erosion and confinement of tungsten in asdex upgrade · ☞ see poster exw/p7-25 (h.höhnle)...

23rd IAEA-FEC-Conference , 14.10.2010, Daejeon, Rep. Korea

ASDEX Upgrade

R. Dux, T.Pütterich, A. Janzer, and ASDEX Upgrade Team

Max-Planck-Institutfür Plasmaphysik

Erosion and Confinement of Tungsten

in ASDEX Upgrade

Main Advantage of Tungsten:Low Erosion Yield for Physical Sputtering

In the relevant temperature range W sputtering is mainly caused by light impurities

since impurities have • a higher mass • and a higher energy gain in the sheath (Z>1)

simple estimate of ion energy E = 2 k T + 3 Z k TB Bi e

10-5

10-4

10-3

10-2

10-1

100

W s

putte

r yi

eld

Yeff for 1% C4+Yeff4+

C4+

D+

1 10 100T (eV)

0

1

2

3

YD

/(f C

YC

)

W erosion yieldsfor sputtering by D and C

High tungsten influx expectedfor large temperatures → ELMs

23rd IAEA-FEC, 14.10.2010, Daejeon Ralph Dux

Main Disadvantage of Tungsten:Large radiative power at high temperatures


-3

10 100T [keV]

1022

1023

n T

τ E [k

eVm

s]

c = 3.0E-5

W

c = 1

.2E-4

W

Ignition Curves with He and He+W

τ = 5τHe E

DT

+He

+W+W

• Ignition condition requests low central W concentration

• W-concentration of 3E-5 leads to an increase of the minimum triple product for self sustained burn by 20% .

• Operation window quickly closes for higher W-concentrations.

Elements Defining the Central Tungsten Density


Central Tungsten Concentration depends on

• rate of eroded tungsten atoms at plasma facing components (PFC) - ionised very close to the surface in the scrape-off layer (SOL)

• ion transport - back to the surface

- across SOL into confined area

- in the edge transport barrier (ETB)

- inside of ETB especially in the very center of the plasma (control of neo-classical tungsten accumulation)

SOL

ETB

ASDEX Upgrade with W Plasma Facing Components

☞ see Poster EXW/P7-25 (H.Höhnle)

Measurement of Tungsten Source and Confined Tungsten Density

Source of neutral W from allmajor erosion areas• WI line at 400.9nm - heat shield: 9 lines-of-sight - outer limiters: 17 lines-of-sight - outer divertor: 12 lines-of-sight

W density in confined plasma• W quasi-continuum at 5nm emitted by ion stages existing around T≈1.5keV typ. r/a ≈0.8

1.0 1.2 1.4 2.0 2.2R[m]

-1.0

-0.5

0.0

0.5

1.0

z[m

]

# 22978 t=2.3s


ELM resolved measurementsof total W erosion on: • inner column (∆t=1.2ms) • outboard limiters (∆t=0.5ms)

During ELMs drastic increase of W source at ouboard limiters by more than a factor of 10

mean temporal shape• rise time ≈ 0.35 ms• FWHM ≈ 0.85ms

Effect of Type-I ELMs on the W Erosion at the Limiters

2.5 2.6 2.7t (s)0

2

4

6

ΦW

1019

s-1

Total limiter source (10 LOS with ∆t=0.5ms)#25059 f_ELM = 20Hz,

ELM contrib.=72%


-2 -1 0 1 2 3 4 5∆tELM (ms)

0

1

2

3

4

ΦW

10

19s-1

outer limiterΦW~ t/τc Exp[-t/τc]

τc=0.35ms

#25095t=1.9-2.9s

FWHM

W erosion at main chamber PFCs releases 10-20% of the W atoms eroded in the divertor.

Balance of W Erosion during ELMs: Main Chamber versus Divertor


0 1•1017 5•1017NW,div

0

2•1016

1•1017

NW

,HS

+N

W,li

m

20%

10%

W per ELMmain chamber vs divertor

W source balance between heat shield and outboard limitersdepends on distance to separatrix

1.0 2.0R[m]

-1.0

-0.5

0.5

1.0

z[m

]

#22978 t=3.5s

∆R

limiters

heat shieldat inner column

flux surface label ∆R- distance to separatrix measured on the outer equator

Expect the first limiting surface withlowest ∆R to be the predominant W source


• between ELMs the balance is in acordance with a parallel loss onto the first limiting element (within uncertainties of the magnetic equilibrium and the erosion measurements)

• during ELMs the balance is shifted to the outboard side (filaments)

W Source Balance between Heat Shield and Outboard Limiters


ratio of W influx limiter/heat shieldversus radial position of

the plasma column

-2 -1 0 1 2 3 4∆Rlim-∆RHS [cm]

0.1

1.0

10.0

ΦW

,lim

/ΦW

,HS

[1]

during ELMsbetween ELMsduring ELMsbetween ELMs

10-5

cW

0.1

1

10

ΦW

[1019

s ]-1

lim.HS

div.

0

5

cm ∆Rlim ∆RHS

0

10ms

t∆ ELM

0

2

4

1022

-1 s

ΦD,puff

1 2 3 4 5t [s]

0

5

P[M

W]

NBI

ECRH

#22978radial sweeps of outer plasma radiusat two puff levels

divertor source dominant

tungsten concentration modulates with the limiter source

mean tungsten concentrationincreases with the ELM period(ELM period is controled by puff level)

Comparison of different PFCs: Source Strength and Effect on W Concentration


Modelling of W Edge Transport with 1D Impurity Transport Code STRAHL

STRAHL• solves coupled radial transport equations for all ion stages of several impurites (AUGD: C,O,W) • ne, Te, Ti are input parameters

• anomalous transport is set ad hoc (equal for all impurities)

• neo-classical transport from NEOART (including impurity-impurity collisions)

• in SOL parallel losses to divertor and limiter are approximated by volume loss rates (strong increase of loss rate when entering limiter shadow)

• W erosion rate at limiters calculated from impurity losses onto limiters times erosion yield

• account for promptly redeposited W 23rd IAEA-FEC, 14.10.2010, Daejeon Ralph Dux

τ SOL→div

core

divertor

τ lim

limite

r

SOL

Y ΓlimW

Γlim

rlim rbndrsep0

Peaking of impurity density in ETB due to collisional inward pinch


CXRS measurements of impurity profile evolution in ETB during ELM cycle:

• He, C, Ne, and Ar • peaking of impurity density in ETB between ELMs• peaking increases with Z• flattening of gradient during ELM

• transport coefficients D and v in accordance with collisional transport• collisionality ν* of impurities in the Pfirsch-Schlüter regime

W has even higher ⟨Z⟩ in ETB:• higher ν* and collisional diffusion coefficient• stronger peaking

1ms before ELM

1ms after ELM

ELM

10.9 ρpol 10.9 ρpol

sepa

ratr

ix

all C

8

0

C-d

ensi

ty [1

0 m

]

17-3

C6+

-200

neoclass.

-100

0

He C Ne Ar

at ρ = 0.99pol

v/D [m ]-1

☞ see Poster EXC/P3-03 (B.Kurzan)

Radial Impurity Transport Parameters during ELM Cycle

0 2 4 6 8 10∆t (ms)

0

5

10

15

20

m2 /s

Diffusion Coefficient("anomalous")

ELM off:• low turbulent diffusion coefficient in ETB• neo-classical v and D dominate

ELM on:• high diffusion coefficient around ETB• no anomalous drift• neo-classical drift unimportant

Timing:• sudden switch-on of ELM transport and linear decay within 1ms


0.01

0.1

1

10

D [m

2 /s]

WOC

an. ELM

an. no ELM

-8 -6 -4 -2 0 2 4Rlfs-Rlfs,sep (cm)

-300-200-100

0100

v/D

[m-1

]

COW

#22895

W profile evolution during an ELM cycle


∆t=0ms

∆t=0.09ms

∆t=0.25ms

n W [1

015m

-3]

∆t=0.53ms

∆t=1.2ms

∆t=2.2ms

0

1

2

0

1

2

0

1

2

∆t=3.9ms

Rlfs-Rsep,lfs (cm)

∆t=6.9ms

-20 -20-20 -10 0 -10 0 -10 0 5

average

#22895(fELM=100Hz)

10

1

2

∆t [ms]

[10

19s-1

]

ΦΦ(1-fredep)

0 2 4 6 80

quasi-equilbirum = ELM averaged densities are constant in time

• flattening of W density gradient in ETB during ELM

• increase of SOL density due to W erosion

• re-build of strong gradient up to next ELM

Increase of Prompt Redeposition during ELM:immediate return to the surface during the first gyration

Increase of electron density and temperature in SOL causesdecrease of ionisation length

→ increase of prompt redeposition

⊗ Bλion

rL


W Influx and Source

0 2 4 6 8 10∆t [ms]

0

1

2

[10

19s-1

]

⟨Φ⟩=3.3x1018s-1

ELM contrib.=70.%⟨1-fprompt⟩=36.%

ΦΦ(1-fprompt)

Study dependence of confinement time of W


=0.6-7.5 ms

=25-200 Hz

=0.1-2 m s

independent scan of

• ELM frequency:

• parallel loss time to divertor:

• diffusion coefficient in SOL:

• turbulent diffusion coefficient in ETB:

-8 -6 -4 -2 0 2 4Rlfs-Rlfs,sep (cm)

0.01

0.1

1

10

D [m

2 /s]

WOC

=0.01-0.1 m s

confinement time of W:

τ increases about linearly with• parallel loss time from SOL to divertor• ELM repetition time

very weak dependence on diffusioncoefficient in SOL• drives radial flux from the source location to the inside and to the the outside• set DSOL to 1m^2/s

Confinement time of W depends on Elm frequency and parallel loss time


10-4 10-3 10-2 10-1

τp,reg[s]

10-4

10-3

10-2

10-1

τ p[s

]

: temporal average over ELM cycle

=0.6-7.5 ms

=25-200 Hz

=0.1-2 m s

=0.01-0.1 m s

p

ms

Increasese of ELM frequency• invoked by increase of D puffing rate• decrease of W-confinement

Code results for SOL diffusioncoefficient = 1 m^2/s

Fit of τ (c ) by adjusting τ :• peaking across ETB ↓ for f ↑ • W loss per ELM ≈10%• τ ↑ for Φ ↑• prompt redeposition about 60%

Code results for three AUG H-mode plasmas with fELM=50,100,200Hz


0

4

8

12 τp[ms]0

2

4ΦW,lim[1018s-1]

0

2

4cW[10-5 ]

0

22 -1

1

2 ΦD,puff[10 s ]

0

4

8τSOL,div[ms]

0 100 200fELM [Hz]

0

10

20

nW,0.9/nW,sep

0 100 200fELM [Hz]

p W

SOL→div D

ELM

→ measurements of parallel losses needed

SOL→div

In ITER mucher lower collisional transport coefficients in ETBdue to higher temperatures and larger magnetic field strength


0.0010.01

0.1

1

10

D [m

2 /s] W

ArHeO

an. ELM

an. no ELM

-30 -20 -10 0r-rsep [cm]

-60

-40

-20

0

v/D

[m-1

]

HeOArW

Assume W Limiters in ITER

Reference scenario for inductive operation (Q=10)

Collisional diffusion coefficient in ETB:• about 20 times below AUG values due to higher T and B

• W, Ar still in PS regime

• He: banana-plateau term contributes 60% at pedestal top an 10% near separatrix

apply transport model and assume turbulent diffusion in ETB to be a factor of 10 below collisional values

T =T = 4.8keVe,ped i,ped

e,pedn = 7.8 x10 m19 -3

TB = 5.3T, I =15MAp

Hec = 2%, c =0.9%, c =0.05%O Ar

In ITER low W peaking in ETB in relevant ELM frequency range

-30 -20 -10 0r-rsep (cm)

0

1

⟨nW

⟩ [10

16m

-3]

⟨Φ(1-fredep)⟩=4.4x1019s-1

5.0Hz10.0Hz16.7Hz25.0Hz33.3Hz


NIP = 40MW, P =80MWα

heatP = 120MW, P =30MW, P =90MWrad,core sep

sep,ELMP = (1/3) P =30MWsep

ELM∆W ≤ 1MJ ⇒ f ≥ 30HzELM

For type-I ELMs:low maximum permissible ELM energy loss ∆W requires high ELM frequency

Effect on W peaking in pedestal:• time to build up strong gradient in ETB longer than typical ELM repetition time

• substantial peaking only for 5Hz if there is still sufficient ELM flushing

In ITER: global He confinement time not affected by low ETB transportat relevant range of ELM frequencies


Exhaust of He across ETB

no divertor recycling• global confinement time limited by perp. transport accross ETB

s

with divertor recycling• realistic case but recycling model in STRAHL very crude

ELM

global confinement time of He doesnot strongly increase for f ≥10Hz.

τ SOL→div

core

divertor

limite

r

SOL

Γrec

Γlim

τ pump

τ div→SOL

0 10 20 30fELM [s-1]

05

101520253035

τ*H

e[s

]

02

4

6

8

10

1214

ρ=τ*

He/τ

E

perp. transport

+div. recycling

Conclusion


Fast Measurements of W Erosion in Asdex Upgrade• For the divertor and the outboard limiters, the W erosion is mainly due to ELMs. • In the main chamber, the ELM erosion is mainly on the outboard limiters.

W Confinement → W Edge Transport during ELM Cycle• in-between ELMs: build-up of steep W density profile in ETB due to neo-classically dominated impurity transport• ELMs: impurity flushing from confined plasma • weak dependence of W confinement on diffusion coefficient in SOL • loss time to divertor and ELM frequency are the important parameters

ITER ETB with dominant collisional transport • combination of low collisional diffusion coefficient and high ELM frequencies leads to low impurity peaking and good He exhaust

erosion and confinement of tungsten in asdex upgrade · ☞ see poster exw/p7-25 (h.höhnle)...

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