 25 30 – n sep /n ped ~25% - 50% in existing Pedestal Density (10 19 m -3 ) experiments P. Snyder NF 2011 • Progress needed in two areas – Pedestal density transport to predict n ped from n sep and Γ n0 – Divertor design techniques to optimize n sep and Γ n0 for dissipative divertor operation 6 IAEA-TM Div. Concepts, Vienna Sept. 2015

Pedestal Density Transport Progress Will Require More Attention ASDEX-Upgrade • Uncertain role of edge recycling in pedestal density profile – Conflicting evidence of pedestal density pinch – Sets possible range of n sep /n ped – Lack of pedestal ionization profile measurements hampers testing of emerging pedestal transport models • Pedestal ionization source – High poloidal asymmetry M. Willensdorfer NF 2013 – Difficult to measure 0.3 1.0 DIII-D D eff • Pedestal ionization profile 0.8 measurements a key capability Density (10 20 m -3 ) 0.2 D eff (m 2 s -1 ) 0.6 for progress 0.4 0.1 0.2 0.0 0.0 0.90 0.95 1.00 1.05 ψ A. Leonard JNM 2013 7 IAEA-TM Div. Concepts, Vienna Sept. 2015

Pedestal Pressure Often Degrades with Divertor Heat Flux Control and Detachment JT-60U (a) 1.2 • Confinement and pedestal Ar puff ( δ =0.36) degradation with high density 1 operation widely reported H H -factor 0.8 • Mechanisms for pedestal 0.6 degradation must be understood to D 2 puff ( δ =0.36) D 2 puff ( δ =0.16) scale results to future tokamaks 0.4 0.2 0.3 0.4 0.5 0.6 0.7 0.8 – Excessive core radiation n e /n GW – Lower MHD stability and increased (b) 2.5 transport at high collisionality Ar puff 2 – Neutrals, charge-exchange directly W th [MJ] 1.5 degrading transport barrier D 2 puff – Induced turbulence 1 0.5 δ =0.36 n e 0 [10 19 m -3 ] 2 2.5 3 3.5 4 4.5 0.8 n e /n GW 0.4 0.5 0.6 0.7 H. Urano NF 2015 8 IAEA-TM Div. Concepts, Vienna Sept. 2015

Adequate power across separatrix required for robust pedestal Alcator C-mod • Pedestal typically degrades for P sep ≤ P LH • Pedestal degradation for: – Lower P sep due to instrinsic or seeded impurity radiation – Density increase raises P LH • Implications for divertor design – Limited radiated power fraction in main chamber A. Loarte PoP 2011 – SOL impurity density limit – Upper limit to core (and SOL) density LH = 4.9 x 10 4 n e ,20 0.72 B t 0.8 S 0.94 P 9 IAEA-TM Div. Concepts, Vienna Sept. 2015

Excessive X-point Radiation Can Degrade Pedestal and Confinement ASDEX-Upgrade • ASDEX-Upgrade: Achieving complete detachment across N 2 injection D 2 injection divertor target with N 2 injection Inner Detachment Complete Detachment results in X-point radiation – Pedestal pressure degrades ~60% – Profile peaking limits confinement loss to ~10% – Unknown correlation of P ped with P sep à P LH • Is X-point radiation as detrimental as radiating mantle? – On one hand, L || may allow large T e gradient to midplane – On the other, X-point radiation may rob q ⊥ across separatrix F. Reimold Nucl. Fusion 2015 10 IAEA-TM Div. Concepts, Vienna Sept. 2015

Impurities can also Improve Pedestal Through Z eff and ν * JET • Optimal impurity density can be > 0 – Low collisionality can lower pedestal pressure stability limit through excessive bootstrap current – Similar effect to pedestal density – Z eff and ν * may also affect local pedestal transport • Ideal impurity level for DEMO will depend on operational scenario – Best plasma and impurity densities will depend on overall performance optimization – Ideal pedestal collisionality may require some ‘leak’ of seeded divertor impurities G. Maddison Nucl. Fusion 2014 11 IAEA-TM Div. Concepts, Vienna Sept. 2015

Low Pedestal Ionization May Lead to Higher Pedestal Pressure and Confinement JET • Reduced pedestal fueling – Lower density and pressure gradients – Wider pedestal with higher pressure limit – Uncertain transport relationship between n e and T e • Effect of pedestal ionization in future devices will require development and validation of pedestal transport models – Low appears better. Is zero the best? – Measurement of 2D pedestal ionization profile a key capability needed for this development C. Challis NF 2015 NSTX Lithium Injection J. Canik PoP 2011 12 IAEA-TM Div. Concepts, Vienna Sept. 2015

SOL Turbulent Transport May Limit Compatibility of Divertor Detachment with H-mode • As density increases: DIII-D f GW GW ~ 0.4 ~ 0.4 f GW GW ~ 0.27 0.27 – Far SOL turbulence with rapid radial transport f GW GW ~ 0.5 ~ 0.5 f GW GW ~ 0.35 0.35 100 moves inward towards separatrix DSOL (a) LSOL OWS – SOL radial transport correlated with collisionality ~ 3 cm n e ( × 10 18 m -3 ) 10 • If increased SOL turbulence linked with ~ 8 cm ~ 3 cm collisionality at field-line/material ~ 5 cm 1 interface: LCFS ~ 2 cm – Divertor detachment may inherently induce BL 0.1 excessive turbulence at midplane separatrix 223 225 227 229 231 233 235 237 R (cm) – Potentially linked to density limit D. Rudakov NF 2005 D. Carrarelo JNM 2015 13 IAEA-TM Div. Concepts, Vienna Sept. 2015

Pedestal Degradation Not Inherently Linked with Divertor Detachment in DIII-D 12.0 Pedestal Pressure • Full detachment across target before Pedestal Pressure (kPa) 10.0 degradation of pedestal pressure 8.0 • Reduction of pedestal pressure gradient 6.0 6.2 MW consistent with MHD stability 4.0 4.9 MW • Further work needed on pedestal 2.0 response to high collisionality 0.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Pedestal n e,ped (10 19 m -3 ) n e,ped =7.0 × 10 19 m -3 Normalized Pedestal Current (j*w 1/4 ) -0.9 Unstable 0.30 Stable 19 n ped =3.8x10 -1.0 19 n ped =4.7x10 0.20 19 n ped =5.3x10 -1.1 19 Z (m) n ped =6.0x10 19 γ =0.5 ω Dia n ped =6.7x10 0.10 γ =0.05 ω A -1.2 19 n ped =7.0x10 19 n ped =7.1x10 0.00 1.35 1.40 1.45 1.50 1.55 1.60 1.65 0.0 0.5 1.0 1.5 2.0 2.5 Major Radius (m) Normalized Pressure gradient (p’*w 1/4 ) 0.1 1.0 10.0 100.0 1000.0 14 IAEA-TM Div. Concepts, Vienna Sept. 2015 T e (eV)