Operation status of KTX By Wenzhe Mao on behalf of KTX team 2015-10-26 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

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

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.

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

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

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)

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.

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.

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

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

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

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

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.

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 1.43. 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

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

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

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. ………….

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.

Thank you for your attention!