球状トカマクに適用する多機能センターソレノイドコイル電源の開発

概要

九州大学学術情報リポジトリ
Kyushu University Institutional Repository

Development of a versatile central solenoid
power supply for spherical tokamaks
張, 逸凡

https://hdl.handle.net/2324/7157366
出版情報：Kyushu University, 2023, 博士（工学）, 課程博士
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氏

名

：張 逸凡

Name

論 文 名

：Development of a versatile central solenoid power supply for spherical

tokamaks (球状トカマクに適用する多機能センターソレノイドコイル電源の開発）
Title
区
分 ：甲
Category

論

文 内 容 の 要
Thesis Summary

旨

A spherical tokamak (ST) is one of the magnetic plasma confinement devices to realize a
compact fusion reactor. For tokamaks, ohmic heating (OH) is the fundamental method to
initiate plasma and to start up the current. An OH is performed, according to Faraday’s law,
by swinging the current in the central solenoid (CS). In STs, the size of CS is limited since a
space in the central part of the machine is narrow. Accordingly, various non-inductive plasma
start-up techniques have been established. After a long-term research, even though noninductive start-ups are successful in many ST devices, it may fairly be presumed that
removing CS coil from ST is not practical. To establish the plasma start-up method of ST, in
this thesis, the hybrid scenario with CS current control is proposed for ST reactors. With
versatile power supply for the CS, developed in this work, tokamak parameters have been
improved and a variety of plasma experiments have been provided in the QUEST spherical
tokamak. Utilizing stacks of insulated gate bipolar transistors (IGBT), two high power current
sources (8 kA and 375 V) have been combined so that it works as a bipolar current source.
Tokamak start-up solely by electron cyclotron heating (ECH), as one of the representative
non-inductive plasma start-up methods, can achieve plasma current of 100 kA level. In the
QUEST experiment, this method has been adopted for the tokamak start-up. By the ECH,
energetic electrons are mass-produced. Accelerating energetic electrons by auxiliary OH,
plasma current is more driven. The value of plasma current is an essential parameter to
confine plasma finely. In QUEST, a CS coil whose 217 turns has been installed in the narrow
central space. To drive higher plasma current, OH through a double current swing from
negative to positive polarity in the CS current was proposed. Namely, change of the poloidal
flux in the CS is also doubled. This double swing property is realized through IGBT bipolar
switching with developed real time current monitor and interlock system using FieldProgramable-Gate-Array (FPGA). With 28 GHz-ECH, a plasma current over 100 kA can be

achieved stably through the double flux swing. The electron density exceeds the cut-off density
for 8 GHz range electron cyclotron (EC) waves, and hence electron Bernstein wave (EBW) can
be excited in such over-dense plasma. EBW heating/current drive is a promising method in
the QUEST experiment, and furthermore in future ST reactors.
Toroidal electric field is applied by controlling the CS current. Increasing or decreasing the
CS current, the electric field is applied positively and negatively. In the case that the electric
field drives plasma current through OH, energetic electrons are accelerated more efficiently
than bulk electrons in the EC start-up plasma as ECH power also tends to be absorbed in
energetic electrons. It is difficult to heat bulk electrons or ions under such situation. Inversely,
an opposite toroidal electric field can decelerate energetic electrons efficiently so that a hard
X-ray bremsstrahlung is suppressed through the retarding toroidal electric field. Owing to the
EC current drive, plasma current does not change the polarity and the tokamak configuration
is maintained even with the retarding field. Favorably, the temperature of bulk electrons
increases significantly with such retarding electric field. In addition, after applying retarding
electric field through decreasing the CS current, OH is applicable through increasing the CS
current. One distinctive feature is that the OH is repeatable because the ECH maintains the
plasma current with retarding electric field.
A high-voltage capacitor bank has been also installed on the negative side in the versatile
power supply. A fast trigger system for the ignitron switch with small jitter is developed, and
hence high loop voltage could be applied transiently at a specific timing. Our target for the
fast trigger system is co-axial helicity injection (CHI) experiment which is also one of the noninductive start-up techniques. Plasma generated by CHI is resistive with high density, so OH
is likely effective in general. In the experiment, toroidal current is gained from 120 kA up to
170 kA through the OH by the capacitor bank discharged before the CHI. In addition, the OH
is applied just after start-up of toroidal CHI current. The pulse duration in the CHI discharge
is extended from 2 to 3 ms, which contributes a clear formation of the closed flux surfaces in
the CHI discharge.
A versatile central solenoid power supply is developed to encounter the key issue of the
plasma start-up in the spherical tokamaks. It can swing double the coil current to conduct the
effective OH discharges, and can excite the retarding electric field to suppress the highly
energetic electrons evolution in the non-inductive ECH start-up plasma. It also assists the
short-pulse non-inductive CHI plasma start-up. These works and techniques would contribute
to the development of compact spherical tokamak reactors.