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Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system
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Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system Jeehyun Kim a,∗ , Sonjong Wang a , Julien Hillairet b , Hyunho Wi a , Sangwon Seon a , Jongwon Han a , Lena Delpech b a b
NFRI, Daejeon, South Korea CEA, IRFM, 13108,St. Paul-lez-Durance, France
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 28 April 2017 Accepted 4 May 2017 Available online xxx Keywords: LHCD RF heating PAM Antenna KSTAR
a b s t r a c t The major upgrade of KSTAR LHCD system to 4 MW is planned in 2021. 4-MW RF power will be provided by 8 × 0.5 MW klystrons. Highly oversized circular waveguide transmitting TE01 ◦ mode will be adopted for the low loss transmission line longer than 50 m. The design of the 4-MW mid-plane Passive Active Multijunction launcher has been performed. Considering parallel refractive index N|| for the efficient current drive and the maximum power density at the launcher mouth, the launcher is composed of 256 active waveguides whose dimension is 58 mm x 7 mm and 18 mm of spatial period for N||0 = 2.5. Multijunction with Emax < 3.5 kV/cm and high power waveguide components have been designed using HFSS code. The antenna properties were evaluated using ALOHA. Prototype PAM launcher with 32 active waveguides is under development for 0.5-MW pulsed operation. © 2017 Published by Elsevier B.V.
1. Introduction KSTAR LHCD system for the off-axis current drive has adopted the grill type antenna with the parallel refractive index, N|| , 2.0 for 0.5 MW at 5 GHz [1]. Edge electron temperature rise and change of sawtooth period were observed by the 150 kW of delivered power for 1.5 s but significant current drive effect was not observed. The major upgrade is planned in 2021 as illustrated in Fig. 1. 4-MW CW RF power will be provided by eight 5-GHz 500-kW CW klystrons. The prototype klystron is made by Toshiba Electron Tubes and Devices and is being used in KSTAR LHCD system. The length of transmission line will be longer than 50 m thus highly oversized circular waveguide transmitting TE01 ◦ mode will be used for extremely low loss. Since the klystron output is TE10 ⵦ mode in WR187, TE10 ⵦ -to-TE01 ◦ mode converter is necessary. Two types of TE10 ⵦ -to-TE01 ◦ mode converter mock-ups were developed and tested. One was an in-line coupling type using serpentine mode converter and the other was a sidewall coupling type [2]. N|| of the launcher is preferred to be 2.5 from the preliminary study for the efficient current drive by single pass absorption in a KSTAR plasma [3]. For the excitation of LH wave at N|| = 2.5, preliminary design of the 4-MW CW mid-plane Passive Active Mul-
∗ Corresponding author. E-mail address: [email protected] (J. Kim).
tijunction (PAM) launcher has been performed as presented in Section 2 [4,5]. However off mid-plane injection near the upper diverter and high field side launching are also under consideration [6]. The launcher consists of 32 columns × 8 rows of active waveguides. One PAM module is composed of 4 active waveguides in toroidal direction with a fixed 90◦ phasing and passive waveguides are inserted between them. RF design of the multijunction structure has been optimized using HFSS [7]. N|| spectrum, directivity, and the reflection were evaluated using ALOHA [8]. There is no plan to increase power until 2021. Prototype midplane PAM launcher with 32 active waveguides for 0.5-MW pulsed operation is under development. RF design and the resulting N|| of PAM are the same as that of 4 MW PAM launcher. Detailed RF and mechanical design of PAM launcher with preliminary thermal analysis have been performed and summarized in Section 3.
2. 4-MW Passive active multijunction (PAM) PAM launcher is very convenient for steady state operation due to its properties such as extremely low reflection, insensitive to the density profile in front of the launcher, space for active cooling [4,5]. Dimension of the active and passive waveguide was determined, considering the N|| , power density, E field strength, etc. The main peak N||0 of the radiated N|| spectrum is evaluated from the spatial
http://dx.doi.org/10.1016/j.fusengdes.2017.05.021 0920-3796/© 2017 Published by Elsevier B.V.
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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Fig. 1. The sketch of 4 MW KSTAR LHCD system.
period, P of the active waveguides, the phase shift, between the adjacent active waveguide, and the frequency, f
N||0 =
c/f p
·
(1)
2
P of PAM is 18 mm with = −270◦ for N||0 = 2.5. P = bA + bP + 2s, where active waveguide width, bA , passive waveguide width, bP , and the septum width, s as illustrated in Fig. 2(a). s is 2 mm for sufficient mechanical stiffness of the launcher and bA = bP = 7 mm. The height of the waveguide a is 58 mm, thus only the fundamental TE10 mode propagates at 5 GHz. Assuming 15% of overall loss in the transmission line and antenna, 425 kW of RF power from each 500-kW klystron is delivered and supplied to the plasma. From the empirical frequency scaling, the power density at the launcher mouth should be less than 33 MW/m2 at 5 GHz [9]. Hence at least 32 active waveguides of 58 mm × 7 mm are necessary to launch 425 kW CW power. N|| spectrum can be adjusted by varying the phase shift M between multijunction modules as in Fig. 2(b) [11,12]. N||0 = N||0 (1 −
1
NWG
M
(2)
where NWG is the number of active waveguides in a single multijunction. Each antenna module for a klystron is composed of 4 columns x 8 rows of active waveguides to increase the flexibility of N|| as shown in Fig. 3. The RF power is poloidally divided into eight by 3-dB hybridsplitter followed by double-staged 3 dB splitter and then four in toroidal direction by multijunction. 4-MW system comprises of 8 stacks of antenna module in toroidal direction. SF6 pressurized WR187 3-dB hybrid splitter is followed by two 250-kW vacuum RF windows which are the same as those used in klystron. Fourth port of the 3-dB hybrid splitter is connected to compact waterload for the dissipation of the reflected power from the plasmas. In the vacuum, the waveguides are enlarged to WR229 by tapers. The RF power guided by the 2-m WR229 waveguide is coupled to a double-
staged 3 dB. Each output waveguide of double-staged 3 dB splitter is connected to a mutijunction. 2.1. RF design of multijunction and components In this section, detailed RF designs of waveguide components used in PAM launcher will be described such as the multijunctions, 3-dB hybrid splitter, double-staged 3 dB splitter. Multijunction has 90◦ phase shifting by the combination of phase shifters as shown in Fig. 2(b). 180◦ fixed phase shifter (FPS), 90◦ FPS, and two kinds of bijunctions have been designed and integrated together. Fig. 4(a) is the RF model of the multijunction. The detailed dimensions of the FPS are summarized in Table 1. Length of two bijunctions, lbj1 and lbj2 are 68.9 mm and 34.9 mm, respectively. Bijunction corresponding to lbj1 has input port with 29 mm width and output port width 25 mm. Since 500 kW will be supplied to the plasma by eight multijunctions, each multijunction should transmit 63 kW. Fig. 4(b) illustrates the electric field distribution in a multijunction with 63 kW. The maximum electric field is 3.1 kV/cm which is lower than 4.3 kV/cm suggested by Goniche [10]. Scattering parameters representing the RF characteristics of the multijunction have been computed by HFSS code and summarized in Table 2. Port 2–5 corresponds to the active waveguides of the multijunction. The S11 is −34 dB. The splitting ratios are −6.02 ± 0.02 dB. Phase shifts are 90 ± 1 deg. The scattering parameter of the multijunction at 5 GHz calculated by HFSS was used as an input parameter of ALOHA for the evaluation of the antenna characteristics such as N|| spectrum, directivity, reflection. Fig. 5(a) is the normalized radiated
Table 1 Detailed dimension of the 90◦ and 180◦ FPS in multijunction.
a1 a2 l1 l2
90◦ FPS
180◦ FPS
48.0 mm 53.0 mm 181.0 mm 16.0 mm
40 mm 48 mm 133.5 mm 19.5 mm
Table 2 The scattering parameter of the multijunction at 5 GHz and the phase shift between adjacent waveguides. Magnitude [dB]
Fig. 2. (a) The dimension of waveguides at the launcher mouth and (b) the phase shift of multijunction.
Phase shift [deg]
S11
−34.14
S21 S31 S41 S51
−6.01 −6.02 −6.03 −6.04
– S31 –S21 S41 –S31 S51 –S41 S21 –S51
89.3 90.5 89.2 91.0
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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Fig. 3. One module of PAM antenna for 0.5 MW.
Fig. 4. RF model (a) and Electric field distribution (b) of multijunction.
spectrum (the real part of the spectrum) by eight multijunctions in two different electron densities, ne , at the launcher mouth. Tilted angle of magnetic field was not taken into account, which means that nz = N|| . The peak value of N|| is 2.5 in accord with Eq. (1). PAM has better directivity near the 5-GHz cutoff density, 3 × 1017 m−3 . Fig. 5(b) shows the directivity and the peak N|| as a function of phase shift between multijunctions, M in accord with Eq. (2) as well. The definition of the antenna directivity used here is the ratio of the power in negative N|| (where the main N|| peak belongs) to the total radiated power. Fig. 5(c) predicts the reflection in the range of plasma density, ne = 1 × 1017 m−3 –1 × 1019 m−3 in front of the launcher mouth. Reflection is minimum around the cutoff density of 5 GHz. Two-density profile was assumed. The first and second plasma layers from the launcher have decay lengths 1 = 2 mm and 2 = 20 mm, respectively, where = ne /ne . The thickness of the first layer was 2 mm. The design of double-staged 3 dB splitter is based on the Riblet short slot 3-dB hybrid splitter [13]. The detailed dimension is marked in Fig. 6(a). The return loss, S11 , the isolation, S41 are higher than 35 dB and the output are well balanced as seen in Fig. 6(b). One port of the 4-port hybrid splitter was terminated in a short circuit whose position was optimized for the best performance. The resulting 3-port splitter in Fig. 6(c) will be used in 0.5-MW prototype PAM antenna. Double-staged 3 dB splitter is illustrated in Fig. 6(d). Its splitting ratio is −6.02 dB and the reflection is 38 dB. As in Fig. 3, each double-staged 3 dB splitter should transmit 250 kW. Threshold electric field increases as the waveguide height is increased [10].
Fig. 5. Antenna characteristics calculated using ALOHA code. (a) Normalized radiated spectrum N|| by eight multijunction modules, (b) the directivity and the peak N|| as a function of phase shift between multijunctions. ne0 = 3 × 1017 m−3 . (c) Reflection as a function of density.
Multipactoring is suppressed when the height exceeds 26 mm. Thus 6.6 kV/cm in a waveguide with 29 mm height is expected to be less than the threshold value. Only conceptual and RF design of PAM antenna has been performed. In order to design the cooling system and rigid supporting structure, it is necessary to perform the thermo-mechanical analysis which concerns the antenna heat loading by plasma radiation and RF loss and resulting thermal stress; eddy current by poloidal magnetic field swing and plasma disruption, and consequent torque induced on the antenna in conjunction with static magnetic field, etc. The length of the waveguides will be changed
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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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-s RF pulse with 100 kW/m2 plasma radiation was estimated to be ∼355 ◦ C for stainless steel and 102 ◦ C for copper at the mouth of the launcher, facing the plasma. Therefore, it is possible to operate the prototype PAM launcher without cooling for 500 kW 60-s pulse operation. However, more elaborated calculation is necessary. 4. Summary
Fig. 6. (a) Electric field distribution of 3-dB hybrid splitter with detailed dimension (port size = 58 mm x 29 mm) (b) spectrum of S11–S41 (c) 3-dB poloidal splitter and (d) double-staged 3 dB splitter.
Conceptual and RF designs for 4-MW mid-plane PAM launcher have been performed for the upgrade after 2021. The dimensions and number of active and passive waveguides for 4-MW mid-plane PAM launcher were determined considering the parallel refractive index, maximum power density at the launcher mouth. The launcher is composed of 32 columns × 8 rows of active waveguides whose dimension is 58 mm × 7 mm with 18 mm of spatial period for N||0 = 2.5. RF designs of the multijunction and other waveguide components have been performed for 90◦ phasing and maximum electric field in the fixed phase shifters less than 4.3 kV/cm using HFSS code and then the antenna properties were evaluated using ALOHA. 0.5 MW prototype PAM with 32 active waveguides is under development. Conceptual design, RF design of components, detailed consideration on phase error by curved surface of the launcher, and preliminary thermal analysis have been performed. Waveguide dimension and RF design of multijunction is the same as those of 4-MW system. N|| is peaked at a fixed value 2.5. According to the preliminary thermal analysis, it is possible to operate the launcher without cooling for 500 kW 60 s pulse operation. Acknowledgments This work is supported by the Ministry of Science, ICT and Future Planning of Korea. References
Fig. 7. 0.5 MW prototype PAM launcher.
when the launcher mouth will be machined along the plasma curvature. The change of the length will results in the phase error of the RF wave launched from antenna and consequently affect the N|| spectrum and reflection. Thus the phase error should be compensated by adjusting the fixed phase shifter. 3. 0.5 MW prototype PAM 0.5-MW prototype PAM launcher is under development. Multijunction RF design and the consequent N||0 is the same as those of 4-MW system. Prototype PAM is composed of 4 rows x 8 columns of active waveguides as shown in Fig. 7. Instead of double-staged 3 dB splitter, one toroidal splitter and two 3-dB poloidal splitter in Fig. 6(c) were adopted. The launcher mouth will be machined into a curved surface with radius of 900 mm in poloidal plane. The toroidal curvature will not be applied to the curved surface since the depth of the toroidal arc is only 1.3 mm along the 155 mm width. Thermal analysis based on a simplified model was performed for 60-s pulse. Temperature change of prototype PAM launcher by the RF loss and plasma radiation without cooling has been calculated. The multijunction was divided into three parts in the model. The
[1] S. Park, et al., Progress of KSTAR 5-GHz lower hybrid current drive system, Fusion Sci Technol. 63 (1) (2016) 49–58. [2] T. Seong, et al., Mode converters for low-loss transmission-line of KSTAR LHCD system, 29th symposium on fusion technology, prague, Czech Republic 5–9 (2016). [3] J. Kim, et al., Preliminary result of KSTAR LHCD experiment and consideration of N|| upgrade, 21 st Topical Conference on Radiofrequency Power in Plasmas (2015). [4] P. Bibet, X. Litaudon, D. Moreau, Conceptual study of a reflector waveguide array for launching lower hybrid waves in reactor grade plasmas, Nucl. Fusion 35 (10) (1995) 1213. [5] J. Hillairet, et al., Lower hybrid antennas for nuclear fusion experiments, in: 6th European Conference on Antennas and Propagation, Prague, Czech Republic, March 2012, 2017 https://arxiv.org/ftp/arxiv/papers/1503/1503. 05056.pdf. [6] Y. Bae, et al., Simulation study of proposed off-midplane lower hybrid current drive in KSTAR, Plasma Phys. Control Fusion 58 (2016) 075003. [7] ANSYS HFSS: High Frequency Electromagnetic Field Simulation, 2017 http:// www.ansys.com/Products/Electronics/ANSYS-HFSS. [8] J. Hillairet, et al., ALOHA: an advanced lower hybrid antenna coupling code, Nucl. Fusion 50 (2010) 125010. [9] LH4ITER-EFDA Task WP09-HCD-08-03-01 Final Report, 2017. [10] M. Goniche, et al., Modelling of power limit in RF antenna waveguides operated in the lower hybrid range of frequency, Nucl. Fusion 54 (2014) 013003. [11] D. Moreau, et al., Optimization of slow wave multijunction antennae for current drive applications of radiofrequency power to plasmas proc. 7th top. conf. (Kissimmee, FL, 1987), AIP Conf. Proc. 159 (1987) 137. [12] X. Litaudon, D. Moreau, Coupling of slow waves near the lower hybrid frequency in JET, Nucl. Fusion 30 (3) (1990) 471. [13] H.J. Riblet, The short-slot hybrid junction, Proc. I.R.E. 40 (1952) 180.
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
ARTICLE IN PRESS
FUSION-9505; No. of Pages 4
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system Jeehyun Kim a,∗ , Sonjong Wang a , Julien Hillairet b , Hyunho Wi a , Sangwon Seon a , Jongwon Han a , Lena Delpech b a b
NFRI, Daejeon, South Korea CEA, IRFM, 13108,St. Paul-lez-Durance, France
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 28 April 2017 Accepted 4 May 2017 Available online xxx Keywords: LHCD RF heating PAM Antenna KSTAR
a b s t r a c t The major upgrade of KSTAR LHCD system to 4 MW is planned in 2021. 4-MW RF power will be provided by 8 × 0.5 MW klystrons. Highly oversized circular waveguide transmitting TE01 ◦ mode will be adopted for the low loss transmission line longer than 50 m. The design of the 4-MW mid-plane Passive Active Multijunction launcher has been performed. Considering parallel refractive index N|| for the efficient current drive and the maximum power density at the launcher mouth, the launcher is composed of 256 active waveguides whose dimension is 58 mm x 7 mm and 18 mm of spatial period for N||0 = 2.5. Multijunction with Emax < 3.5 kV/cm and high power waveguide components have been designed using HFSS code. The antenna properties were evaluated using ALOHA. Prototype PAM launcher with 32 active waveguides is under development for 0.5-MW pulsed operation. © 2017 Published by Elsevier B.V.
1. Introduction KSTAR LHCD system for the off-axis current drive has adopted the grill type antenna with the parallel refractive index, N|| , 2.0 for 0.5 MW at 5 GHz [1]. Edge electron temperature rise and change of sawtooth period were observed by the 150 kW of delivered power for 1.5 s but significant current drive effect was not observed. The major upgrade is planned in 2021 as illustrated in Fig. 1. 4-MW CW RF power will be provided by eight 5-GHz 500-kW CW klystrons. The prototype klystron is made by Toshiba Electron Tubes and Devices and is being used in KSTAR LHCD system. The length of transmission line will be longer than 50 m thus highly oversized circular waveguide transmitting TE01 ◦ mode will be used for extremely low loss. Since the klystron output is TE10 ⵦ mode in WR187, TE10 ⵦ -to-TE01 ◦ mode converter is necessary. Two types of TE10 ⵦ -to-TE01 ◦ mode converter mock-ups were developed and tested. One was an in-line coupling type using serpentine mode converter and the other was a sidewall coupling type [2]. N|| of the launcher is preferred to be 2.5 from the preliminary study for the efficient current drive by single pass absorption in a KSTAR plasma [3]. For the excitation of LH wave at N|| = 2.5, preliminary design of the 4-MW CW mid-plane Passive Active Mul-
∗ Corresponding author. E-mail address: [email protected] (J. Kim).
tijunction (PAM) launcher has been performed as presented in Section 2 [4,5]. However off mid-plane injection near the upper diverter and high field side launching are also under consideration [6]. The launcher consists of 32 columns × 8 rows of active waveguides. One PAM module is composed of 4 active waveguides in toroidal direction with a fixed 90◦ phasing and passive waveguides are inserted between them. RF design of the multijunction structure has been optimized using HFSS [7]. N|| spectrum, directivity, and the reflection were evaluated using ALOHA [8]. There is no plan to increase power until 2021. Prototype midplane PAM launcher with 32 active waveguides for 0.5-MW pulsed operation is under development. RF design and the resulting N|| of PAM are the same as that of 4 MW PAM launcher. Detailed RF and mechanical design of PAM launcher with preliminary thermal analysis have been performed and summarized in Section 3.
2. 4-MW Passive active multijunction (PAM) PAM launcher is very convenient for steady state operation due to its properties such as extremely low reflection, insensitive to the density profile in front of the launcher, space for active cooling [4,5]. Dimension of the active and passive waveguide was determined, considering the N|| , power density, E field strength, etc. The main peak N||0 of the radiated N|| spectrum is evaluated from the spatial
http://dx.doi.org/10.1016/j.fusengdes.2017.05.021 0920-3796/© 2017 Published by Elsevier B.V.
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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Fig. 1. The sketch of 4 MW KSTAR LHCD system.
period, P of the active waveguides, the phase shift, between the adjacent active waveguide, and the frequency, f
N||0 =
c/f p
·
(1)
2
P of PAM is 18 mm with = −270◦ for N||0 = 2.5. P = bA + bP + 2s, where active waveguide width, bA , passive waveguide width, bP , and the septum width, s as illustrated in Fig. 2(a). s is 2 mm for sufficient mechanical stiffness of the launcher and bA = bP = 7 mm. The height of the waveguide a is 58 mm, thus only the fundamental TE10 mode propagates at 5 GHz. Assuming 15% of overall loss in the transmission line and antenna, 425 kW of RF power from each 500-kW klystron is delivered and supplied to the plasma. From the empirical frequency scaling, the power density at the launcher mouth should be less than 33 MW/m2 at 5 GHz [9]. Hence at least 32 active waveguides of 58 mm × 7 mm are necessary to launch 425 kW CW power. N|| spectrum can be adjusted by varying the phase shift M between multijunction modules as in Fig. 2(b) [11,12]. N||0 = N||0 (1 −
1
NWG
M
(2)
where NWG is the number of active waveguides in a single multijunction. Each antenna module for a klystron is composed of 4 columns x 8 rows of active waveguides to increase the flexibility of N|| as shown in Fig. 3. The RF power is poloidally divided into eight by 3-dB hybridsplitter followed by double-staged 3 dB splitter and then four in toroidal direction by multijunction. 4-MW system comprises of 8 stacks of antenna module in toroidal direction. SF6 pressurized WR187 3-dB hybrid splitter is followed by two 250-kW vacuum RF windows which are the same as those used in klystron. Fourth port of the 3-dB hybrid splitter is connected to compact waterload for the dissipation of the reflected power from the plasmas. In the vacuum, the waveguides are enlarged to WR229 by tapers. The RF power guided by the 2-m WR229 waveguide is coupled to a double-
staged 3 dB. Each output waveguide of double-staged 3 dB splitter is connected to a mutijunction. 2.1. RF design of multijunction and components In this section, detailed RF designs of waveguide components used in PAM launcher will be described such as the multijunctions, 3-dB hybrid splitter, double-staged 3 dB splitter. Multijunction has 90◦ phase shifting by the combination of phase shifters as shown in Fig. 2(b). 180◦ fixed phase shifter (FPS), 90◦ FPS, and two kinds of bijunctions have been designed and integrated together. Fig. 4(a) is the RF model of the multijunction. The detailed dimensions of the FPS are summarized in Table 1. Length of two bijunctions, lbj1 and lbj2 are 68.9 mm and 34.9 mm, respectively. Bijunction corresponding to lbj1 has input port with 29 mm width and output port width 25 mm. Since 500 kW will be supplied to the plasma by eight multijunctions, each multijunction should transmit 63 kW. Fig. 4(b) illustrates the electric field distribution in a multijunction with 63 kW. The maximum electric field is 3.1 kV/cm which is lower than 4.3 kV/cm suggested by Goniche [10]. Scattering parameters representing the RF characteristics of the multijunction have been computed by HFSS code and summarized in Table 2. Port 2–5 corresponds to the active waveguides of the multijunction. The S11 is −34 dB. The splitting ratios are −6.02 ± 0.02 dB. Phase shifts are 90 ± 1 deg. The scattering parameter of the multijunction at 5 GHz calculated by HFSS was used as an input parameter of ALOHA for the evaluation of the antenna characteristics such as N|| spectrum, directivity, reflection. Fig. 5(a) is the normalized radiated
Table 1 Detailed dimension of the 90◦ and 180◦ FPS in multijunction.
a1 a2 l1 l2
90◦ FPS
180◦ FPS
48.0 mm 53.0 mm 181.0 mm 16.0 mm
40 mm 48 mm 133.5 mm 19.5 mm
Table 2 The scattering parameter of the multijunction at 5 GHz and the phase shift between adjacent waveguides. Magnitude [dB]
Fig. 2. (a) The dimension of waveguides at the launcher mouth and (b) the phase shift of multijunction.
Phase shift [deg]
S11
−34.14
S21 S31 S41 S51
−6.01 −6.02 −6.03 −6.04
– S31 –S21 S41 –S31 S51 –S41 S21 –S51
89.3 90.5 89.2 91.0
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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Fig. 3. One module of PAM antenna for 0.5 MW.
Fig. 4. RF model (a) and Electric field distribution (b) of multijunction.
spectrum (the real part of the spectrum) by eight multijunctions in two different electron densities, ne , at the launcher mouth. Tilted angle of magnetic field was not taken into account, which means that nz = N|| . The peak value of N|| is 2.5 in accord with Eq. (1). PAM has better directivity near the 5-GHz cutoff density, 3 × 1017 m−3 . Fig. 5(b) shows the directivity and the peak N|| as a function of phase shift between multijunctions, M in accord with Eq. (2) as well. The definition of the antenna directivity used here is the ratio of the power in negative N|| (where the main N|| peak belongs) to the total radiated power. Fig. 5(c) predicts the reflection in the range of plasma density, ne = 1 × 1017 m−3 –1 × 1019 m−3 in front of the launcher mouth. Reflection is minimum around the cutoff density of 5 GHz. Two-density profile was assumed. The first and second plasma layers from the launcher have decay lengths 1 = 2 mm and 2 = 20 mm, respectively, where = ne /ne . The thickness of the first layer was 2 mm. The design of double-staged 3 dB splitter is based on the Riblet short slot 3-dB hybrid splitter [13]. The detailed dimension is marked in Fig. 6(a). The return loss, S11 , the isolation, S41 are higher than 35 dB and the output are well balanced as seen in Fig. 6(b). One port of the 4-port hybrid splitter was terminated in a short circuit whose position was optimized for the best performance. The resulting 3-port splitter in Fig. 6(c) will be used in 0.5-MW prototype PAM antenna. Double-staged 3 dB splitter is illustrated in Fig. 6(d). Its splitting ratio is −6.02 dB and the reflection is 38 dB. As in Fig. 3, each double-staged 3 dB splitter should transmit 250 kW. Threshold electric field increases as the waveguide height is increased [10].
Fig. 5. Antenna characteristics calculated using ALOHA code. (a) Normalized radiated spectrum N|| by eight multijunction modules, (b) the directivity and the peak N|| as a function of phase shift between multijunctions. ne0 = 3 × 1017 m−3 . (c) Reflection as a function of density.
Multipactoring is suppressed when the height exceeds 26 mm. Thus 6.6 kV/cm in a waveguide with 29 mm height is expected to be less than the threshold value. Only conceptual and RF design of PAM antenna has been performed. In order to design the cooling system and rigid supporting structure, it is necessary to perform the thermo-mechanical analysis which concerns the antenna heat loading by plasma radiation and RF loss and resulting thermal stress; eddy current by poloidal magnetic field swing and plasma disruption, and consequent torque induced on the antenna in conjunction with static magnetic field, etc. The length of the waveguides will be changed
Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021
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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-s RF pulse with 100 kW/m2 plasma radiation was estimated to be ∼355 ◦ C for stainless steel and 102 ◦ C for copper at the mouth of the launcher, facing the plasma. Therefore, it is possible to operate the prototype PAM launcher without cooling for 500 kW 60-s pulse operation. However, more elaborated calculation is necessary. 4. Summary
Fig. 6. (a) Electric field distribution of 3-dB hybrid splitter with detailed dimension (port size = 58 mm x 29 mm) (b) spectrum of S11–S41 (c) 3-dB poloidal splitter and (d) double-staged 3 dB splitter.
Conceptual and RF designs for 4-MW mid-plane PAM launcher have been performed for the upgrade after 2021. The dimensions and number of active and passive waveguides for 4-MW mid-plane PAM launcher were determined considering the parallel refractive index, maximum power density at the launcher mouth. The launcher is composed of 32 columns × 8 rows of active waveguides whose dimension is 58 mm × 7 mm with 18 mm of spatial period for N||0 = 2.5. RF designs of the multijunction and other waveguide components have been performed for 90◦ phasing and maximum electric field in the fixed phase shifters less than 4.3 kV/cm using HFSS code and then the antenna properties were evaluated using ALOHA. 0.5 MW prototype PAM with 32 active waveguides is under development. Conceptual design, RF design of components, detailed consideration on phase error by curved surface of the launcher, and preliminary thermal analysis have been performed. Waveguide dimension and RF design of multijunction is the same as those of 4-MW system. N|| is peaked at a fixed value 2.5. According to the preliminary thermal analysis, it is possible to operate the launcher without cooling for 500 kW 60 s pulse operation. Acknowledgments This work is supported by the Ministry of Science, ICT and Future Planning of Korea. References
Fig. 7. 0.5 MW prototype PAM launcher.
when the launcher mouth will be machined along the plasma curvature. The change of the length will results in the phase error of the RF wave launched from antenna and consequently affect the N|| spectrum and reflection. Thus the phase error should be compensated by adjusting the fixed phase shifter. 3. 0.5 MW prototype PAM 0.5-MW prototype PAM launcher is under development. Multijunction RF design and the consequent N||0 is the same as those of 4-MW system. Prototype PAM is composed of 4 rows x 8 columns of active waveguides as shown in Fig. 7. Instead of double-staged 3 dB splitter, one toroidal splitter and two 3-dB poloidal splitter in Fig. 6(c) were adopted. The launcher mouth will be machined into a curved surface with radius of 900 mm in poloidal plane. The toroidal curvature will not be applied to the curved surface since the depth of the toroidal arc is only 1.3 mm along the 155 mm width. Thermal analysis based on a simplified model was performed for 60-s pulse. Temperature change of prototype PAM launcher by the RF loss and plasma radiation without cooling has been calculated. The multijunction was divided into three parts in the model. The
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Please cite this article in press as: J. Kim, et al., Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.021