High fusion performance at high Ti/Te in JET-ILW
baseline plasmas with high NBI heating power
and low gas puffing
Hyun-Tae Kim, A.C.C. Sips, C. D. Challis, F. Rimini, L. Garzotti,
E. Lerche, L. Frassinetti, M. Beurskens, X. Yuan, S. Kaye, andJET contributors

2018 IAEA-FEC rehearsal
4 Oct 2018, Culham, UK

Key questions on 2016 JET-ILW
baseline discharges
2016 JET-ILW baseline discharges
Record discharge
in 2016

2.0

1.5

Ti/Te2) Previously, high Ti/Te was observed only
in hybrid and advanced discharges wherene is low. How could 2016 baseline
discharges achieve Ti>Te, despite the highne (i.e. ne >7e19m-3)?

Hybrid1.00.501) The fusion performance achieved in 2016
(i.e. #92436) was much better than that
expected based on previous experimentswith the same high heating power in 2014
(i.e. #87412). Why?

Baseline
M. Beurskens NF 2013
nueff3) High Ti/Te was not observed in theprevious discharges with the same highheating power (i.e. #87412). How was it
possible only in 2016?
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 2

Contents

1.Database
2.Setting for statistical TRANSP analysis
3.Answers to the three key questions
4.SummaryHyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 3

Database❖ 115 H-mode discharges from the 2016 baseline scenario
development were selected.
❖ http://users.euro-fusion.org/tfwiki/images/1/1b/M1501_GoodPulsesList_4wiki.xls

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•
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Pulse number : 89062 - 92442
Ip=2.16 – 3.47 [MA]
Bt=2.35 – 3.03 [T]PNBI=7.22 – 27.79 [MW]PICRF = 0.8 – 6.34 [MW]ne0 > 7e19 [m-3]q95= 2.72 – 3.75
Deuterium Gas puffing = 0 – 9e22 [sec-1]
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 4

Setting for statistical TRANSP analysis• Measured Ti is not available in all discharges due to the difficulty of
the CX data analysis in ITER-Like Wall i.e. No Carbon!
• Alternatively, statistical analysis of interpretive TRANSP runs wereperformed.
✓ Ti=Te was assumed in TRANSP runs of all 115 discharges✓ For subset 11 discharges, where CX measurement is available,measured Ti was used for TRANSP runs
• Except the above mentioned, an identical setting was used for all
TRANSP analysis✓✓✓✓✓✓✓✓✓

Fitted HRTS Te and ne
Zeff from Visible Bremsstrahlung assuming uniform profile
100% Be or 100% Ni impurity assumption (used for error bars).
Total bulk Radiation from Bolometry assuming uniform radiation profile
NUBEAM (30000 Monte Carlo ptls)
TORIC (dipole phasing with Nphi=27, H minority, 3% concentration)
q profile from EFIT
Boundary equilibrium from EFIT
Internal equilibrium calculated by TEQ

Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 5

Key questions on 2016 baseline discharges
1) The fusion performance achieved in 2016 (i.e. #92436) was much better thanthat expected based on previous experiments with the same high heating power
in 2014 (i.e. #87412). Why?

2) Previously, high Ti/Te was observed only in hybrid and advanced discharges where
ne is low. How could 2016 baseline discharges achieve Ti>Te, despite the high ne (i.e.
ne >7e19m-3)?
→The heating power (>25MW) was high enough to decrease the collisionality
despite high ne→Ion heating fraction increases with heating power→Ion heat transport was reduced with high Ti/Te3) High Ti/Te was not observed in the previous discharges with the same high heatingpower (i.e. #87412). How was it possible only in 2016?
→High Ti/Te is correlated with high rotation.
→Rotation freq increase as gas puffing is reduced at the same NBI power.
→Low gas puffing was possible in M15-01 thank to the pellet injections.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 6Statistical Investigation on Ti > Te
High neutrons with Ti=Te in TRANSP
Low neutrons with Ti=Te in TRANSP• TRANSP runs with Ti=Te assumption
underestimates the total neutrons athigh heating power (>25MW).

with Ti=Te

The error bars are the calculated neutrons withdifferent impurity assumptions i.e. 100% Beryllium(left bar) or 100% Nickel (right bar)Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 7

Statistical Investigation on Ti > Te
High neutrons with Ti=Te in TRANSP
Low neutrons with Ti=Te in TRANSP
Ti=CX measurement in TRANSP• TRANSP runs with Ti=Te assumption
underestimates the total neutrons athigh heating power (>25MW).

with Ti=Te• TRANSP neutron predictions consistent
with neutron measurements whenmeasured Ti (>Te) were used in thesubset discharges .

The error bars are the calculated neutrons withdifferent impurity assumptions i.e. 100% Beryllium(left bar) or 100% Nickel (right bar)Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 8

Neutron analysiswith Ti=Te

Increase in thermal neutron rate is the main contribution for the highfusion performance, rather than beam-target neutrons.
High Ti, exceeding Te, is the key factor.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 9

Key questions on 2016 baseline discharges
1) The fusion performance achieved in 2016 (i.e. #92436) was much better thanthat expected based on previous experiments with the same high heating power
in 2014 (i.e. #87412). Why?
→Higher thermal neutrons achieved due to higher Ti
2) Previously, high Ti/Te was observed only in hybrid and advanced discharges wherene is low. How could 2016 baseline discharges achieve Ti>Te, despite the high ne (i.e.
ne >7e19m-3)?
→The heating power (>25MW) was high enough to decrease the collisionality
despite high ne→Ion heating fraction increases with heating power→Ion heat transport was reduced with high Ti/Te3) High Ti/Te was not observed in the previous discharges with the same high heatingpower (i.e. #87412). How was it possible only in 2016?
→High Ti/Te is correlated with high rotation.
→Rotation freq increase as gas puffing is reduced at the same NBI power.
→Low gas puffing was possible in M15-01 thank to the pellet injections.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 10

Collisionality

with Ti=TeNeutron rate well
reproduced
in TRANSP withTi=Te assumption• The neutron under-estimation with Ti=Te in TRANSP is more significant at low
collisionality regime i.e. Ti > Te at low collisionality.
 ex  0.32 10−14me  ne [m −3 ] mi  Te [keV ] 3/ 2• High heating → Increase in Te → sufficient decrease in collisionality (despite high ne)→ Low equilibration power → decoupling Ti from Te.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 11

Heating power analysis
J. Wesson,TOKAMAKSHigh beam power increases Te, thereby decreasing εbeam/Te.
→ ion heating fraction increases.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 12

Transport analysis

Ion heat transport is reduced with high Ti/Te,
while electron heat transport isn’t.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 13

Key questions on 2016 baseline discharges1) The fusion performance achieved in 2016 (i.e. #92436) was much better thanthat expected based on previous experiments with the same high heating power
in 2014 (i.e. #87412). Why?
→Higher thermal neutrons achieved due to higher Ti2) Previously, high Ti/Te was observed only in hybrid and advanced discharges
where ne is low. How could 2016 baseline discharges achieve Ti>Te, despite thehigh ne (i.e. ne >7e19m-3)?
→Pheat(>25MW) was high enough to decrease the collisionality despite high ne→Ion heating fraction increases with heating power→Ion heat transport was reduced with high Ti/Te3) High Ti/Te was not observed in the previous discharges with the same high heatingpower (i.e. #87412). How was it possible only in 2016?
→High Ti/Te is correlated with high rotation.
→Rotation freq increase as gas puffing is reduced at the same NBI power.
→Low gas puffing was possible in M15-01 thank to the pellet injections.

Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 14

Requirements for Ti > Te

Requirements for high Ti/Te
1. Low collisionality to decouple Ti and Te
2. High ion heating fraction
3. Low ion heat transport
However, high beam power alone cannot explain the high Ti/Te.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 15

Impact of rotation

Core Ti/Te is correlated with core rotation frequency.

Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 16

Impact of gas puffing• Core rot freq is correlated with edge rot freq, which is correlated with low gas puffing.
• Thanks to the use of ELM pacing pellets, low gas injection operation was possible,
while keeping ELMs frequent enough.
• For high rot freq, high beam power is required for high momentum source, but low
gas fuelling should be also important as neutrals at the edge may be a strong
momentum sink.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 17

#92436 and #92432
Identical control room parameters in thetwo discharges, except gas fuelling
92436

92432

Ip

3MA

3MA

Bt

2.8T

2.8T

PNBI

27.5MW

27.5MW

PICRH

5MW

5MW

Gas

7e21 #/sec

2e22 #/sec

H98

1.1

1.0

NeutronYield

2.8e16#/sec2.1e16#/sec

Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 18

#92436 and #92432
Similar Te and ne profiles
Similar fast ion distribution
Similar heating profiles
Rotation is clearly higherin #92436 (low gas)than in #92432 (high gas)

Higher core Ti in #92436

Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 19

Possible explanation on high Ti/TeHigh PNBI
in #92436, #92432, and #87412

 ex  + PiNBIPeNBITi
Te RTi Ti
3R(1 + ) Te Ti c 2 2 /  Ls

ITGstabilisation
 RTi   Ti cT.S Hahm Phys. Fluids B. Vol. 1. No6 1989

• High PNBI , reducing collisionality and increasing ion heating ratio,
enabled Ti > Te.
• High Ti/Te is also correlated with low ion heat transport i.e. positive
feedback between high Ti/Te and ITG stabilisation
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 20

Possible explanation on high Ti/TePellet injection

Low gas puffing

High edge rotation

New! only in #92436

High core rotation

High PNBIin #92436, #92432, and #87412

 ex  + Pi
NBI

PeNBITi
Te RTi Ti
3R(1 + ) Te Ti c 2 2 /  Ls

ITGstabilisation
 RTi   Ti cT.S Hahm Phys. Fluids B. Vol. 1. No6 1989

• Low gas puffing increases rotation freq, helping further ITG stabilisation.
• Use of low gas puffing was possible by pellet injections, which were the only
main difference in #92436, compared to #92432 and #87412
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 21

Key questions on 2016 baseline discharges1) The fusion performance achieved in 2016 (i.e. #92436) was much better thanthat expected based on previous experiments with the same high heating power
in 2014 (i.e. #87412). Why?
→Higher thermal neutrons achieved due to higher Ti
2) Previously, high Ti/Te was observed only in hybrid and advanced discharges
where ne is low. How could 2016 baseline discharges achieve Ti>Te, despite thehigh ne (i.e. ne >7e19m-3)?
→Pheat(>25MW) was high enough to decrease the collisionality despite high ne→Ion heating fraction increases with heating power→Ion heat transport was reduced with high Ti/Te3) High Ti/Te was not observed in the previous discharges with the same highheating power (i.e. #87412). How was it possible only in 2016?
→High Ti/Te is correlated with high rotation frequency.
→Rotation frequency increases with low gas puffing.
→Low gas puffing was only possible in 2016 baseline discharges, thank to thepellet injections.
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 22

Summary• 2016 baseline discharges with high beam power (max. 28MW) achieved avery good fusion performance. This comes from core confinement
improvement (not only from the pedestal).
• High Ti, exceeding Te, was observed in baseline discharges despite thehigh ne. This enabled the high neutron yield.
• High Ti/Te was possible because of
1. Low collisionality
2. High fraction of beam heating to ions
3. Reduced ion transport, which should be due to the positive feedback
between high Ti/Te and ITG stabilisation.
• High Ti/Te is correlated with high rotation freq, implying ExB flow shear
may improve the positive feedback.
• High rotation is not only correlated with high beam power, but alsowith low gas puffing, which was possible by pellet injections in 2016.
Neutrals at the edge could be a momentum sink.
• Further details available in Hyun-Tae Kim et al, NF 58 (2018) 036020
Hyun-Tae Kim | TF meeting in Oct 2018 | Culham, UK| Page 23