Study of transient heat flux deposition for various ELM types in NSTX

Study of transient heat flux deposition for various ELM types in NSTX

J-W. Ahn, R. Maingi, and T.K. Gray (ORNL)

In collaboration with K.F. Gan (ASIPP)

Work done through NSTX / ASIPP collaboration

NSTX Monday Physics MeetingAugust 27, 2012

NSTXNSTX Supported by

Columbia UCompXGeneral AtomicsFIUINLJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UORNLPPPLPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

NFRIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech Rep

NSTXNSTX J-W. Ahn: ELM heat flux study in NSTX (NSTX Monday Physics Meeting) 27 August 2012 2

Motivation

• Engineering limit of steady state peak heat flux is generally ~10 MW/m2

• Transient heat flux from large ELMs is a great threat to the protection of divertor plates for ITER and other future devices

It is possible that only small ELMs may be allowed

• NSTX-U will be in higher Ip and heating power with longer pulse length,

therefore ELM heat flux problem will be much more serious

Need to understand heat deposition for various ELM types in NSTX

Possibility to extrapolate to or predict ELM heat flux in NSTX-U and

future ST device

First systematic investigation of ELM heat flux from IR measurement in NSTX


IR camera diagnostics for heat flux measurement

• Divertor surface temperature is monitored by fast IR camera

• Single band (8-12 μm) fast IR camera in 20091

– Spatial resolution: 1.7 mm

– Temporal resolution: 1.6 – 6.3 kHz

• Dual band (4-6μm and 7-10 μm) IR adapter in 20102

– For lithiated PFC surface, 1.6kHz frame speed

• Heat flux calculation from the measured surface temperature

– 1-D radial heat flux profile from THEODOR3

– 2-D (r, Φ) heat flux profile from TACO4,

improved to incorporate thin surface layer effect5

1J-W. Ahn, RSI 81 (2010), 023501, 2A.G. McLean, RSI 83 (2012), 0537063Collaboration with IPP Garching, A. Hermann 4G. Castle, COMPASS Note 97.16, UKAEA Fusion (1997) 5K.F. Gan, submitted to RSI (2012)


Outline

• Study of transient ELM heat flux1 – temporal and spatial evolution of peak heat flux and heat flux width

- Type-V ELMs

- Type-III ELMs with high and low p

- Type-I ELMs

- Relationship between qpeak and q

• 2-D heat flux data produced by TACO (through collaboration with MAST Team), now upgraded to take account of thin surface layer effect

1 K.F. Gan, submitted to NF


TACO calculates 2-D heat flux distribution at divertor surface

1

1 0

0)(

))()((

2)(

jl

jxyo

xyjxy

jxy C

CtQ

C

tTtTktQ

Surface heat flux

• 2-D (r, Φ) heat flux data are obtained from TACO

• Analysis results from 1-D radial profile data are presented here but 2-D data are also being analyzed for the ELM and 3-D physics study

Measured surface temperature Calculated 2-D heat flux

Negative heat flux;Non-physical


The effect of thin surface layer on heat flux calculation

• Thin layer of hydrocarbon composite often forms on the surface of PFC material

• Low thermal conductivity with poor thermal contact with the PFC substrate underneath IR emission is much brighter when a heat pulse hits the surface than it should be w/o thin layer interpreted as much higher temperature rise than real Excessively high heat flux computed to explain this Tsurf

• Also, the surface layer makes the drop of surface IR emission excessively rapid after the heat pulse interpreted as a rapid Tsurf drop Solution of heat conduction equation yields negative heat flux to accommodate this temperature change

Bulk PFC material (k, ρ, cp)

Thin surface layer

Incident heat flux


Implementation of heat transmission coefficient in TACO alleviates negative heat flux problem

dklayer / : Heat transmission coefficient1 klayer: surface layer thermal conductivity, d: surface layer thickness

0

1

1 0

0

21)(

)()()(

Ck

CC

tQC

tTtTtQ

j

jxyo

xyjxyjxy

Surface heat flux with incorporated in heat conduction equation

• With implemented in the heat conduction equation, negative heat flux problem is alleviated. This also lowers the computed peak heat flux

1 A. Herrmann, PPCF (1995)

International collaboration on THEODOR with IPP-Garching

Calculated 2-D heat flux with = 60 (kWm-2K-1) Calculated 2-D heat flux with no

s

measuredsurface

qTT

NSTXNSTX J-W. Ahn: ELM heat flux study in NSTX (NSTX Monday Physics Meeting) 27 August 2012

NSTX

Choice of value is made by comparing several values of deposited energy from different

• The value makes a large influence on the heat flux calculation, and different values lead to very different results

• In NSTX, we choose the value that keeps the deposited energy constant after the discharge

8


Comparison of new TACO calculation and THEODOR with same value

• Comparison of 1-D radial profile results from TACO to those from THEODOR

Good agreement for both the temporal evolution of peak heat flux and the

spatial heat flux profile at a specific time slice

9

Peak heat flux evolution Heat flux profile


Outline

• Study of transient ELM heat flux1 – temporal and spatial evolution of peak heat flux and heat flux width

- Type-V ELMs

- Type-III ELMs with high and low p

- Type-I ELMs

- Relationship between qpeak and q

• 2-D heat flux data produced by TACO (through collaboration with MAST Team), now upgraded to take account of thin surface layer effect

1 K.F. Gan, submitted to NF


Characteristics of several ELM types in NSTX

• Type-I ELMs:

plasma stored energy (WMHD)

drop by up to ~10 %

• Type-III ELMs:

1-3 % drop of WMHD

• Type-V ELMs:

Less than 1 % drop of WMHD,

individual D peaks are

usually undiscernible

11

Maingi, NF 2005

Type-I

Type-III

Type-V

Mixed

Type-I /

Type-V


Summary of qpeak and int change

during ELMs and ELM-free phase

ELM type qpeak λint

Type-V Increase Increase

Type-III (low p) Increase Increase

Type-III (high p) Increase Decrease

Type-I Increase Decrease

ELM-free Increase Decrease

• int increases for all ELM types relative to the value for ELM-free phase

12


• Heat flux profile during ELM has multiple local peaks exponential fitting can’t characterize heat flux width well Integral heat flux width int

• Evolution of heat flux profile shows that additional heat flux by ELM filaments mainly occurs in a zone displaced into SOL from original strike point profile broadens

Transient heat flux for type-V ELMs

13

)2/( ,,int x

outdivpeak

outdivpeak

outdiv fqRP

Integral mid-plane heat flux width(1)

(2)

(3)

(1)

(2)

(3)

Separatrix



14

• Heat flux profile during ELM has multiple local peaks exponential fitting can’t characterize heat flux width well Integral heat flux width int

• Evolution of heat flux profile shows that additional heat flux by ELM filaments mainly occurs in a zone displaced into SOL from original strike point profile broadens

• Each type-V ELM increases qpeak by 20 – 50% and int by ~20%

(1)

(2)

(3)

(1)

(2)

(3)

SeparatrixType-V ELMs


Comparison of heat deposition pattern during type-V ELMy to ELM-free phases

L-H transition

ELM-free

Type-V ELMs

• During the ELM-free phase, qpeak increases and λint rapidly decreases

total deposited power is

somewhat constant

• During the type-V ELMy phase, qpeak decreases and λint increases

deposited power is again

constant

15

132406


Transient heat flux for type-III ELMs

• 132401

Ip=600kA, PNBI=4MW High p

• 132460

Ip=700kA, PNBI=2MW Low p

16


Heat deposition for type-III ELMs with low p

• ELM filamentary deposition of heat flux is distributed farther (4-5 cm) from the strike point. The profile then slowly reverts to the peak near SP during the ELM recovery phase

• During inter-ELM period, int grows and qpeak decreases

• Each type-III ELM increases both qpeak (by ~100%) and int (by ~20%)

17

(1)

(2)

273.279ms273.438ms273.597ms273.756ms273.915ms275.505ms(3)

(1)

(2)

(3)


Heat deposition for type-III ELMs with high p

• ELM filaments increase the heat flux 7-8 cm away from original strike point but the peak heat flux reverts back to the strike point afterwards

• During inter-ELM periods, int rebuilds slowly to the nominal value of ~3 cm

• Type-V ELMs are present between ELM peaks and they increase int

• Each type-III ELM increases qpeak by a factor of 4 – 6 and decreases int by ~50%

18

(1)

(2)

(3)

(1)

(2)

(3)


Heat deposition for type-I ELMs

• ELM filaments increase the peak heat flux a few cm away from the original strike point but the peak heat flux reverts back to the strike point afterwards

Similar behavior to type-III ELM with high p

• Each type-I ELM increases qpeak by up to 1000% and decreases int by 30-40%

19

(1)(2)

(3)

(1)

(2)

(3)


Summary of qpeak and int change

during ELMs and ELM-free phase

ELM type qpeak λint

Type-V Increase (x 0.2 – 0.5) Increase (x ~0.2)

Type-III (low p) Increase (x ~1) Increase (x ~0.2)

Type-III (high p) Increase (x 4-6) Decrease (x ~0.5)

Type-I Increase (x 10)Decrease (x 0.3-

0.4)

ELM-free Increase (x 5) Decrease (x 0.7)

• int increases for all ELM types relative to the value for ELM-free phase

20


Type-V

Type-III

Type-I

Relationship between qpeak and int for all ELM types

• Relationship between qpeak and int at ELM peaks for multiple shots

• Overall inverse relationship observed between qpeak and int not good trend for future machine

• No clear correlation for Type-V alone

• Type-V and type-III appear to follow same trend together

• Some type-III ELM int values for large qpeak (> 5-6 MW/m2) are smaller than for type-I ELMs!!

21

• A strong inverse relationship is observed in the lowest range of qpeak for type-I

Acceptable regime might exist below qpeak~5 MW/m2 ??

• int saturates for largest values of qpeak (> 10MW/m2) for type-I


Summary and conclusions

• TACO has been implemented to generate 2-D heat flux data with surface layer effect taken into account – Choice of surface heat transmission coefficient () greatly affects heat

flux calculation. A value to keep the deposited energy constant after discharge is chosen in NSTX

• Temporal and spatial variation of qpeak and q during three types of ELMs (type-V, type-III, and type-I) have been investigated− Type-V ELMs: Both qpeak and q increases during each ELM.

qpeak decreases and q increases during type-V ELMy phase

− Type-III ELMs: Both qpeak and q increase during each ELM for low p. qpeak increases and q decreases for high p.

− Type-I ELMs: qpeak increases and q decreases during each ELM

• Inverse relationship is generally observed between q and qpeak. More work needed to characterize ELM heat flux behavior with underlying physics

• 2-D ELM heat profile analysis is underway using TACO data


MAST

G De Temmerman, PPCF, 52 (2010) 095005 (14pp)

Choice of value is made by comparing several values of deposited energy from different

• Several techniques to determine optimal value

In MAST, comparison of deposited energy from IR measurement with the energy derived from the power balance is used

23


Transient heat flux for type-I ELMs

• Type-I ELMs:

Up to 10% loss of plasma stored energy and large transient heat load

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Type-V ELMs

Type-III ELMs

• Type-V ELMs:

Small ELM with < 1% drop in plasma stored energy

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Study of transient heat flux deposition for various ELM types in NSTX

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