1. IC IDRM2: 18 May 2011 P. Dumortier et al. Slide 1 Validation of the Electrical Properties of the ITER ICRF Antenna using Reduced-Scale Mock-Ups P. Dumortier, F. Durodié, D. Grine, V. Kyrytsya, F. Louche, A. Messiaen, M. Vervier, M. Vrancken LPP-ERM/KMS, Brussels, Belgium, CYCLE Work supported by F4E-2009-GRT-026 grant

2. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 2 Outline ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions

3. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 3 What does IO request ? ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions

4. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 4 ITER ICRH antenna – key RF requirements Nominal power: 20 MW per antenna (2 antennas) Frequency range: 40 – 55 MHz Phased antenna array (6 poloidal x 4 toroidal array) for radiated power spectrum control: Control of toroidal phase differences Control of current ratio between columns of straps Maximum allowed voltage: V max =45kV Maximum allowed electric field: Torus vacuum:E max =2kV/mm perpendicular to B tor ; E max =3kV/mm parallel to B tor Private vacuum: E max =3kV/mm Quasi CW operation Location: equatorial port plug

5. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 5 What is the ITER antenna looking like ? Why ? ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions

6. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 6 Short current straps Short circuit 24 straps grouped in triplets → 6x4 array Port Plug Flange RF grounding Port plug wall Antenna box Neutron shield 3640 2160 1708 Actual Reference Design – ICRH Antenna B17 M. Nightingale Faraday screen 4-Port Junction (arms: Z 01 =15Ω) RF vacuum windows Feeding line (Z 02 =20Ω) Service stub (Z 0SSt =15Ω)

7. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 7 Design choices and features Short low-inductance straps Lower voltage on straps, better radiation efficiency → high power density Z 0F =15Ω Trade-off between maximizing coupling, minimizing VSWR and minimizing E max Segmentation (3 straps) Minimizing E max and V max Passive 4-port junction (4PJ) Connects 3 straps in parallel to one feeding line Reduction of the number of feeding lines No active/moving component in the antenna Currents are in phase → triplet of straps is seen as a long strap with uniform current by plasma Service stub Broad-banding of the RF response curve Outside antenna: 20Ω-50Ω transition at V max to reduce VSWR Decoupling and matching network (Double Stub Tuner) Reduction of mutual coupling effects Control current distribution of array to impose required current spectrum

8. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 8 In all regions: and Assumption for optimization: critical parameter is V max  Improve achievable V max by design (rounding edges,…)  For given V max and I max,lines → Maximize G min to maximize P and minimize S (SWR) for given Z 0 Example: if G min ↑ by 20% → S  by 20% For given |V max |: → P ↑ by 20% → same |I max, lines | and For given P: → V max  by 10% → |I max, lines |  by 10% and Antenna triplet RF model

9. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 9 Triplet frequency response G min1 determined by Z F and Z 01 If ideal TL 4PJ is at V anti-node for all frequencies: G min2,max = 3G min1

10. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 10 Triplet frequency response 4PJ fixed in space → acts as a single frequency filter Maximum at f opt, when electrical junction point is at V anti-node Tune response by choosing Z 01,, Z 02 G min2,max = 3 G min1 f opt solution of tan(β )= Z 01 /X F Bandwidth function of Z 01 /X F and Z 02 /Z 01 f opt

11. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 11 Triplet frequency response Broad-banding → Band-pass filter response in feeding line G min3 response shape determined by Z 0SSt, L 4PJ-SSt and L SSt G min3 response curve turns around a turning point and its slope is determined by L SSt f TP determined by L 4PJ-SSt (Turning point remains on G min2 curve) Turning point f TP

12. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 12 By acting on front face geometry of the antenna (↔ G min1 ) → Strap width, box depth, vertical septum recess, profiling… But |I F | ↑ when X F ↓ because  Trade-off between I ant,max and V max Modeling (MWS, Topica, Antiter II) + Mock-Up Phase 1 By acting on the 4-port junction (↔ G min2 ) → Optimal frequency solution of tan(β )= Z 01 /X F → Bandwidth function of Z 01 /X F and Z 02 /Z 01 → Optimize 4PJ geometry By acting on Service Stub: Z 0SSt, L 4PJ-SSt and L SSt (↔ G min3 ) Modeling (MWS, TL) + Mock-Up Phase 2 Mainly due to antenna box geometry, weak dependence on plasma conditions Partly due to external medium and partly due to antenna box geometry Prematching No impact on I ant Coupling Impact on I ant Frequency response can be optimized… B14 – F. Louche B16 – F. Durodié

13. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 13 RF properties validation using RF mock-ups ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions ITER ICRH antenna - RF requirements Actual reference antenna Design choices & features Optimization of the antenna Frequency response Reduced-scale mock-ups Rationale for the use of reduced-scale mock-ups Phase 1: Optimization of one triplet Validation of antenna box optimization Phase 2: Validation of optimized model Validation of optimized front face and 4-port junction Broadbanding by service stub Phase 3: Mock-up of full antenna Performance evaluation Grounding Matching and Decoupling system test Conclusions

14. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 14 Why using reduced-scale RF mock-ups ? Relatively cheap way to validate the RF simulations results Same impedances as full scale model if ratio between dimensions and vacuum wavelength kept constant (except for skin effect losses) →Operating frequency needs to be multiplied by reduction scale factor Realistic plasma-like load conditions obtained by putting a medium with a large dielectric constant, such as water, in front of the antenna →Load variations obtained by moving water load in front of antenna mock-up No need for large water load →Small concentration of salt added to water allows wave absorption (avoid reflections on walls leading to standing waves)

15. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 15 Phase 1 – RF optimization validation Based on October 2007 design (1 strap triplet and triangular 4PJ) Measurements/simulations performed: Scan in distance mock-up – water load Scan in strap width and antenna box depth 3 different strap widths 3 different box depths → 9 sets of straps Impact of Faraday screen Impact of vertical septum recess

16. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 16 Phase 1 – Set-up

17. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 17 Scan in load conditions Good agreement with simulations (except when load against the antenna) but MWS: importance of BC and meshing to obtain quantitative agreement Measurements: importance of correct de-embedding of 20Ω-50Ω transition Expected frequency response - not centered in ITER band because fixed 4PJ Only slight frequency shift with change in loading

18. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 18 Scan in strap width and antenna box depth Scan in strap width W : X F ↓ when W ↑ → shift towards higher f Scan in antenna box depth D : X F ↑ when D ↑ → shift towards lower f Good agreement with numerical simulations

19. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 19 Numerical optimization Optimization of strap width and antenna box depth Not very sensitive to W and D when close to optimum

20. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 20 Phase 2 – Optimized geometry and service stub Optimized geometry (reference June 2008) Measurement/simulations performed: Set of spacers to scan: 4-port junction arms length Service stub insertion point Scan in 15Ω service stub length Scan in distance mock-up – water load No Faraday screen

21. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 21 Scan in 4-port junction arms’ length Frequency response can be centered in band by acting on 4PJ arms’ lengths But this affects G min2 as G min2,max = 3 G min1 (in case of ideal TL 4PJ)

22. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 22 Scan in 4-port junction arms’ length Comparison Measurements – MWS and TL simulations

23. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 23 Broad-banding by service stub

24. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 24 Comparison measurement – TL model Excellent representation of service stub insertion by Transmission Line modeling

25. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 25 Impact of service stub insertion point Change L SSt → Turn around “turning point” Change L 4PJ-SSt → Move “turning point” along G min curve

26. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 26 40MHz 55MHz Voltage pattern For V max in the MTL of V max3 = 45 kV the voltage can be higher: in 4PJ in section between 4PJ and SSt in SSt → Need to operate at V max3 < 45kV for some frequency ranges

27. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 27 Power limitation Active power for 1 triplet and given experimental load condition Power from G min and V max =45kV in all regions of antenna Infinite extent regions

28. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 28 Power limitation But regions 1, 2 and service stub of finite extent → V max corresponding to G min may not be reached → Voltage margin Power constrained to V max =45kV reached in every region

29. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 29 Power limitation Active power for 1 triplet and given experimental load condition Maximum power constrained to V max =45kV in all regions of antenna Other power limitations exist (electric fields, current) Very sensitive to L SSt, less to L 4PJ and rather insensitive to L 4PJ-SSt = Minimum of dotted lines B16 – F. Durodié

30. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 30 Phase 3: Full antenna RF characterization RF characterization of full array Impact of Faraday Screen on coupling Effect of vertical septa recess Effect of grounding

31. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 31 RF performance – Preliminary measurement Expected RF frequency response (relative) Skew in 0π0π response due to too low K D,water for low f Total radiated power for V max3 =45kV in feeding line and fixed water load position B18 – S. Champeaux

32. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 32 Power distribution amongst triplets Array currents controlled but strong variation in active radiated power to straps for the different triplets due to mutual coupling Active radiated power can be negative for some triplets  Crucial importance of good decoupling network

33. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 33 Comparison with modeling Preliminary analysis show fair agreement between measurements and modeling

34. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 34 Impact of vertical septum recess Significant gain in coupling by recessing further the vertical septa Less gain for dipole (0π0π) Mutual coupling between strap triplets increased → check whether level is tolerable by decoupling network → evaluate impact on tuning elements (range, current rating, …) Frequency shift towards lower frequencies → coupling will increase further when centering in the frequency band Different positions of service stub for internal and external triplets Note: slight uncertainty on exact position on temporary set-up Note: full VS recess, i.e. all vertical septa recessed ReferenceReference + 20mmReference + 40mm

35. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 35 Effect of Faraday Screen Limited decrease of coupling observed Slight shift of towards higher frequencies

36. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 36 Effect of grounding on RF frequency response A54 – V. Kyrytsya Mind the gap: 20mm clearance gap between the antenna plug and the vacuum vessel may lead to mode excitation in the gap Frequency response is essentially affected for monopole phasing → avoid monopole excitation due to unequal anti-node voltage distribution

37. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 37 Performed on design 2003 mock-up at present CT option (back-up) Adjacent poloidal triplets are connected in shunt in the circuit via a T-junction Matching stubs to adjust the conjugate pairs 6 Toroidal decouplers are preset (vacuum load) capacitors 11 feedback actuators for phase control of voltage anti-nodes (other parameters preset) Tuning stubs: 8 Generator relative phase: 3 Fully simulated, implemented and tested on mock-up Hybrid option (reference) Adjacent poloidal triplets are connected to 3dB hybrid splitter Double stub tuning on each triplet line 23 active feedback actuators for full antenna (other parameters preset) Double Stub Tuners: 8 x 2 = 16 actuators Poloidal decouplers: 4 actuators Toroidal (CD phasings) or Poloidal-Toroidal (Heating phasings) decouplers: 3 actuators Fully simulated, implemented and tested (CD case) on mock-up Decoupler tuning by voltage anti-node on adjacent lines comparison Starting conditions important for stability → in practice, starting from the vacuum conditions is OK Phase 5: Matching and Decoupling B15 – D. Grine D. Grine – RF2009

38. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 38 Left: 10 decouplers (green) between the ports A-H and 16 matching stubs (red) on the 8 heating lines; Right: feedback system with software-based controller and associated hardware Mock-up of the ITER antenna and the 3dB hybrid matching circuit Strap array Decouplers Double Stub Tuners 3dB Hybrids & DST probes Water load (removed) Voltage anti-node ports 3dB Hybrid Mock-up Implementation

39. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 39 Impedance tuning is done via one of three developed algorithms: Bang-Bang: same as CT. Fast Bang-Bang: improves on former by simultaneously steering the two tuning stubs. Real/Imag: steers the double stub tuner using analytically derived formulas based on the measured reflection coefficient at the hybrid outputs, both in magnitude and in phase. Resilience study started 3dB Hybrid Simulation of R A,eff excursion from 2.25Ω/m to 5Ω/m and current drive. |  HO | for the heating lines A-H as a function of the normalized iterations n/N BB, where N BB is the number of iterations required for the Bang-Bang algorithm to converge B15 – D. Grine Bang-BangFast Bang-BangReal/Imag

40. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 40 Experimental measurements on mock-up validated simulation results Gain confidence in design optimization and expected performance Frequency response and broad-banding by service stub confirmed Coupling loss due to presence of Faraday screen is limited Coupled power very sensitive to vertical septum position Beneficial to recess further the vertical septum Need integration with decoupling and matching network Vital importance of decoupling network confirmed Grounding Importance of correct grounding (essentially for monopole) Matching and decoupling Suitable algorithms found and implemented Tested on CT and hybrid options on full array (CD case only for hybrid option) Conclusions

41. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 41 Some related contributions R02 – R. D’Inca – Arc detection for the ICRF system on ITER I06 – R. Maggiora – Mitigation of parallel RF potentials by an appropriate antenna design using TOPICA I17 – E. Lerche – ICRF scenarios for ITER’s half field phase A54 – V. Kyrytsya – Detailed modeling of grounding solutions for the ITER ICRH antenna B11 – A. Mukherjee – Status of R&D activity for ITER ICRF power source B12 – D. Rasmussen – ITER ICH transmission line and matching system prototype development B14 – F. Louche – 3D modeling and optimization of the ITER ICRH antenna B15 – D. Grine – Results of the implementation on a mock-up of the full 3dB hybrid matching option of the ITER ICRH system B16 – F. Durodié – Optimization of the layout of the CYCLE ITER antenna port plug and its performance assessment B17 – M. Nightingale – Design of the ITER ICRF Antenna B18 – S. Champeaux – High dielectric dummy loads for ITER ICRH antenna laboratory testing: numerical simulation of one triplet loading by ferroelectric ceramics B19 – JM. Bernard – TITAN: a test bed facility for ICRH antenna and components of ITER B25 – D. Rathi – A simple coaxial ceramic based vacuum window for Vacuum transmission line of ICRF system B30 – A. Messiaen – Influence of the edge plasma profile and parameters on the coupling of an ICRH antenna. Application to ITER. B31 – D. Milanesio – Analysis of the impact of antenna and plasma models on RF potentials evaluation

42. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 42 Scan in load conditions Estimate of strap input impedance Effective strap input resistance significantly varies with load (distance mock-up – water load) Effective strap input reactance almost insensitive to load (distance mock-up – water load)

43. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 43 4-port junction passive distribution Estimate of input strap voltage and current on the 3 straps of one triplet Excellent passive power distribution operated by 4-port junction

44. 19 th RF Top Conf: 3 June 2011 P Dumortier et al Slide 44 Impedance tuning for the CT is done via the Bang-Bang algorithm: an ad-hoc (trial&error) approach using the magnitude of the reflections after the T and steering only one stub at a time for each CT Phase feedback on mock-up Matching feedback on mock-up Simulation of load-resilience at the generators for R A,eff ≈ 2.25Ω/m and Current Drive Measurement of load-resilience at the generators for R A,eff ≈ 2.25Ω/m and Current Drive Initial wrong decisions Conjugate-T