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HT-7

HT-7. HIGH POWER MICROWAVE TEST SYSTEM AND EXPERIMENTS WANG Mao, LIU Yue-xiu, SHAN Jia-fang, LIU Fu-kun, XU Han-dong, YU Jia-wen Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031,China. Abstract. HT-7.

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HT-7

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  1. HT-7 HIGH POWER MICROWAVE TEST SYSTEM AND EXPERIMENTS WANG Mao, LIU Yue-xiu, SHAN Jia-fang, LIU Fu-kun, XU Han-dong, YU Jia-wen Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031,China

  2. Abstract HT-7 The HT-7 LHCD experiments use a phased multi-junction grill to deliver LHW power coupled to the plasma from 12 klystrons. High microwave reflection or arc occurs on the experiments sometimes, which threatens the klystrons and the wave-guide windows. A high power microwave test system is built to do some simulations. On the test system the klystron and wave-guide can be protected perfectly with optimized microwave monitor and PIN switch. This paper describes the layout of the test system and the experiments on it and HT-7 tokamak. Key words: LHCD, microwave monitor, arc, PIN switch

  3. Outline HT-7 • Introduction • Layout of the high power microwave test system • Experiments 3.1 Measuring the output power value 3.2 High reflection protection 3.3 Waveguide arc research 4. Conclusion

  4. 1. Introduction HT-7 Lower hybrid current drive (LHCD) is an effective mean to sustain Tokamak steady state operation. The HT-7 LHW system with a total power of 1.2MW is built up with 12 klystrons at a frequency of 2.45 GHz. On the LHCD experiments, some protection signals (high reflection, over current and arc) are needed to protect the klystrons and the wave-guide windows. In order to obtain some useful data for LHCD experiment, a high microwave power test system (test line) is founded. This paper describes the layout of the test system whose characteristic is using a water load to absorb the microwave power and a segment of vacuum wave-guide to simulate the condition of Tokamak. Some experiments on the system have been done and these experiments lay a foundation for the LHCD system improvement.

  5. 2. Layout of the high power microwave test system HT-7 The main part of the layout of the high power microwave test system. The maximum power of 100KW can be generated by klystron. The isolator, which can absorb the reflected power, is used to protect the klystron. On the coupler two detectors are used to measure the output power and reflected power. After the coupler it’s a vacuum waveguide with pumping and puffing system and arc detector. At the end of the line there is a matched load with cooling water to absorb the output power.

  6. The schematic diagram of the input power generating system HT-7 It is composed of a master oscillator, a PIN switch and an amplifier. The PIN switch is used for protection at emergency. When high reflection or arc occurs, the corresponding control voltage will be sent out and the PIN switch switches off the input power of the klystron.

  7. The schematic diagram of the controlling system HT-7 It is also a data processing system. All data are sent to the computer for processing. On the experiments, when the computer detects that high reflection or arc occurs, it will output a protection signal immediately. This signal will startup the drive circuit and sent a control voltage to the PIN switch. From the display screen we can see all the experiment data.

  8. 3. Experiments HT-7 HT-7 is a superconducting tokamak with circular poloidal limiter. Main Goal : Steady-state advanced operation and related physics. The main parameters : R = 1.22 m , a = 27 cm B = 1 ~ 2.5 T Ip = 100 ~ 250 kA ne = 0.8 ~ 6  1019 m-3 Te = 0.6 ~ 2 keV Ti = 0.3 ~ 1 keV Duration = 0.5 ~ 20 s ICRF: f = 24 ~ 30 MHz Power = 0.3 MW LHCD: f = 2.45 GHz Power = 1.2 MW

  9. 3.1 Measuring the output power value HT-7 There is only a bi-directional coupler used for measuring during the LHCD experiment. At the coupler a power detecting device (including a cymoscope and a voltage amplifier) is used for power monitor. In order to get the accurate power a water load is applied for verifying the coupler on the test line, after that the coupler can be applied on LHCD system. The experiment of using a water load to verify the coupler has been done. It’s well known that the water load is a matched one. In the experiment the flexible cooling water absorbs the microwave power and it’s temperature rises. When the energy balance state is reached, the outlet water temperature retains at a degree, and the microwave power can be calculated from the risen temperature and the water flux. The temperature difference between the outlet water and the inlet water is defined as △t (0C). M (kg/s) is used to represent the water flux and C (J/kg·0C) to water specific heat capacity. The power can be calculated with the formula P=CM△t (W), where C, a constant of 4.18×103 J/kg·0C, is water specific heat capacity. The measured water flux M is 5200 kg/h (44 kg/s). Thus the microwave power is obtained, P=CM△t (W)=4.18×103×1.44△t (W)=6.02△t (KW)

  10. The result curves of the verifying experiment HT-7

  11. 3.2 High reflection protection HT-7 In order to protect the ceramic windows and the LHCD system, formerly, protection signal is used to cut off the master oscillator’s power which is divided to feed the 12 klystrons. Therefore all the 12 klystrons will be switched off when high reflection or arc occurs in one chain. Recently, 12 PIN switches are considered to be used for protection in 12 chains, and the experiments have been done. In the experiments, when high reflection or arc occurs, the computer will output a protection signal which is driven to the PIN switch, the PIN switch will cut off the power in 10 µs with an attenuation of 20 dB. The experiments are successful on the test system. In 2003 HT-7 experiments, 12 PIN switches are used and the effect is satisfying.

  12. 3.3 Waveguide arc research HT-7 On the LHCD system, there are some reasons of resulting in waveguide arc and high reflection. The first is the load (plasma). On tokamak experiments, the plasma is a relative matching load by controlling the plasma displacement, density and other parameters. But sometimes the microwave power can’t be coupled well because the plasma parameters are not a good matching ones. The second is the vacuum condition. High power microwave can go through the waveguide safely when the vacuum inside the waveguide is high. But in the TOKAMAK condition, the vacuum can’t be always high enough for the wave to pass through the waveguide. On the test system some experiments have been done to find which condition is easy for arc occurring. The vacuum inside the waveguide can be controlled by the pumping and the puffing systems (see fig.1). When arc occurs, the arc detector will send a signal to the computer and the arcing time is recorded. At the same time the protection signal is sent to the PIN switch. The experiments can be divided to two parts. One is scanning power. The other is scanning vacuum.

  13. Vacuum before wave (Pa) Vacuum after wave (Pa) Power（KW） Arcing time （s） 5.0x10-4 6.7x10-4 50 ∞ 5.0x10-4 2.2x10-3 55 38 4.9x10-4 2.4x10-3 38 4.9x10-4 2.3x10-3 60 30 4.8x10-4 2.7x10-3 65 17.6 4.8x10-4 2.4x10-3 73 12 4.8x10-4 2.8x10-3 79 6.4 4.7x10-4 2.7x10-3 8 Results by scanning power HT-7 The experiments can be divided to two parts. One is scanning power. On these experiments, the wave power is scanned and the arc time is recorded while the vacuum inside the waveguide keeps at a constant value (about 4.8x10-4 Pa). The wave pulse length is all the same of 60s.

  14. Vacuum before wave (Pa) Vacuum after wave (Pa) Power （KW） Arcing time （s） 5.7x10-5 1.0x10-4 60 ∞ 7.10x10-5 1.0x10-4 60 ∞ 9.0x10-5 1.9x10-4 60 ∞ 1.1x10-4 2.8x10-4 60 ∞ 2.9x10-4 2.5x10-3 57 59 2.8x10-4 2.2x10-3 55 5.0x10-4 2.2x10-3 58 38 4.9x10-4 2.4x10-3 38 7.2x10-4 2.8x10-3 57 44 9.6x10-4 3.9x10-3 57 29 1.1x10-3 5.1x10-3 57 17 3.3x10-3 1.4x10-2 62 11 5.4x10-3 1.6x10-2 62 9 7.5x10-3 1.5x10-2 62 7.5 1.6x10-2 63 8 9.7x10-3 1.7x10-2 62 6 Results by scanning vacuum HT-7

  15. 1. Conclusion HT-7 The HT-7 LHCD experiments use a phased multi-junction grill to deliver LHW power coupled to the plasma from 12 klystrons. High microwave reflection or arc occurs on the experiments sometimes, which threatens the klystrons and the wave-guide windows. A high power microwave test system is built to do some simulations. On the test system the klystron and wave-guide can be protected perfectly with optimized microwave monitor and PIN switch. This paper describes the layout of the test system and the experiments on it and HT-7 tokamak.

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