New Power Modulator for High Voltage Accelerators - IEEE Xplore

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ITER or CFTER (China fusion engineering test reactor). It is implemented with high frequency insulation transformer for powering loads of the accelerator floating.
New Power Modulator for High Voltage Accelerators Ge Li1, Yingui Zhou2, Hua Li1, Lu Qu1, Jing Zhang2, Ying Wang1 1

Institute of Plasma physics, Chinese Academy of Sciences, PO Box 1126, Hefei, Anhui 230031, PRC 2

National Synchrotron Radiation Lab, University of Sci. & Tech. of China, Anhui Hefei 230029, PRC

ABSTRACT

crosstalk problems of its 40MW RF power which is also faced by EAST and ITER-like CFETR. New power modulator with less stray capacitance is proposed for high-power ion accelerators which are used as actuators of tokamak fusion device, such as EAST, ITER or CFTER. Presently ITER accelerator use 50 Hz/-1 MV/5 MW High Voltage (HV) Insulation Transformers to power its low voltage loads on its high voltage pole, such as its extraction grid, plasma grid, RF system, bias plate and snubbers for fast fault protection as shown in Figure 1 [4]. The -1MV power supply is cascaded by five -0.2 MV dc generators—its equivalent fault-snubber circuit is shown in Figure 2. Due to selection of 50 Hz frequency in the 1 MV insulation transformer and its 90-120 m HV transmission line to the accelerator, its total stray capacitance for its snubber design is now as high as 5.9 nF, which challenge its present safety design for fast-fault-protection within ITER EDH [1]. Simulation shows it is difficult to satisfy IEC 61000-4-16 and ITER Electric Design Handbook (EDH) due to its too high fault voltage and current even in 100kV SPIDER case [5].

New power modulator with less stray capacitance is proposed for HVDC accelerators which are used as actuators of tokamak fusion device, such as EAST (Experimental Advanced Superconducting Tokamak), ITER or CFTER (China fusion engineering test reactor). It is implemented with high frequency insulation transformer for powering loads of the accelerator floating on its high voltage pole, such as its extraction grid, plasma grid, RF system, bias plate and snubbers for fast-faultprotection (FFP). It facilitates its FFP design to satisfy IEC EMC standards implemented by ITER or ITER-like CFETR. Based on our former work [Ge Li et al., "Compact Power Suppliers for Tokamak Heating.", IEEE Trans. on DEI, Vol. 19, No. 1 (2012)], a 3 kA power module is designed for powering the plasma grid, as might be configured a new roadmap with less stray capacitance for reliable engineering design of HVDC accelerators in CFETR. Index Terms — CFETR (China Fusion Engineering Test Reactor), Experimental Advanced Superconducting Tokamak (EAST), Electric Design Handbook (EDH), Fast fault protection; IEC standards, EMC, HVDC, Heating Neutral Beams (HNB), International Thermonuclear Experimental Reactor (ITER), Ion accelerator, Power Modulator, RAMI, RF tubes, Tokamak

1 INTRODUCTION THE China Fusion Engineering Test Reactor (CFETR) is an ITER-like machine with more duty factor and RAMI requirements carried out within the framework of Chinese fusion energy development based on enabled technologies of EAST (Experimental Advanced Superconducting Tokamak) and ITER ü the International Thermonuclear Experimental Reactor. Crosstalk problems within above roadmap must be solved before CFETR could be constructedüall ITER-like sub-system should agree with ITER Electric Design Handbook (EDH), i.e. IEC standards for safety of their integration. ITER is driven by three 1 MV & 40 A accelerator in its heating neutral beam (HNB) and 40MW RF power as its heating and current drive (H&CD). In which 120MW ion accelerators are the largest heating power of ITER facility, three negative ion accelerator-based neutral injectors will be implemented in its H&CDübut its 5.9 nF stray-capacitance challenges its electrical safety design [1-3], together with 978-1-4799-4047-9/14/$31.00 ©2014 IEEE

Figure 1. The ITER HNB Configuration with Power Suppliers

In order to mitigate above electric safety problem, the new power modulator is designed with high frequency insulation transformer (HFIT) for powering plasma grid of the accelerator floating on -1MV high voltage pole, where all loads may be implemented in the way by scaling up our former work in [6] to power level of about 3MW with structure of multi-module inputs in the ground level and multi-loads in the HV pole insulated by HFIT. The surface 308

area of HV shields of the transformer could then be scaled down significantly which decrease its stray capacitance at the same insulation distance with the same transferred power. This will facilitate its snubber design to satisfy IEC EMC standards implemented by ITER or ITER-like CFETR, i.e. the fault current is less than 0.6kA. The circuit of above design is the same as suggested in [6], but scaled up with 800A IGBT for powering plasma grid.

be inserted in EAST or CEETR while one vacuum tube is sparking and the other is working normally.

2 REQUIREMENTS OF HV ACCELERATOR MODULATORS Table 1 lists typical electric parameters of negative ion accelerators used in ITER and CFETR [1-7]. The stray capacitance parameter has dominated its effects on its snubber design [1]. As shown in figure 1, the ion source and extraction power supply of ITER accelerator includes its power suppliers for RF ion sources and plasma grid, power suppliers for extracting beams and power suppliers for its three bias as listed in table 2. Power suppliers for RF ion sources are composed by four 250kW PS and plasma grid PS. Their functions are supplying electricity to these sub systems of ion source by floating on the accelerated PS. Thus, all these power supplies or modulators require HV insulation to the ground. They are presently designed to be powered by a 50Hz/1 MV/5 MW HV Insulation Transformer which has too much stray capacitance owing to its large area shields in its design. No. 2 Plasma Grid power modulator is sorted out to be designed with 800A IGBT for powering plasma grid with the same power structure as in [6].

Figure 2: Schmatic HVDC circuit of the accelerators where spark frequently occurs at -1MV parameters. The amplitude of the spark induced short current is limited by core snubbers.

Table 1: typical electrical parameters of HV accelerators

In order to solidify its electrical viability of the complex engineering system of CFETR for long pulse and more duty factor, as that of ITER with 3600s long pulse requirement [13], one test platform is being constructed for developing its fast protection and crosstalk elimination technologies. This includes the power supply of plasma grid, the HV snubber and its bias power supplier—but the most urgent one is the crosstalk elimination in multi-terminal HVDC for RF current drive (CD) of plasma due to that 6 RF tubes shared a HVDC PS for its 4.6G RF CD in EAST case due to economic reasons üRF tube is generally priced as over 1M USD per MW in long pulse case and presently only about 1.2 MW RF tubes within 1000s could be fabricated and tested well in near term. Multi-RF tubes shared a common HVDC PS might be the only cheap and reliable solution within 10 years in which crosstalk problem must be solved. It might be a right selection to insert fault-protection facility as that of DIII-D [7] for operating high parameter plasma in safe for over 30 years with cost of only 10% of these vacuum tubes. For CFETR accelerator [8], its HV snubber will be inserted between its ion source and 500 kV HV PSM power supplier, which means for protecting its expensive ion source and power supply while vacuum spark occurred in operation. For its RF tubes, crosstalk among them must be eliminated by inserting core snubbers for reliable tokamak CD while extending its pulse width. All above requires a specially HVDC fault testing bed with at least two loads to validate the protection performance of HV snubber and related power supply to be installed in the real environment, such as EAST or CEETR. Thus, the crosstalk problem can be detected and solved to be suitable to

Electrical Parameters

ITER DNB

ITER HNB

CFETR HNB

HVDC Voltage

100kV

1000kV

500kV

7 MW

75 MW

39 MW

2.6 MW

5 MW

5 MW

Stray capacitance

N/A

5.9 nF