Korea-Japan Workshop on Physics and Technology of Heating and Current Drive 2016 PAL, Pohang, Korea / Dec. 14-16, 2016, Construction of 0.5-MW prototype PAM for KSTAR LHCD system Jeehyun Kim a, Sonjong Wang a, Jongwon Han a, Hyunho Wi a, Taesik Seong b, Sangwon Seon a, M. H. Cho b, and W. Namkung b a National Fusion Research Institute, Daejeon, Korea b POSTECH, Pohang, Korea E-mail : jeehkim@nfri.re.kr 1
Contents Basic configuration of the 4-MW system in 2021 Preceding study on 4-MW system using 0.5 MW system - Detailed design of prototype PAM RF design of multijunction and splitter using HFSS N// spectrum, reflection properties of the launcher, Model for construction of Prototype PAM - Mode converter study for a low loss circular waveguides system. Two types of mode converter designs were developed to transform the rectangular output of klystron to circular waveguide of the transmission line. In-line coupling vs. Sidewall coupling Prototype is under construction Summary and Future Work 2
4-MW system basic configuration of the system 8 x (5-GHz 0.5 MW CW klystrons) Low-loss transmission line with oversized circular waveguide - Total loss <15% Passive Active Multijunction (PAM) launcher for the mid-plane injection - PAM is ideal for steady state operation due to extremely low reflection, insensitive to the density profile in front of the launcher, active cooling - Off mid-plane injection near the upper diverter is also under consideration. Ref. Y. Bae et al., "Simulation study of proposed off-midplane lower hybrid current drive in KSTAR, Plasma Physics Control Fusion 58, 075003 (2016) TE 01O -to-te 10 mode converter 16 m 20 m TE 10 -to-te 01 O mode converter X 8 4 m 12 m 50-kW waterload X 8 250-kW RF window X 16 Mid-plane PAM Low loss transmission line transmitting TE 01O mode in oversized circular waveguides 120mmΦ X 50 m X 8 5-GHz 500 kw CW klystron X 8 3
PAM design N// > 2.5 for the efficient current drive by a single pass absorption from the previous study Dimension of the active and passive waveguide was determined, considering the N //o, power density, E field strength, etc. - Toroidal pitch, p=18 mm for N //o =2.5 b A =b P =7mm, s=2mm - a=58.2 mm (same as WR229): maximize a to reduce power density but <60 mm to prevent TE 20 mode excitation - Power density at the launcher mouth should be <33 MW/m 2 15% loss in MTL ~ 425 kw/module P forward b A s b P The number of active waveguides, 0.425MW/(58.2x7mm 2 )/ (33MW/m 2 ) ~31.6 ~ 32 32/4 = 8 multijunctions / PAM module a p P reflected to dummyload WR187 3 db hybrid splitter x 8 One PAM module for 500 kw WR187 RF window (commercial) WR187-WR229 Taper 4 columns x 8 rows = 32 active waveguides 4-way splitter Multijunction -180-90 0-90 -180-90 -180-270
Requirement for RF design of multijunction module Two bijunctions and two fixed phase shifters were integrated after individual optimization. Then total length was optimized again to maximize the transmission by adjusting the length of the straight sections. Requirement for phase shifter design - Maximum power density in the waveguide < 40 MW/m 2 - Maximum E-field in the waveguide < 4 kv/cm -180 o FPS l bj2-270 o FPS l bj1 a 1 a 2 l 1 l 2 a 2 l 2 l 1 a 1-270 o FPS 180 o FPS Electric field threshold normalized to the frequency. Ref. M. Goniche, et al., Nucl. Fusion 54 (2014) 013003 Modelling of power limit in RF antenna waveguides operated in the lower hybrid range of frequency a 1 a 2 l 1 l 2 48.0 mm 53.0 mm 181.0 mm 16.0 mm 40 mm 48 mm 133.5 mm 19.5 mm 5
HFSS model of multijunction module (N //0 =2.5) P in = 80 kw, E max = 3.5 kv/cm -180 o FPS -270 o FPS E norm E 0 o -270 o -180 o -90 o S 11 <-34 db Max Power density = 39.5 MW/m 2 Reflection coefficient [%] 50 45 40 35 30 25 20 15 10 5 0 5-GHz cutoff density λλ_1=2 mmmm, λλ_2=20 mmmm 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Density, n e0 (x10 19 m -3 ) Grill64C PAgrill32C FAM64C PAM32C Comparison of reflection coefficient of various types of antenna, i.e. grill vs. passive active grill vs. FAM vs. PAM as a function of density in front of the launcher (N// =2.5). PAM has lowest reflection around the cut off density of 5 GHz. (Evaluated using ALOHA code) S 21 =-6.02±0.02 db S 21 =90±1 deg 6
0.5 MW Prototype PAM 0.5 MW PAM is under construction for the preceding study on 4-MW system The design of multijunction is the same as the 4 MW system (N //0 = 2.5). RF characteristics of ½ PAM P2 P9 8 columns x 4 rows of active waveguide P10 P17 S 11 < -35 db RF balance = -12.1 +/-0.1 db E max in the phase shifter < 4 kv/cm 7
RF model of toroidal and poloidal splitter E max ~4.6 kv/cm with 125 kw E max ~5.7 kv/cm with 250 kw WR229 58.2mm x 29.1 mm
3D model for 0.5 MW Prototype PAM antenna Toroidal splitter Poloidal splitter Directional coupler Vacuum port Taper Straight waveguide PAM 330 mm X 2 X 2 X 2
Machining and Joint of PAM plate Material : OFHC Cooling line Cutaway view 9 Diffusion bonding Joint surface has the same physical and mechanical properties as the base material. a high quality joints with no discontinuity and porosity in the interface. joining either similar or dissimilar materials. Easy and inexpensive
Bonding technology for PAM construction Brazing, Explosive bonding, Welding, Diffusion bonding, etc. PAM in WEST SS Cu Explosive bonding for dissimilar metal joining Limitations Wave height depends of ignition location Copper At 300 mm from the IP At 800 mm from the IP 2700 mm from the IP Stainless steel.6 m m 1 mm 2 mm Available SS Copper Brazing - Leakage of filler material can contaminate the inside of PAM and even can cause arc in high power transmission when it s not properly processed. Diffusion bonding - Solid-state diffusion by applying high pressure at an elevated temperature - Pros: no impurities - Cons: Deformation (or contraction) by pressure 11
Phase error due to curved surface (X,Y,Z) R1C1/C8 C2/C7 C3/C6 C4/C5 R2C1/C8 C2/C7 C3/C6 C4/C5 X Y Z ΔX ΔΦ R [%] 8.48 63 117 8.07 45 117 0.42 2.1 o 0.14% 7.79 27 117 7.66 9 117 0.14 0.71 o 0.015% 1.69 63 39 1.28 45 39 0.41 2.1 o 0.14% 1.00 27 39 0.86 9 39 0.14 0.71 o 0.015% Curved surface of the active waveguide of the launcher. RF 180 R 1-R 90 90 Plasma 350 mm Passive waveguide ~750 mm Curvature shaping Passive waveguide 17.6 26.5 17.6 34.0 R900 17.6 34.0 17.6 26.5 12
Thermal analysis with a simplified model stainless steel 25 kw/m 2 355 300 12.5 kw/m 2 200 100 Temperature [ o C]400 102 Copper Stainless steel 100 kw/m 2 50 kw/m 2 Temperature distribution of prototype PAM made of stainless steel by 60 seconds pulse by 125-kW RF power and plasma radiation without cooling 0 0 10 20 30 40 50 60 Time [s] The comparison of the maximum temperatures of the PAM made of copper and stainless steel. Thermal analysis based on a simplified model was performed for 60 sec pulse. Temperature change of PAM prototype by the RF loss and plasma radiation without cooling was calculated. The multijunction was divided into three parts in the model. The peak RF loss in each part was adopted as an average RF loss in each to maximize the temperature rise for the conservative calculation. The materials were compared in stainless steel and copper cases. The peak temperature after 60 second RF pulse with 100 kw/m 2 plasma radiation was estimated to be ~ 355 o C and 102 o C at the mouth of the launcher, facing the plasma, made of stainless steel and copper, respectively. Therefore it is possible to operate the prototype PAM without cooling for 500 kw 60 seconds pulse operation. 13
Low loss transmission line system %Loss/10m 10 9 8 7 6 5 4 3 2 1 0 WG radius~57 mm TE01 mode TE11 mode Rect WG 0 200 400 600 800 1000 Circumference of WG [mm] Comparison of Resistive Loss on the copper waveguide wall depending on the waveguide circumference. TE 11O mode TE 01O mode Easy to make mode converter Polarization can be rotated during propagation. No polarization due to circular symmetric field Extremely low loss in highly oversized WG Complicated to make mode converter Oversized circular waveguide should be used as long as the length of TL > 50 m. It is difficult to lower the loss <5% with rectangular WG. Circular waveguide radius, R R>57 mm: loss of TE 11O > TE O 01 R<57 mm: loss of TE 11O < TE O 01 Resistive loss in the 50-m long circular waveguide with 60 mm radius TE 10 in WR284 TE 01O in 60 mm radius TE 11O in 60 mm radius Material 80 m 50 m Cu 25% 17% Al 32% 21% Cu 5.9% 3.7% Al 7.6% 4.8% Cu 6.6% 4.2% Al 8.5% 5.4% 14
Rectangular to circular mode converter is necessary Since the klystron output is TE 10 mode in a WR187 rectangular waveguide, TTEE 1100 to TTEE OO 0000 mode converter is necessary. Two types of mode converters were developed; In-line coupling & sidewall coupling 10 20 30 40 50 60 70 80 R [mm] Cutoff radius of the several TE and TM mode in a circular waveguide at 5 GHz The radius of circular output of mode converter was chosen to be 40 mm not to excite higher order mode since 10 modes can propagate in a circular waveguide of 60-mm radius. The cutoff radius of TE O 01 mode = 36.6 mm. The R40-to-R60 taper will extend the waveguide size. Mode conversion scheme K K WR187 R40 R40 mode mode converter converter WR187 mode converter R40 Taper R60 Taper R60 In-line coupling (Serpentine) Sidewall coupling 15
RF designs for mode converters Sidewall coupling Poles to supress TE 21 and TM 11 mode In-line coupling S 21 (TE 01 )=-0.028 db Diffraction loss ~0.6% E norm E max with 500 kw = 9.85 kv/cm TE 10 -TE 11 O WR187-to-R32 TE 11O -TE 01O (R40) [Taper] Z p 2A 487 mm (Radius total length) ~ 1500 mm when R60 S 21 (TE 01 )=-0.061 db Diffraction loss = 1.4% 16
Construction of mode converter mock-ups TE O 10 to TE 11 TE O O 11 to TE 01 Two types of mode converter mock-up were developed and their performances were compared. Since the TE O 01 mode has a circular symmetric field, azimuthal dependence of the transmission through one pair of mode converters were measured. Serpentine mode converter Port 1 Port 2 α α (Serpentine) Sidewall MC, Measure Sidewall coupling mode converter 17
RF measurement of mode converter mock-ups Sidewall coupler has excellent azimuthal symmetry but low S 21 due to surface condition. The serpentine mode converter showed sudden decrease at +/- 80 o. HFSS calculation showed that the poor azimuthal symmetry was due to the length of straight section, 200mm (---), connecting two mode converters. The sudden change disappeared when the length is 300 mm (---). (Serpentine) Sidewall MC, Measure HFSS calculation and experimental measurement showed excellent agreement. 18
Refinements for prototype construction Refined design for machining with better performance. Sidewall coupling Perfect conductor boundary, S 21 = -0.007 db= 99.8% Diffraction Loss = 0.2% Serpentine Z p =115.8 mm, A= 4.4 mm, R=39.5 mm S 21 = -0.0465 db ~ 98.9% Diffraction Loss = 1.1%
Prototype is under construction Design of Circular Taper The radius of the mode converter circular output is 40 mm and the radius of the circular waveguide for transmission is 60 mm. R40-R60 taper is necessary for connection. Various shapes of profiles were compared. R40 Linear R60 R40 Two slope linear R60 R40 Arc R60 R40 Parabola R60 R40 Sinusoidal R60 Low diffraction loss for TE 11O mode Sidewall coupling type was adopted as a prototype. TE 01O mode Transmission efficiency with copper is predicted to be 99.5% from the HFSS calculation. Copper boundary S21 = 99.5% Diffraction loss ~0.2% Resistive loss ~ 0.3% 20
Summary and Future Work 0.5 MW Prototype PAM is under development. - RF and mechanical design for construction was completed - Diffusion bonding will be used for construction of PAM - No phase correction of curved shape - Cooling on launcher mouth facing the plasma Two types of mode converter mock-ups for TE 01O mode were developed and their characteristics were measured. - Side wall coupling type showed better azimuthal symmetry. - Prototype mode converter adopting sidewall coupling type is under construction. Future work - Manufacturing and installation of PAM - Rigid supporting structure design eddy current by PF swing and plasma disruption, and consequent torque induced on the antenna in conjunction with static magnetic field, etc. - Part of transmission line (~16 m) made of WR284 rectangular waveguide will be changed to oversized circular waveguide made of aluminum, which are lighter and cheaper 21
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Circular waveguide 16 m Part of transmission line will be changed to oversized circular waveguide made of aluminum which is cheaper and lighter 16-m WR284( ) Cu loss =5.7% 16-m R60(O) Al loss =1.6%
HFSS design of multijunction module (N //0 =2.5) WR229 3-dB hybrid splitter E norm 44.7 mm 68.3 mm 50.4 mm WR229 4-way splitter design based on 3-dB hybrid splitter E max = 6.6 kv/cm with 250 kw 123.9 mm E max ~9 kv/cm with 500 kw 160.3 mm R40 Linear R60 R40 Two slope linear R60 R40 Arc R60 R40 Parabola R60 R40 Sinusoidal R60 Low diffraction loss for TE 11O mode TE 01O mode