R. Dumont Association Euratom-CEA, CEA/DSM/IRFM, Centre de Cadarache

1. R. Dumont Association Euratom-CEA, CEA/DSM/IRFM, Centre de Cadarache 13108 Saint-Paul-lez-Durance, France The Tore SupraICRF program - Long pulses and steady state issues - Contributions: L. Colas, A. Ekedahl, X. Litaudon, P. Mollard, K. Vulliez… AssociationEuratom-CEA

2. General context

3. Tore Supra in the fusion roadmap • Preparation of steady-state operation is vital • Control of a self-organised state for economical reactor. • Technology development for continuous operation. Exploration of continuous operation: ″Terra Incognita″ INTEGRATION Physics of non-inductive regimes & model validation on long time scale Technology of long-pulse operation Training

4. Physics / Technology for ITER / DEMO • Test bed for technology development • Actively cooled PFC operation at 10MW/m2 • Integrated real time control • Optimisation of RF mix & RF-plasma interaction • RF development (gen., PAM, ILA, ECRH mirror) • Integrated burn control experiment for ITER & DEMO • ICRH/ECRH simulate the a-power →Pa; Pfus • fboot~80%: ECCD/LHCD mixture (+real bootstrap) simulate BS • complementary ECRH/ICRH to insure MHD & burn control →Padd1 • Ip & remaining LHCD/ECCD to control J(r) at Vloop=0 →Padd2 • → Qeff = Pfus /(Padd1 + Padd2)~10-20 (prescribed) • Tore Supra 20MW: access to experimental ‘simulation’ of burning steady state plasmas: ‘Qeff ~15’ & ‘fboot~80%’ • 12MW ICRH/ECRH to simulate alpha heating • 4MW LHCD/ECCD + real bootstrap → fboot~80% • Padd1+ Padd2 ~4MWfor burn & CD control → Qeff = 15

5. LHCD 3.7GHz • 8MW 16 klystrons 500kW/60s • 2 multijunction antennas ICRH ECRH LHCD • ECRH 118GHz • 2 gyrotrons ~700kW/10s • 1 antenna ITER-like antenna • ICRH 40-70 MHz • 12MW/30s • 3 resonant double loop antennas + ITER-like antenna prototype or Tore Supra RF equipment (2008) ECRH APL TPL

6. Radiofrequency systems test rigs ICRF test facility (SEO) (500kW CW – 2MW, 30s) High power test under vacuum of large components (Picture of a Tore Supra antenna under test in the SEO)

7. The ICRF program on Tore Supra • Exploitation/ developmentof ICRF scenarios • D(H), He-4(H), D(He-3) minority heating • Second harmonic H heating • FWEH, FWCD • ITER-like prototype ICRF antenna • ICRF power coupling issues

8. ITER-like ICRF antenna

9. Project history • Concept :Modificationof an existing antenna (ORNL Resonant Double Loop). • Project initiated in 2004. A first campaign on plasma ended by failure of a tuning component. • Since then, several modifications/improvementshave been performed on the mechanical structureandRF diagnostics. • The enhanced prototype has been qualified in vacuum in June 2007 and assembled on TS in October for 13 days of experiments on plasma.

10. TS ITER-like antenna: pictures

11. ITER-like antennas: family snapshots TS prototype :4symmetric straps, with small poloidal / toroidal mutual coupling. Internal matching system. JET-EP antenna :asymmetric arrays of 8 straps, with mutual coupling (mostly poloidal). Internal matching system with second stage. ITER antenna :asymmetric array of 24 straps (8 triplets), with mutual coupling. External matching system, thick FS bars, CW operation.

12. Load resilience studies • Strong VSWRvariations (increase of the reflected power) • Power limitationon the generator outputs • Voltage increaseon the generators(risk of arcs) • Power cut-off(reflected power safety system trigger) Classical TS antenna Strong and fast variations of plasma conditions (density, position, ELMs …) • Slow VSWR variations • Steady operation of the generator Prototype Antenna

13. Compared load resilience properties Theoretical model TS classical antenna (Resonant Double Loop) Classical TS antenna Prototype Antenna Prototype antenna (ITER-relevant circuit) 42MHz 48MHz 57MHz 63MHz [S. Brémond]

14. First experimental observations • The studies of the load resilience properties started with a series of shots at 48MHz and at a power level not exceeding 700kW. • First observation  low coupling resistance(Rc) of the prototype compared to the TS RDL antenna • Rc =1.4 W/m on prototype / Rc = 3 W/m on TS antennas • The classical RDL located in an adjacent port was placed in a recessed position (30mm) to get the same level of coupling resistance (as a comparison exercise)

15. Coupling Resistance (Ω/m) Classical antenna Prototype Prototype Classical Distance density cut-off layer/ antenna(m) [F. Clairet] Loading resistance • The difference in Rc between the two antennas is due : • Mostly • Differences in the strap-to-Faraday Screen distance • Partly • Presence of a thick vertical septum (reduces mutual toroidal coupling) • Lowfrequency and low power level of operation • Electric scheme (systematic difference ~0.2 W/m)  In steady conditions only a limited range of Rc was accessible (due to the intrinsic difficulty of strongly modifying the edge density)

16. Plasma variations • The load resilience of the prototype antenna was validated by : • Slow and fast variations of plasma condition: fast current ramp-up 1.6MA/s, changesof plasma position  only a limited range of Rc was accessible • Fast transient increase of the edge density during Supersonic Molecular Beam Injection (SMBI) Classical antenna (Q2) ITER-Like antenna (Q1) SMBI Ip

17. Slow plasma variations Theoretical response (match at Rc = 0.6 W/m) Theoretical response (match at Rc = 0.85 W/m) Measurements (match at Rc ~ 0.65 W/m)

18. Fast transient: improved load resilience Prototype antenna Classical antenna • Supersonic molecular beam injection • Density in front of the antenna varies by a factor 3 • Coupling resistance Rc ranges from 0.8 to 3 /m Classical antenna (exceeds the safety threshold) Vr/Vi (x10) ITER-like

19. Summary (ITER-like antenna) In comparison with the RDL antenna, the prototype demonstrates : • Strong insensibility w/r to sudden changes of the edge density and coupling conditions. • A far lessspikymatching array • Variations of ±0.5pF around a matched point are sufficient to mismatch the RDL (especially at low Rc value) • Variations of ±3pF on the prototype capacitors lead to voltage/current unbalance but do not prevent the coupling of the power to the plasma.

20. RF Power coupling issues Tore Supra discharges are of interest to study plasma-antenna interactionsin view of ITER, because of: • Long pulse capability • Water-cooled plasma facing components • Presence of fast particles, and hot spots • Good diagnostic capability and security IR cameras

21. Real time control based on IR security Real time control of ICRF phase #38741 Incorrect phasing #38742 But, when accidental wrong antenna phasing, the IR security helped to reduce the heat load on the antenna. Correct dipole phasing

22. SOL and coupling modification • In ITER, the LHCD and ICRH antennas are foreseen to be located close to each other, magnetically connected. • Experiments in JET and Tore Supra show that the LH coupling can be strongly modified in the presence of ICRH. Problem in ITER ? • Tore Supra experiments (L-mode plasmas) also show that the heat load by electron acceleration is modified.

23. Magnetic connections IR image, antenna Q5 LH launcher     IRF (0,p)  B0 q(a)=5.2 • Follow fast LH e- beam • LH laucher is connected to upper corner of ICRH antenna Q5.

24. LH + ICRF coupling TS 38073 LH grill LH grill             t=12s t=19s • Thermal steady state: DTIRQ// • LH alone (1MW) : similar evolution of 4 waveguide rows. • LH + ICRH Q5 (1+1MW) : opposite evolution  poloidal asymmetry

25. Fast ion interactions • Two loss mechanisms for fast ions: • Direct ripple losses  towards the bottom of the tokamak • Stochastic diffusion of large orbits  low field side, below or at the mid-plane  heat load on antennas • Efast PICRH * Te3/2 / (ne*nH) • p~Efast1/3 b~Efast1/2 • potato banana • Parametric dependences: • PICRH • ne • IP • nH/(nH+nD) p  ne beneficial  IP beneficial

26. Influence of IC power and density LHCD launcher B A 1 1 IP constant. nH/(nH+nD) roughly constant. T = TA - TB • Temperature increase scales with Efast PICRH*Te3/2/ne2 • Higher density is beneficial for decreasing the heat load

27. +81°C +350°C Heat load on ITER-like antenna # 40848 / t=10s P=1.1MW nl=81019m-2Dr = 1cm • Limited DTIR compared to classical antennas. State of surface ? • Highest DTIR => upper/left part. Density convection ?

28. LH launcher        LH grill – ITER-like antenna connections Upper part of LH launcher magnetically connected to lower corner of ITER-like antenna. IR image, ITER-Like antenna B0 q(a)=5.2 [A. Ekedahl]

29. Studies of RF sheaths 2D mapping of the plasma Scrape Of Layer in the vicinity of the antenna with reciprocating probe. 2D map with reciprocating probe • Radial resolution: probe reciprocation • Vertical resolution: scan q(a) viaIp [L. Colas, J. Gunn]

30. RF sheath modelling Probe measurements Simulation (tilted FS) • RF sheath modelling effort: • Comparison with probe measurements • Quantify respective contributions of various physical process • Support to ICRF antenna design (Faraday screen…)

31. Summary (ICRF coupling) • Tore Supra experiments allow to study issues related to SOL modifications and heat load on antennas due to fast electrons, fast ions and sheath effects. • These issues are studied in long pulses, at high power levels, using security based on IRmeasurements. • Understanding the LH coupling modifications in the presence of ICRH requires understanding of the density profile around the ICRH antenna sheath effects. • Challenge is to design antennas (both ICRH and LHCD) wich reduce the presence of hot spots.

32. Conclusions • Tore Supra: a unique tokamak to explore technologyand physics issues of long pulse operation at Vloop = 0. • Routine long pulse operation with actively cooled plasma facing components. • Major upgrade of RF heating systems is underway to further explore issues in relation with steady state regimes on ITER and DEMO. • The Tore Supra ICRF system is a central part of this endeavour: • Scenario development • Test of antenna concepts in relevant operational conditions. • Extensive studies of power coupling issues.