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Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus
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Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus Sun-Ho Kim a,∗ , Seung-Ho Jeong a , Hyunwoo Lee b , Byungje Lee b , Jong-Gab Jo c , Hyun-Young Lee c , Yong-Seok Hwang c a
Korea Atomic Energy Research Institute, Daejeon, Republic of Korea KwangWoon University, Seoul, Republic of Korea c Seoul National University, Seoul, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx Keywords: Lower hybrid wave Fast wave Current drive Coupling Inter-digital antenna
a b s t r a c t An efficient heating and current drive scheme in central or off-axis region is required to realize steady state operation of tokamak fusion reactor. And the fast wave in lower hybrid resonance range of frequency could be a candidate for such an efficient scheme in high density and high temperature plasmas. Its propagation and absorption characteristics including current drive and coupling efficiency are analyzed for Versatile Experiment Spherical Torus and it is shown that it is possible to drive current with considerable current drive efficiency in central region. The RF system for the fast wave experiment including klystron, transmission systems, inter-digital antenna, and RF diagnostics are given as well in this paper. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The steady state operation of fusion reactor is based on the advanced scenario by using bootstrap current through active density profile control [1]. However, an external current drive scheme is still needed to control the profile and to meet the insufficient current in total current. Most efficient current drive is to use slow wave in lower hybrid resonance frequency range, i.e., Lower Hybrid Current Drive(LHCD) [2]. However, it has been observed to drive current off-axis or outer region in high density plasmas [3,4] though it is recently reported that it could drive current in the reactor grade high density plasmas at higher magnetic field and lower N|| on Tore Supra, Alcator C-Mod and FTU [5]. Meanwhile, there is another possibility that the fast wave in the same lower hybrid resonance range could be used for the reactor grade plasmas because it can propagate into high density and high temperature plasmas with considerable current drive efficiency [6]. In this context, the fast wave in frequency of two times greater than lower hybrid resonance frequency was suggested and it is planned to prove experimentally the current drive scheme on Versatile Experiment Spherical Torus (VEST) [7].
The propagation and absorption of the fast wave on VEST are analyzed in section 2.1. The coupling characteristic is investigated by the 1D full wave code in section 2.2 because higher density is required for the fast wave to propagate than slow wave or fast waves in other frequency range. And the propagation and absorption on VEST is calculated by using GENRAY ray tracing code in section 2.3. Finally, the preliminary design and status of the RF system for the fast wave experiment on VEST are given in section 3. The overall RF system design including klystron, power transmission lines, fast wave antenna, diagnostics are described in this section. 2. Propagation, absorption, and coupling of LHFW on VEST 2.1. Dispersion relation The propagation and absorption characteristics of LHFW can be understood through the real and imaginary part of the dispersion relation. It is represented as Eqs. (1) and (2) [3].
N⊥r ∗ Corresponding author. E-mail address: [email protected] (S.-H. Kim).
⎧ 1/2 ⎪ P(N||2 − S) ⎪ ⎪ SW ⎪ ⎨ − S ∼ = 1/2 ⎪ ⎪ (N||2 − R)(N||2 − L) ⎪ ⎪ FW ⎩ −
(1)
(N||2 − S)
http://dx.doi.org/10.1016/j.fusengdes.2016.02.014 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 1. Real and imaginary refractive index of LHFW and LHSW on VEST.
Fig. 2. Density profile model for coupling calculation.
where S, P, R, L are Stix parameters, N is a parallel refractive index. SW and FW represent Slow Wave and Fast Wave, respectively.
⎧ N⊥r,SW 1/2 3 exp −2 LHSW ⎪ ⎪ ⎨ N⊥i ∼ = 1/2 3 exp −2 ⎪ ⎪ LHFW ⎩ N⊥r,FW
temperature increases the imaginary part of LHFW increases dramatically in central high density region as given in Eq. (2). 2.2. Coupling of LHFW
(2)
2 N 2 /ω2 −1 + ωce pe ||
where is the ratio of wave phase velocity to electron thermal velocity. LHSW and LHFW represent Slow Wave and Fast Wave in Lower Hybrid resonance frequency range. The real perpendicular refractive index of SW is usually very large and attains to several tens and hundreds depending on the plasma parameters and magnetic field. Therefore, LHSW could be absorbed very efficiently once if the Landau condition is roughly satisfied as given in the imaginary wave number of LHSW in Eq. (2). Meanwhile, the real wave number of FW is usually one or two orders lower than that of SW so that the damping is weak unless the Landau damping is satisfied precisely. However, if the density is large enough to make the denominator approach zero, it can be absorbed efficiently as LHSW. The real and imaginary part of the perpendicular refractive indices of FW and SW for the VEST plasma and device parameters are shown in Fig. 1. The central and edge density are 3 × 1018 #/m3 and 4 × 1017 #/m3 , respectively. And the assumed N|| is 4.0. As the
The launching density at which LHFW starts to propagate can be obtained from the dispersion relation (1) by equating R to zero. And it can be represented as Eq. (3).
me ε0 2 nlaunch ∼ N|| − 1 ωωce = e2
(3)
where ω is a operating frequency, ωce is a electron cyclotron resonance frequency. The launching density of LHFW is usually several hundred times greater than that of LHSW and it means that the coupling to LHFW can be more difficult than to LHSW. Therefore, the coupling efficiency should be investigated in detail to apply the LHFW current drive scheme to reality. In this study, the coupling efficiency is calculated with 1D full wave simulation code developed for the study of XB mode conversion efficiency [8]. The density profile to calculate the coupling is modeled as Fig. 2. The magnetic field is 0.2 T at the plasma center and the operating frequency and parallel refractive index are 500 MHz and 4.0, respectively. The parameters including density profile parameters are summarized in Table 1.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 4. Coupling efficiency and ratio for Ez excitation. Fig. 3. Coupling efficiency and ratio for Ey excitation.
The coupling efficiency and coupled power ratio for Ey antenna excitation to launch LHFW and for Ez excitation to launch LHSW are shown in Figs. 3 and 4, respectively, where the bulk density is varied from 6 × 1015 #/m3 to 5 × 1017 #/m3 to estimate the density and density gradient effect. The coupling efficiency by Ey excitation is much lower than that by Ez excitation if the bulk density is less than about 3 × 1017 #/m3 which is the LHFW launching density. However, it increases almost 10 times if LHFW start to propagate. It means that LHFW can be efficiently coupled to plasmas in high density and high density gradient plasmas above LHFW launching density. Interesting thing is that the considerable power is coupled to LHSW in total injected power. It attains to almost 40% as the bulk density reaches 5 × 1017 #/m3 . It may be explained in three points of view as follows. First, the electric field of LHSW has considerable Ey component though it is less than Ez component. Second, the launching density of LHSW is much lower than that of LHFW. Third, if the density gradient increases, the two wave branch cannot be separated exactly and a portion of LHFW power can be transferred to LHSW branch. Inversely, the coupled power ratio to LHFW appears also for Ez excitation case as the bulk density increases though it is just about 10% as shown in Fig. 4.
plotted together at the same plasma and antenna parameters for a reference. The rays and driven currents for the LHFW are shown in Fig. 5. The LHFW propagates into central region and the drive current is comparable to that of LHSW as predicted in Section 2.1. 3. LHFW RF system 3.1. Overall RF system including diagnostic
The propagations and driven currents of LHFW on VEST are calculated with GENRAY code. The parameters for the calculation are summarized in Table 2. The core density is 3 × 1018 #/m3 . And the edge density is set to be 4 × 1017 #/m3 for LHFW launching. The parallel refractive is 4.0 which satisfies the accessibility condition for the given magnetic field and RF frequency. LHSW case is also
A LHFW RF system is designed based on an old 10 kW UHF broadcasting system at Seoul National University (SNU) and the antenna is being developed in inter-digital type through collaboration with Kwang-Woon University (KWU). The system schematic of including diagnostic is shown in Fig. 6. The input signal is amplified in two steps through 39 dB solidstate amplifier and 48 dB klystron. The RF power of about 10 kW is transmitted to inter-digital antenna through 3-1/8 coaxial transmission line, 10 kV rating DC breaker, matching internal transformer with VSWR < 1.05 in wide band and high vacuum compatible feed-thru. The parameters of main RF system components are summarized in Table 3. The RF diagnostics are consisted of directional coupler for the forward and reflected power measurements in front of amplifiers and voltage probes for arcing detection and current measurement at the transmission line and the inter-digital antenna. The power and voltage signals are digitized and monitored through peakphase detector and ADC. The voltage signal measured at antenna is transmitted to ADC via optical fiber because the reference potentials of main VEST chamber and RF power system are different each other due to DC breaker which protects the RF system from the induced voltage of VEST main vacuum chamber.
Table 1 Parameters for coupling calculation on VEST.
Table 2 Parameters for ray tracing calculation on VEST.
2.3. Ray tracing simulation
Parameters
Values
Parameters
Values
B0 Frequency N N0 Nb d(X2 − X1 ) X1
0.2 T 500 MHz 4.0 6 × 1015 –5 × 1017 #/m3 4 × 1015 #/m3 3 cm 2 cm
B0 Frequency N Core density Edge density Core temperature Edge temperature
0.2 T 500 MHz 4.0 3 × 1018 #/m3 4 × 1017 #/m3 3 keV 0.2 keV
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 5. Ray tracing: (a) ray, (b) driven current profile.
Fig. 6. Schematic of LHFW RF system.
3.2. Klystron and antenna A klystron is being prepared by refurbishing an old UHF broadcasting system. The maximum power is 30 kW, and the frequency
ranges from 470 to 700 MHz. The degree of vacuum of the klystron was confirmed to be satisfactory through the vacuum measurement. The specification of the klystron is summarized in Table 4. Since the beam power for the klystron operation is absent, it
Fig. 7. Curved antenna for LHFW RF system on VEST.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 8. Parallel refractive index spectrum (a) and S-parameters (b) of curved inter-digital antenna designed.
Table 3 Specification of LHFW RF system components. Components
Values
Signal generator Solid state amplifier Circulator(SSA) Klystron Circulator(Klystron) DC breaker Internal transformer Vacuum feed-thru Power line Inter-digital antenna
500 kHz–1.3 GHz (13 dBm) 35 dB (max 29 dBm) 14 dB isolation, 75 W 10 kW UHF Harris system for broadcasting 20 dB isolation, 15 kW Rating 10 kV, VSWR <1.05 VSWR < 1.05,for matching High vacuum< 10−7 mbar 3-1/8” EIA coaxial N : 3 –5, VSWR <3
Table 4 Specification of klystron. Parameters
Values
Frequency Output power Gain Beam voltage Beam current Electrode voltage Heater voltage Heater current Body current Magnet voltage Magnet current Collector cooling Body cooling Magnet cooling Gun cooling
470–700 MHz 37.5 kW 48 dB 19.5 kV 5.4 A 19.5 kV 7V 17 A 50 mA 145 V 32 A Water 2.0 gal/min Water 1.5 gal/min Water 2.0 gal/min Forced air 50 ft3 /min
was lent from Korea Accelerator and Plasmas Research Association (KAPRA) and refurbishing is currently completed. The repair of magnet and filament power is progressing and the klystron operation will be started after the completion of the repair in near future. The preliminary curved design of inter-digital antenna for VEST LHFW RF system is shown in Fig. 7. The power fed to input port is transmitted sequentially to neighbor current straps with /2 phase difference and the current straps radiate RF field to plasmas with asymmetric parallel refractive index spectrum. The uncoupled power is dissipated in dummy load through output port. The radiated antenna parallel spectrum and S-parameters were calculated with HFSS commercial code for a vacuum load. In 480–496 MHz frequency range, the parallel refractive index is between 3.5 and 4.5 and the S-parameters S11 and S12 are less than
−10 dB as shown in Fig. 8. They satisfies the requirement for LHFW experiment on VEST and more specific design and optimization will be carried out considering the plasma density profile in front of antenna in next step. 4. Summary The feasibility study of heating and current drive by using Lower Hybrid Fast Wave(LHFW) is progressing aiming to prove the concept on VEST and to apply it to tokamak fusion reactor. The propagation, absorption, current drive, and coupling characteristics are analyzed and simulated for VEST and it was shown that it is possible to heat and current drive by using LHFW in central region. For the LHFW experiment, overall LHFW RF system including diagnostics has been designed and the key components such as antenna and klystron are being prepared successfully in collaboration with KWU and SNU. Acknowledgments This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2014M1A7A1A03045372). The authors would like to thanks Dr. Harvey and Dr. Petrov of CompX for the help concerning GENRAY code use and also appreciate KAPRA for the granting of klystron beam power use. References [1] A.C.C. Sips, Advanced scenarios for ITER operation, Plasma Phys. Control. Fusion 47 (A19) (2005). [2] J. Wesson, Tokamaks, 3rd ed., CLARENDON Press-OXFORD, 2004, pp. p. 144. [3] S.C. Luckhardt, et al., Generatin of rf-driven currents by lowr hybrid wave injection in the Versato II tokamak, Phys. Rev. Lett. 152 (48) (1982). [4] F. Alladio, et al., Density limit for lower hybrid wave-electron interaction and minority hydrogen absorption in FT, Nucl. Fusion 24 (1984) 725. [5] P. Bonoli, Review of recent experimental and modeling progress in the lower hybrid range of frequencies at ITER relevant parameters, Phys. Plasmas 21 (2014) 061508. [6] K. Theilhaber, A. Bers, Coupling to the fast wave at lower hybrid frequencies, Nucl. Fusion 20 (1980). [7] K.J. Chung, et al., Plasma Sci. Technol. 15 (2013) 244. [8] S.H. Kim, et al., One-dimensional full wave simulation on XB mode conversion in electron cyclotron heating, Phys. Plasmas 21 (2014) 062108.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
ARTICLE IN PRESS
FUSION-8531; No. of Pages 5
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus Sun-Ho Kim a,∗ , Seung-Ho Jeong a , Hyunwoo Lee b , Byungje Lee b , Jong-Gab Jo c , Hyun-Young Lee c , Yong-Seok Hwang c a
Korea Atomic Energy Research Institute, Daejeon, Republic of Korea KwangWoon University, Seoul, Republic of Korea c Seoul National University, Seoul, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx Keywords: Lower hybrid wave Fast wave Current drive Coupling Inter-digital antenna
a b s t r a c t An efficient heating and current drive scheme in central or off-axis region is required to realize steady state operation of tokamak fusion reactor. And the fast wave in lower hybrid resonance range of frequency could be a candidate for such an efficient scheme in high density and high temperature plasmas. Its propagation and absorption characteristics including current drive and coupling efficiency are analyzed for Versatile Experiment Spherical Torus and it is shown that it is possible to drive current with considerable current drive efficiency in central region. The RF system for the fast wave experiment including klystron, transmission systems, inter-digital antenna, and RF diagnostics are given as well in this paper. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The steady state operation of fusion reactor is based on the advanced scenario by using bootstrap current through active density profile control [1]. However, an external current drive scheme is still needed to control the profile and to meet the insufficient current in total current. Most efficient current drive is to use slow wave in lower hybrid resonance frequency range, i.e., Lower Hybrid Current Drive(LHCD) [2]. However, it has been observed to drive current off-axis or outer region in high density plasmas [3,4] though it is recently reported that it could drive current in the reactor grade high density plasmas at higher magnetic field and lower N|| on Tore Supra, Alcator C-Mod and FTU [5]. Meanwhile, there is another possibility that the fast wave in the same lower hybrid resonance range could be used for the reactor grade plasmas because it can propagate into high density and high temperature plasmas with considerable current drive efficiency [6]. In this context, the fast wave in frequency of two times greater than lower hybrid resonance frequency was suggested and it is planned to prove experimentally the current drive scheme on Versatile Experiment Spherical Torus (VEST) [7].
The propagation and absorption of the fast wave on VEST are analyzed in section 2.1. The coupling characteristic is investigated by the 1D full wave code in section 2.2 because higher density is required for the fast wave to propagate than slow wave or fast waves in other frequency range. And the propagation and absorption on VEST is calculated by using GENRAY ray tracing code in section 2.3. Finally, the preliminary design and status of the RF system for the fast wave experiment on VEST are given in section 3. The overall RF system design including klystron, power transmission lines, fast wave antenna, diagnostics are described in this section. 2. Propagation, absorption, and coupling of LHFW on VEST 2.1. Dispersion relation The propagation and absorption characteristics of LHFW can be understood through the real and imaginary part of the dispersion relation. It is represented as Eqs. (1) and (2) [3].
N⊥r ∗ Corresponding author. E-mail address: [email protected] (S.-H. Kim).
⎧ 1/2 ⎪ P(N||2 − S) ⎪ ⎪ SW ⎪ ⎨ − S ∼ = 1/2 ⎪ ⎪ (N||2 − R)(N||2 − L) ⎪ ⎪ FW ⎩ −
(1)
(N||2 − S)
http://dx.doi.org/10.1016/j.fusengdes.2016.02.014 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 1. Real and imaginary refractive index of LHFW and LHSW on VEST.
Fig. 2. Density profile model for coupling calculation.
where S, P, R, L are Stix parameters, N is a parallel refractive index. SW and FW represent Slow Wave and Fast Wave, respectively.
⎧ N⊥r,SW 1/2 3 exp −2 LHSW ⎪ ⎪ ⎨ N⊥i ∼ = 1/2 3 exp −2 ⎪ ⎪ LHFW ⎩ N⊥r,FW
temperature increases the imaginary part of LHFW increases dramatically in central high density region as given in Eq. (2). 2.2. Coupling of LHFW
(2)
2 N 2 /ω2 −1 + ωce pe ||
where is the ratio of wave phase velocity to electron thermal velocity. LHSW and LHFW represent Slow Wave and Fast Wave in Lower Hybrid resonance frequency range. The real perpendicular refractive index of SW is usually very large and attains to several tens and hundreds depending on the plasma parameters and magnetic field. Therefore, LHSW could be absorbed very efficiently once if the Landau condition is roughly satisfied as given in the imaginary wave number of LHSW in Eq. (2). Meanwhile, the real wave number of FW is usually one or two orders lower than that of SW so that the damping is weak unless the Landau damping is satisfied precisely. However, if the density is large enough to make the denominator approach zero, it can be absorbed efficiently as LHSW. The real and imaginary part of the perpendicular refractive indices of FW and SW for the VEST plasma and device parameters are shown in Fig. 1. The central and edge density are 3 × 1018 #/m3 and 4 × 1017 #/m3 , respectively. And the assumed N|| is 4.0. As the
The launching density at which LHFW starts to propagate can be obtained from the dispersion relation (1) by equating R to zero. And it can be represented as Eq. (3).
me ε0 2 nlaunch ∼ N|| − 1 ωωce = e2
(3)
where ω is a operating frequency, ωce is a electron cyclotron resonance frequency. The launching density of LHFW is usually several hundred times greater than that of LHSW and it means that the coupling to LHFW can be more difficult than to LHSW. Therefore, the coupling efficiency should be investigated in detail to apply the LHFW current drive scheme to reality. In this study, the coupling efficiency is calculated with 1D full wave simulation code developed for the study of XB mode conversion efficiency [8]. The density profile to calculate the coupling is modeled as Fig. 2. The magnetic field is 0.2 T at the plasma center and the operating frequency and parallel refractive index are 500 MHz and 4.0, respectively. The parameters including density profile parameters are summarized in Table 1.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 4. Coupling efficiency and ratio for Ez excitation. Fig. 3. Coupling efficiency and ratio for Ey excitation.
The coupling efficiency and coupled power ratio for Ey antenna excitation to launch LHFW and for Ez excitation to launch LHSW are shown in Figs. 3 and 4, respectively, where the bulk density is varied from 6 × 1015 #/m3 to 5 × 1017 #/m3 to estimate the density and density gradient effect. The coupling efficiency by Ey excitation is much lower than that by Ez excitation if the bulk density is less than about 3 × 1017 #/m3 which is the LHFW launching density. However, it increases almost 10 times if LHFW start to propagate. It means that LHFW can be efficiently coupled to plasmas in high density and high density gradient plasmas above LHFW launching density. Interesting thing is that the considerable power is coupled to LHSW in total injected power. It attains to almost 40% as the bulk density reaches 5 × 1017 #/m3 . It may be explained in three points of view as follows. First, the electric field of LHSW has considerable Ey component though it is less than Ez component. Second, the launching density of LHSW is much lower than that of LHFW. Third, if the density gradient increases, the two wave branch cannot be separated exactly and a portion of LHFW power can be transferred to LHSW branch. Inversely, the coupled power ratio to LHFW appears also for Ez excitation case as the bulk density increases though it is just about 10% as shown in Fig. 4.
plotted together at the same plasma and antenna parameters for a reference. The rays and driven currents for the LHFW are shown in Fig. 5. The LHFW propagates into central region and the drive current is comparable to that of LHSW as predicted in Section 2.1. 3. LHFW RF system 3.1. Overall RF system including diagnostic
The propagations and driven currents of LHFW on VEST are calculated with GENRAY code. The parameters for the calculation are summarized in Table 2. The core density is 3 × 1018 #/m3 . And the edge density is set to be 4 × 1017 #/m3 for LHFW launching. The parallel refractive is 4.0 which satisfies the accessibility condition for the given magnetic field and RF frequency. LHSW case is also
A LHFW RF system is designed based on an old 10 kW UHF broadcasting system at Seoul National University (SNU) and the antenna is being developed in inter-digital type through collaboration with Kwang-Woon University (KWU). The system schematic of including diagnostic is shown in Fig. 6. The input signal is amplified in two steps through 39 dB solidstate amplifier and 48 dB klystron. The RF power of about 10 kW is transmitted to inter-digital antenna through 3-1/8 coaxial transmission line, 10 kV rating DC breaker, matching internal transformer with VSWR < 1.05 in wide band and high vacuum compatible feed-thru. The parameters of main RF system components are summarized in Table 3. The RF diagnostics are consisted of directional coupler for the forward and reflected power measurements in front of amplifiers and voltage probes for arcing detection and current measurement at the transmission line and the inter-digital antenna. The power and voltage signals are digitized and monitored through peakphase detector and ADC. The voltage signal measured at antenna is transmitted to ADC via optical fiber because the reference potentials of main VEST chamber and RF power system are different each other due to DC breaker which protects the RF system from the induced voltage of VEST main vacuum chamber.
Table 1 Parameters for coupling calculation on VEST.
Table 2 Parameters for ray tracing calculation on VEST.
2.3. Ray tracing simulation
Parameters
Values
Parameters
Values
B0 Frequency N N0 Nb d(X2 − X1 ) X1
0.2 T 500 MHz 4.0 6 × 1015 –5 × 1017 #/m3 4 × 1015 #/m3 3 cm 2 cm
B0 Frequency N Core density Edge density Core temperature Edge temperature
0.2 T 500 MHz 4.0 3 × 1018 #/m3 4 × 1017 #/m3 3 keV 0.2 keV
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 5. Ray tracing: (a) ray, (b) driven current profile.
Fig. 6. Schematic of LHFW RF system.
3.2. Klystron and antenna A klystron is being prepared by refurbishing an old UHF broadcasting system. The maximum power is 30 kW, and the frequency
ranges from 470 to 700 MHz. The degree of vacuum of the klystron was confirmed to be satisfactory through the vacuum measurement. The specification of the klystron is summarized in Table 4. Since the beam power for the klystron operation is absent, it
Fig. 7. Curved antenna for LHFW RF system on VEST.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014
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Fig. 8. Parallel refractive index spectrum (a) and S-parameters (b) of curved inter-digital antenna designed.
Table 3 Specification of LHFW RF system components. Components
Values
Signal generator Solid state amplifier Circulator(SSA) Klystron Circulator(Klystron) DC breaker Internal transformer Vacuum feed-thru Power line Inter-digital antenna
500 kHz–1.3 GHz (13 dBm) 35 dB (max 29 dBm) 14 dB isolation, 75 W 10 kW UHF Harris system for broadcasting 20 dB isolation, 15 kW Rating 10 kV, VSWR <1.05 VSWR < 1.05,for matching High vacuum< 10−7 mbar 3-1/8” EIA coaxial N : 3 –5, VSWR <3
Table 4 Specification of klystron. Parameters
Values
Frequency Output power Gain Beam voltage Beam current Electrode voltage Heater voltage Heater current Body current Magnet voltage Magnet current Collector cooling Body cooling Magnet cooling Gun cooling
470–700 MHz 37.5 kW 48 dB 19.5 kV 5.4 A 19.5 kV 7V 17 A 50 mA 145 V 32 A Water 2.0 gal/min Water 1.5 gal/min Water 2.0 gal/min Forced air 50 ft3 /min
was lent from Korea Accelerator and Plasmas Research Association (KAPRA) and refurbishing is currently completed. The repair of magnet and filament power is progressing and the klystron operation will be started after the completion of the repair in near future. The preliminary curved design of inter-digital antenna for VEST LHFW RF system is shown in Fig. 7. The power fed to input port is transmitted sequentially to neighbor current straps with /2 phase difference and the current straps radiate RF field to plasmas with asymmetric parallel refractive index spectrum. The uncoupled power is dissipated in dummy load through output port. The radiated antenna parallel spectrum and S-parameters were calculated with HFSS commercial code for a vacuum load. In 480–496 MHz frequency range, the parallel refractive index is between 3.5 and 4.5 and the S-parameters S11 and S12 are less than
−10 dB as shown in Fig. 8. They satisfies the requirement for LHFW experiment on VEST and more specific design and optimization will be carried out considering the plasma density profile in front of antenna in next step. 4. Summary The feasibility study of heating and current drive by using Lower Hybrid Fast Wave(LHFW) is progressing aiming to prove the concept on VEST and to apply it to tokamak fusion reactor. The propagation, absorption, current drive, and coupling characteristics are analyzed and simulated for VEST and it was shown that it is possible to heat and current drive by using LHFW in central region. For the LHFW experiment, overall LHFW RF system including diagnostics has been designed and the key components such as antenna and klystron are being prepared successfully in collaboration with KWU and SNU. Acknowledgments This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2014M1A7A1A03045372). The authors would like to thanks Dr. Harvey and Dr. Petrov of CompX for the help concerning GENRAY code use and also appreciate KAPRA for the granting of klystron beam power use. References [1] A.C.C. Sips, Advanced scenarios for ITER operation, Plasma Phys. Control. Fusion 47 (A19) (2005). [2] J. Wesson, Tokamaks, 3rd ed., CLARENDON Press-OXFORD, 2004, pp. p. 144. [3] S.C. Luckhardt, et al., Generatin of rf-driven currents by lowr hybrid wave injection in the Versato II tokamak, Phys. Rev. Lett. 152 (48) (1982). [4] F. Alladio, et al., Density limit for lower hybrid wave-electron interaction and minority hydrogen absorption in FT, Nucl. Fusion 24 (1984) 725. [5] P. Bonoli, Review of recent experimental and modeling progress in the lower hybrid range of frequencies at ITER relevant parameters, Phys. Plasmas 21 (2014) 061508. [6] K. Theilhaber, A. Bers, Coupling to the fast wave at lower hybrid frequencies, Nucl. Fusion 20 (1980). [7] K.J. Chung, et al., Plasma Sci. Technol. 15 (2013) 244. [8] S.H. Kim, et al., One-dimensional full wave simulation on XB mode conversion in electron cyclotron heating, Phys. Plasmas 21 (2014) 062108.
Please cite this article in press as: S.-H. Kim, et al., Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.014