J. Plasma Fusion Res. SERIES, Vol. 5 (2ffi2) 47U77

lmproving the Performance of a Negative-ion Based
Neutral Beam Injector for JT-60U
YAMAMOTO Takumi, UMEDA Naotaka, KURIYAMA Masaaki, LEI Guangliul,
GRISHAM Larry2, KAWAI Mikito, OHGA Tokumichi and JT-60 NBI-TeamNaka Establishment, Japan Atomic Energy Research Institute, Naka, Ibaraki 319-1101, Japan
1
Southwestern Institute of Physics, China
2

Princeton Plasma Physics Laboratory, USA(Received: 11 December 2001 / Accepted: 2 October 2002)

Abstract
The performance of a 500 keV negative-ion based neutral beam injection system for the JT-60U has
been enhanced significantly. By improving the beam convergence, a beam injection pulse duration ofup

to 10 seconds has been attained. This is the maximum duration defined by the system design
specification. Small peaks were observed in the beam current density profile measured in the near field.
These peaks are clearly resulted from the deflection of beamlets due to an unwanted electric field
generated at an extractor. Correction ofthe beamlet deflection improved the beam convergence, resulting

in a reduced temperature rise of the beam limiter at the port of the JT-60U by more than 5O Vo. ADoppler shifted spectrum of the Da line radiated from the negative-ion beam extracted from the ion
source was measured. The Doppler shifted spectrum indicated that electron stripping of the negative ion

beam occurs primarily inside the extraction grid and in the first acceleration gap, as expected fromtheoretical considerations. Moreover, the electron stripping contributed to half of the heat load on thegrounded grid at the operating pressure of the ion source.

Keywords:
negative-ion, NBI, JT-60U, heat load of grids

acceleration voltage are all somewhat lower than thedesign specifications. To achieve optimal performance,
there are key issues that must be resolved, namely,
excess heat load of accelerator grids, low-convergence

1. lntroduction
The negative-ion based neutral beam injection (N-NBI) system for the JT-60U has been in operation since1996 for plasma core heating and non-inductive current
drive in higher density plasma [1]. The design capabilityof the N-NBI system is a beam energy of 500 keV withan injection power of 10 MW for a duration of up to 10seconds. Practically, the beam power has been increased

beams and low holding acceleration voltages. A lowconvergence beam yields an excess heat load on thebeam limiter near a beam focusing position of 24 mfrom the ion source, as shown in Fig. l. Consequently,
the temperature rise of the beam limiter restricts theinjection beam power and the pulse duration. On theother hand, it is thought that the main heat load on thegrids is due to electron stripping of the negative-ion

gradually through optimization of the operation
parameters of the ion source. The injection beam powerof the JT-60U plasma has been reported previously tohave reached 5.8 MW at 400 keV for 0.86 seconds [2].
To date, the beam power, pulse duration and operating
C o rre sp ondin g autho r' s e - mail : y amamo to @ naka

j ae r i. g o j p474

@20O2 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchYamamoto T. et al., lmproving the Performance of a Negative-ion Based Neutral Beam Injector JT-60U> Spectrorn eter(Dcr line)

Bendingcoillon dumpFig. 1 Cross section view of the beamline and the set up for the measurement of the beam profileand Dd line.
(a)beam in their passage through the grid structure bycollision with residual gas molecules [3]. In this paper,
we describe the improvement in performance of thegroup_

neutral beam injection and investigation on the heat loadon grids.

2. lmproving Convergence of the Negativelon BeamCharacteristics of a beam profile were investigatedby measuring an infrared image on a beam target
located 3.5 m downstream from a grounded grid. Thisdistance is still within the near field of the negative ionsource, where the beam profile shows a footprint of grid

(Bofiom+ile) <i+segments. The infrared image, which reveals a twodimensional profile of the temperature on the target
surface, is thus a measure of the corresponding power
density profile of the negative-ion beam extracted fromthe ion source. Intensification along rows of apertures,
which adjoin boundaries of individual grid segments(Upper€ille)<--2m--+

Apeilure

formed each accelerator grid, was observed in theinfrared image for some operation parameters of the ionsource. A beanr orbit analysis has indicated that the peakis a consequence of the overlap of outermost beamlets
deflected by an undesirable electric field with thebeamlets radiated from the second row apertures of theneighboring segment. This electric field was generated

18unit mm

Details of section AFig. 2 (a) Cross-section of the negative ion source and(b) detail of copper pieces installed on an extrac-by small groove downstream of the extraction grid

tor.
475

Yamamoto T. et al., Improving the Performance of a Negative-ion Based Neutral Beam Injector JT-60U

segment boundaries. In order to remove the undesirable
electric field, copper pieces were embedded along thegroove as illustrated in Fig. 2.
Figures 3 (a) and (b) show vertical profiles of thetemperature rise at a center of the target in the absence
and presence of copper pieces, respectively. The dottedlines in Fig.3 indicate the corresponding boundaries offive grid segments. As shown in Fig. 3 (a), a peak
appears at an inner side of each segment boundary.
Incorporation of copper pieces into the extractor resulted
in a shift of both the peaks to the center of the gap

50 Vo compared with the uncorrected arrangement. This
subsequently led to a remarkable improvement in beam
convergence. Until now, a l0 second operation with23
MJ of injection energy had been achieved, as shown by
the double circle in Fig. 4. In addition, no appreciablereduced heat load on the grids by the correction of thebeamlet was observed.

between the boundaries of each segment, as shown inFig. 3(b). This is understood in the way that theoutermost beamlets on the neighboring segments aresteered inward and overlapped at the center of the gap

stripping of negative ion beams in their passage through3. Measurement of Beam Stripping andHeat Load on the Grids
An intense heat load on the grid results from thethe grid structure due to collision with residual gas
molecules [4]. Since the negative-ion beams areneutralized by stripping the electrons after passing

on the target. It is indicated that the installation ofelectric field which steers the outermost beamlets of thegrid segment to correct undesirable deflection.
The beamlet deflection also influenced the heatthrough differing lengths of the accelerating electrostatic
field, some of them have energies lower than thenominal accelerating voltage, producing a lower energy
continuum in the beam energy spectrum. The stripping
arises from a steeply increasing pressure in theload on the beam limiter at the port of the JT-60U,

accelerator.

whose cross-sectional area is the smallest in the path of

Doppler shifted spectra of Da line radiated fromthe neutral beam by collision with residual gas wererecorded for the different filling pressures in the arc
chamber. A pressure range of 0.19 to 0.44 Pa wasinvestigated using the experimental setup as illustrated

copper pieces at the extractor is useful for generating thethe beam. Figure 4 shows the temperature rise of thebeam limiter as a function of injection energy using abeam energy of 360-390 keV and an injection power of
2-5 MW operating with one or two ion sources, as
appropriate. The injection energy is defined as theinjection power multiplied by pulse duration. Figure 4
revealed that correction of the beamlet deflection
resulted in a temperature rise that was reduced by over400bba300

9l200ph0r00
E'n0)

o0oF400c)

300o200

F-t000510ls2025
Injection energy (MI)0-450 -300 -r50 0
Bottomr50300 450

Fig. 4 Temperature rise of the beam limiter at the injection port of JT-60U. The squres and circles aredenoted for the data before and after correction ofTopposition [mm]Fig. 3 Temperature rise on the beam target (a) without
the copper pieces, Vacc = 350 kV, lacc = 19 A andVext = 6.6 kV, and (b) with the copper pieces, Vacc
= 380 kV, lacc = 19 A and Vext = 7 kV.

the beamlet deflection, respectively. The double
circle is for 10 second pulse duration. The injection energy is referred for injection power to multiply by pulse duration.
4',16

'famamoto T. et al., lmproving the Performance of a Negative-ion Based Neutral Beam Injector JT-60U

in Fig. 1. Figure 5 shows typical Doppler shifted spectra
recorded at three different arc chamber pressures. Peaks

negative ions inside the extraction grid and the firstat Doppler-shifted wavelengths corresponding to theextraction and the first acceleration voltages in addition
to the full acceleration voltage were observed. The
spectra display clear evidence of stripping of thedependence of the grid heat loading on the intensity ofacceleration gap. Figures 6 (a) and (b) show theDa line at wavelengths corresponding to beam energiesof the extraction and the first acceleration, respectively.

Da line was normalized to the intensity of noaccelerated beam at the Doppler-unshifted wavelength .

A water calorimeter was used to measure the total heat

Vu=378 kV, VexF6.8V, lu=2v-24 A

loading of the grids. All grid heat loads were found toincrease with increasing intensity of Da line. Thegrounded grid heat load extrapolating to zero intensity is500 kW corresponding to 6 Vo of the total acceleration
power. This indicates the heat loading due to directinterception of the negative ion beam. This was also a
half of the total heat load of 1 MW, which was obtainedon the grounded grid at the operating pressure of the ion

Unshifted=0keV E6""r=3g5ke2)!

Stripping€6 zoO r5h.^ tu.=osource. Moreover, it is noted that the direct interceptionwith the first and the second acceleration grids was very

655

660

66ssmall.

670

Wavelength (nm)4. Gonclusions

Fig. 5 Doppler-shifted spectrum of Da line for typicalThe performance of the N-NBI has been enhanced

three different filling pressures in the arc chamber. The symbols of triangle, square, and circle
are denoted for the filling pressure of 0.19, 0.3
and 0.4,1 Pa, respectively. E"", E"gr and E0"". are
the bearn energy of the extraction, the first acceleration, and the full acceleration, respectively.

| 200Ilooo

J.

4,7 I,7800bo6ooo:c)400200operation for the JT-60 super-conducting tokamak thatis under design studies. The direct interception of thenegative ion beams contribute to half of the total heatload on the grounded grid, which needs to be reduced tofurther augment the performance.

It(b)

(e)

significantly by improving the convergence of theinjected beam. The successful operation duration of 10
seconds has illustrated the prospect of a longer pulseA.t,[,f,Acknowledgementsl-continuous support and encouragement. They are also
grateful to the members of NBI facility division and JT-The authors would like to thank Drs. M.Shimizu,A.Kitunezaki, H.Ninomiya and S.Matsuda for theirar-60 group.

References

0 c.02 0.04 0.06 0.08 0tll M. Kuriyama et al., J. Nucl. Science and Technol.

0.02 0.04 0.06

35,739 (1998).

I2l T. Ohga et al., "Present status of the negative ion

Normalized intensity of Do line

based neutral beam injector for JT-60U", Proc. 9thFig. 6 Heat load on the grids as a function of the normalized intensity of Da line at the Doppler-shifted

Int. Conf. on Ion Sources, Oakland, Calf. USA, 3-7
Sept. (2001).

wavelength corresponding to the beam energy ofthe extraction voltage (a) and the first acceleration

voltage (b). The normalized intensity of Da isdivided by the intensity at the unshifted wavelength. 'fhe symbols of triangle, square and diamond are denoted for the grounded, second andfirst acceleration grids, respectively.

t3l

L. Grisham et al., "Progress in Development oft4l

Negative Ion Sources on JT-60U", Fusion Energy,2000 (IAEA, Vienna) FIP1/I7(R) (2000).
M. Hanada et al., Rev. Sci. Instrum. 69, 947(l998).
4'.t7