J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 456459

Steady State Diagnostic Neutral Beam Injector
DAVYDENKO Vladimir* and IVANOV AlexanderBudker Institute of Nuclear Physics, Novosibirsk, 630090, Russia(Received: 18 January 2000 / Accepted: l0 May 2000)

Abstract
To provide a beam for active beam emission spectroscopy measurements in large fusion devices adedicated neutral beam injector was developed. It is operated at energy 50 keV, equivalent beam current(for hydrogen) up to I A, pulse duration of up to 4 s. The injector is also capable of providing beam
modulation with frequency of up to 500 Hz. In the injector ion source the emitter plasma is produced byan inductively exited RF-discharge. Distinctive feature of the ion source is that in order to simplify
injector design, a thermal inertia-type ion optics system with "thick" electrodes is used. With the grids
formed to focus the beam 4 m downstream from the source the integral angular divergence of the beam,measured at l/e level in the focus, of 10-2 rad was obtained. At present two injectors are successfully put
into operation at TEXTOR and TCV tokamaks.
The developed injector, in principle, can be operated in steady state regime after several
nonprincipal changes in design. The RF-plasma generator allows significantly increased pulse duration
without considerable changes. To increase further pulse duration new versions of ion optics system withmore intense water cooling are being developed.

Keywords:
plasma diagnostic, neutral beam injector, ion beam, plasma emitter, ion optics system

1. Introduction

these beams are sufficiently large current and energy ofup to 500 Hz. Plasma emitter in the injector is provided
by a radio frequency (RF) discharge in hydrogen. The
beam parameters were experimentally measured andcompared with those predicted by simulations. Possible
ways to evolve the developed injector to steady state
operation are also considered below in the paper.

the particles, so that the beam could penetrate to plasmacore. At the same time, duration of the beams should belong enough, close to that of a plasma shot, amounting

2. Injector Description
In the injector the neutral beam is provided byLow-divergent, quasi-stationary neutral beams areoften applied in modern magnetic fusion devices as adiagnostic tool providing unique information aboutplasma parameters. The most important requirements toto, at least, a few seconds for large machines. We
developed neutral beam injector which is capable ofextraction of charged ions from the RF plasma and theiracceleration to the desired energy with subsequent
neutralization by charge exchange with gas target.
General layout of the diagnostic neutral beam injector ismeeting above mentioned requirements. The maximal
beam energy is set to 50 kV, the source delivers ion
current is up to 2 A, and pulse duration of the beam isup to 4 s. The low divergent beam (-10-2 rad) is
geometrically focused 4 m downstream from the source.
The beam can be modulated with a frequency variable

presented in Fig. l.

It includes a neutralizer,

calorimeters, ion deflection magnet, residual ion dump,
pumps and other major components which are housed@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research*Corresponding author's e-mail: davydenko@ inp.nsk.su
456

Davydenko Y. et al., Steady State Diagnostic Neutral Beam Injector

Fig. 2 RF-discharge ion source. 1 - soft metal case, 2 -gas feeding capillary tube, 3 - inner magnetic
shield, 4 - trigger, 5 - ceramic wall, 6 - RF-antenna, 7 - pull stud (insulator), 8 - ceramic spacers, 9 - beam duct, 10 - grid cooling manifold, 11- grounded grid, 12 - accelerating grid, 13 - extracting grid, 14 - plasma grid, 15 - permanentFig. 1 General view of one of the version of diagnostic
neutral beam injector

magnets, 16 - co-axial feedthroughinside or at a cylindrical vacuum tank.

Gas is partly supplied into the beamline
neutralization duct by puff from the discharge chamber,indium ring sealings that allow for effective heat

the rest amount is puffed into the duct through a

conduction to the backplate and front side flange bothwater cooled. A thermocouple monitored surface

manifold by making use of a pulse gas valve. Bending
magnet and residual ion dump are placed inside thevacuum vessel at a special servicing platform. Eachpump installed on the top of the injector tank has anominal hydrogen pumping speed of 24 x 103 l/s intemperature of the tube revealed temperature rises at themolecular flow regime.

tube center of less than 50oC under conditions of l0 s
operation at maximal power.
The gas is introduced at the backplate by a pulsed
gas valve through a flexible capillary tube 0.5 mm in

3. Plasma Emitter

inner diameter and 150 mm long that simultaneously
provide electrical insulation of the valve, installed on theThe injector ion source (Fig. 2) comprises an RF
plasma source feeding a multi-aperture ion optics
system. The plasma source consists basically of agrounded flange, from the plasma source. For -2 A
extracted current gas puffing rate for hydrogen was set

to 1.5 Torr Us.
The discharge is initiated by applying a highvoltage pulse between the trigger electrode and thedischarge chamber rear flange at which the trigger ismounted. To improve the particle confinement in thevacuum-tight cylindrical alumina ceramic chamber andan external coil. For the ion source, the ion current
density of 10G-130 mAlcm2 was chosen.
The emitter plasma is produced by an inductively

excited RF discharge in hydrogen (or helium). The
external RF coil is made of six turns of copper wire
wound on a teflon frame. The RF coil is fed byalternating current with a frequency of 4.65 MHz fromsource, two NdFeB pemanent magnets are installed atthe backplate. Typical ion current density profile
measured at the plane of the first (plasma) grid is shownin Fig. 3. The ion current is homogenious with +107o

an amplifier with an output power variable up to l0 kW.

accuracy within the plasma grid surface. The emittercurrent density was found to be changed almost linearlywith the RF power coupled to the plasma.

Typically, to provide the required current density (130mA/cm2) -2.5 kW of RF-power is to be coupled to theplasma.

The copper backplate of the source vacuum

4. lon Optics Systemchamber vessel has water channels for active cooling.
The ceramic tube is vacuum tiehten from both end with

The ion optics system consists of four molibdenum
grids with the 163 apertures 4 mm in diameter forming457

Davydenko V. et al., Steady State Diagnostic Neutral Beam Injectorj,mNcri

60, 10-2rad160

ta0
120

t00

200t.01.4

4-3-2-101234

1.8I,AFig.4 Beam divergence vs current for the extractingr, cm

voltage 6.5 kV and 6.75 kV (upper curve)
Fig. 3 Typical ion current density profile of RF plasma

emitter
agreement with the simulation of the beam formation inelementary cell of ion optics system [].

an hexagonal structure with step of 5 mm. To provide

Experimental tests indicate that the grids areenhanced thermocapacity the extracting, accelerating
and ground grids are 4 mm in thickness. Thickness ofthe plasma grid is 2 mm. Enlarged thickness of the grids

capable to handle a pulse thermal load when operated atrequired computer simulations of elementary beams, with a duty cycle of about l.5Vo for cooling.

formation for optimization of geometry of thein [] have shown that the angular beam divergence
better than l0-2 rad can be achieved. According to thesimulation the gaps between the plasma and extracting
and between extracting and accelerating electrodes are

The species composition has been investigated for
50 keV hydrogen beam. The beam was analyzed by amagnetic mass spectrometer located - 4 m from thesource. Mass analysis of ion beam constituents indicatesthat H*, H2*, and H3* percentages are 7l,5Vo, l3Vo, and15.5Vo, respectively, when the ion source is operating

2.6 mm and 7.4 mm, respectively. Thermal

with beam of 1.9 A.

50 kV and extracted ion current of. at least. 1.7 Afor 4

elementary cell. The results of the simulations presented

deformations and mechanical stability of the grids atheating by secondary particles have been accessed in

6. Steady State Regime
The developed injector, in principle, can beoperated in steady state regime after non-significant[2]. The grids are mounted on the water-cooled flanges
enabling the full heat removal from pulse to pulse as
well as partial heat removal during the injection pulse.
In order to focus the beam on to the desired point inside
the plasma, thp grids are formed to be sphericalsegments with the required curyature radius of 4 m.

changes in design. The RF-plasma emitter allows

significantly increased pulse duration without
considerable changes. The thermal loads to ceramicwalls of the plasma generator are moderate and can beremoved by the water cooling from the ends incontinuous regimes. Estimation of the stationary
temperature rise of the ceramic tube is 100'C.
To provide steady state beam formation new

5. Beam Tests
Beam profiles were measured by making use of anarray of secondary emission detectors near the focal
plane at distance 4 m downstream from the ion source
and, alternatively, by a segmented retractable
calorimeter installed 2.2 m ftom the ion source. The
results of these two diagnostics reasonably coincide toeach other. The V-shape dependencies of an angulardivergence upon the beam current and voltage on theextracting electrode (Fig. ) have been obtained. The

7 is thermal conductivity and h is effective thickness ofthe grid. Taking e = 24 Wcm2 [2] we obtain for gridexperimental results on the beam divergence and valueof optimal extracting voltage are in reasonable= 300"C for beam formation without
modulation. The next approach is based on use ofversions of intense water cooling of the grids are being
developed. In case of use of intense peripheral watercooling the radial profile of stationary temperature riseof grid is expressed by parabolic law 6Z = q1a2 - r2)l47h, where q is thermal load to grid, a is radius of grid,center tf458

Davydenko V. et al., Steady State Diagnostic Neutral Beam Injector

additional water cooling of the grid central part. In thiscase the radial profile of the temperature can be written

the ion optical system is capable of handling a pulse
thermal load when operated at 50 kV and maximal
extracted ion current of 1.7 A for 4 s, with a duty cycleof about l.5vo for cooling. The beam is focused atdistance 4 m from the source having - 35-40 mm radialextent at the focal point. Simple versions of watercooling of the ion optics system with "thick" grids canbe used for evolving the source to steady state regime ofas 67 = q((a2 - b2)ln(r/a)lln(a/b) + a2 - r2y+yn F1,

where b is the radius of grid central part with withcooling. For a/b = 5 maximum of temperature riseachieved at r = 2.7 b is 6T = 100"C. In version of grid
system with more intense cooling water channels arearranged in place of each third row of holes. Maximumof temperature rise for this case is dZ = 25'C but
integral grid transparency then decreases from 54Vo tooperation.

References

36Vo.

tll V.I. Davydenko et al.,Rcv. Sci. Instrum.6E, 1418

Another parts of the injector such as neutralizer,

cryopumps, bending magnet, beam dump can be(1997).

operated in steady state regimes without significant[2] V.I. Davydenko et al.,Rev. Sci. Instrum. 69,2007

problems.

(1998).

[3] H.S. Carslow and J.C. Jager, Conductivity of Heat

7. Conclusions

in Solids, (Clarendon Press, Oxford, 1959).

Experimental tests of the ion source indicate that459