H. D. Pacher 1, A. S. Kukushkin 2, G. W. Pacher 3, V. Kotov 4, G. Janeschitz 5, D. Reiter 4, D. Coster 6 1 INRS-EMT, Varennes, Canada; 2 ITER Organization, Cadarache, France; 3 Hydro-Québec, Varennes, Canada; 4 FZ Jülich, Germany; 5 Forschungszentrum Karlsruhe, Germany; 6 Max-Planck IPP, Garching, Germany presented at PSI th Int. Conf. on Plasma-Surface Interactions Toledo, Spain May 2008 Impurity seeding and scaling of edge parameters in ITER

Outline 1. Results for nonlinear neutral model with carbon Update scaling (PSI2006, IAEA 2006) 2. Edge/divertor simulation: Impurity-seeded carbon-free divertor 3. Core simulations: Effect of impurity seeding on ITER operating diagram Conclusions 1

Edge/Divertor Model B2-Eirene (SOLPS4.3) Now routinely nonlinear neutral model neutral-neutral collisions D 2 molecular kinetics Parallelized 2 domes 2 1. Full carbon wall C sputtering: phys. + const. Y ch 2. Carbon-free wall with neon: (wall same as 1. but no C erosion) 3. Variant: Full Be wall with neon: (=> with Ne small difference from 2., Be concentration small, Be radiation small)

Edge density limit Density analogue of from n scaling Scaling Update 3 Scaling results PSI2006: Key parameter is normalised pressure is at detachment of either divertor Same curve for qpk from JET at 16 MW to DEMO at 500 MW (2006) Previous was linear neutral model except for some points => Update required. PowerDT pressure at PFR (Throughput) FactorsSize

DT flux - Scaling with  and S 4 With nonlinear neutral model Both domes, both S: DT neutral influx to core: linear model was: i.e. stronger variation Value at is 2.4 times that previously Total influx is still small: i.e gas puffing provides little core fuelling (opacity to neutrals)

Value at is about 1/3 previous (linear) Helium small, rises less strongly toward lower pressures  helium does not constrain operation unless pumping reduced strongly or dome removed linear model was: Helium - Scaling with  and S 5 With nonlinear neutral model Helium : slight difference between domes "bump" at related to detachment Scaling:

But: Impurity radiation in inner divertor volume is smaller than with C: Neon seeding without carbon - power 6 With neon seeding: Impurity radiation for similar to but smaller than for C varies little with (self-consistent carbon is):

Ne - T 7 With neon: Temperature at inner target higher ( C chemical erosion, low Ne radiation at < 10 eV) No additional factor in for detachment With carbon-free and neon at inner target compared to C: plasma power higher radiation lower, total power a bit higher power load higher (peaking) => peak power load shifts from outer to inner target (see next)

Ne - peak power 8 With neon: Peak power shifts to inner target Only points for which larger load is at divertor plotted below Peak power has same scaling but is 30% lower than with C (but flux expansion and angle are not same)

Ne - n DT and n e 9 The strongest effect of neon is: As neon density increases, => DT density decreases strongly => n e decrease ~80% of n DT decrease over range varies little => explains why neon radiation varies little with concentration Factor 40 broadly consistent with ratio of ionisation energy ~100 for 8<Z<9 - Less power available for DT recycling

Ne - helium 10 As neon increases to relative to carbon helium density at separatrix progressively decreases by 2.2 helium neutral influx to core progressively decreases by 7.5 Tentatively attribute to: lower DT and electron densities in divertor plasma => lower opacity of inner divertor plasma to neutrals => more efficient pumping => lower He densities and fluxes upstream Details to be worked out (He reduction stronger in inner divertor)

Core/edge model 11 Core transport in Astra MMM, stabilised by ExB and magnetic shear, time-average ELMs, sawteeth, fitted to JET and AUG, also fits Sugihara pedestal scaling (EPS2008) Because of the opacity of the ITER SOL, core fuelling controls mostly the core density, gas puffing controls mostly the edge (including peak power load via  )  profiles, self-consistent pedestal width and height  operating window cf e.g IAEA2006, also paper submitted to Nucl. Fusion linked to edge via scaling relations from B2-EIRENE

At constant alpha particle power has maximum as n increases =>Low temperature limit of alpha power from fusion cross-section Operating diagram 12 Peak power load set given value across window by varying gas puffing (throughput and 

Operating diagram limits 13 For q pk always within limit, a point lies within the operational window if for that point: Max. attainable alpha power (roll- over of P  with min Q for ITER mission (5 for ITER) Edge density limit (full detachment) H-L transition Available heating power (73MW)

Operation at q pk = 5 MW/m 2 ; C vs. C-free+Ne 14 If very low peak power load required: With carbon: Excessive gas puff (up to 300 Pa-m3/s) would be required but density limit dominates, =>limiting the actual throughput used =>limiting alpha power With carbon-free and neon: Q is lower less gas puff needed => alpha power is higher

Operation at q pk = 10 MW/m 2 ; C vs. C-free+Ne 15 The operating window is smaller in both Q and alpha power with neon Core fuelling: a bit higher at the same alpha power with neon because of increased core radiation (lower T) Gas puffing: much lower with neon (60 vs 120 Pa-m 3 /s) since favourable scaling of peak power in previous section demands less gas puffing in addition. If moderate peak power load required:

Operating diagram summary 16 Superposition shows: For Ne relative to C: low peak power: advantage with neon in alpha power, disadvantage in Q same throughput at limit moderate peak power: disadvantage both in Q and alpha power throughput lower by a factor 2 (but lower S would do the same thing) edge density limit plays strong role with C, less so with neon

Conclusions Scaling from edge modelling with nonlinear neutral model updated: - DT neutral influx to core higher than previous but remains small -> gas puffing ineffective for core fuelling - more benign helium scaling toward lower pressure -> helium is not strong constraint for ITER unless pumping strongly reduced 2. Edge: carbon-free+neon relative to carbon: - radiated power similar, peak power load 30% lower, but with same scaling - peak power load shifted to inner divertor - helium density and helium influx to core even lower - DT density decreases as neon density increases because less power is available for DT 3. Operating window: carbon-free+neon relative to carbon: - Operating window increases relative to carbon only when stringent low peak power is specified, which would require excessive gas puffing - For moderate specified peak power loading: - neon reduces operating window - but also reduces throughput (as does reducing pumping speed, with less impact) =>Mutually consistent modelling of edge and core shows that for ITER operation carbon-free operation with neon seeding offers only limited advantage over carbon operation Carbon-free operation at low seeded impurity levels needs to be examined further, as does W