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Estimation of Expected Power Fluxes on ITER PFCs Caused By RF Sheaths .

Estimation of Expected Power Fluxes on ITER PFCs Caused By RF Sheaths. L. Colas 1 , M. Goniche 1 , C. Portafaix 1 , G. Agarici 1 D. Milanesio 2 , E. Faudot 3 , Ph. Jacquet 4 A. Loarte 5 1. Association Euratom-CEA, CEA/DSM/IRFM, Centre de Cadarache, 13108 Saint-Paul lez Durance, France

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Estimation of Expected Power Fluxes on ITER PFCs Caused By RF Sheaths .

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  1. Estimation of Expected Power Fluxes on ITER PFCs Caused By RF Sheaths. L. Colas1, M. Goniche1, C. Portafaix1, G. Agarici1D. Milanesio2, E. Faudot3, Ph. Jacquet4 A. Loarte5 1. Association Euratom-CEA, CEA/DSM/IRFM, Centre de Cadarache, 13108 Saint-Paul lez Durance, France 2. Dipartimento di Elettronica, Politecnico di Torino, 10129 Torino, Italy 3. LPMIA, UMR 7040 CNRS, BP 239, F-54506 Vandœuvre cedex, France 4. Euratom – UKAEA association, Culham science center, Abingdon OX14 3DB, UK 5. ITER Organization, Fusion Science and Technology Department Cadarache - Building 523,13108 St. Paul-lez-Durance

  2. Goal & methodology • ITER review (WG8): urgent request ! • Estimate heat loads on PFCs surrounding the ITER ICRF antenna due to specific edge ICRF processes => • RF sheaths + RF-induced convection considered here. • 2 approaches : • Modelling from RF field maps + simple RF sheaths models + EB0 convection. • Simple extrapolation from measured surface temperatures on Tore Supra & JET • Limits: • ITER far SOL: terra incognita • Simple physics models • Precision of measurements Only qualitative answers & orders of magnitude

  3. Modelling Flowchart RF antenna 3 phasings ne profiles 4 scenarios Field line Pitch angle 2D map E//RF @ antenna Integration //, Slow Wave evanescence TOPICA Torino T profiles, L// profiles, B0 Cells code Cadarache 2D map DC potential 2D map RF potential vx SEM code Nancy 2D map density T profiles 4 scenarios 2D map Q//

  4. ITER far SOL is largely unknown Main plasma • Solution: 4 ne profiles (A. Loarte) • Near SOL: B2-Eirene • Scenario 2 & 4 • Far SOL extrapolation: 2 pinch velocitiesvx • Short SOL vx~30m/s • Long SOL vx~90m/s ~factor 100 @ Antenna mouth

  5. Antenna geometry in TOPICA Radiating structure recessed in wall

  6. TOPICA field maps @ aperture Sc. 2 short, [0,0,p,p] phasing • 12 casesperformed • 4plasmascenarios • [00pp], [0pp0], [0p0p] phasing • Various vacuum gaps between antennas & plasma • Poloidal & toroidal extension • E// negligible at boundaries. • Field lines from map corner do not intercept antenna. • Poloidal & toroidal resolution: • Resolve small-scale antenna features • Recessed antenna: E// emerges from aperture. • Toroidal phasing clearly visible. B0

  7. E// integration => VRF • Flux tubes labelled in 2D by coordinate (x,y) of their strike point in reference poloidal plane z=0. Polo. B0 Toroidal Radial

  8. |VRF| @ antenna mouth Poloidal [00pp] [0pp0] • For 1V @ feeders, VRF @ antenna mouth weakly dependant on scenario. • VRF for 20MW depends on RF coupling • Pessimistic value (sc2 short) 0.15V/1V  8kV @ 20MW coupled • Poloidal structure: peaks • Depends on strap phasing RF currents on front face. [0p0p]

  9. Near RF field radial penetration • Skin depth (Slow Wave): kp-1~lskin=c/wpe • LongSOL : lskin~4-6mm; short SOL : lskin~33-35mm

  10. RF sheaths with coupled flux tubes Tranverse map Flux tube Antenna box B0 • SLAB geometry, straight field lines • B0= cst and n= cst • Flute approximation: • only 2D^B0. • Coupling strength  L//1/2 • electrostatic model; • Boltzmann electrons Sheath dynamics with transverse current DI^ Transverse RF currents: f + RF conductivity E. Faudot, S.Heuraux & L. Colas, Physics of Plasmas13 042512 (2006)

  11. RF sheaths with coupled flux tubes |VRF| Sc. 2 long [00pp] • AmplitudeVDC: |VRF|/p |VRF|/2 • Typical values: several kV / 20MW • Poloidalpeak structure ~ preserved. • Radialbroadening: • increases with connect. length L// •  extension of power losses VDC, L//=2.8m VDC, L//=20m Polo. Radial

  12. Density maps: cells code • Flute approximation  2D density map in poloidal plane. • Transport processes : • Parallel losses(t//(x)=L//(x)/cs) • Diffusion (small D^= const.) • radial density pinch vx0 (fit input profiles) • RF-induced EB0 convection in VDC map • No volume source / sink (atomic physics) • Boundary conditions : • Plasma & wall side : n prescribed (unperturbed prof.) • No convection on top & bottom.   vd -Vrect M. Bécoulet & Al. Phys. Plasmas, 9 (6) 2002

  13. RF-induced EB0 convection vEB Sc. 2 long w/o convection [00pp] • Centre of convective cells (high VDC zones) depleted  convection reduces overall power losses. • Radial penetration  VDC extension • Thin fingers of high n brought to antenna front face on top of cells  localized hot spots expected. Location  poloidal shape VDC  phasing • Quantitative features sensitive to badly known transport coefficients !

  14. Heat loads: 2D mapping Sc. 2 short [00pp] Sc. 2 long [00pp] • LongSOL: • large heat loads • localized in peaks • @ antenna side Q//=enVDCcs • shortSOL: • large heat loads • slightly above cells • on plasma side

  15. Heat loads & density convection With convection vEB=0 • Convection depletes high VDC zones. • Amount of depletion  uncertain transport coeff. • Sc. 2 long, [0,0,p,p], 20 MW coupled • no RF: edge losses 1013kW • VDC & no convection 2018kW • VDC + convection 1416kW 403 kW attributed to RF

  16. Edge power loss (20MW coupled) w/o RF with RF, w/o n convection Long SOL with RF & n convection Losses attributed to RF Short SOL

  17. Many uncertainties remain Profiles (n,T) in far SOL : factor 100 uncertainty. Determines magnitude of Q// and SW penetration. Non self-consistent E// VDC peak interpretation Sheath model, radial broadening SLAB geometry How to reduce them SOL modelling needed closer to wall SW @ k//=0 in FELICE j// on antenna front face Test predictions against plasma measurements on existing antennas More self-consistent model (sheath BC in plasma ?) ?? Discussion of modelling • Qualitative indications on RF sheath topology • Heat loads concentrated where VDC penetrates => few cm radially • Located poloidally in peaks => j// on antenna frame ? • Heat flux, power losses: quantitative estimates, expect large errorbars

  18. Vfloat : radial structure + x o Ip=1.2MA => ZQ5~-21cm connected side

  19. Saturation current Jsat Radial structure Connected side Poloidal structure • Depending on ZQ5, Jsat « hole » (-85% !) or over-density observed

  20. Assumptions on far SOL • T profiles: • near SOL: B2-Eirene, c^~3m2/s • constant in far SOL • Sc2 ~ 12eV • Sc4 ~ 23eV • Ti~2Te • Connection length L//(r) • between upper & outer wall, • calculated from equilibrium and wall configuration • no limiters assumed

  21. Plasma needed @ antenna mouth • Vacuum gap (length xgap): • slow wave @ k//=0 propagative • l0~c/w0>>xgap linear decay • Plasma: • slow wave @ k//=0 evanescent • lskin~c/wpe  exponential decay • Connection: E// & dE///dx continuous • Consequence: E//radial penetrationlE

  22. CELLS w/o RF-induced convection • Reproduce input n profiles in far SOL • Radial transport w/o RF: phenomenological radial pinch vx0 • vx0 fitted from input profiles  long/short SOL • Parallel lifetime t//(x): Te, Ti and L//(x) profiles • Numerical stability: residual D^~0.04m2/s • Far SOL well reproduced; near SOL => discrepancy

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