JAERI-Conf 95-021

PA09
Development of a High Brightness Negative Hydrogen Ion Source
H. Oguri, Y. Okumura, N. Miyamoto, M. Mizumoto and J. Kusano
Japan Atomic Energy Research InstituteTokai-mura, Naka-gun, lbaraki-ken 319-11, JAPANABSTRACT

2. SINGLE APERTURE EXTRACTION

Development of a high brightness negativehydrogen ion source has been performed for a highintensity proton linear accelerator. Negative ions aregenerated in a magnetically filtered multi-cusp
plasma generator. The negative ion production isenhanced by seeding a small amount of cesium in theplasma generator. As the result of the ion source
beam test, negative hydrogen ion beam of 36 rnA (23mA/cm2) was extracted from a single aperture at anacceleration voltage of 50 kV. To obtain higher ion
beam current, the focusing of beamlets extracted
from multi-aperture grid has been demonstrated withaperture displacement technique.

1. INTRODUCTION
At JAERI, construction of a 1.5 GeV/10 rnA
proton linear accelerator has been proposed forengineering tests of accelerator-based nuclear wastetransmutation and for various basic science
researches [1]. A high brightness positive hydrogenion source was fabricated for the accelerator. The ion
source has been successfully operated at the full
design value of 100 keY and 140 rnA peak [2]. At
the next stage of the ion source development, anegative hydrogen ion source has been newly
designed and fabricated. Negative ion beam isrequired mainly for basic researches to inject thebeam into the storage ring which produces certain
specific pulse duration and repetition rate at the highenergy portion of the accelerator.
The basic performance of single aperture
beam extraction system was investigated with thevolume negative ion source which has beenoriginally developed for the neutral beam injector forfusion application [3]. The multi-aperture beam
extraction with aperture displacement technique wasalso tested with the modification of the positive ion
source which was used for previous beam
performance experiments [2].

Ion Source
Figure 1 shows a cross-sectional view of thevolume production type ion source. The plasma
generator, whose dimensions are 340 mm indiameter and 340 mm in length, has a large semicylindrical volume and strong magnetic cusp field.
The source plasma is produced by arc dischargeusing eight tungsten filaments. The filaments aresupported by molybdenum chips on coaxial
feedthroughs. The plasma is confined by strong
multicusp magnetic field. A magnetic filter, which isformed by Sm-Co permanent magnets, divides thegenerator into two regions and modifies electronenergy distribution so as to produce negative ion.
Negative ion production rate is enhanced by seedinga small amount of cesium in the plasma generator.

Extractioo Electrode

Grounded ElectrodeFig.l Cross sectional view of the negative ion sourceThe beam extractor consists of four electrodes
such as a plasma electrode (PE), an extraction
electrode (EXE), an electron-suppression electrode(ESE) and a grounded electrode (GE). Negative ion
is extracted from a single aperture of 14 mm in-46-JAERI-Conf 95-021

diameter. The gap length are adjusted to 5.1 mm
between the PE and EXE, 2.5 mm between EXE andESE, and 30 mm between ESE and GE. The plasma
electrode is made of molybdenum plate. There was astrong dependence of the negative ion production
rate on the plasma electrode temperature. This isbecause the cesium coverage is optimized by thetemperature rise to give a minimum work function of
the plasma generator surface. The plasma electrodewas heated up at 200 - 300 oc by pulsed arc
discharge power and its temperature was controlled
by the duty cycle of the power. The temperature wasmonitored by a thermocouple. The extraction
electrode is made of a 11 mm thick copper plate witha water cooling channel and magnet grooves. In theextraction electrode, Sm-Co permanent magnets areinserted so as to produce a dipole magnetic field.
This field deflects the extracted electron and preventsthe leakage of the electron to the acceleration gap.
The electron-suppression electrode is installed fortrapping the leakage electron escaping from theextraction electrode. By applying deceleration
voltage of a few kV, electron leakage is suppressedefficiently.
Extracted Ion Current
For measurements of the beam current andprofile, a multichannel calorimeter was installed at
1.4 m downstream from the ion source in the beamdiagnostics chamber, where no electrons reached tothe calorimeter. Figure 2 shows the negative ioncurrent as a function of the arc power at fillinghydrogen gas pressure of 10 mTorr. The beam
current increased with the arc power and reached to36 rnA at 45 kW. The arc efficiency which is definedby the ratio of the beam current to the arc discharge
power is calculated to be 0.9 mA/kW.

50
Ps=10mTor~

Pvac=6x1o· Torr
TpG=290C~.s 40-Cs seeded
00

1::
Q)
.....
.....

:J30(.)

1::..Q20Q)

~cuC)
Q)

z10002010304050

Arc Power [kW]Fig.2 Negative ion beam as a function of arc
discharge powerelectrode, respectively. These two components havedifferent intensity distributions as a function of thebeam divergence. The normalized emittance of thebeam from the central region was calculated to be 1.1:rcmm.mrad. Such a large amount of the beam fromthe peripheral region is due to aberration of beam
optics and may be decreased by optimizing the beam
extractor,

Vacc=SOkV

60

IH-=20mA·5

Pvac=8x1 0 Torr

40!:0Beam Optics~ 20The emittance of the negative ion beam wasmeasured by using a double slits with a Faraday cup
system. To prevent the beam from spreading by spacecharge force, the vacuum pressure in the beamdiagnostics chamber was kept at 8 x 10-5 Torr where
beam neutralization effect is expected for negative
ion beam. Figure 3 shows a typical emittancediagram at an acceleration voltage of 50 kV and abeam current of 20 rnA. As shown with solid anddotted lines in the figure, there are two beamcomponents corresponding to those which areextracted from central and peripheral region of the.§.

>- 0
E=1.1Jtmm.mrad(norm.)-20-40-40-200204060

Y [mm]Fig.3 Emittance diagram at an acceleration voltageof 50 kV-47-JAERI-Conf 95-021

3. MULTI APERTURE EXTRACTIONREFERENCES

The multi-aperture beam extraction withaperture displacement technique was tested. Thebasic design of the ion source is the same as thepositive hydrogen ion source [2] except for theexistence of the transverse magnetic field, which iscreated by changing the polarity of the cusp magnets
near the plasma grid. A conceptual illustration of theaperture displacement technique is shown in Fig. 4.
The peripheral apertures on electron-suppression
grid and grounded grid are slightly displaced, andthe negative ion beams extracted from peripheral
apertures are steered by electrostatic lens. Eachbeamlet is merged into a single beam at the entranceof the accelerator.
In the preliminary beam test, the beam
trajectories were measured by observing the Balmeralpha light emission from the negative hydrogen ion
with CCD camera. It has demonstrated that thebeamlets from seven apertures were successfully
merged into single beam at the focusing point of
about 1 m downstream of the ion source.

[1] M. Mizumoto et al., "High Intensity Proton
Accelerator for Nuclear Waste Transmutation",
Proceedings of the 1992 Linear Accelerator
Conference, Ottawa, Canada, AECL-10728 (1992)
749[2] H. Oguri, et al., "A High Brightness Hydrogen
Ion Source for the BTA at JAERI", Proceedings ofthe 1994 International Linac Conference, Tsukuba,
Japan (1994) 381[3] T. Inoue et al., " A Merging Preaccelerator forHigh Current H- Ion Beams", Rev. Sci. Instrum., 66(7), 3859 (1995)~Plasma Grid~t=I

=1==r:::::pFI••

Extraction Grid
Electron-Supp. GridII•

F.•••

Grounded Grid

Fig.4 A conceptual illustration of the multi-aperture
displacement technique

4.SUMMARY
The beam test of the negative hydrogen ion
source has been performed. The extracted ion beam
current (density) from a single aperture of 14 mm indiameter was 36 rnA (23 mNcm2) at the arc
discharge power of 45 kW. The test of the aperture
displacement technique has also been performed.
The preliminary test proved that the technique has apossibility to produce the high brightness beam. TheR&D work of the technique is continued.

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