A filament is a critical component of the klystron used to heat the cathode. There are totally 44 klystrons in experimental advanced superconducting tokamak (EAST) lower hybrid current drive (LHCD) systems. All klystron filaments are powered by AC power suppliers through isolated transformers. In order to achieve better klystron preheat, a klystron filament power supply control system is designed to obtain the automatic control of all filament power suppliers. Klystron filament current is measured by PLC and the interlock between filament current and klystron high voltage system is also implemented. This design has already been deployed in two LHCD systems and proves feasible completely.
Klystron filament is one crucial component used for warming up the cathode to emit electrons and the heating performance will affect the klystron’s output ability.1 In experimental advanced superconducting tokamak (EAST), there are two sets of lower hybrid current drive (LHCD) systems, 4 MW/2.45 GHz (consisting of 20 klystrons) and 6 MW/4.6 GHz (consisting of 24 klystrons) including 44 klystron filament totally. Every klystron filament in two LHCD systems is powered by an AN61800T,2 a low-distortion AC power supply with 500 VA output capacity. As klystron filament’s work voltage is floating on cathode with tens of thousands kilovolts DC high voltage that is an unsafe factor in case of high voltage leak especially, it is reasonable to isolate the cathode high voltage from filament power supply. Thus klystron filament is not feed directly by the power supply but through an isolated transformer with a turns ratio 12:1 as shown in Figure 1. AN61800T necessarily works in voltage source mode due to the adoption of the transformer.
The resistance of the klystron filament grows as its temperature rises, vice versa. The cold filament with small resistance is subject to the fact that a large overcurrent will occur if the rated voltage is directly applied on filament, which is the leading factor to filament breakdown. In order to protect the klystron filament, the filament voltage is usually gradually increased to the rating at a slow speed. Generally, the AN61800T firstly outputs a small voltage to make a weak current on the corresponding filament. This current heats the filament and may diminish as filament getting warm and finally gets stable. After a stable filament current is established, the AN61800T’s output voltage can be increased by a certain voltage step. This procedure, called klystron filament preheat process, will be repeated until the rated filament current is reached and keeps steady. In the past the klystron filament preheat in two LHCD systems was carried out manually and it cost several operators at least an hour, which was a great waste of human resources. Moreover, the manual operation cannot ensure the time step and voltage step proper enough to avoid the overcurrent. Consequently two klystron filament power supply (KFPS) control system for both LHCD system are designed to automatically manipulate these filament power suppliers. This note will revolve around the 6 MW/4.6 GHz LHCD KFPS control system because two control system are identical inherently.
The 4.6 GHz LHCD KFPS control system consists of an industrial personal computer (IPC), a network switch, two serial device servers, NPort 5650 from MOXA and 24 AN61800Ts as shown in Fig 2. Each AN61800T is physically connected to NPort 5650 via a COM-to-RJ-45 cable.3 Two NPort 5650 and IPC connected to one network switch via network cables form a small LAN for communication through TCP/IP protocol. The KFPS control software running on the IPC centrally controls all power suppliers remotely.
The AN61800T merely provides a single serial port and a special command set for status query and parameter configuration with standard RS-232 protocol. As RS-232 is a simple point-to-point communication protocol not supporting interconnection between multi-ports, it is impossible for the IPC to centrally control all AN61800Ts with the serial ports through one RS-232 filed-bus. So the centralized management of all AN61800Ts is achieved with the assistance of a kind of serial device server, NPort 5650, which is designed to change the filed-bus protocol to TCP protocol for data transmission and make serial device Internet ready instantly.4 A NPort 5650 has an IP address and 16 port numbers for 16 serial ports in RJ-45 form actor in the rack panel. With protocol conversion, each port in NPort 5650 actually represents the serial device connected to it and can be identified by a <IP address, port number> tuple that is a network address supporting standard TCP/IP communication. Eventually all AN61800Ts are able to be controlled remotely by the IPC over the LAN.
KFPS control software running on the IPC is developed in the object oriented method and its class diagram can be seen in Figure 3. There are two core classes in the control software, NPort class and FilaPowerUnit class. A NPort class instance representing a serial port in NPort 5650 encapsulates all methods for network communications while the FilaPowerUnit class inheriting NPort class is an abstraction of AN61800T containing common operations such as increasing output voltage, decreasing output voltage and voltage automatic increase/decrease. The FilaPowerConSys takes charge of 24 FilaPowerUnit objects and provides human machine interface (HMI) for operators to manipulate all AN61800Ts with the software.
Figure 4 is the GUI of 4.6 GHz KFPS control software that’s able to control all power suppliers separately or in a unified way. In EAST daily experiment, operators turn on the power supply firstly by click the start button and set the AN61800T’s output voltage to automatic increasing mode. In this mode, each AN61800T’s output voltage will be increased at a 1 V/20 Sec speed to its maximum output voltage at which filament current reaches the rating. When experiment is finished, AN61800T’s output voltage will be diminished to 0 V by 1 volt per second.
To avoid the interference from the PSM high voltage in filament current measurement, current in the primary winding instead of the secondary windings of the isolated transformer pass through a Hall sensor that outputs a voltage signal in proportion to the passing current. See Figure 1. These 24 voltage signals are transmitted to PLC for real time monitor with a 500 milliseconds scanning period. According to the voltage signal and turns ratio, PLC is able to calculate the current value in the secondary side of the isolated transformer namely the filament current. PLC compares the actual filament current with protection thresholds and would not give the PLC Ready light signal to disable the klystron PSM high voltage system in case that the filament current is too low or too high. Besides, the klystron filament is in series with a 20 Ω resistance for current-limiting. This current-limiting resistance is parallel with a PLC controlled normally open contact closed only when klystron is working. It can be seen that the filament current is always under the rated current at the most time which is helpful to extend filament’s life.
Figure 5 presents a typical volt-amp relation in No.4 klystron filament preheat process during which voltage step and time step are separately set to 1 volts and 20 seconds. The klystron filament current grows almost linearly to rated current with the AN61800T’s output voltage and there is hardly overcurrent through the whole preheat process due to the small voltage step and large time step. By now the KFPS control system, successfully deployed in 4.6 GHz LHCD and expanded to 2.45 GHz LHCD, has not only save much more manpower but also provided a better and more precise control of the voltage and time step during preheat to further enhance the safety of klystron filament.
This work is supported by the Science Foundation of Institute of Plasma Physics Chinese Academy of Sciences (Grant No. DSJJ-14-GC01) and the author would thank all staff in EAST LHCD team.
