JAERI-Conf 95-021

1a-2
DEVELOPMENT OF HIGH POWER NEGATIVE ION SOURCESFOR FUSION AT JAERIY. Okumura

Japan Atomic Energy Research InstituteNaka-machi, Naka-gun, lbaraki-ken, 311-01, Japanin JAERI/CEA Joint Experiment at an low operating gas
pressure of 0.3Pa [4,5]. Long pulse o~eration wasdemonstrated at 50keV, 0.3A (>lOrnA/em ) for a pulse
duration of 24 hours in a small H- ion source [6], which
has a same design concept as the lOA source. Highenergy H- ion beams of 350keV, 0.5A were produced
using a three-stage electrostatic accelerator [7]. Beamlet
divergence of as low as 5mrad was obtained at 400ke Vwith a current density of 13mNcm2 [7].
Based on these successful results, two R&D
projects have been initiated at JAERI. One is to develop
a 22A, 500keV, lOs D- ion source for the N-NBI system
for IT-60U [8]. The other is the development of a lMeV
negative ion source/accelerator for the demonstration ofhigh energy acceleration of ampere-class negative ion
beams to lMeV [9], the energy required for ITER.
In the present paper, after a brief description ofthese two R&D projects, present status of thedevelopment is reviewed together with a future plan ofthe development.

ABSTRACTTechnologies producing high power Illegative ion
beams have been highly developed in these years at
JAERI for use in neutral beam injectors for heating thethermonuclear fusion plasmas. At present, it is possible
to produce multi-ampere H-ID- ion beams quasicontinuously at energies more than a few hundred ke Vwith a good beam optics of beamlet divergence of a few
mili-radian. Based on these technologies, two R&D
projects have been initiated; one is to develop a
22N500keV/10s D- ion source for the neutral beam
injector for IT-60U, and the other is to develop a
lN1MeV/60s H- ion source to demonstrate high·current
negative ion acceleration up to the energy of lMeV, theenergy required for the neutral beam injector forInternational Thermonuclear Experimental Reactor(ITER).
1. INTRODUCTION
Negative ion based neutral beam injector (N-NBI)
is one of the most promising candidates for the current
drive and heating system in steady state operation of ·reactor-grade tokamak fusion devices, such as theInternational Thermonuclear Experimental Reactor(ITER) [1]. The most crucial issue in realizing theinjector is to develop a high power negative ion source
which can produce negative deuterium ion beams with acurrent of several tens of amperes at an energy of 0.5-2
MeV. The ion source required for ITER, for example,
has to produce 40A D- ion beams at an energy of lMeV
for a pulse duration of more than 1OOOs with a beam
divergence of less than 5mrad [1,2]. The current densityshould be more than 20mNcm2 to make the size of theN-NBI system reasonable. In addition, the source shouldbe operated at a low operating gas pressure of less than
0.3Pa to reduce the stripping loss of negative ions in theaccelerator and to have a reasonable acceleration
efficiency.
Although these specifications are far beyond thespecifications of existing negative ion sources, basic
performances have already been achieved individuallyby the negative ion sources developed at JAERI. Namely,
H- ion beams of lOA, 50keV were extracted with anenough current density of 37 mNcm2 [3], and D- ionbeams of 2.2A, lOOkeV, 5s (10 mNcm2) was produced-2. D- ION SOURCE FORJT-60U
A 500keV, lOMW N-NBI system has been
constructed for studying of mega-ampere level NB
current drive and plasma core heating experiments witha high density reactor-like plasma in IT-60U. It is thefirst neutral beam injector in the world using negativeions as the primary ions. The system is designed for
injecting lOMW deuterium or hydrogen neutral beams
for lOs beam duration with a duty cycle of 1/60. TheNNBI system consists of a beamline ·equipped with two
ion sources, a power supply, a control system, and an
auxiliary sub-systems that includes a cooling watersystem, cryogenic refrigeration system, and auxiliary
pumping system. Each ion source has a cesium-seededvolume negative ion generator, a multi-aperture
extractor and a three-stage electrostatic accelerator.
Figure 1 shows a cross-sectional view of the ion source.
The source is designed so as to produce 22A, 500keV,
lOs D- ion beams with a current density of 13mNcm2[10,11]. The dimensions of the plasma generator are
64cm in diameter and 122cm in length. Produced
negative ions are extracted from 1080 apertures of
14mm in diameter, which distributed within the area of
45cm x llOcm.
Out of the two ion sources, first ion source has been

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designed and fabricated. After successful performance
tests in test stands, the ion source was installed in thebeamline of the N-NBI system and full power test has
just started. Up to now, the source has been successfully
operated and produced a 400keV, 5.9A, O.ls D- ionbeam [12], the world highest D- current and beam power.
The full power test will be finished by the end ofOctober 1995, and the second ion source is to beinstalled in the beamline by January 1996. NB injection
experiment is scheduled to start from April1996.
3. PROTOTYPE ACCELERATOR FOR ITER
The negative ion beam energy required for ITER is
as high as lMeV to penetrate the beam into the highdensity, large core plasma of ITER. Although multiMeV beams have been produced in the electrostatic
accelerators used for high energy physics, the current isless than an order of rnA. There is no experiment toaccelerate· ampere-class particle beams, including
positive ion beam and electron beam, above the energyof several hundred ke V. A demonstration of the ability
to accelerate the ampere-class negative ion beams is,therefore, the most critical and urgent R&D item for
ITER. Electrostatic acceleration will be employedbecause of its high electrical efficiency.
To develop the lMeV negative ion accelerator, wehave constructed a new test stand called MeV Test
Facility (MTF) [9]. MTF has an acceleration power
supply which has a capacity of lMV, lA, 60s, the world
biggest Cockcroft-Walton circuit, and the power supplies
for negative ion production and extraction. These highpotential power supplies are installed in the SF6 gas
vessel with the a lMe V prototype accelerator for ITER.
In the prototype accelerator, negative ions createdin the 'KAMABOKO' type negative ion generator [13]are accelerated by five-stage electrostatic accelerator.
Up to now, the accelerator was conditioned up tothe voltage of 7 50 kV. Hydrogen negative ion beamswere accelerated after the accelerator conditioning. The
H- beam was successfully accelerated up to an energy of700 keV with an acceleration drain current of 230 rnA
for a duration of ls. The highest drain current of 360
rnA was obtained at 600 keV.

REFERENCES[1] R. Hemsworth et al.,"Neutral Beams for ITER", 6thInt. Conference on Ion Sources, Whistler, B.C., Canada,Sept.l0-16, 1995.
[2] Y. Okumura et al., JAERI-Tech 95-018 ,1995[3] Y. Okumura et al., Proc. 16th Symp. on Fusion
Technology, London, 1990, p.1026.
[4] K. Watanabe et al., Proc. 6th International Symp. onProduction and Neutralization of Negative Ions andBeams, BNL, 1992, p.326.
[5] J. Pamela et al., Nuclear lustrum. and Methods B73(1993) 289.
[6] Y. Okumura et al., Rev. Sci. lustrum. 63 (1992)
2708.
[7] K. Miyamoto et al., Proc. 18th Symp. on Fusion
Technology, 1994, p.625.
[8] M. Kuriyama et al., Fusion Engineering and Design,
26 (1995) 445.
[9] M. Hanada et al., Proc. 18th Symp. on Fusion
Technology, 1994, p.617[10) Y. Okumura et al., Proc. 15th Symp. on Fusion
Engineering, Hyannis, 1993, p.466.
[11] Y. Okumura et al., Proc. 6th International Symp.
on Production and Neutralization of Negative Ions andBeams, BNL, 1992, p. 839.
[12] Y. Okumura et al., "Development of a 500keV, 22A,
D- Ion Source for the Neutral Beam Injector for IT-60U",
6th Int. Conference on Ion Sources, Whistler, B.C.,
Canada, Sept.l0-16, 1995.
[13] Y. Okumura et al., "High Power Negative Ion
Sources for Fusion at JAERI", ibid.

4.SUMMARY
Figure 2 shows the negative ion beam currents andenergies achieved in the existing ion sources.
Specifications at which the N-NBI system for IT-60Uand MTF aim are also shown in the figure together withthe final target for ITER. The large negative ion sourcefor IT-60U and the prototype accelerator in MTF are thetwo critical steps for lMeV, 40A, CW ion
source/accelerator for ITER.

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Plasma Generator/Filament~~~~.-.~~'
111-'-.....--nf'--J

PG Filter
FeedthroughInsulator

2m

Cross-sectional view ofD- ion source for JT-60U N-NBI

Fig. I

100A........

cQ)''::J0E10AroQ)

Illc-0Q).>........
ro1AC>Q)z

10 keV

100 keV

1 MeV

10MeV

Beam Energy
Fig.2

Status of high power negative ion beam development-4-