Proceedings of ITC/ISHW2007

Development of radial neutral beam injection system on LHD
M.Osakabe, K. Nagaoka, K.Tsumori, Y.Takeiri , K.Ikeda, Y.Oka,M.Sato, T.Kondo, M.Shibuya, E.Asano,S.Komada, O.Kaneko and LHD experimental group
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan
A radial Neutral Beam Injector (NBI) is newly installed on the Large Helical Device (LHD). The aims of the NBI are its
usage as a diagnostic NB for charge exchange recombination spectroscopic measurement and using the NB as a heating
source for ions in plasmas. A new positive-ion source was developed for this NBI at NIFS. The structure of the cusp
field of the source was determined by the numerical code and its performances were verified by experiments. The
performances of the developed source fulfill its specification. Especially, the maximum beam current of 102[A] exceeds therequirement of 75[A] about 33[%].
Keywords: NBI, large positive ion sources, radial beam injection

1. Introduction
A positive-ion based Neutral Beam Injector (NBI) is
newly installed on the Large Helical Device (LHD).
The major purposes of installing this NB on LHD are; (1)to provide a tool for ion temperature profile measurement
and for electric field profile measurement by using thisNB as a diagnostic beam for Charge eXchange
Recombination Spectroscopic (CXRS) measurement, (2)to achieve the high ion temperature regime on LHD by
producing peaked density profiles with central beamfueling and by an intensive ion heating with low energy
and high power beam, and (3) to use this NB as a probe
beam for investigating the confinement property ofperpendicular fast ions on LHD.
In this article, we will show our research anddevelopment results for the new positive ion-source forthe radial NBI. In designing the ion-source, thethickness of a plasma-electrode, which is an electrodefacing to the ion-source plasma, was the most important
issue since it significantly affects the optics of extracted
ion-beam. Considering the beam-optics, the thicknessof the electrode is preferred to be as thin as possible.
But, in the actual case, the electrode needs to have finite,i.e., non-zero, thickness so that it can have enoughstrength to avoid the distortion by the gravity and by
thermal heat loads. The required thickness depends on
the heat loads onto the electrode, and also on the materialand cooling structure of it.
The arc-efficiency is also an important parameter ofion-sources, where the efficiency is defined by the ratioof extracted beam current to the required arc-power insustaining ions-source plasmas. Since the heat load
onto a plasma electrode can be considered to be
proportional to the arc-power, the increase of thearc-efficiency will reduce the heat load onto the electrode.
Thus, achievement of high arc-efficiency will help toauthor’s e-mail:osakabe.masaki@lhd.nifs.ac.jp

reduce the thickness of a plasma electrode.
The specification of the radial-NBI on LHD and its
ion-sources are briefly discussed in Sec.2. The designof cusp-field configuration for the ion-source is shown in
Sec.3.
The determination of its plasma-electrode
thickness is shown in Sec.4. The operation of thesource at the NBI are shown in Sec.5. The section 6 is asummary.
2.

Specification of the radial-NBI and its ion-sources
Figure 1 shows the schematic drawing of theradial-NBI on LHD. Its specifications are shown in
Table 1.
As shown in this table, the required
performances on the ion-source is almost comparable tothose of the giant positive ion sources for NBI [1-3].
a)Coilcenter

Focal
length
8.3mb)

Fig.1 Schematic drawing of the radial-NBI on LHD.
(a) Top-view and (b) side –view.

Proceedings of ITC/ISHW2007
Table 1 Specification of LHD radial-NBI
Beam species
HydrogenInjection energy
40 [keV]
Total Injection power
6 [MW]
Beam pulse duration
10 [s]
Injection port-size
φ0.8[m]Size of NB-protection(W)1[m] x (H)1 [m]armor at the injection
counter wall
Focal length and pivot
8.3[m]
length of ion-sourcesType of ion source
Positive ion-source
Accele-decel-ground
Electrode systemelectrodes system.
Number of ion sources
4
Beam current required for75[A]single ion source
Beam extraction area(W)0.21[m] x (H)0.5 [m]Transparency of the35[%]
electrodeThe injection energy was chosen to be 40-keV with
Hydrogen beam ion species.
This energy wasdetermined by the requirement of CXRS measurement.
To have enough penetration of NB into the core-region of
LHD plasmas, the beam was injected radially to the LHD
torus, thus the injected fast ions has its kinetic energy
mostly on the perpendicular direction to themagnetic-field lines.
One of the largest limitations in designing aradial-NB injector on LHD is the small-protection area ofits injection counter wall and the limitation on the size ofinjection-port. The former limitation comes from thefact that there is no enough space left in the inboard sideof LHD vacuum vessel to install protection armor tiles
Port-Through Efficiency, Armor-In Efficiency

1 .2

10 .80 .60 .40 .2

05 .5

66 .5

77 .5

88 .5

P iv o t a n d F o c a l L e n g th [m ]Fig.2 Calculated port-through efficiency (solid lineswith closed circles) and armor-in efficiency(dashed lines with open circles)of the radial-NB.

with cooling structures at the toroidal section whereplasmas are vertically elongated. At this location, thevacuum vessel walls, which cover the helical coils,
significantly project to the LHD-plasma. Therefore, itis very important to consider the injectors’s configuration
which maximize the armor-in efficiency as well as
port-through efficiency. Here, armor-in efficiency isdefined by the ratio of the beam power which goes into
the inside of the protection armor plates to the total
neutral beam power. The pivot-length and focal-lengthof the ion-sources were determined to maximize thearmor-in efficiencies and to minimize the difference of
these two efficiencies, simultaneously. It was foundthat the focal length is better to be equal to the pivot
length of the ion-sources to maximize the armor-inefficiency. From Fig.2, the pivot and focal length of
8[m] is determined to be the best, but they were set to8.3[m] at the final design to avoid the interference withthe diagnostic ports for CXRS-measurements.
3.

Design of an arc-chamber of the ion-source for theradial-NBI
Several structures of cusp magnetic-field weresurveyed for the ion-source of the radial-NBI using acode developed by Tsumori. et.al.[4,5]. Among them,two of the configurations were chosen as candidates forthe ion-source. The calculated distributions of cusp
field lines and those of primary electrons from filaments
are shown in Fig.3. The calculation shows that type-A
configuration has better confinement than type-B, i.e.,the 63% of electrons launched from filament locations
are confined for type-A, while only 15% are confined for
type-B. On the other hand, type-B configuration has
more electrons in the region close to the plasma electrodeand better uniformity in this region than type-A as shownin Fig.3(c) and (d).
A series of experiments to compare thearc-efficiencies of these two configurations was performed
at NIFS-NBI test-stand since it was very difficult todetermine which cusp configurations are better in theefficiency just from the simple orbit calculations. The
arc-efficiency of 0.44[A/kW] were obtained for type-A
configuration, while that of 0.35[A/kW] are for type-B as
shown in Fig.4. We must note that the half of theplasma-electrode was masked to limit the beam current inthese experiments since the acceleration power supply at
the test-stand can only handle the beam current of 40[A].
Thus, the arc-efficiencies in the test-stand experimentbecame about a half of those for non-masked configuration.
No significant differences were observed on beam-profiles
between the two cusp configurations. It was confirmedby experiments that the confinement of primary electrons
was more important than the uniformity and the numbers

Proceedings of ITC/ISHW2007a)

2515I

beam

[A]

20105b)00

102030

P

arc

40

5060

[kW]

Fig.4 The comparison of arc-efficiency for type-A(closed-blue-circles) and type-B (open-redcircles) cusp configurations.
4.

Determination of plasma electrode thickness of theion-source for the radial-NBIc)d)

As a plasma electrode of the ion-source, we haveadopted a oxygen-free copper electrode withwater-cooling channels which is placed to each raw ofbeam extraction holes. The thickness of the electrode isrequired to be greater than 4-mm from the view point of
mechanical strength and heat load handling, while this is
to be less than 3.3-mm from the view point ofbeam-optics. To overcome the conflict, experiments ofbeam extraction using electrodes of both thicknesses
were also performed at the NBI test-stand. For these
two electrodes, the perveances, which express the beam
currents normalized by acceleration voltages according to40Fig. 3 Calculated magnetic filed lines for (a)type-Aand (b)type-B cusp configurations. Magnetic
field lines are shown by red lines and thelocation of permanent magnets are by green
lines in these figures.
Primary electron
distribution are shown by red-dots in (c)type-Aand (d)type-B.
The z=0 lines show thelocation of plasma electrode.
of them in the region near plasma electrode by theexperiment. Thus, the type-A configuration is adopted as
the cusp configuration of the ion sources for the radial
NBI.

Beam 1/e Width [cm]35302520
1510500 .0 40 .0 60 .0 80 .1

P erve a nc eFig.50 .1 2
[A /k V0 .1 41 .50 .1 6]

Perveance dependence of beam profile width.
The beam widths for a 4-mm thickplasma-electrode are shown by the open-redcircles, while those for a 3.3-mm thick electrodeare by closed-blue-circles.

Proceedings of ITC/ISHW2007200 .2

150 .1 5

100 .1500 .0 40 .0 50 .0 50 .0 60 .0 70 .0 800 .0 9H e a t L o a d o n to p l a s m a -e le c tr o d e [M J ]Fig.6 Dependence of beam widths on the heat loads ofplasma-electrode. The beam widths are shown by
open-red-circles, the perveances of the beam arealso shown by closed-blue-circles.
Operation of ion-source at the radial-NBI
The acceleration power supply of the radial-NBI is
designed to operate two ion-sources with single powersupply. With the operatin of single ion-source with thispower supply, the maximum beam current can beevaluated (Fig.7). The arc-efficiency of the source was

100

250

beam

[A]

8020060

150

40

100

205000204060
Pa rc

80

100

2300

Current Density(avg.) [mA/cm ]1200120[k W ]

Fig.7 Arc-discharge power dependence of extracted
beam current of the source at the radial-NBI.
6.

Summary
A radial-NBI using positive ion-sources was installed
and is successfully operated on LHD. To fulfill thespecifications of the NBI, a giant positive ion source was
newly developed at NIFS. The performances of thedeveloped source fulfill its specification. Especially, themaximum beam current of 102[A] exceeds therequirement of 75[A] about 33[%].

Acknowledgement

1.5

Beam Width

1/e[cm]0 .2 5Perveance [A/kV ]25

0.78[A/kW] in thses experiments. The maximum beam
current of 102[A] was achieved at the acceleration voltageof 40[kV].
The averaged current density exceeds
2250[mA/cm ] with this maximum beam current. Themaximum of the current was limited by the operational
limit of arc-discharge, i.e., arc-discharges of the ion-source
became unstable when the arc-power exceeded 120[kW].
The optimum perveance was examined using thecalorie-meter array in the injector. It was 0.28[A/kV1.5]
when the gap distance (dgap) was set to 5.5[mm]. The
total injection power of 7.2[MW] was achieved withsimultaneous four ion-sources operation.

Ithe Child-Langmuir law, were scanned to find theiroptimum value where the beam widths have theirminimum in the experiments (Fig.5). The distance of thegap between a plasma-electrode and a decel-electrode was6.5[mm] at the experiments for the 3.3[mm] thickelectrode, and was 6[mm] at those for 4[mm] thick one.
As shown in Fig.5, the optimum perveance for theelectrode of 3.3[mm] thickness is larger than that for the4[mm] thickness.
Thus, the electrode of 3.3[mm]thickness has preferable feature, as is expected. Taking
the gap(dgap) dependence of the Child-Langmuir law,where the beam current is proportional to dgap-2, into anaccount, this tendency becomes more significant.
The distortion of plasma-electrode by the heat loads
can be evaluated by the change of beam profile with theheat loads. The dependence of beam widths on the heatloads of the plasma electrode are shown for the 3.3[mm]thick electrode in Fig.6. The heat loads were measured
by a water calorie-metric method using the cooling waterof the electrode. Scan of the heat loads are done bychanging the duration time of arc-discharges precedingwith the beam extraction. These heat loads in the figure
correspond to the beam duration time of 0.3[s] to 4[s].
To minimize the dependence of beam widths onperveances, beams are extracted at around the optimum
perveance condition. As shown in Fig.6, the dependenceof beam widths on the heat loads are almost negligible.
Therefore, it was concluded from these experiments thatthe 3.3[mm] is thick enough as a plasma-electrode of theion-source.

Authors wish to thank the continuous encouragementby the Director general (Prof. Motojima) and the project
leader of LHD (Prof. Komori). This work is supportedby the NIFS05ULBB501.

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S.Tanaka,et.al., Rev. Sci. Instrum. 57, 145(1986)
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