Steady state tokamak research ( Power and particle handling –

Is H-mode relevant for fusion reactor?)M. Kikuchi

Supreme Researcher, JAEAChairman, Nuclear Fusion Board of Editors

Guest Professor, ILE Osaka UniversityVisiting Professor, Fudan University, SWIP

Guest Lecturer, the University of Tokyo

Acknowledgement: A. Fujisawa for turbulence & measurement

L. Villard, A. Fasoli and TCV team R. Goldston for SOL heat flow scaling

J. Rice, B. Lipschultz for C-Mod, I-mode H. Sugama for NC polarization

Wulu Zhong/X. Duan for ITG/TEM work Pat Diamond for discussion (WCI symposium)

Lecture 3 at ASIPP, May 15, 2013

Motivation of this talk

1.“Tokamak” is a most promising concept with its excellent energy confinement.

2.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).

Motivation of this talk

1. 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 handling for lower Pf~0.5GW and short pulse tduration~400s by temporary measures such as RMP, pellet pace making, etc.

3. But DEMO/Commercial requires Pf~3GW & tduration~10Months. This may require fundamental change in design philosophy for tokamak reactor configuration.

“Optimize CORE” -> “Optimize power handling”.

0.1

1.0

10

100

1000

10-3s 1 year

Fossil

Fission

Fusion Divertor(even with RRC)

Fusion 1st wallHeat

Flu

x (M

W/m

2 )

~1MW/m2

~0.3MW/m2

Surface / Volume ratio is small in Fusion but large in Fission

Present Fusion power handling scenario is very challenging

RRC=Remote Radiative Cooling

Duration

w/o RRC

High thermal efficiency may be possible only at low heat flux!!

Any energy system (Fusion) must have reliable heat exhaust scenario

• Tokamak configuration is optimized for good confinement, but not for power handling.

[1] 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[2] D-shape leads to X-point toward small R region. This makes power handling more difficult. Temporary measure : Snow flake, Super X

Do we see significant progress in these 20 years?DEMO : Strong D and impurity puffs at divertor, shallow pellet at SOL

SOL transport : Sophisticated control is required to reduce q~7MW/m2 even with Bohm diffusion (L-mode)

High Z : sheath acceleration (important even for He)Stable semi-detach is challenging In reactor : one failure is serious !!

Fe puff = 0.01Gp

Ueda, Kikuchi NF1992

Q=600MWGp=2.5x1023/s

Gas puff 7Gp

Imp. puff 0.01Gp

tE=1.4stp=0.5s

Kajita, NF2009 (Top10) W nano structure

Divertor Plasma Control (Fluid simulation)

Albedo=0.96

Particle balance

Ion force balance

Ion energy balance

Electron energy balance

Imp. force balance

Ueda, Kikuchi, et al. NF1992Bohm diffusion is assumed for SOL particle transport perpendicular to flux surface.

Should be kinetic at SOL !!

Where is question on power handling?

Figure (Federici, NF2001)

Previous estimate for ITER:5mmRecent estimate for ITER:1mm

Div heat flux e-folding length lq-div is larger by flux expansion ratio for attached plasma.

R. Goldston NF2012. H-mode SOL

Note: L-mode is governed by different physics , empirical scaling 1cm for ITER

SOL heat flux e-folding length lq-SOL

R

1mm

5mm

lq~rp

What is key physics of Goldston scaling?

ionelectron

(neo)classical particle transport in H-mode

Assumed as same order

<vd> <vd>l//l

0.5cs

✪ Grad /curvature B drift into SOL

✪ Parallel flow connect top and bottom

✪ PSOL is Spitzer thermal conduction

2nd Goldston scaling(l~rp )Fast parallel SOL flow reduces l to 1mm!!

A. Chankin NF2007: Fast parallel flow ~ 0.5Cs comes not from fluid simulation, unresolved issue.

B. Lipschultz, FESAC meeting July, 2012“ Goldston scaling needs more check.”

C-Mod (Bp~BpITER) SOL e-folding length~1mm

Key evidences :

1. H-mode particle flux from separatrix ~ neoclassical drift flux.

2. Particle flux GpELM free H-mode ~0.1 Gp

L-mode

is too low and, Required flux multiplication factor G becomes larger. Tdiv ~ q//div / (GGp/ln)

3. Scale length difference ln>>lq

especially in H-mode

4. ELM to enhance Gp : ELM must be minute. Controllability of ELM Gp << L-mode

Experimental result seems in agreement with Goldston scaling

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 !! -

Key questions :

1. 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!

3. If not, shall we kill H-mode? L-mode is best but not sufficient I-mode as an alternative path?

4. High edge pedestal is good choice? Shall we reduce edge beta limit for small ELM?

Modify H-mode to more high recycling?

[1] Wall saturation is natural consequence of steady state tokamak reactor.[2] Ti at mid-plane SOL is order of 500-700eV, strong gas puff at mid-plane produces energetic neutrals to erode wall a few cm/year.[3] DEGAS simulation in typical JT-60U condition showing non-negligible population of fast neutrals (100-1000eV). [4] Therefore control of neutral around main first wall is important.

Kikuchi, FED2006

Gas puffing at main chamber is prohibitive!!

Issues in present reactor design philosophy

(A) : Optimization of Core plasma

(B) : Divertor design to match (A)

(C) : consistency of (A)& (B)

D-shape/H-mode is thought as optimum for CORE.

1. D-shape : Rdiv << Rp : bad for power handling !2. 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.

SSTR1990

Rp

Rdiv Level of problem : D-shaped > H-mode

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) mode High 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 orbit

Takizuka CPP2010

Whyte NF2010 I-mode geometry has even faster SOL flow-> leads to lower edge density?

(A)： Configuration optimization on power handling

(1) Core to match (A)

(2) Divertor to match (A)

(3) Integration to match (A)

(B)

Think different !

‘Core the first’ is not a good design philosophy

First priority

We have rich knowledge

First Step : Divertor priority higher than core!

Stay foolish !A choice - negative D

Make 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 precession Negative D reduce TEM growth.

- S. Jobes -

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=9.7m

Factor of 2.5 for Rdiv

Negative D makes DN possible Factor of 1.5 - 2

(care on up-down asymmetry, controllability)

Snow flake at Rx : Factor of 2-3

Factors : 2.5 x 2 x 2 =10 !!!

4.3m

Note: - 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 coils NbTi is possible at low field.

9.7m

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

Structure of SOL flow in negative DHigh field side:There is no trapped particles across Separatrix. -> Absence of parallel acceleration mechanism-> Absence of subsonic flow?

Low field side: SOL is almost vertical -> No NC drift across separatrix. -> No change in pressure anisotropy -> Do we see parallel viscous force?

Larger local pitch -> shorter connection L Near X-point -> lower local pitch by snow flake

Ip, Bt

Banana orbit loss in negative DConfined 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

2nd Step : Consistent core plasma!

There are two paradigm to suppress turbulent transport

1. Flow shear/zonal flow suppression 2. De-resonance of trapped particle precession with TEM

Operationally, we have 3 core improved regimes(See my RMP paper)

3. Weak positive shear (High bp mode, optimized shear, improved H, etc)2. Negative shear (NS, RS, NCS, etc)3. Current Hole See Fujita NF review paper.

B.B. Kadomtsev, NF 1971

Connor, NF 1983

Negative d and Shafranov shift

Precession drift

Good for high bp scenario since Shafranov shift increases with bp

Shafranov shift can change precession drift

Negative d can reduce TEM growth rate

G. Rewoldt, PF 1982

Dispersion relation for TEM/ITG modes in strong ballooning limit.

Weiland textbook, 2000

Wulu Zhong, 2nd APTWGTore Supra expl.

Increasing experimental evidence of TEM/ITG transition

Also, J. Rice, FEC2012 bifurcation of intrinsic rotation TEM/ITG

Shaping effect of Residual Zonal Flow (RZF)

Xiao-Catto PoP2006, 2007 Belli, Hammett, Dorland, PoP2008

Elongation increases RZFNegative d may weakly reduces RZF.

Radial profile of d - dd/dr is key to RZF -

Understanding of RZF in negative triangularity (k,-d, D) is necessary

Xiao PoP2007

Xiao PoP2007

(1)

Key is to reduce NC polarization

(1)GS2

GS2

NC polarization ~ (Banana width)2

Negative delta : strong outboard Bp -> smaller banana width!!

Kikuchi NF1990, PPCF1993 Ozeki IAEA1992

FujitaPRL2001,05OzekiEPS2011FujitaNF2011

Wall stab.q(0) up

Reduce dp/dr at qmin

Core improved confinements

WS regime NS regime CH regime

TCV negative triangularity experiment

Negative triangularity produces large Shafranov shift, which changes precession drift of trapped electron. This leads to a change in TEM stability.

Camenen NF2007

More tilted

Less tilted

Non-locality will be reduced in Reactor

Large tilting in negative delta Similar effect like Er’ ?

Summary• The 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.

Prof. P.H. Rebut : Best Scientist in engineering and physics

He 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.