Erosion and Confinement of Tungsten in ASDEX Upgrade

Transcription

1 ASDEX Upgrade Max-Planck-Institut für Plasmaphysik Erosion and Confinement of Tungsten in ASDEX Upgrade R. Dux, T.Pütterich, A. Janzer, and ASDEX Upgrade Team 3rd IAEA-FEC-Conference, 4.., Daejeon, Rep. Korea

2 Main Advantage of Tungsten: Low Erosion Yield for Physical Sputtering simple estimate of ion energy E = k T + 3 Z k T B i B e 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>) High tungsten influx expected for large temperatures ELMs W sputter yield W erosion yields for sputtering by D and C C 4+ Y for % C 4+ eff D + T (ev) 3 Y D /(f C Y C ) 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

3 Main Disadvantage of Tungsten: Large radiative power at high temperatures 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 %. Operation window quickly closes for higher W-concentrations. n T τ -3 E [kevm s] 3 Ignition Curves with He and He+W τ He = 5τE +He W +W c = 3.E-5 W +W c =.E-4 DT T [kev] 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

4 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) ASDEX Upgrade with W Plasma Facing Components SOL ion transport - back to the surface ETB - 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) see Poster EXW/P7-5 (H.Höhnle) 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

5 Measurement of Tungsten Source and Confined Tungsten Density Source of neutral W from all major erosion areas WI line at 4.9nm - heat shield: 9 lines-of-sight - outer limiters: 7 lines-of-sight - outer divertor: lines-of-sight W density in confined plasma W quasi-continuum at 5nm emitted by ion stages existing around T.5keV typ. r/a.8..5 z[m]. -.5 # 978 t=.3s R[m].. 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

6 Effect of Type-I ELMs on the W Erosion at the Limiters ELM resolved measurements of total W erosion on: inner column ( t=.ms) outboard limiters ( t=.5ms) During ELMs drastic increase of W source at ouboard limiters by more than a factor of mean temporal shape rise time.35 ms FWHM.85ms Φ W 9 s - Total limiter source ( LOS with t=.5ms) 6 4 #559 f_elm = Hz, ELM contrib.=7%.5.6 t (s).7 Φ W 9 s #595 t=.9-.9s outer limiter Φ W ~ t/τ c Exp[-t/τ ] c τ c =.35ms FWHM t (ms) ELM 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

7 Balance of W Erosion during ELMs: Main Chamber versus Divertor W erosion at main chamber PFCs releases -% of the W atoms eroded in the divertor. 7 N W,HS +N W,lim 6 W per ELM main chamber vs divertor % % 7 N W,div 5 7 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

8 W source balance between heat shield and outboard limiters depends on distance to separatrix flux surface label R - distance to separatrix measured on the outer equator Expect the first limiting surface with lowest R to be the predominant W source..5 limiters #978 t=3.5s R z[m] -.5 heat shield at inner column -. 3rd IAEA-FEC, 4.., Daejeon Ralph Dux.. R[m]

9 W Source Balance between Heat Shield and Outboard Limiters 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) Φ W,lim /Φ W,HS [].. ratio of W influx limiter/heat shield versus radial position of the plasma column during ELMs between ELMs R - R [cm] lim HS 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

10 Comparison of different PFCs: Source Strength and Effect on W Concentration radial sweeps of outer plasma radius at two puff levels divertor source dominant tungsten concentration modulates with the limiter source mean tungsten concentration increases with the ELM period (ELM period is controled by puff level) Φ W [ 9 s - ] cm ms -5 c W P[MW]. 5 3rd IAEA-FEC, 4.., Daejeon Ralph Dux 5 #978 t ELM R lim Φ D,puff NBI ECRH R HS t [s] div. HS lim. 4 s -

11 Modelling of W Edge Transport with D 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 r sep r lim r bnd anomalous transport is set ad hoc (equal for all impurities) Y Γ lim W neo-classical transport from NEOART (including impurity-impurity collisions) core SOL Γ lim τ lim limiter in SOL parallel losses to divertor and limiter are approximated by volume loss rates (strong increase of loss rate when entering limiter shadow) divertor τ SOL div W erosion rate at limiters calculated from impurity losses onto limiters times erosion yield account for promptly redeposited W 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

12 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 see Poster EXC/P3-3 (B.Kurzan) 3rd IAEA-FEC, 4.., Daejeon Ralph Dux C-density [ 7 m -3 ] 8 C 6+ all C ms before ELM separatrix ELM ms after ELM.9 ρ pol.9 ρ pol - - neoclass. - v/d [m ] at ρ pol=.99 He C Ne Ar

13 Radial Impurity Transport Parameters during ELM Cycle 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 ms m /s 5 5 Diffusion Coefficient ("anomalous") t (ms) 3rd IAEA-FEC, 4.., Daejeon Ralph Dux D [m /s] v/d[m - ] an. ELM W O C an. no ELM C O W # R lfs -R lfs,sep (cm)

14 W profile evolution during an ELM cycle quasi-equilbirum = ELM averaged densities are constant in time flattening of W density gradient in ETB during ELM #895(f ELM =Hz) increase of SOL density due to W erosion re-build of strong gradient up to next ELM [ 9 s - ] Φ Φ(-f redep ) t [ms] n W [ 5 m -3 ] t=ms t=.9ms t=.5ms t=.53ms t=.ms t=.ms t=3.9ms t=6.9ms average R -R (cm) lfs sep,lfs 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

15 Increase of Prompt Redeposition during ELM: immediate return to the surface during the first gyration r L B λion W Influx and Source Φ =3.3x 8 s - ELM contrib.=7.% -f prompt =36.% Increase of electron density and temperature in SOL causes decrease of ionisation length [ 9 s - ] Φ Φ(-f prompt ) increase of prompt redeposition t [ms] 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

16 Study dependence of confinement time of W independent scan of ELM frequency: =5- Hz parallel loss time to divertor: = ms diffusion coefficient in SOL: =.- m s turbulent diffusion coefficient in ETB: =.-. m s D [m /s].. W O C R lfs -R lfs,sep (cm) 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

17 Confinement time of W depends on Elm frequency and parallel loss time confinement time of W: : tempor al average over ELM cycle τ p increases about linearly with parallel loss time from SOL to divertor ELM repetition time very weak dependence on diffusion coefficient in SOL drives radial flux from the source location to the inside and to the the outside set DSOL to m^/s τ p [s] ms τ p,reg [s] =5- Hz =.- m s = ms =.-. m s 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

18 Code results for three AUG H-mode plasmas with felm=5,,hz Increasese of ELM frequency invoked by increase of D puffing rate decrease of W-confinement Code results for SOL diffusion coefficient = m^/s Fit of τ p (c W ) by adjusting τ SOL div : peaking across ETB for f ELM W loss per ELM % τ SOL div for Φ D prompt redeposition about 6% measurements of parallel losses needed ΦD,puff [ s - ] 4 c W [ -5 ] n W,.9 /n W,sep f ELM [Hz] 4 Φ W,lim [8 s - ] τ p [ms] τ SOL,div [ms] 4 f ELM [Hz] 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

19 In ITER mucher lower collisional transport coefficients in ETB due to higher temperatures and larger magnetic field strength Assume W Limiters in ITER Reference scenario for inductive operation (Q=) Collisional diffusion coefficient in ETB: about times below AUG values due to higher T and B W, Ar still in PS regime He: banana-plateau term contributes 6% at pedestal top an % near separatrix apply transport model and assume turbulent diffusion in ETB to be a factor of below collisional values D [m /s] v/d[m - ] T e,ped =T i,ped = 4.8keV n e,ped = 7.8 x 9m-3 B T = 5.3T, I p =5MA c He = %, c O=.9%, c =.5% Ar an. ELM an. no ELM He O Ar W W Ar He O r-r sep [cm] 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

20 In ITER low W peaking in ETB in relevant ELM frequency range P NI = 4MW, P α =8MW P heat = MW, P rad,core =3MW, P sep =9MW Φ(-f redep ) =4.4x 9 s - For type-i ELMs: low maximum permissible ELM energy loss W requires high ELM frequency P sep,elm = (/3) P sep =3MW W MJ f 3Hz ELM ELM Effect on W peaking in pedestal: time to build up strong gradient in ETB longer than typical ELM repetition time n W [ 6 m -3 ] 5.Hz.Hz 6.7Hz 5.Hz 33.3Hz substantial peaking only for 5Hz if there is still sufficient ELM flushing r-r (cm) sep 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

21 In ITER: global He confinement time not affected by low ETB transport at relevant range of ELM frequencies Exhaust of He across ETB no divertor recycling global confinement time limited by perp. transport accross ETB with divertor recycling realistic case but recycling model in STRAHL very crude global confinement time of He does not strongly increase for f ELM Hz. core Γ rec SOL Γlim limiter τ* He [s] f [s - ] ELM +div. recycling perp. transport s ρ=τ* He /τ E τ SOL div divertor τ div SOL τ pump 3rd IAEA-FEC, 4.., Daejeon Ralph Dux

22 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 3rd IAEA-FEC, 4.., Daejeon Ralph Dux