Transcript ht-7 high power microwave test system and experiments

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
The HT-7 LHCD experiments use a phased multijunction 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
Outline
HT-7
1. Introduction
2. Layout of the high power microwave test
system
3. Experiments
3.1 Measuring the output power value
3.2 High reflection protection
3.3 Waveguide arc research
4. Conclusion
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.
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.
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.
The schematic diagram of
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the controlling system
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.
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
3.1 Measuring the output
HT-7
power value
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)
The result curves of the
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verifying experiment
3.2 High reflection protectionHT-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.
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.
Results by scanning power HT-7
Vacuum before
wave (Pa)
5.0x10-4
5.0x10-4
4.9x10-4
4.9x10-4
4.8x10-4
4.8x10-4
4.8x10-4
4.7x10-4
Vacuum
after wave (Pa)
Power
（KW）
6.7x10-4
2.2x10-3
2.4x10-3
2.3x10-3
2.7x10-3
2.4x10-3
2.8x10-3
2.7x10-3
50
55
60
65
73
79
Arcing time
（s）
∞
38
38
30
17.6
12
6.4
8
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.
Results by scanning vacuumHT-7
Vacuum before wave (Pa)
5.7x10-5
7.10x10-5
9.0x10-5
1.1x10-4
2.9x10-4
2.8x10-4
5.0x10-4
4.9x10-4
7.2x10-4
9.6x10-4
1.1x10-3
3.3x10-3
5.4x10-3
7.5x10-3
9.7x10-3
Vacuum
wave (Pa)
1.0x10-4
1.0x10-4
1.9x10-4
2.8x10-4
2.5x10-3
2.2x10-3
2.2x10-3
2.4x10-3
2.8x10-3
3.9x10-3
5.1x10-3
1.4x10-2
1.6x10-2
1.5x10-2
1.6x10-2
1.7x10-2
after
Power
（KW）
60
60
60
60
57
58
57
57
57
62
62
62
63
62
Arcing time
（s）
∞
∞
∞
∞
59
55
38
38
44
29
17
11
9
7.5
8
6
1. Conclusion
HT-7
The HT-7 LHCD experiments use a phased multijunction 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.
Contact Form
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