22P160-P
Production and beam extraction of H- ions in a RF negative ion source
高周波負イオン源における水素負イオン生成とビーム引き出し特性
Takuya Funaoi1, Tetsuro Matsuno1, Nozomi Tanaka1, Akira Ando1,Katsuyoshi Tsumori2 and Yasuhiko Takeiri2
船生拓矢1，松野哲郎1，田中のぞみ1，安藤 晃1，津守克嘉2，竹入康彦2
1

Department of Electrical Engineering, Tohoku University
6-6-05, Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan
東北大学工学研究科 980-8579 仙台市青葉区荒巻字青葉6-6-05
2
National Institute for Fusion Science322-6, Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所 509-5292 土岐市下石町322-6A high density negative hydrogen/deuterium ion source based on RF plasma is required for ITER-NBI
reference source. We utilized a MOSFET-based inverter power supply with lower frequency (~0.3 MHz)and high density RF plasma was produced. Produced H- ions are extracted by DC high voltage up to 15 kV
with cesium seeding into the source. Extraction and acceleration current increased with extraction voltage
and RF power. Characteristics of high density negative hydrogen ion production and high voltage H- beam
extraction are presented.

1. Introduction
High density negative hydrogen ion sources are
successfully developed for Neutral Beam Injection(NBI) sources for fusion devices [1,2]. For heating
and current drive, ITER requires a 1 MeV negative
deuterium (D-) beam with pulse length of 3600 s
and current intensity of 40 A with the sourcepressure of 0.3 Pa. RF ion sources are chosen as
ITER reference sources for maintenance free, longpulse operation and low cesium (Cs) consumption
operation.
Plasma production by using RF waves has beenutilized in various industrial applications, where RFwave with the frequency of 13.56 MHz is oftenused. Relatively lower frequency of around 1 MHz
is feasible for high density plasma production
because of large skin depth. A RF ion source using
around 1 MHz RF frequency has been operated
successfully at IPP Garching [3,4]. Vacuum tube
oscillator is conventionally used for the RF power
supply for these high power operations. Recent
development of metal oxide semiconductor field
effect transistors (MOSFET) enables us to use an
inverter circuit for RF power supply. This FET
switching inverter power supply has the advantagesof higher switching efficiency and easier frequency
matching system compared to the vacuum tube
based RF power supplies [5,6]. RF plasma
production using a RF frequency lower than 1 MHz
is somewhat difficult in low gas pressure because
passing length of the accelerated electrons during
one cycle of RF field becomes larger than thedevice scale. Validation of plasma discharge in low
gas pressure using lower RF frequency is one of themain subjects in the experiment. We havesuccessfully obtained an RF plasma in the pressureof 0.3 Pa without any help of gas puffing andfilament emission for startup the discharge. In thispaper, characteristics of H- beam extraction fromthe RF ion source are presented.
2. Experimental set up
Figure 1 shows the schematic of the experimental
setup for the experiment of a FET-based negative
ion source. The ion source consists of a driver
region and an expansion region. In the driver region
RF plasma is produced in an Al2O3 ceramic tube
(inner diameter: 70 mm, outer diameter: 80 mm,length: 170 mm). RF power (~0.3 MHz, ~20 kW, 5ms) is applied to an antenna coil wound around theceramic tube. RF matching is adjusted by LC circuitby changing the RF frequency and capacitance. Inaddition, an outer Helmholtz coil forms an axial
magnetic field up to 22 mT. High density plasma

Fig. 1 Schematic of the experimental setup

3. Experimental results
RF hydrogen plasma was produced successfully at
low pressure of 0.3 Pa. ne was measured by theLangmuir probe and ne increased with increasing
RF power and axial magnetic field. The effect ofmagnetic filter field was confirmed. Withoutmagnetic filter field, ne was up to ~1018 m-3 and Te
was ~5 eV. With magnetic filter field, Te decreased
to 1 eV and ne was about 1017 m-3.
Produced H- was successfully extracted at the highvoltage of 15 kV without any breakdown.
Extraction current (Iext) and acceleration current(Iacc) increased with the increase of the extraction
voltage (Vext) and RF power. Fig. 2 shows thedependences of the extraction current and theacceleration current on the extraction voltage. WithCs vapor seeding, each current increased. In thisoperation P.G. was heated up about 200 degrees.
With the Cs seeding, extraction current was about
400 A/m2 and acceleration current was about 200
A/m2 (extraction aperture area is ~6.4 x 10-5 m-2).
The e-/H- ratio evaluated from the ratio of (Iext Iacc)/Iacc was ~1 and the ratio decreased with Cs
vapor seeding.
I (w/o Cs)

I(w/o Cs)

I (with Cs)

I(with Cs)

500

acc

ext

acc

250
200

300

150

200

100

10050acc

I

ext

22400(A / m )ext

I (A / m )can be produced with an electron density ne of ~1019 m-3 in the driver region with axial magnetic
field of around 20 mT. It expands into theexpansion region, where a magnetic filter field (~8mT in the center) is formed by a set of permanent
magnets attached at the outside of the expansion
chamber. The filter field decreases an electron
temperature Te near the plasma grid (P.G.). Plasma
parameters near the extraction apertures affect thecharacteristics of H- production and beam extraction.
A Langmuir probe is placed near the extraction
apertures and ne and electron temperature Te aremeasured.
Produced negative hydrogen ions are extracted bythree electrodes consists of molybdenum plasma
grid, copper extraction and acceleration grids. Eachgrid has 9 extraction apertures of 9 mm in diameter.
The distances between the plasma grid and theextraction grid and between the extraction grid andthe acceleration grid are 2 mm and 5 mm,respectively. The extraction grid has a set of
permanent magnets and up to ~50 mT of magnetic
field is applied for the deflection of co-extracted
electron beam. The extraction voltage andacceleration voltage can be applied up to 10 kV and15 kV, respectively. The RF source is grounded.
The ion source chamber and power supplies are seton the DC high voltage stage. RF power is applied
via an isolated transformer (1:1). A Faraday cup isattached downstream of the source. To evaluate theH- beam intensity accurately, calorimetricmeasurements are going to be started.
A Cs oven is equipped on the expansion chamber
to seed a small amount of Cs vapor. This oven andP.G. temperature are heated up by sheath heater. P.G.
is covered by thin Cs layer and the work function of
the grid surface decreased for the effective surface
production of H-.

022.533.5 4 4.5V (kV)

505.5

ext

Fig. 2 Dependences of the extraction and acceleration
current on the extraction voltage

4. Summary
We have fabricated a high voltage beam extraction
system in a FET-based compact RF negativehydrogen ion source. High density RF plasma wassuccessfully produced with low gas pressure. A Hbeam was extracted with the high voltage of 15 kV.
Extracted H- beam currents were measured
electrically and the increase of beam currents byseeding a small amount of Cs vapor was confirmed.
Evaluations of the extracted H- beams using acalorimeter are going to be started.
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