IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

Impact of High Field & High Confinementon L-mode-Edge Negative
Triangularity Tokamak (NTT) Reactor
M. Kikuchi, T. Takizuka, S. Medvedev, T. Ando, D.
Chen, J.X. Li, M. Austin, O. Sauter, L. Villard, A. Merle,
Y. Kishimoto, K. Imadera

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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

1. Motivation: Fusion Power Handling is quite challenging!

10MW/m2 : short time fusion / 0.3-1MW/m2 : Reliable power system
Disruption
Runaway electron~1MW/m2
Fission1000
Heat Flux (MW/m2)

ELM
100
10w/o Radiative cooling

Radiation heat flux on the Sun

Fusion Divertor(with Radiative cooling)

1.0
0.1

Fossil

~0.3MW/m2

Fusion 1st wall
10-3s

Duration

1 year2

IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

2. Fusion Target to Months of operation

There is 7 order of gap between present-day highperformance plasma to DEMO in the power load duration.
Most critical is long pulse power handling
7 order difference3IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

3. L-mode Edge Negative Triangularity Tokamak (NTT)
1. Geometrical Factor ~2.8 @ Grazing angle ~1o
2. H-mode edge << L-mode edge (suitable for reactor)a) D-shaped+ H-modeb) Negative δ+ L-mode Edge
Better stability for TEM

Grad B
drift

Ip, BtL-mode profile

1.0

Γp

ne

Rdiv=5m

H-mode profile
SOL flowR

Rdiv=14.5m

ΓpH ~NC level

ΓpL >>NC level

Difficult to pump

Easier to pump

R 0.0
0.0r/a1.04IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

4. There are increasing Evidence of L-mode Edge High Confinement
Y. Camenen, NF2007

H

τEe (NTT)~ 2 times(L-mode Edge)

M. Austin, EX/P6-6
High confinement
inNegative triangularity
discharges in DIII-D

L

CamenenNF 2007

ne(0)=6x1019m-3
Te(0)=4keV
Pheat=10.5MW

TCV (ap=25cm,Bt=1.44T)
Kadomtsev NF1971
Precession drift reversal
for TEM stability

More stable

DIII-D (ap=59cm,Bt=2T)

More unstable

Higher Bp

δ=-0.3δ=+0.3Riva PPCF2017：
Larger magnetic pitch stabilize
resistive ballooning
SauterPoP2014: De-stiffening
MelroPPCF2015: Higher Tcrit
Fontana2018: smaller turbulence
CodaFEC2018: smaller λq??
5IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

5. Optimized Pressure Profile in NTT
NTT is magnetic Hill configuration.
- Central Region limited by Mercier stability.
- Outer half limited by Ballooning stability.
b) Optimized pressure gradient profile in KINXa) Optimized pressure profile0.04

1.0pp’

Mercier limiting edge0.02Ballooninglimiting region

Mercierlimiting
region0.5

w/o pedestal

00.5

0.0
1.0

R/Rp

1.500.5ψ0.5

1.0

βN (stability limit) ~3.3
Within this limit, reactor design is possible βN=2.3.

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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

6. Reducing risk for large ELM energy loss
Large pedestal pressure is the source of risk of huge ELM energy loss
NTT tends to stay at L-mode edge but still has risk to be H-mode and ELM.
NTT edge pedestal pressure limit is at least 4 times smaller than D-shaped.
Thus reducing risk of large ELM energy loss.

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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

7. Double and Single Null NTT in FEC 2014 and 2016
IAEA FEC 2014 S. Medvedev, M. Kikuchi, et al.
Rp=7m, a=2.7m (A=2.6), Ip=15MA, Bt=6.2T,
κx=1.5,δx=-0.9, βN lim>3, γn=0=95/s

IAEA FEC 2016 S. Medvedev, M. Kikuchi, et al. Rp=9m,Ip=21MA, 3GW fusion power at βN=2.1, Bmax=13.6T, HH=1.12

κx=2OK8

IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

8. Parameter scan of SN-NTT Configuration for compact reactor
A=3, HHIpA= 69.3MA : Given- Solid line : HH=1.2 fixed,- Broken line : HH varied (1.1-1.5) with Rp

9IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

9. Reactor Configurations in NTT with Race-track shaped TF coilsa)

IAEA FEC 2016 S. Medvedev, M. Kikuchi, etal. Rp=9m, Ip=21MA, 3GW fusion power at
βN=2.1, Bmax=13.6T, HH=1.12b)

Significant size reduction is possible byenhanced confinement HH=1.5.
Rp=7m, Ip=15.4MA, 2GW fusion power at
βN=2.7, Bmax=13.5T
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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

10. Flux Tube Expansion Divertor in NTT
15MA equilibrium a) without FTE,b) with FTE,c)Expanded magnetic structure with FTE

IFTE=2.5MA

α=2.1 o

GFTE=4.2
GFTE=4.2

GFTE=2.0

IFTE=2.5MA

GFTE=2.0-FTE coil current to expand flux tube to satisfy a=1-1.5o is only 2.5MA α=2.6 o
Active control of flux expansion for both legs are possible.
Coils can be sector coils by NbTi wires
L-mode edge ay enable power handling more easier than H-mode edge.

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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

11. Magnet Designs in NTT
Magnet design of NTT is technically feasible (Jc=1000A/mm2)

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IAEA-CN/258/FIP/3-4 , 27th IAEA Fusion Energy Conference, Ahmedabad, India

12. Summary
1. D-shaped H-mode is the standard operation mode in tokamak.
2. It brings many critical issues (ELM, narrow heat channel, small particle flux,
heat and particle exhaust at small R)
3. Efforts has been made to resolve these issues but still challenging.
4. Divertor for Negative triangularity provides maximum wetted surface withflux tube expansion.
5. L-mode edge observed in NTT experiments enhances its merit much further.
6. Low edge pedestal beta limit is safer for ELM damage.
7. Long-pulse power handling is 1st priority for 1st generation tokamak reactor.
8. L-mode edge NTT may provide possible solution.
9. We are ready for medium to large test NT tokamak.
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