IAEA-CN-258/512

NEGATIVE TRIANGULARITY EFFECTS ON
TOKAMAK MHD STABILITY

L. J. Zheng, M. T. Kotschenreuther, F. L. Waelbroeck,
M. E. Austin, W. L. Rowan, P. Valanju, and X. Liu
Institute for Fusion Studies,
University of Texas at Austin
Troyon limit

Negative triangularity tokamak:Ø Not only good for divertor designØ But also good for MHD stability

10/22/2018

IAEA FEC 2018

1Outline
1.

Motivations: why negative triangularity
Ø Design philosophy prioritizes solution of divertor heat load issue

2.

Concern about the MHD Stability of negative triangularity tokamaks
Ø H mode confinement is poor
Ø L mode gets the H mode level confinement, beta limit is lower, but acceptable

3.

Our NEW results: L mode with high bootstrap current fraction can achieve evenhigher beta than H mode in the positive triangularity caseØØ
ØØ

4.

High beta confinement: 8-10 Li (I/aB), beta limit doubled for low n modes!
ELM free, no major concern about RWMs, kink disruption, etc.
Steady state confinement, “soft” beta limit (high n ballooning)Experiments show low turbulence levelConclusions and discussion
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Outline
1.

Motivations: why negative triangularity
Ø Design philosophy prioritizes solution of divertor heat load issue

2.

Concern about the MHD Stability of negative triangularity tokamaks
Ø H mode confinement is poor
Ø L mode gets the H mode level confinement, beta limit is lower, but acceptable

3.

Our NEW results: L mode with high bootstrap current fraction can achieve evenhigher beta than H mode in the positive triangularity caseØØØØ

4.

High beta confinement: 8-10 Li (I/aB), beta limit doubled!
ELM free, no major concern about RWMs, kink disruption, etc.
Steady state confinement, “soft” beta limit (high n ballooning)Experiments show low turbulence level

Conclusions and discussion
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H-modes are good, but ELMs are unacceptableD3 experiment results

Kichuchi, et al, EPS 2014
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For foreseeable materiallimit, divertor heat load is achallenging problem for
positive triangularity
tokamaks, especially forfusion reactors
4Non core-the-first design philosophy:
Negative triangularity tokamaks (Kikuchi,et al.)Original thoughts: Negative triangularity can gain for divertor design,
but may give up in the beta limit
Ø a larger separatrix wetted area,
Ø wider trapped particle-free scrape-off layer,
IAEA FECthe
2018 divertor room.
Ø10/22/2018
larger pumping conductance from5

Outline
1.

Motivations: why negative triangularity
Ø Design philosophy prioritizes solution of divertor heat load issue

2.

Concern about the MHD Stability of negative triangularity tokamaks
Ø H mode confinement is poor
Ø L mode gets the H mode level confinement, beta limit is lower, but acceptable

3.

Our NEW results: L mode with high bootstrap current fraction can achieve evenhigher beta than H mode in the positive triangularity caseØØØØ

4.

High beta confinement: 8-10 Li (I/aB), beta limit doubled!
ELM free, no major concern about RWMs, kink disruption, etc.
Steady state confinement, “soft” beta limit (high n ballooning)Experiments show low turbulence level

Conclusions and discussion
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IAEA FEC 20186Earlier TCV negative triangularity experiments

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7Recent DIII-Dexperiments

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Important development of DIII-D experiment

Ø the H-mode-levelconfinement (H98yp=1.2)
with L-mode-like edge
behavior without ELMs
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Stability: DIII-D experiment interpretation

Ø Equilibrium: use the g file from experimental data reconstruction

Ø Ideal MHD Stability is confirmed with critical wall position 1.11,experiments
10/22/2018consistent with the D3D limiter
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2018

10

Numerical exploration of D3D type of L-mode equilibria

Ø Three types of triangularity cases: δ = 0.4, 0, and - 0.4 are investigatedØ L mode profile is assumed (close to DIIID experiments)
Ø Results: Positive triangularity: best
Zero triangularity: stay at middle
Negative triangularity: worst, but acceptable
10/22/2018Ø Negative triangularity is bad for
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FEC 2018
H mode11

Outline
1.

Motivations: why negative triangularity
Ø Design philosophy prioritizes solution of divertor heat load issue

2.

Concern about the MHD Stability of negative triangularity tokamaks
Ø H mode confinement is poor
Ø L mode gets the H mode level confinement, beta limit is lower, but acceptable

3.

Our NEW results: L mode with high bootstrap current fraction can achieve evenhigher beta than H mode in the positive triangularity caseØØØØ

4.

High beta confinement: 8-10 Li (I/aB), beta limit doubled for low n modes!
ELM free, no major concern about RWMs, kink disruption, etc.
Steady state confinement, “soft” beta limit (high n ballooning)Experiments show low turbulence level

Conclusions and discussion
10/22/2018

IAEA FEC 201812Profile comparison between
positive and negative triangularity cases
Positive triangularity

Negative triangularity

Ø Observation: the safety factor q at edge is smaller for negative triangularity caseØ Motivate us to reduce the Ohmic current to consider the advanced tokamak
scenario with high bootstrap currentIAEA
fraction
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FEC 2018

13

Negative triangularity tokamak in advanced scenario
with high bootstrap current fraction
0.1414
δ = - 0.40

0.12

120.1

10

β0.08

q

β80.06

6

q

0.04

40.02

20000.2

0.4

0.6

0.81

χ

Given density and temperature profiles, the current is computed self consistently
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Low n (1-5) kink mode stability for
negative triangularity tokamak in advanced scenario

Low n kink modes stablecases:
Ø n=1-5 stableØ No-wall limit
Ø Stability confirmed
both by AEGIS andDCON
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Troyon limit

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High n ballooning mode stability for
negative triangularity tokamak in advanced scenario
0.14

0.12

120.1

10

β

Local ballooning
unstable radial region

0.06

8q0.08βØ Because of peaked pressure
profile, high n ballooning
modes tend to give lower
betaN limit: 4 li (I/aB)Ø Further profile optimizationis still in process14δ = - 0.40

6q0.0440.02

20000.2

0.4

0.6

0.8

1

χ

Ø High n ballooning mode theory keeps only lowest order, global calculationshows that the n=5-10 stability can be achieved.
Ø Possible FLR stabilization for high n modes
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Ø “Soft” beta limit

16

Further D3D experiments, guided by our calculations, yieldinteresting results based on the preliminary analyses

In the PT case, reducing current leads to the confinement time worsen (~I/aB).
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In thisD3D NT case, when the current isIAEA
reduced,τe remains unchanged --- good 17signOutline
1.

Motivations: why negative triangularity
Ø Design philosophy prioritizes solution of divertor heat load issue

2.

Concern about the MHD Stability of negative triangularity tokamaks
Ø H mode confinement is poor
Ø L mode gets the H mode level confinement, beta limit is lower, but acceptable

3.

Our NEW results: L mode with high bootstrap current fraction can achieve evenhigher beta than H mode in the positive triangularity caseØØØØ

4.

High beta confinement: 8-10 Li (I/aB), beta limit doubled!
ELM free, no major concern about RWMs, kink disruption, etc.
Steady state confinement, “soft” beta limit (high n ballooning)Experiments show low turbulence level

Conclusions and discussion
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Conclusions: Negative triangularity& L - mode & low Ip and high bootstrap current• The benefits of negative triangularity are not limited to divertor
Ø NEW: negative triangularity also improves MHD stabilityü Steady state confinement with high bootstrap current fraction
ü ELM free
ü high resistive wall mode beta limitü Low n stability, reduce the kink type disruption possibilityü soft instabilities (high n ballooning modes) to avoid high inevitablebeta state, that eventually causing disruption, FLR stabilization? Global effects
ü Experiments already show a reduced anomalous transport
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