Ion Cyclotron Radio Frequency Antenna Desing for the Thermonuclear FAST Experiment

1. Ion Cyclotron Radio Frequency Antenna Desing for the Thermonuclear FAST Experiment Riccardo Maggiora, Daniele Milanesio, Marco Sorba

2. FAST EXPERIMENT The International Thermonuclear Experimental Reactor (ITER) is the world greatest experiment towards to the nuclear fusion. ITER requires a parallel R&D activity on devices that can investigate burning plasma conditions with high flexibility. The FAST (Fusion Advanced Studies Torus) project is a new facility with the important task of preparing ITER scenarios. FAST will work with Deuterium plasma 98% D 2% 3He B = 7.5 T The predominant auxiliary heating scheme is ICRH Each ICRH antenna will provide 5 MW and will work in the range [60 - 90] MHz (due to minority heating)

3. REFERENCE DESIGN 8 current straps Faraday Shield composed of 30 bars of square section • 48 kV on the coaxial lines; • steady state plasma profile; • 5 cm of SOL length. Power delivered to plasma: 3.035 MW

4. OPTIMIZATION STRAPS • Reference design: • strap width = 14 cm; • distance between strap = 34 cm; 3.641 MW Distance to box: [10 cm  1 cm] Strap width: [12 cm  16 cm] • Adopted plasma: • steady state plasma profile; • 5 cm of SOL length. • Optimized design: • strap width = 14 cm • distance between straps = 28 cm Power delivered to plasma: 3.641 MW

5. OPTIMIZATION FARADAY SCREEN Reference design rod section Reduction of the radial thickness Reduction of the distance between the radiating elements and the plasma edge 2 cm 1.5 cm 2 cm 2 cm Smoothing of the rod section Power delivered to plasma: 3.944 MW

6. OPTIMIZATION FINAL SMOOTHING Strap refinement Feeder refinement Mesh structuration Electric field distribution in front of the antenna Power delivered to plasma: 3.935 MW

7. Overall power improvement 3.935 MW + 29.7 % 3.035 MW

8. PLASMA ANALYSIS H-mode profile Two different plasma profiles Steady state profile Analysis of antenna performances and input impedance behavior varying the plasma profile and the SOL length in the range [3 cm – 8 cm] SOL High density plasma SOL: scrape off layer. Tokamak center Separatrix Plasma edge Separatrix: last closed flux surface. SOL length: distance between antenna and separatrix. SOL length

9. PLASMA ANALYSIS POWER PERFORMANCES • The steady state plasma profile is more suitable to reach high power values 6.665 MW • The performances degrade rapidly when the separatrix is far from the antenna 4.578 MW A reliable knowledge of the separatrix position is very important

10. PLASMA ANALYSIS Steady state profile: INPUT IMPEDANCE Z11 Toroidal coupling Poloidal coupling Z12, Z21 Z13, Z31

11. SPECTRUM ANALYSIS Steady state H-mode The maximum amount of power is delivered at npar = ± 5 npar npar nperp nperp npar is the normalized spectral frequency: npar = kpar / k0 nperp=0 nperp=0 npar npar

12. RF POTENTIALS For high RF power system, it is important to study the RF potentials, generated by the electric field parallel to the magnetic field responsible of the confinement. In order to compute the RF potentials, it is required the evaluation of the electric field distribution in front of the antenna and the integration of the parallel component along tilted field lines. The RF potentials cause the acceleration of particles which can seriously damage the antenna (hot spot).

13. RF POTENTIALS NEW COMPUTATIONAL METHOD The electric field map over a radial plane is computed in the spectral domain It is possible to exploit the knowledge of the spectral electric field avoiding a computationally expensive two-dimensional inverse Fourier transform. Electric field: RF potential: The RF potentials can be computed performing a one-dimensional inverse Fourier transform on spectral samples satisfying kx = 0, instead of an entire distribution of samples in the spectral kx,ky domain.

14. RF POTENTIALS EVALUATION FOR THE DESIGNED ANTENNA H-mode profile: magnetic field inclination = 16° Overviews of RF potentials between the aperture and 5 cm inside the plasma RF potentials at the first radial plane y [m] y [m] Radial position VRF (V) Computations performed considering 4 cm of SOL length, 1 V for each port and dipole phasing.

15. RF POTENTIALS EVALUATION FOR THE DESIGNED ANTENNA • because of the inclination of the magnetic field, the RF potentials are not poloidally symmetric; • the radial decay results to be fast; • the absolute values of the RF potentials are not strong enough to be considered dangerous for the antenna itself; There is no need to further refine the antenna geometry in order to mitigate the RF potentials

16. ANALYSIS OF TWO ANTENNAS This is the first carried out study in order to characterize the reciprocal interaction between two IC antennas displaced at different locations around the tokamak

17. ANALYSIS OF TWO ANTENNAS POWER PERFORMANCES Steady state profile 3 cm of SOL length H-mode profile 3 cm of SOL length Power delivered by the single antenna 4.578 MW 6.665 MW Power delivered by the double antenna 11.246 MW 9.406 MW Overall performance decrease: strong mutual coupling Lower mutual coupling

18. MAIN CONCLUSIVE OUTCOMES Fundamental role played by TOPICA simulation tool An IC antenna design procedure requires the management of: • antenna geometry with high level of detail • peculiarities of different plasma profiles • simulation parameters Design of the best antenna geometry in term of coupled power Development of a TOPICA post-processing tool for the evaluation of the RF potentials in front of the antenna Achievement of the deepest knowledge of all antenna features Accurate description of a design and refinement procedure for a generic ion cyclotron radio frequency antenna