A design method of optimized impedance transformer for the ICRF antenna in LHD Kenji SAITO, Tetsuo SEKI, Hiroshi KASAHARA, Ryosuke SEKI, Ryuhei KUMAZAWA, Goro NOMURA, Fujio SHIMPO, Takashi MUTOH National Institute for Fusion Science Japan-Korea Workshop on Physics and Technology of Heating and Current Drive Hanhwa Resort, Haeundae, Busan, Korea January 28-30, 2013 1/21
Outline 1. Introduction 2. Method of optimization of impedance transformer 3. Optimization of impedance transformer for HAS antenna 4. Performance in various antenna impedances 5. New antenna with the optimized impedance transformer 6. Summary 2/21
1. Introduction Antenna head side Outer conductor HAS antennas in LHD Hasuu Seigyo (wave number control in Japanese) antenna or Hand Shake type antenna HAS antennas have high heating efficiency by adjusting current phase. Filled with N2 Ceramic Oscillator side Inner conductor Ceramic Feed-Through (FT) 3/21
Problems in present antenna and plan for the solution (1)Problems Loading resistance of HAS antenna is low. Typically R 3.5Ω (R net 3.2Ω). Power is limited by the interlock of voltage on transmission line. R net =3.2Ω 0.78MW V max =35kV(interlock level) Increasing of loading resistance is necessary. In 2010, Ceramic in the feed-through (FT) was cracked caused by the arcing. Decreasing of voltage around FT is necessary. During long pulse operation, temperature around FT was kept increasing. (5min, 30kV in transmission line T 10 C) Decreasing of current around FT is necessary. (2)Plan for the solution Impedance transformer between antenna head and FT is useful. Simpler than the pre-stub tuner in vacuum vessel. However, operating frequency must be fixed. In LHD f=38.5mhz is mainly used. This frequency is used for minority ion heating (H minority and He majority). With planed D neutral beam injection, high efficiency of second harmonic heating is expected. Optimization of the impedance transformer was done at f=38.5mhz. 4/21
2. Method of optimization of impedance transformer HAS antenna (in 3.5L port) Definition point for current, voltage on FT Port FT 1 FT 2 Antenna head Pivot (148mm) FT Optimization of radius of two inner conductors head side:1980mm port side: 360mm 5/21
Typical impedance of antenna head is calculated with the experiment and the simulation using HFSS (High Frequency Structure Simulator, ANSYS). Transmission line between antenna head and FT is divided to several segments. The radii of inner conductors (a i ) are independently and discreetly scanned. (i:segment number) Optimization is done so as to increase the effective loading resistance R eff within the following limitations. 1. Maximum electric field in transmission line between antenna head and FT 2. Maximum voltage in FT 3. Maximum current in FT These are normalized by square root of power to antenna head. P port R eff =ηr η: Power transmission efficiency between antenna head and port R: Loading resistance at the port 2 2 1 Vmax 1 Vmax = R Phead R eff 2 = Z c0 2 Z Higher R eff lower V max c0 V max : Maximum voltage in transmission line outside the vacuum vessel Z c0 : Characteristic impedance of the line 6/21
Electric field in transmission line is estimated at the center of segments and the both ends in each segment. Voltage and current in FT is determined at one position in FT. Simulated S matrix of FT 1, FT 2 and pivot are used for the optimization. Around the optimum a i, ranges of radius are narrowed and the calculation is repeated several times for the precise optimization. Maximum R eff a 2 a 1 Radii are smoothed with 3 order polynomials. Slope is continuous to prevent electric field from converging. Finally HFSS simulation is done to check the convergence of electric field. (Convergence of electric field cannot be estimated with the above method) 7/21
Wave number k and characteristic impedance Z c α = 1 2 Re(Z where, c ) fµ 1 + πσ a Re(Z c 1 b 1 ) = 2π k = 2πf / c jα αc Zc = Re(Zc )(1 j ) 2πf µ ε 0 0 (attenuation constant) b ln a Impedance, electric field, etc Γ(x) = V(x) = V e 2 jkx ant E (x) = Z Z ant ant + Z Z c c {1 + Γ(x)}/{e V(x) a ln(b / a) jkx a: radius of inner conductor b: radius of outer conductor σ: electric conductivity Stainless steal: σ=1.35 10 6 (S/m) ant means antenna side end in a segment. x is the distance from the antenna side end. + Γ(x)e jkx } Z(x) electric field on inner conductor, where the electric field is the maximum = Z c 1+ Γ(x) 1 Γ(x) 8/21
3. Optimization of impedance transformer of HAS antenna Determination of antenna impedance Typical discharge R ax =3.6m, B t =-2.75T Distance between Faraday shield and Last Closed Flux Surface:6cm n e 1 10 19 m -3, T e T i 1keV at t=2s 5 #111353_3.5L 0.55 voltage 4 0.54 R(Ω) 3 2 L=0.5204m (vacuum) 0.53 0.52 L(m) antenna L 1 0.51 L: position of minimum voltage 0 0.5 0 1 2 3 4 5 6 Time(s) Loading resistance and position of minimum voltage measured with directional coupler attached between antenna and impedance matching device. Impedance changed, maybe due to heat load on antenna after 3s. 9/21
Around t=2s R=3.6Ω (approximately 25m far from port) R=3.5Ω (calculated loading resistance at the port with the electric conductivity of transmission line) Position of minimum voltage is the same with the vacuum condition, which is calculated using HFSS. Impedance of antenna head Z a is calculated using S parameters between port and head simulated by HFSS. Z a =54.1+190.3j Ω (typical plasma discharge) optimization is done with this impedance. 10/21
Before optimization Radius of inner conductor a=0.0508m(fixed) (Radius of outer conductor b=0.10165m) Effective loading resistance R eff : 3.35 (Ω) Power transmission efficiency η (port head): 0.956 (power loss: 4.4%) Normalized current on FT I FT :0.884 (A/W 0.5 ) (Normalization was done with the square root of power to antenna head.) Normalized voltage on FT V FT :13.3 (V/W 0.5 ) Maximum normalized electric field on transmission line except pivot and FT: 1.10 (kv/m/w 0.5 ) Determination of limitations for optimization Normalized current on FT < 0.884 /3.5 0.5 =0.472 (A/W 0.5 ) Normalized voltage on FT < 13.3 /3.5 0.5 =7.12 (V/W 0.5 ) (FT will be tolerable with 3.5 times of power) Maximum normalized electric field < 1.10 (kv/m/w 0.5 ) Present maximum normalized electric field is low enough for arcing, assuming the arc voltage > 15kV/cm 11/21
After optimization Segment lengths:7 90mm+1350mm(head side)+5 40mm+160mm (port side) Initial range of radius a i =0.025 0.08m Smoothed with 3 order polynomials Effective loading resistance R eff : 13.3 (Ω) Power transmission efficiency η (port head): 0.985 (power loss: 1.5%) Normalized current on FT I FT : 0.441 (A/W 0.5 ) Normalized voltage on FT V FT : 6.61 (V/W 0.5 ) Maximum normalized electric field on transmission line except pivot and FT: 1.10 (kv/m/w 0.5 ) Effective loading resistance: 4 times of original value Current and voltage on FT: half of original values Power loss: one third of original value 12/21
Distributions in impedance transformer Original coaxial line Optimized stepped transmission-line transformer Smoothed transformer Dots: voltage and current at the definition point of FT They decreased by the optimization. 13/21
Smith chart (Smoothed final transformer) Reflection coefficient Γ (assuming port impedance of 50Ω) Port side 0.5 1.0 1.5 2.0 Head side Im(Γ) 0 0.5 1.0 Normalized resistance -0.5-1.0-1.5-2.0 Normalized reactance Re(Γ) From head to port, Γ decreases. increase of loading resistance 14/21
Analysis of typical discharge with HFSS Head side S 11 = 0.871, phase(s 11 )= 96.0 R=3.44 (Ω) Power to head:1mw Electric field before optimization (a=0.0508m) intense electric field Port side S 11 Electric field after optimization S 11 =0.565, phase(s 11 )=107.4 R=13.9 (Ω) Definition point of I FT, V FT intense electric field was not seen before optimization R eff =ηr=3.28 (Ω) η=0.955 (power loss: 4.5%) I FT =0.875 (ka) V FT =13.4 (kv) after optimization R eff =ηr=13.7 (Ω) η=0.985 (power loss: 1.5%) I FT =0.426 (ka) V FT =6.73 (kv) Current and voltage on FT, power efficiency and effective loading resistance are improved and the values are almost the same with the former calculation. Intense convergence of electric field is not seen. 15/21
4. Performance in various antenna impedances Variation of R, L (Poloidal array antenna, not HAS antenna) 0.1 Change of R, L during long pulse discharge 0 L [m] -0.1-0.2 Vacuum n =0-4x10 19 m -3 e n =4-8x10 19 m -3 e n =8-11x10 19 m -3 e -0.3 0 2 4 6 8 R [Ω] Journal of the Korean Physical Society, Vol. 49, December 2006, pp. S187~S191 Change is maybe due to the deformation of antenna caused by heat load on antenna. Normally -0.1< L<0.05 16/21
Calculation of antenna performance with various antenna impedances R eff (updated) / R eff (original) η(updated) / η(original) E max (updated) / E max (original) R(Ω) L(m) L(m) R(Ω) R(Ω) I FT (updated) / I FT (original) V FT (updated) / V FT (original) R(Ω) R(Ω) 17/21
Ordinate and abscissa are L and R before optimization, respectively. In the typical discharge, L=0, R=3.5(Ω) as indicated by +. Effective loading resistance and power efficiency increase by the update of the antenna. Electric field on transmission line slightly increase. (small enough for arcing) Current and voltage on FT decreases in wide region by the update of antenna. In the region of L<-0.2-0.25 (m), the voltage on FT increases by the update. However, V FT for the updated antenna is smaller than the original value of 13.3 (V/W 0.5 ). 18/21
5. New ICRF antennas with the optimized impedance transformer Faraday shield area: w340mm h348mm Expected injection power for the typical discharge: 1.6MW/antenna (steady-state) 19/21
impedance transformers Ceramic feed-through Pivot for antenna drive upper port antenna Bellows Outer conductor Inner conductor Z=3895mm Z=3200mm Z=1000mm lower port antenna Z=-3745mm Z=-3200mm Z=-1000mm Expected loading resistance 10Ω for the typical discharge 20/21
6. Summary Optimization method of impedance transformer was developed. By the optimization, effective loading resistance of 3.35Ω will increase to 13.3Ω voltage on the transmission line will decrease. Current and voltage at ceramic feed-through will decrease to the half. Therefore, possibility of damage of the feed-through will decrease. Power transmission efficiency will increase. Intense convergence of electric field was not seen in the simulation by HFSS. High performance will be kept in the wide region of antenna impedance. New ICRF antennas with the optimized impedance transformer are steadily ongoing. 21/21