Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 1 Mitigation of parallel RF potentials by an appropriate antenna design using TOPICA R. Maggiora and D. Milanesio Special thanks to ENEA. ASDEX Upgrade and CYCLE teams

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 1.Motivations and background 2.ITER-like IC antenna 3.ASDEX Upgrade IC antenna 4.Plasma sensitivity 5.Conclusions 2 Outline

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport The successful design of an IC antenna is based not only on the capability to deliver enough power to the plasma (feature that has been extensively investigated in recent years), but also on the reduction of those unwanted phenomena, such as rectification discharges or hot spots, that are naturally associated with the required power levels. In these conditions, an accurate design tool such as TOPICA, i.e. able to account for a realistic 3D antenna geometry and an accurate plasma model, is essential! 3 Motivations and background

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 4 Outline 1.Motivations and background 2.ITER-like IC antenna 3.ASDEX Upgrade IC antenna 4.Plasma sensitivity 5.Conclusions

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 5 TOPICA modeling: reference geometry Constant working frequency: 53MHz Full horizontal septa Constant plasma profile: Sc2short (~17cm SOL), B angle =15° “V-shaped” curvature in the poloidal direction only Electric field computed at the plasma edge, located at constant 5mm distance from the FS 2cm gap all around the antenna box to account for the grounding, located 1m in the back Constant 4cm distance between the straps and the plasma edge Realistic ITER-like antenna geometry based on 2010 TOPICA model, not tilted FS

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 6 TOPICA modeling: retracted horizontal septa Constant working frequency: 53MHz Constant plasma profile: Sc2short (~17cm SOL), B angle =15° 2cm gap all around the antenna box to account for the grounding, located 1m in the back “V-shaped” curvature in the poloidal direction only Constant 4cm distance between the straps and the plasma edge Fully retracted (4cm) top and bottom horizontal septa Realistic ITER-like antenna geometry based on 2010 TOPICA model, not tilted FS Electric field computed at the plasma edge, located at constant 5mm distance from the FS

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 7 Top and bottom horizontal septa have been retracted of 4cm respect to the their original position in the reference geometry The horizontal central septum has not been moved at this stage Reference geometry TOPICA modeling: retracted horizontal septa

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 8 RF potentials are shown 5mm in front of FS bars Poloidal phasing: 0π 20MW coupled to plasma Tilted magnetic field lines (15°) RF potentials: retracted horizontal septa Lower horizontal septum RF potentials strongly depend on the input phasing. Visible reduction localized on the magnetic lines crossing the upper and lower horizontal septa

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 9 Constant working frequency: 53MHz Full horizontal septa Constant plasma profile: Sc2short (~17cm SOL), B angle =15° 2cm gap all around the antenna box to account for the grounding, located 1m in the back “V-shaped” curvature in the poloidal direction only Constant 4cm distance between the straps and the plasma edge Realistic ITER-like antenna geometry based on 2010 TOPICA model, tilted FS according to magnetic field lines TOPICA modeling: tilted FS bars Electric field computed at the plasma edge, located at constant 5mm distance from the FS

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 10 RF potentials are shown 5mm in front of FS bars Poloidal phasing: 0π 20MW coupled to plasma Tilted magnetic field lines (15°) RF potentials: tilted FS bars Remarkable reduction of RF potentials on the entire poloidal section Power coupled to plasma is 15% lower

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 11 RF potentials are shown 5mm in front of FS bars Poloidal phasing: 0π 20MW coupled to plasma Tilted magnetic field lines (15°) RF potentials: tilted FS bars & fields misalignment Field misalignment could determine, depending on the poloidal position, a not negligible increase of the RF potentials

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 12 RF potentials are shown 5mm in front of FS bars Poloidal phasing: 0π 20MW coupled to plasma Tilted magnetic field lines (15°) RF potentials: tilted antenna The alignment of the whole antenna to B (instead of the FS alone) remarkably reduces the RF potentials for 0ππ0 toroidal phasing, i.e. in case of symmetrical feeding

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 13 Outline 1.Motivations and background 2.ITER-like IC antenna 3.ASDEX Upgrade IC antenna 4.Plasma sensitivity 5.Conclusions

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 14 TOPICA modeling: reference geometry Constant working frequency: 30MHz Two plasma profiles: a small gap and a large gap measured in 2010 campains Electric field computed 3mm in front of the limiters Constant 4.5cm distance between the straps and the plasma edge Approximated flat model based on the “reference” ASDEX Upgrade IC antenna recently installed

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 15 The “broader limiter” approach Radial plates Return currents are induced on the two lateral radial plates and then screened by the FS bars Tapered straps Broader limiters

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 16 Reference vs. Broader Limiter Reference antenna BL antenna Plasma1 (small gap) Plasma2 (large gap) 1MW coupled to plasma Tilted magnetic field lines (11°) Average reduction (global) Plasma1 : 1.85 Plasma2 : 1.59 Average reduction (local) Plasma1 : 2.09 Plasma2 : 1.88 Peak reduction Plasma1 : 1.77 Plasma2 : 2.56

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 17 An alternative approach The basic concept is to have a two-layers recess to protect the back connections to the straps and to exploit all the possible symmetries in the plasma facing components. Two-layers concept Broader limiter Recessed horizontal limiter (to FS level) Radial plate Tapered simpler straps

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport An alternative approach Reference antenna BL antenna Alternative design Plasma1 (small gap) Plasma2 (large gap) Substantial RF potential reduction despite slightly higher absolute electric field. Average reduction (global) Plasma1 : 2.89 (1.85) Plasma2 : 2.63 (1.59) Average reduction (local) Plasma1 : 4.20 (2.09) Plasma2 : 3.65 (1.88) Peak reduction Plasma1 : (1.77) Plasma2 : 7.63 (2.56) BL antenna 1MW coupled to plasma Tilted magnetic field lines (11°) 18

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport To be continued … Are flat models a reliable approximation of the real curved geometries, both in terms of coupled power and RF potentials? Milanesio D. et Al. - “Analysis of the impact of antenna and plasma models on RF potentials evaluation” poster session B31 Do the differences between flat models hold true also for curved geometries? Krivska A. et Al. - “Density profile sensitivity study of ASDEX Upgrade ICRF Antennas with the TOPICA code” poster session A55 Are 3 straps or 4 straps geometries better than the 2 straps solutions? Come and check it at the poster session!

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 1.Motivations and background 2.ITER-like IC antenna 3.ASDEX Upgrade IC antenna 4.Plasma sensitivity 5.Conclusions 20 Outline

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 21 Plasma sensitivity Antenna position Cutoff density The plasma sensitivity is tested for a number of plasma profiles (varying the density gradient and the antenna-cutoff distance) both in terms of power transferred to plasma and of RF potentials Antenna position Cutoff density

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 22 Plasma sensitivity Antenna position Cutoff density The plasma sensitivity is tested for a number of plasma profiles (varying the density gradient and the antenna-cutoff distance) both in terms of power transferred to plasma and of RF potentials Antenna position Cutoff density 2cm vacuum layer

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 23 Plasma sensitivity Antenna position Cutoff density The plasma sensitivity is tested for a number of plasma profiles (varying the density gradient and the antenna-cutoff distance) both in terms of power transferred to plasma and of RF potentials Antenna position Cutoff density 4cm vacuum layer

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 24 Plasma sensitivity Antenna position Cutoff density The plasma sensitivity is tested for a number of plasma profiles (varying the density gradient and the antenna-cutoff distance) both in terms of power transferred to plasma and of RF potentials Antenna position Cutoff density 4cm vacuum layer

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 25 Plasma sensitivity: density gradient 25 Provided the same antenna-cutoff distance, a negligible dependence on the density gradient is observed 1MW coupled to plasma Tilted magnetic field lines (11°) Test1a Test2a Test3a No vacuum 2cm vacuum 4cm vacuum

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 26 Plasma sensitivity: antenna-cutoff distance 26 Provided the same density gradient, the RF potentials notably increase with a vacuum layer insertion 1MW coupled to plasma Tilted magnetic field lines (11°) Test1a Test2a Test3a No vacuum 4cm vacuum

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport The RF potentials are considerably different if, instead of adding a vacuum layer, a full plasma profile is loaded 1MW coupled to plasma Tilted magnetic field lines (11°) Vacuum vs. full plasma profile Antenna position Cutoff density Effect of the slow wave…

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport Antenna position Cutoff density 28 Plasma sensitivity: edge density 28 1MW coupled to plasma Tilted magnetic field lines (11°) Provided the same core-cutoff density profile, the increase of the edge density determines an RF potentials reduction

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 1.Motivations and background 2.ITER-like IC antenna 3.ASDEX Upgrade IC antenna 4.Plasma sensitivity 5.Conclusions 29 Outline

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport Localized modifications of the geometry can be rather effective in terms of RF potentials mitigation. As a consequence, the precise knowledge of the antenna geometry (in particular of the front part) is mandatory to carefully predict the electric field distribution in front of it and, therefore, to compute the RF potentials. At least two approaches can be pursued to reduce RF potentials, i.e. to lower the electric fields absolute value and to reduce geometrical asymmetries. A plasma loading should be adopted in order to be as realistic as possible. Moreover, a parametric study of the plasma profile (above all of the SOL) is extremely important to provide accurate predictions. Is there a perfect antenna? Retracted horizontal septa A B angle tilted antenna and FS A toroidally symmetric antenna Additional feeding lines with phase and amplitude flexibility 30 Conclusions

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 31

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport 32 Reference vs. Broader Limiter Reference antenna BL antenna ε= i Plasma1 (small gap) Plasma2 (large gap) 1MW coupled to plasma Tilted magnetic field lines (11°) Average reduction (global) Dielectric : 2.19 Plasma1 : 1.85 Plasma2 : 1.59 Average reduction (local) Dielectric : 2.89 Plasma1 : 2.09 Plasma2 : 1.88 Peak reduction Dielectric : 1.71 Plasma1 : 1.77 Plasma2 : 2.56

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, Newport An alternative approach 1MW coupled to plasma Tilted magnetic field lines (11°) Reference antenna BL antenna Alternative design Plasma1 (small gap) Plasma2 (large gap) Plasma1 (small gap) Plasma2 (large gap) Substantial RF potential reduction despite the higher total electric field. Average reduction (global) Plasma1 : 2.89 (1.85) Plasma2 : 2.63 (1.59) Peak reduction Plasma1 : (1.77) Plasma2 : 7.63 (2.56) BL antenna