1. Operation status of KTX
By Wenzhe Mao
on behalf of KTX team
Good evening, ladies and gentleman, I am Wenzhe Mao from USTC. My presentation is about the operation status of KTX
17th IEA Reversed Field Pinch Workshop, Hefei, China

2. Outline Power supply and magnetic coils of poloidal field
Vacuum pumping and gas puffing system
Wall baking system
Wall cleaning and Pre-ionization
Conditioning of KTX plasma
Those are the main content that I want to talk about, which is relative to operating KTX. First, I will talk about the power supply and magnetic coils of poloidal field. Then I will introduce the vacuum pumping and gas puffing system. Next is about wall baking system. Then the wall cleaning and pre-ionization system. At last, I will give a short description about the conditioning state of KTX plasma

3. Operation diagram
This slice show the operation diagram. KTX operation system consists of power supply of magnetic field coils, vacuum and gas puff system, wall baking system and pre-ionization system.
All those systems are controlled by the central control system.

4. Comparison between Measured and simulated magnetic field coil current waveforms
plasma
shell
OHA, EQ1, EQ6
First thing we need to check is that if the coil current flow as the way we design them, so we made a comparison between measured and simulated magnetic field coil current waveforms. The figure on the left shows the cross section of the poloidal field coils. The poloidal field coils include the ohmic field coils and equilibrium coils. Those coils are connected with the same capacitor power supplies. Equilibrium coils are divided into 6 gourps and ohmic field coils are divided into three groups. Each oh group is connected in parallel with two groups of equilibrium coils. The right figure shows the waveforms of measured pf coils. We can see that the measured poloidal field coils current direction is consistent with simulated results.
OHC, EQ3, EQ4
OHB, EQ2, EQ5
Measured poloidal coil current direction is consistent with simulated results

5. Vacuum Pumping and gas puffing
Design vacuum Target: 5e-6Pa
Design vacuum pumps:
1 roots pump and 1 dry compressing pump for fore vacuum pumping
5 turbo molecular pumps and 3 cryopumps for high vacuum pumping
Vacuum duct around the torus
Gas puffing:
Helium is used for glow discharge cleaning
Hydrogen is used for plasma
Deuterium has been prepared for the next step discharge
This diagram show the design of vacuum pumping and gas puffing system. The design target is 5e-6Pa. There are 1roots pump and 1 dry compressing pump for fore vacuum pump, and five turbos and three cryo pumps for high vacuum pumping。 Those vacuum pump are all connected with the vacuum duct around the torus.
Design diagram of KTX vacuum pumping and gas puffing

6. Vacuum Pumping Fore vacuum pumps:
Now we have 1roots pump and 1dry compressing pump as fore vacuum pump, one of the cryopump is under test. Four turbo molecular pumps have been put into use as high vacuum pumps. The pumping speed can reach 4800 litres per second for hydrogen and 3600litres per second for Nitrogen. The best vacuum achieved until now is 1.2 multiply 10 to minus 5 pascal. A residual gas analyzer is installed for gas composition analysis.
Fore vacuum pumps:
1 roots pump and 1 dry compressing pump
High vacuum pumps:
4 turbo molecular pumps
Pumping speed:
4800l/s for H2 and 3600l/s for N2
Best vacuum achieved: 1.2e-5Pa
 Residual gas analyzer(RGA)

7. Gas puffing Number of valves 2 Valve type Piezoelectric crystal
Flow range(Per Valve)
0-500sccm
Gas feeding control
Programmed pulses or manual
Response time
2ms
Gas type
Ar, He, H2, D2
There are two small windows for gas puffing, and piezoelectric ( [paɪ,izoɪ'lɛktrɪk]) crystal valves are used to control gas feeding. Programmed pulses or manual control can be applied. The response time of the valves can reach 2ms. And the gas type can be used include argon, helium, hydrogen, and deuterium.

8. RGA shows H2, H20, O2, and N2 peaks
RGA signals shows hydrogen, water, oxygen, and nitrogen peaks before and after Hydrogen plasma discharge. We can see that there is still much space for us to do on wall conditioning and gas puffing control.

9. Wall baking is achieved by inductive current
Toroidal field coils
Wall baking power supply
Wall baking is achieved by inductive current on KTX. A independent power supply is developed to drive alternating current through toroidal field coils. No current is induced on the copper shell due to the horizontal gaps, so eddy current is induced on vacuum vessel and heats the metal wall directly. The power supply can provide 40 kilo watt power. The penetration time of the vacuum vessel is about 2ms, so the oscillation frequency is chosen around 800Hz to drive current effectively.
Power supply is developed to drive alternating current through toroidal field coils
No current induced on the copper shell due to the horizontal gaps
Eddy current is induced on vacuum vessel for wall baking
Power~40kW, Frequency~800Hz
Bench test has been performed

10. Wall baking bench test result
Time(min)
Temperature(℃)
Temperature waveform of the wall during wall baking
This slice shows the wall baking bench test result. A quarter of the vacuum vessel is used as the testing platform as shown in the left picture. The right figure shows the temperature evolution during the wall baking. During the initial few minutes, the output current of the power supply was adjusted to 200A, and the temperature rise very quickly. Then the current was set to about 50A, and the temperature decrease a little and become stable around 220 degrees for a long time, which is good enough for wall baking.
A quarter of the vacuum vessel:
Testing platform for wall baking power supply

11. Pre-ionization——Hot filament
Material
Thoriated tungsten
Structure
4 pins
Diameter of filament
0.4mm
Effective length
10cm
Effective surface area
1.57cm2
Pre-ionization is realized by hot filament. The 4 pin structure filament is made from thoriated tungsten. The diameter of the filament is 0.4mm, the effective length is around 10cm, and the effective surface area is 1.57 square centimeter. Single pin has been tested and can provide 9A current, which will help improve breakdown.
Single pin can provide 9A current, which will help improve gas breakdown

12. Glow discharge cleaning has been conducted
Glow discharge cleaning has been conducted on KTX. Two sets of GDC cathode were installed on the vacuum vessel, and can work under pressure~ 1Pa. The operating voltage of GDC power is between 200 and 400 volts. The current is between 0.5 and 2 ampere. The two pictures show glow discharge captured by camera inside and outside the vacuum vessel.
Glow discharge captured by camera inside and outside the vacuum vessel
Two sets of GDC , work under pressure ~1Pa, voltage 200~400V, and current 0.5~2A

13. Breakdown pressure（Pa）
First plasma of KTX
Ip Max（kA）
108
Loop voltage Max 86 Bt
~0.2
Pulse time
~4ms
Gas He
Breakdown pressure（Pa）
After the power supply , the vacuum pumping and gas puff system test work were completed, we made a try to generate the first plasma on august 15th，and we get it. This picture shows the light from the transparent window during the pulse. The maximum plasma current is 108kA. The maximum loop voltage is 86V. The toroidal field is stable during the discharge, and is around 0.2T. The pulse time is around 4ms. And the discharge gas is Helium with breakdown pressure about 3e-2 pa.

14. Scanning Toroidal field in stable TF mode
q(a)=1.43
q(a)=0.74
We try to explore the operation zone of KTX in stable toroidal field mode, so we fixed the voltage of poloidal field power supply, and scan the toroidal magnetic field. The figure show the waveforms of plasma current , loop voltage, toroidal field and edge safety factor for one shot. For all those shots, the maximum plasma current is between 50kA and 65kA. This figure shows the toroidal field versus minimum edge safety factor. We can see that the minimum edge q is 0.74 and the maximum edge q is Next step we will increase the voltage of toroidal field power supply to try to produce tokamak plasma with higher q.
Minimum edge q is 0.74 and the maximum edge q is 1.43

15. Forced reversal Bt has been attempted
Lowest F achieved so far is around 0.7
Forced reversal toroidal field has been attempted. Those figures show the waveforms of plasma current ,loop voltage, pinch parameter theta, reversal parameter F, toroidal field near the wall and the average toroidal field. The blue waveforms represents the preliminary attempt to force Bt reversal. We can see that the toroidal field decrease very slow, so for the second shot we decrease the capacitance of the TF power supply to make edge toroidal field decrease more quickly. Then for the third shot as the yellow curves show, we increase the loop voltage to make plasma current rise more quickly. The lowest F achieved so far is around 0.7.
First shot
Second shot : Decrease the capacitance of TF power supply to make edge Bt decrease more quickly
Third shot : Increase the voltage of PF power supply to make plasma current rising more quickly

16. Plasma conditioning is underway
Now Plasma conditioning is underway. This plot shows the plasma current waveforms with increasing bank voltage of poloidal field power supply up to 30kV. The maximum plasma current increase with the voltage, and the maximum plasma current achieved until now is 140kA. The maximum PF bank voltage is around 48kV, so we still have rising space during plasma conditioning process.
The maximum plasma current and pulse time rise with the increasing voltage of poloidal field power supply
Maximum plasma current achieved until now is 140kA

17. Summary
Power supplies work well and Poloidal field coil current waveforms are consistent with designed results
Base vacuum pressure reaches 1.2e-5Pa
Glow discharge cleaning system has been tested
Pre-ionization system has been tested
Inductive wall baking system has been newly developed and tested
First plasma was generated two weeks after completion of KTX installation
Conditioning and optimization are underway
This slice show the summary of my presentation. ………….

18. Future plan
Apply wall baking system and GDC system to achieve the design vacuum pressure and more clean wall
Apply pre-ionization filament to improve gas breakdown
Apply single chord THz interferometer system for density control to lengthen pulse time
Through we are moving fast, however the operation of KTX is still in start up phase, we still have a long way to go. Until now, we have got 134 shots with plasma except for the testing shots. Our future plan include applying wall baking system and GDC system to achieve the design vacuum pressure and more cleaning wall. Also, we will apply pre-ionization filament to improve gas breakdown and apply single chord THz interferometer system for density control to lengthen the discharge pulse time.

19. Thank you for your attention!