22P153-P
Characteristics of a RF negative hydrogen ion sourcewith a long pulse operation
高周波水素負イオン源の長時間動作特性
Tetsuro Matsuno1, Takuya Funaoi1, Nozomi Tanaka1, Akira Ando1,Katsuyoshi Tsumori2, Yasuhiko Takeiri2
松野哲郎1, 船生拓矢1, 田中のぞみ1, 安藤 晃1, 津守克嘉2, 竹入康彦2
1
Tohoku University
6-6-05, Aoba, Aramaki, Aoba, Sendai, 980-8579, Japan
東北大学 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
2
National Institute for Fusion Science322-6, Oroshicho, Toki, 509-5292, Japan
核融合研 〒509-5292 土岐市下石町322-6

A radio-frequency (RF) based negative hydrogen ion (H-) source is investigated for the NBI system infusion experiments. The RF power supply utilizes a FET inverter circuit operated with the RF frequency
lower than 0.5 MHz, which enables a compact and efficient RF system. Until now, we operated RF power
supply with the pulse length of several milliseconds. Recently, we introduced new CW RF power supply
and cooling systems were attached to the transmission line. They were successfully operated and RF
plasma was operated during several seconds. Simultaneously, H- ion beam was extracted and accelerated
up to 15 kV and the beam current was measured with several plasma parameters.

1. Introduction
Neutral beam injection (NBI) system is one of
the most powerful and successful heating tools infusion researches. A hydrogen negative ion (H-)
source has been developed as a beam source withthe beam energy of more than 100 keV [1-3].
Recently, negative ion source using RF plasma
was chosen for ITER NBI system due to its
maintenance free and long pulse (~3600 s)
operation [4-6].
Conventionally, RF power was supplied byusing a vacuum tube oscillator with MHzfrequency range. Recent progress of MOSFET
devices enables us to use a FET switching fullbridge type inverter circuit, which realizes acompact and flexible RF power supply [7].
Although the operation frequency is relatively
low for plasma production (~500 kHz), it can be
operated with high efficiency of DC to AC
conversion without any high voltage (more than0.5 kV) part in the source. We have developed theFET based compact RF negative ion source andmeasured basic plasma parameters. H- ion beam
were accelerated up to 15 kV. The pulse lengthwas limited in several milliseconds because of thepower supply’s capacity. We introduced a new
CW RF power supply and started to operate itwith pulse length of several seconds. In this paper,
the experimental results of measured basic
plasma parameters and accelerated H- ion beamwith longer pulse operation are presented.

2. Experimental setup
Experiments were performed using a small ion
source with a cylindrical driver section. An RF coil
was wound around the cylindrical tube made of
Al2O3 ceramic (inner diameter: 70 mm, outer
diameter: 80 mm, length: 170 mm). The ceramic
tube (driver region) was attached to a cylindricalchamber (expansion region). The schematic of theexperimental setup is shown in Figure 1. Axial
magnetic field up to 200 G can be applied by
Helmholtz magnetic coils attached at the driver
region. Electron density and temperature weremeasured by a Langmuir probe at the driver and theexpansion region. The H- ion beam was extracted
and accelerated with extractor attached to theexpansion region up to 15 kV.

Fig. 1. Schematic of experimental setup.

3. FET inverter power supply
A CW RF power supply with a full bridge typeinverter circuit is used for RF plasma production(Fig. 2). A dozen of MOSFET (PDM755HA) isused as a switching device in the inverter circuit.
An operation frequency can be adjusted by PLL(phase-locked loop) circuit in the range of 0.2-0.5
MHz. As a matching frequency is chosen by thePLL circuit automatically, a matching circuit is notnecessary. Total RF power of more than 20 kW canbe delivered to the antenna.

OutputCooling

FET Invertercircuit1mFig. 2. Photo of inverter unit of the CW RF power
supply.

4. Experimental Results
Firstly we tried to produce plasma and extract thebeam with the pulse length of 1 second. Fig. 3
shows the time traces in an RF operation. When theRF power was applied, the PLL circuit
automatically chose the matching frequency byfeedback system using the signal of switching
timing of current and voltage. Subsequently, the1 sec

Fig. 3. Time traces of RF operation with beam extraction.
p=0.78 Pa, Vext=3 kV, Vacc=5 kV, Hydrogen plasma.

plasma was produced and H- ion beams wereextracted and accelerated with 3 kV of theextraction voltage (Vext) and 5 kV of theacceleration voltage (Vacc). The RF source weregrounded and isolated from the source by anisolation transformer. The beam extraction andacceleration can be operated with total applied
voltage of 15 kV, which is limited by the isolation
voltage of the transformer. The pulse length of thebeam can be extended with improvement of watercooling system.
5. Summary and future plan
The long pulse operation of FET based RF Hion source is on going by using a new CW source.
In near future, the pulse length is going to beextended to about 100 seconds by improving thecooling system. The relation between basic plasma
parameter, cesium behavior and production
efficiency of H- in the plasma discharge are going
to be evaluated in detail with OES [8] and CRD
system [9].
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