3.10 Lower Hybrid Current Drive (LHCD) System

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1 3.10 Lower Hybrid Current Drive (LHCD) System KUANG Guangli SHAN Jiafang Purpose of LHCD program Introduction Lower hybrid waves are quasi-static electric waves propagated in magnetically confined plasmas. The lower hybrid wave may transfer its energy to electrons by Landau damping, leading to acceleration of the electrons motion at the direction along with the wave vector, additional electrical current is thus driven, that is called lower hybrid current drive (LHCD). In tokamak plasmas, the lower hybrid range of frequency is usually within microwave frequency range, therefore, an LHCD system is actually a microwave system that can couple its microwave energy into tokamak plasmas in a lower hybrid mode. The LHCD system on the EAST mainly includes: (1) Microwave generators; (2) high voltage power supply (HVPS); (3) transmission system & antenna; (4) monitor & control system; and so on. The LHCD system on EAST will be used for the following purposes: (1) To maintain long duration or even steady state plasma current in the tokamak, and supply its energy to the plasma continuously; (2) To achieve steady state high performance Background The studies on LHCD in the world began at the early 1980's. So far most of the large tokamaks have their own LHCD systems, the parameters are listed in the following table 1. In China, the studies on LHCD experiments have been performed for more than 10 years. We have already achieved lower hybrid current drive in HT-6B, and then in the HT-6M and HL-1M tokamaks. From those experiments, we have obtained a lot of results, accumulating some experiences. A full non-inductive plasma current for more than 20 seconds has been achieved in HT-7 superconducting tokamak by means of LHCD, that successfully demonstrated the reliability of LHCD technology Purpose To operate a tokamak in steady state mode is very significant for developing a tokamak fusion reactor. It needs two fundamental conditions: steady magnetic field and steady toroidal plasma current. Since the toroidal and poloidal coils of EAST tokamak are made of superconducting materials, they can sustain a steady magnetic field. However, the duration of the induced plasma current by means of the OH transformer in EAST is very limited (about 10 sec, as designed). Therefore, to achieve steady performance of EAST needs a non-inductive current drive that can be operated in a continuous mode. The 237

2 LHCD system on EAST is just designed mainly for the purpose. It is also hopeful to achieve high performance for long pulse by means of LHCD, according to the experience obtained from HT-7. Name of Device Microwave power(mw) No. Of Klystron and their kinds Input power (MW) Frequency (GHz) Maximum density MW/m 2 normal JET UK 12(20s) 15(10s) 24 TH2103A Tore Supra France 16 JT-60U Japan 8 24 TH E LD4444 TdeV Canada 1.0(30s) 3(10s) 2 TH2103B TRIAM-1M Japan HT-7 China TH VA876 s 12 KU (20) 38 (25) 21 (13) 50 (25) 11 (4) 65 (47) η cd AW -1 m I p [MA] (V l =0,n e m -3 ) Maximum pulse[s] 3 (1.6) 1 (n eo =3) (0.3) 10(60s at P<0.2MW 0.19 (1.9) 0.02 (0.2) 0.06 (2.6) (1.2) Design of LHCD System and Its General Features Design of LHCD system In order to sustain steady plasma current and actively control its profile, the input power, spectrum and coupling of LH wave should meet some special requirements. According to the experimental scaling rule, it is possible for LHCD on EAST to achieve a current drive efficiency η CD = I CD ner / P of Am -2 /W. Thus, a lower p hybrid wave power of 2.43 MW is required, provided that the line averaged electron density is at m -3 and the wave driven current is 1MA(the plasma radius R p 1.7m). Based on theoretical calculation and experimental scaling, the transmission efficiency of the wave system can reach at 95%, and the wave coupling into the plasma can be more than 80%. Taking account of all the factors, 3.2MW of output power from the wave generator will be required so as to have 2.43MW net lower hybrid wave in the tokamak plasma. Therefore, the designed output power of the LHCD system on EAST is 3.5MW, that is expected to sustain 1MA non-inductive current in EAST plasmas with a central line averaged electron density at about m -3. LH 238

3 To design the coupling spectrum, the accessibility criterion, drive efficiency and distribution of power deposition will be considered. For example, if the toroidal magnetic field is at 3T,and the center density at N e (0)= cm -3,the LH wave can still penetrate well into the core of plasma. In this case, when the wave frequency is at 2.45GHz, its coupling spectrum should be at N N acc =1.855; but at 3.7GHz, the spectrum should be N N acc =1.943; when the center density is down to cm -3, the coupling spectra at 2.45GHz and 3.7GHz should be N N acc =1.645 and N N acc =1.702 respectively. To adjust wave coupling spectrum can change LH wave deposition region in plasmas, accordingly changing the plasma current profile. To optimize the structure of antenna and to control the wave phase of each unit can satisfy the requirements of wave spectra for experiments General features The LHCD system should have the ability to deliver the required wave power with adjustable wave spectra. Meanwhile, it should utilize the existing LHCD devices from the present tokamak HT-7 as much as possible to reduce the cost. The designed LHCD system will have the following general features: (1) LHCD system on EAST is composed by two sub-systems, one of them delivers 2MW at 2.45GHz and another delivers 1.5MW at 3.7GHz. Both have wave pulse length over 1000 sec. (2) The wave source of the 3.7GHz LHCD system is composed of 2 high power klystron amplifiers. The 3.7GHz system will be mainly used to sustain plasma current, especially it is expected to drive current in the plasma central region. (3) The wave source of the 2.45GHz LHCD system is composed of 20 klystron amplifiers (each can deliver 100KW). The coupling wave spectrum of the system can be flexibly adjusted in time. This system will be mainly used to control plasma current profile GHz LHCD system A LHCD system with an output power of 1.2MW (12 klystrons are used as the wave generators) at 2.45GHz was constructed on HT-7 tokamak in The system will be rebuilt to deliver 2.0MW on EAST tokamak. The layout of the designed 2.45GHz LHCD system on EAST is show in Fig.1. The main parts of the system are described in the following: Klystron amplifiers The klystron amplifiers (type KU-2.45) were designed and constructed by a Russian company (ISTOK). The main parameters are listed in the Table 2. Table 2 Frequency Output power 2.45GHz 100KW,cw 239

4 Efficiency >50% Gain >50dB The wave exciter consists one master oscillator, one PIN modulator, a 1 20 power divider, and 20 identical wave drive chains. Each of the chains consists of a digital phase shifter, a semiconductor wave balance amplifier, a voltage control attenuator, a power detector, etc. The master oscillator has three outputs: one delivers 100 mw for phase reference; either of the other two delivers 1W. The PIN modulator can switch off wave power in 10 s with a cut-off level of 20dB. The phase adjustment range of the digital phase shifter is with an adjustment step The semiconductor amplifier has a constant output of 3 W when its input changes in a range of 5dB. The attenuator has a dynamic range of 20 db. Each of the 12 wave drive chains connects to the input of one klystron amplifier Transmission lines and antenna The wave power from each klystron is transmitted to the grill antenna through a standard rectangular waveguide (WR430). The klystrons are protected from the reflected wave power by high power isolators. The isolator is composed of a four-port differential phase shift circulator and two water dummy loads. Its isolation is over 25 db and inserted loss less than 0.3 db. A grill-type coupler is used to couple the wave from 10 klystrons to EAST plasma, where the coupler is composed of 4X20 sub-waveguides (4 rowsx20 columns), the wave from each klystron is transported to the coupler by 1:8 divider then to the 8 sub-waveguides. The grill coupler is made of stainless steel and has guard limiters (shaped carbon tiles) around its mouth. The entire grill is cooled by circulating pure water, where its mechanical structure can withstand any kinds of stress produced by thermal cycling and electromagnetic pulse during its operation Wave monitoring and phase feedback control system There are 2 bi-directional couplers installed in each transmission line, one closed to the window of klystron and the other to the grill. Since all klystrons are excited by sole master oscillator, their output waves are coherent. The phase signals are compared with their preset value by a computer that generates the control digital signals for 20 phase shifters in the input circuit prior to the klystrons. By this method, the coupling spectrum can be adjusted. The input and output wave power from each klystron, as well as reflected wave from the antenna and transmission lines, are detected and then to be used in the high VSWR protection Power supplies Each klystron in the LHCD system (2.45GHz) has several kinds of low voltage power supplies (for solenoid, filament, ion pump, etc). The HVPS are used in the LHCD system, each feeds 5 klystrons in parallel, as shown in Fig.2. Each HVPS has the ability to supply 35kV/40A with a pulse length from sec. The ripple and stability of the high 240

5 voltage can be controlled to lower than 1%. Ignitrons are used for fast switching of the high voltage DC voltage on Klystrons Main parameters of LHCD system (2.45Ghz) Maximum output power Maximum pulse length 2MW 1000s Efficiency >50% Number of grill and grill structure 2 4X20 The range of spectrum 1.4 N p 3.0 The width of spectrum N 0.4 DC voltage 33kV Ripple of DC voltage <1% Fast switch-off time 150 µs Normal switch-off time 50 ms GHz LHCD system The layout of the LHCD system operating at 3.7GHz is show in Fig.3, and main parts will be described in the following: Klystron amplifiers The klystron amplifiers (type TH2103C) are supposed to be used. The main parameters could be as listed in Table 3, Table 3 Frequency Output power, pulse length 3.7GHz 750KW, 1000 sec Efficiency 45% Gain >50dB No. Of klystrons 2 The wave exciter consists mainly one master oscillator, two positive PIN modulator, one 1 2 power divider, and 2 identical wave drive chains. Each of the chains consists of a digital phase shifter, a semiconductor wave balance amplifier, a voltage control attenuator, a power detector, etc Transmission lines and antenna The wave power from each klystron is transmitted to the grill coupling through a standard rectangular waveguide (WR284). The klystrons are protected from the reflected wave power by high power isolators of hybrid type. A grill-type antenna is used to couple the wave from 2 klystrons to EAST plasma, where the grill is composed of two 241

6 2X32 waveguides (2 rowsx32 columns), the wave from each klystron is transported to 2X8 sub-waveguides of the coupler through eight 3dB dividers. The phase difference between adjacent sub-waveguides can be adjusted through wave phase shifter, so that the launched wave spectrum can be changed. The grill type antenna is made of stainless steel and has guard limiters (shaped carbon tiles) around its mouth. The grill is cooled by means of circulating pure water. Its mechanical structure can withstand any kinds of stress produced by thermal cycling and electromagnetic pulse during its operation Wave monitoring and phase feedback control system There are 2 bi-directional couplers installed in each transmission line, one closed to the window of klystron and the other to the grill. The phase signals are compared with their preset value by a computer that generates the control digital signals for phase shifters in the wave exciter. The input and output wave power from each klystron, as well as reflected wave from the antenna are to be detected and then to be used for high VSWR protection Power supplies Two TH2103C klystrons will have their HVPS and LVPS respectively. HVPS have nominal parameter of -70kV/25A/1000sec. LVPS will include magnet power supplies, filament power supplies and pump power supply etc. The HVPS will adopt star-point controller as regulated rectifier, which turned out to be the most appropriate and economic circuit. The required insulation level of the inductance to the earth is much smaller if it was on the secondary side of the transformer. In addition, the HVPS will include power switch, rectifier transformer, high voltage uncontrolled rectifier, crowbar protection system, high voltage capacitor and its control system. The stability and harmonic will be limited to within 1%, and HVPS can be switched off in 150 µs through triggering the crowbar at emergency Main parameters of LHCD system (3.7Ghz) Maximum output power Maximum pulse length 1.5MW 1000s Efficiency >45% Number of grill and its structure 1 4X32 The range of spectrum 1.35 N p 3.0 The width of spectrum N 0.23 DC voltage 65kV Ripple of DC voltage <1% Fast switch-off time 150 µs 242

7 Normal switch-off time 50 ms Main Devices of LHCD Systems on EAST The 2.45GHz LHCD system Device Type Number Parameters Notes Klystron KU GHz,100kWC W Ready High power isolator FV kW (25dB) Ready Windows VO kW, VSWR=1.15 Ready Grill coupler 2 1MW Upgrade HVPS 4 35kV/40A Upgrade LVPS 2 groups - Upgrade Power Monitor and phase control system 2 - Upgrade Protection system 2 - Upgrade Control system 1 Upgrade GHz LHCD system Device Type Number parameters Notes Klystron TH2103C 2 750kW, 3.7GHz, 1000sec Thomson Company High power isolator 2 groups VSWR<1.3 (20dB) Ceramic windows 20 VSWR<1.2 Thomson Grill coupler 1 1.5MW HVPS 2 70kV/26A LVPS 2 groups - Power monitor & phase control system 1 - Protection system 1 - Exciter 2 groups - Thomson Control system 1 243

8 FIG.1 The layout of LHCD system (2.45GHz) 244

9 FIG.2 The layout of LHCD system (3.7GHz) 245

Development Status of KSTAR LHCD System

Development Status of KSTAR LHCD System September 24, 2004 Y. S. Bae,, M. H. Cho, W. Namkung Plasma Sheath Lab. Department of Physics, Pohang University of Science and Technology LHCD system overview Objectives