Presentation on theme: "Steady state tokamak research"— Presentation transcript:
1 Steady state tokamak research Lecture 3 at ASIPP, May 15, 2013Steady state tokamak research( Power and particle handling –Is H-mode relevant for fusion reactor?)M. KikuchiSupreme Researcher, JAEAChairman, Nuclear Fusion Board of EditorsGuest Professor, ILE Osaka UniversityVisiting Professor, Fudan University, SWIPGuest Lecturer, the University of TokyoAcknowledgement:A. Fujisawa for turbulence & measurementL. Villard, A. Fasoli and TCV teamR. Goldston for SOL heat flow scalingJ. Rice, B. Lipschultz for C-Mod, I-modeH. Sugama for NC polarizationWulu Zhong/X. Duan for ITG/TEM workPat Diamond for discussion (WCI symposium)
2 Motivation of this talk “Tokamak” is a most promising concept with its excellent energy confinement.Tokamak with D-shaped, H-mode is optimized for core confinement.3. Steady state operation needs more work (see my Reviews of Modern Physics (2012).
3 Motivation of this talk Recent papers by Goldston (NF2012) and Eich(PRL2011) casted important question on reactor power handling in H-mode. Prediction for ITER heat flux 1/e length lq-SOL=5mm -> 1mm.2. ITER may be able to manage power handlingfor lower Pf~0.5GW and short pulse tduration~400sby temporary measures such as RMP, pellet pace making, etc.3. But DEMO/Commercial requiresPf~3GW & tduration~10Months.This may requirefundamental change in design philosophyfor tokamak reactor configuration.“Optimize CORE” -> “Optimize power handling”.
4 Present Fusion power handling scenario is very challengingSurface / Volume ratio is small in Fusion but large in Fission1000Fission100w/o RRC~1MW/m210Fusion Divertor(even with RRC)Heat Flux (MW/m2)Fossil1.0Fusion 1st wall~0.3MW/m2High thermal efficiency may be possible only at low heat flux!!RRC=Remote Radiative Cooling0.110-3s1 yearDuration
5 Any energy system (Fusion) must have reliable heat exhaust scenario Tokamak configuration is optimized for good confinement, but not for power handling. D-shape is good (MHD) for high pedestal pressure with H-mode (ETB), leading to large DW loss during ELM.Temporary measure : RMP, Pellet pacing/SMBI D-shape leads to X-point toward small R region. This makes power handling more difficult.Temporary measure : Snow flake, Super X
6 Do we see significant progress in these 20 years? DEMO : Strong D and impurity puffs at divertor, shallow pellet at SOLUeda, Kikuchi NF1992SOL transport : Sophisticated control is required to reduce q~7MW/m2 even with Bohm diffusion (L-mode)Fe puff = 0.01GpQ=600MWGp=2.5x1023/stE=1.4stp=0.5sGas puff 7GpImp. puff 0.01GpHigh Z : sheath acceleration (important even for He)Stable semi-detach is challengingIn reactor : one failure is serious !!Kajita, NF2009 (Top10) W nano structure
7 Divertor Plasma Control (Fluid simulation) Should be kinetic at SOL !!Imp. force balanceParticle balanceIon force balanceAlbedo=0.96Ion energy balanceElectron energy balanceUeda, Kikuchi, et al. NF1992Bohm diffusion is assumed for SOL particle transport perpendicular to flux surface.
8 Where is question on power handling? SOL heat flux e-folding length lq-SOLR1mm5mmlq~rpPrevious estimate for ITER:5mmRecent estimate for ITER:1mmR. Goldston NF2012. H-mode SOLNote: L-mode is governed by different physics , empirical scaling 1cm for ITERFigure (Federici, NF2001)Div heat flux e-folding length lq-div is larger by flux expansion ratio for attached plasma.
9 2nd Goldston scaling(l~rp ) What is key physics of Goldston scaling?(neo)classical particle transport in H-modeGrad /curvature B drift into SOLParallel flow connect top and bottomPSOL is Spitzer thermal conductionAssumed as same order<vd><vd>l//l0.5cs2nd Goldston scaling(l~rp )Fast parallel SOL flow reduces l to 1mm!!ionelectronA. Chankin NF2007:Fast parallel flow ~ 0.5Cs comes not from fluid simulation, unresolved issue.
10 Experimental result seems in agreement with Goldston scaling C-Mod (Bp~BpITER) SOL e-folding length~1mmKey evidences :H-mode particle flux from separatrix ~ neoclassical drift flux.Particle flux GpELM free H-mode ~0.1 GpL-modeis too low and,Required flux multiplication factor Gbecomes larger.Tdiv ~ q//div / (GGp/ln)Scale length difference ln>>lqespecially in H-mode4. ELM to enhance Gp :ELM must be minute.Controllability of ELM Gp << L-modeB. Lipschultz, FESAC meeting July, 2012“ Goldston scaling needs more check.”
11 Why SOL flow is so fast as 0.5Cs ? Takizuka, NF2009 showed PARASOL PIC simulation reproduces correct SOL flow pattern and fast SOL flow but not Er effect.Trapped & Circulating ion excursion across the separatrixcomparably kick parallel ion flow to be 0.5Cs like a NC parallel viscous force!! Takizuka, CPP2010 (PET12)- It is ion convective flux !! -
12 Can we increase GpH-mode? Key questions :Can we increase GpH-mode?High recycling at main SOL is prohibitive!2. Can we reduce SOL flow speed?Drift across flux surface is key!If not, shall we kill H-mode?L-mode is best but not sufficientI-mode as an alternative path?4. High edge pedestal is good choice?Shall we reduce edge beta limit for small ELM?
13 Modify H-mode to more high recycling? Gas puffing at main chamber is prohibitive!! Wall saturation is natural consequence of steady state tokamak reactor. Ti at mid-plane SOL is order of eV, strong gas puff at mid-plane produces energetic neutrals to erode wall a few cm/year. DEGAS simulation in typical JT-60U condition showing non-negligible population of fast neutrals ( eV). Therefore control of neutral around main first wall is important.Kikuchi, FED2006
14 Issues in present reactor design philosophy (A) : Optimization of Core plasmaSSTR1990(B) : Divertor design to match (A)(C) : consistency of (A)& (B)D-shape/H-mode is thought as optimum for CORE.D-shape : Rdiv << Rp : bad for power handling !H-mode : Large Pedge -> Large ELM energy loss !3. H-mode : Low particle flux !4. D shape : huge Amp Turn for “snow flake”.5. D-shape : inboard blanket design not easy.RpRdivLevel of problem :D-shaped > H-mode
15 I-mode (MIT) with peaked ne may be better, but -- I-mode : Grad B away from X-point and need high power L -> I (H) modeHigh edge Te (low collisionality).L-mode like tp but at lower edge ne. Note : Reactor needs high SOL ne.[ NSTX Li discharge has high Te and low ne]Trapped ion orbitTakizuka CPP2010Whyte NF2010I-mode geometry has even faster SOL flow-> leads to lower edge density?
16 ‘Core the first’ is not a good design philosophy Think different !First priority：Configuration optimizationon power handling(B)(1) Core tomatch (A)(2) Divertor tomatch (A)Integration tomatch (A)We have rich knowledge
17 First Step : Divertor priority higher than core! Stay foolish !- S. Jobes -A choice - negative DMake edge pedestal b limit low!Stay in L-mode edge or I-mode?Find new transport reduction physics!Ex.Reactor core is more collisionless.Optimization of TEM- Trapped electron precessionNegative D reduce TEM growth.
18 Make power handling easier by an order of magnitude R=7m, a=2.7m (A=2.6)Standard D shape : Rx=4.3mInverted D shape : Rx=9.7mFactor of 2.5 for RdivNegative D makes DN possibleFactor of(care on up-down asymmetry, controllability)Snow flake at Rx : Factor of 2-3Factors : 2.5 x 2 x 2 =10 !!!4.3mNote:- DN in D-shape is difficult for piping to inboard blanket.- Snowflake needs internal PF coil to reduce AT.- Outboard is much easier to install internal PF.Field becomes stiff by near-by PF coilsNbTi is possible at low field.9.7m
19 MHD stability of negative triangular plasma Negative delta has higher frequency ELM.Strongly shaped negative delta has higher edge pressure limit at low J///<J> due to large shear.Pochelon PFR2012Courtesy : TCV team
20 Ip, Bt Structure of SOL flow in negative D High field side: There is no trapped particles across Separatrix.-> Absence of parallel acceleration mechanism-> Absence of subsonic flow?Ip, BtLow field side:SOL is almost vertical-> No NC drift acrossseparatrix.-> No change inpressure anisotropy-> Do we see parallelviscous force?Larger local pitch-> shorter connection LNear X-point-> lower local pitchby snow flake
21 Ip, Bt Banana orbit loss in negative D Confined Banana : Larger than banana width from separatrix, trapped ions will be confined.Lost Banana:Near the separatrix,we have lost banana orbit.-> This may induce Er<0 and resultant counter toroidal rotation >> standard D.-> Effective RWM stabilization.-> Nullify parallel flow acceleration in low field SOL.Ip, Bt
22 2nd Step : Consistent core plasma! There are two paradigm to suppress turbulent transportFlow shear/zonal flow suppressionDe-resonance of trapped particle precession with TEMOperationally, we have 3 core improved regimes(See my RMP paper)Weak positive shear(High bp mode, optimized shear, improved H, etc)2. Negative shear(NS, RS, NCS, etc)3. Current HoleSee Fujita NF review paper.
23 Negative d and Shafranov shift Good for high bp scenario since Shafranov shift increases with bpPrecession driftNegative d can reduce TEM growth rateB.B. Kadomtsev, NF 1971Shafranov shift can change precession driftConnor, NF 1983G. Rewoldt, PF 1982
24 Increasing experimental evidence of TEM/ITG transition Wulu Zhong, 2nd APTWGTore Supra expl.Dispersion relation for TEM/ITG modes in strong ballooning limit.Weiland textbook, 2000Also, J. Rice, FEC2012 bifurcation of intrinsic rotation TEM/ITG
25 Shaping effect of Residual Zonal Flow (RZF) Xiao-Catto PoP2006, 2007Belli, Hammett, Dorland, PoP2008Key is to reduce NC polarizationRadial profile of d- dd/dr is key to RZF -NC polarization ~ (Banana width)2Negative delta : strong outboard Bp-> smaller banana width!!Elongation increases RZFNegative d may weakly reduces RZF.Xiao PoP2007GS2GS2(1)(1)Xiao PoP2007Understanding of RZF in negative triangularity (k,-d, D) is necessary
27 TCV negative triangularity experiment Camenen NF2007Negative triangularity produces large Shafranov shift, which changes precession drift of trapped electron. This leads to a change in TEM stability.Large tilting in negative deltaSimilar effect like Er’ ?More tiltedLess tiltedNon-locality will be reduced in Reactor
28 SummaryThe power system should have reliable power handling but fusion power handling is challenging in divertor.H-mode with D-shaping “Optimize Core choice” seems enhancing its challenge.Tokamak physics is ready for new innovation. Good knowledge in core physics will make innovation possible.Power handling-driven Tokamak optimization needs good core physics innovation.We proposed “Negative D” as a candidate of this challenge.
29 We probably need order of magnitude change to solve this issue. Prof. P.H. Rebut : Best Scientist in engineering and physicsHe is in favor of Fusion-Fission Hybrid.I asked him why?P.H. Rebut : There is no solution for power handing in pure fusion, right now. Stay low fusion power. We have to boost fusion energy to have net energy. Fission is most effective to boost.His word is important from engineering point of view on pure fusion.We probably need order of magnitude change to solve this issue.