3.6MW Power Supply System of the 170GHz ECH\&CD System in KSTAR

Abstract. A 3.6 MW (-66 kV/55 A) Gyrotron power supply system was developed for the 170 GHz ECH&CD gyrotron system in KSTAR. This power supply system consists of Cathode Power Supply(CPS), Anode Power Supply(APS) and Body Power Supply(BPS). The CPS is using the Pulse Step Modulation by the 32set of IGBT choppers. The respons time of Chopper is very fast. So the cathode voltaget is able to be controlled rapidly. The APS is a sort of voltage devider using zener and switch component. It was achieved 3kHz modulation operation. The BPS is combined the commercial power supply and special high voltage switches. It is very simple topology but 5kHz modulation was accomplished easily. Theses power supply system were installed and commissioned successfully in 2011. This paper presents the topology of the each power supply and test result for 170 GHz gyrotron in KSTAR.


Introduction
The high voltage power supply system consists of a HVDC generator for supplying cathode voltage , an anode voltage controller (mod-anode), a body power supply (BPS). The HVDC generator is capable of supplying a maximum voltage of -66 kV and a current of 55 A to the cathode with respect to the collector and its DC voltage generation is based on the Pulse Step Modulation (PSM) principle which is a series connection of elementary rectifier and IGBT switching powermodules (called as chopper). The DC generation consists of 20 high voltage (HV) chopper stacks and 12 low voltage (LV) chopper stacks. The voltage regulation unit of individual chopper module of each HV chopper stacks and LV chopper stacks is 3 kV and 0.5 kV, respectively. The highvoltage switching system for the cathode is made by parallel and series connections of many MOSFET fast solid-state switch of which switching time is less than 1 microsecond. It can turn on the high voltage pulse to the cathode with a rise time of 30 microseconds from 0 kV to 60 kV and the opening time of 4 microseconds. The anode voltage with respect to the cathode is controlled in a range of 0 to 60 kV by using of voltage divider circuit for the beam current control between the cathode and the anode composing of a fixed resistor and many zener diodes which has function of variable resistor. Each zener diode has voltage holding of 200 V, and it is shorted by closing the solid-state switch each parallel connected to each zener diode. The precise control of the anode voltage with respect to the cathode is required for the triode gun operation of the gyrotron which has advantage of pitch factor control of the electron beam without changing the beam voltage. The BPS supplies the positive voltage to the body with respect to the ground for the depressed collector of the gyrotron. The maximum voltage and current of BPS is 50kV and 150mA. The voltage output is regulated with stability of 1%. For the higher frequency modulation, the parallel discharge switch is introduced between the cathode and anode in order to clamp the charged voltage in the stray capacitance between the cathode and anode. The series solid-state switch is also introduced in the BPS and on-off modulation up to 5 kHz synchronized with mod-anode circuit is possible. This paper

Single Line Diagram
The single Line Diagram is shown in Fig 3. The Incoming is 22.9kV. There is VCB for the 22.9kV and the transformer which makes the voltage down to 6.6kV. the 6.6kV VCB is installed after the transformer. The VCS with pre-charging VCS and pre-charging resistor in parallel for the HV chopper and the LV chopper is used. After VCS, there are 4set of HV chopper transformers and 1set of LV chopper transformer that supply the proper voltage for the choppers. The HVDC generator consists of 20 high voltage (HV) choppers and 12 low voltage (LV) choppers. The HVS is installed at the end stage of output for the openning the gyrotron from the HV circuit at the arcing. This output is connected to the cathode of gyrotron. The APS is the a sort of voltage divider using zeners and resistors. The APS controls the voltage between the catode and anode of gyrotron in a range of 0 to 60kV. The discharging switch is equiped for high frequency modulation up to 5kHz.
The BPS consist of high voltage source and high voltage switches. It is connect to the body and collector.and supply the positive voltage. The resistor, Rs is connected at output stage. The function of Rs is the surge suppression at the arc situation.

Power Supply Topology
This power supply system is composed of three power supplies. Cathode power supply(CPS), anode power supply(APS) and body power supply(BPS).

CPS
The CPS is full chopper type power supply. It consist of 20set HV choppers and 12set LV choppers. All chopper is connected in series each other. The chopper uses the two kinds of IGBTs. The 6500V/400A IGBT is for HV chopper. The 1200V/400A IGBT is for LV chopper. The output of HV chopper and LV chopper is 3kV and 0.5kV. The total output voltage is 66kV. Full chopper type HVDC generator has a following advantages.
-fast compensation of the voltage drop -fast rising time without HV switch

HVS system
The CPS has the high-voltage switching system at the final output stage. The HV switching system for the cathode is made by parallel and series connections of many MOSFET fast solid-state switch of which switching time is less than 1 microsecond. It can turn on the high voltage pulse to the cathode with a rise time of 30 microseconds from 0 kV to 60 kV and the opening time of 4 microseconds. The main function of HVS is the openning the CPS from the cathode of gyrotron at the arcing. The HVS system has the reactor for limitation of current rising rate during the arcing time. But this reactor cause the voltage drop of CPS at the moment of beam current starting. So proper value of reactance has to be seleted to minimise the voltage drop. The freewheeling diode is connected to the reactor in parallel. The resistance of freewheeling loop has to be minimised for less voltage drop.

APS
APS is using the zener and the resistor to divide the voltage There are 2ea large voltage zener of 20kV and 96ea small voltage zener of 200V. The all zener have the switch in parallel to bypass the zener. The voltage of the each zener become zero when the switch is on. Therefore APS voltage is adjusted by the number of switch on . Fig 8. shows the APS circuit diagram. APS has the function of modulation that frequency is up to 5kHz. Modulation is charging and discharging of the parasitic 04011-p.4 capaci 9. show

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