Nuclear Instruments and Methods in Physics Research A309 (1991) 37-42North-Holland

A high-current low-emittance dc ECR proton sourceTerence Taylor and John S.C . WillsChalk River Laboratories, AECL Research, Chalk Ricer, Ontario, KOJ IJO, Canada

Received 18 June 1991

A simple high-current low-emittance do ECR proton source has been developed for a cw RFQ accelerator injector . Protonfractions of up to 90% have been obtained at beam current densities as high as 350 mA/cmz. The normalized rms emittance of a25 mA beam from a single 4.0 mm diameter extraction aperture was 0.07 Tr mm mrad corresponding to a brightness of 5 A/(Tr mmmrad)Z . Similar results were obtained with multiple aperture extraction systems .

1 . Introduction

The resonant absorption of microwaves by electronsorbiting in a magnetic field is a convenient means ofexciting a cold plasma suitable for the production of ahigh-quality ion beam . Electron cyclotron resonance(ECR) ion sources have many advantages over theirarc-discharge competitors : the mass spectrum, as wellas the charge state distribution, of the beam from anECR ion source is modified relatively easily by adjust-ing either the plasma generator design or the operatingparameters ; an ECR plasma generator is more de-pendable than an arc-discharge plasma generator witha short-lived cathode; the ionization efficiency of anECR ion source is very high and, consequently, the gasload on the vacuum system is very low; in principle, allof the power supplies for an ECR ion source can be atground potential, avoiding the isolation transformersassociated with most arc-discharge ion sources andsimplifying the control system .

The advantages of ECR plasma generators in theproduction of multiply-charged ions have been recog-nized for many years [1] . However, the designers ofhigh-current ion sources have hardly begun to exploitmicrowave and, especially, ECR plasma generators .Sakudo et al . [2] have generated a few hundred mA ofseveral different ions with microwave ion sources de-signed for ion implantation . An ECR ion source devel-oped by Torii et al . [31 has produced up to 200 mA ofoxygen ions for the fabrication of buried SiO 2 layers .Nevertheless, high-current accelerator injectors are stillpredominantly arc-discharge ion sources. The presentarticle describes a very simple high-current low-emit-tance de ECR proton source developed at Chalk RiverLaboratories as an injector for a ew RFQ accelerator .

Elsevier Science Publishers B.V .

2. Design

Several factors must be considered in developing adesign for a high-current low-emittance ECR ionsource .

Firstly, the ions are, of necessity, created in a sub-stantial magnetic field . The divergence of an ion beamextracted from an axially symmetric magnetic field canbe deduced from [41

1 qBr

2 p

l gmcer

NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

Section A

where q is the charge of the ions, B is the magneticinduction at the extraction aperture, r is the radius ofthe aperture and p is the momentum of the extractedions . The contribution of the thermal velocities of theions is neglected . The magnetic induction that satisfiesthe ECR condition is given by

where m and e are, respectively, the mass and thecharge of the electron and to is the drive frequency.Assuming that the magnetic induction is constant, Bccan be substituted for B in eq . (1) . The ion beamdivergence then becomes

Thus, the quality of the beam from an ECR ion sourcemay be limited by the frequency of the microwavegenerator. (For example, a 35 keV proton beam ex-tracted from a 30 GHz ECR ion source through a 5mm diameter aperture would have an unacceptablyhigh divergence of at least 50 mrad . On the other

38

hand, at a microwave frequency of 2.45 GHz, theinfluence of the magnetic field would be so small thatthe divergence would be determined by the opticalaberations .)

Secondly, although a low microwave frequency isessential for a low-divergence ion beam, low-frequencymicrowaves cannot propagate through a high-densityplasma . To be precise, microwaves of frequency m areexponentially attenuated above a critical density givenby

eoMW

(The critical density for 2.45 GHz microwaves, forexample, is only 7 X 10" ) cm-3 , far lower than thedensity required in a high-current ion source .) As aconsequence, low-frequency microwaves can be cou-pled to a high-density plasma at the surface only . Inother words, in an ECR ion source with a plasmadensity greater than nc, the resonance condition mustbe satisfied by the magnetic induction at the pointwhere the microwaves are introduced to the plasma .

Finally, the magnetic induction must be essentiallyconstant throughout the plasma generator so that theplasma will be confined radially . If the magnetic fielddeclines significantly anywhere between the pointwhere the plasma is generated and the point where theions are extracted, the plasma will diffuse along thediverging magnetic field lines, proportionately reducingthe density at the extraction aperture .

RF WINDOW

T. Taylor, J.S.C. Wills / A dc ECR proton source

KLASMA ELECTRODE -,

ACCEL ELECTRODE

An ion source that roughly conforms to the aboveconsiderations was assembled largely from existingcomponents for a preliminary demonstration . It is de-picted schematically in fig . 1 .

The WR284 drive line incorporates a 1000 W cwmicrowave power supply based on a 2.45 GHz mag-netron, a dual directional coupler for power monitor-ing and a motorized three stub tuner for impedancematching . Active tuning elements are essential becausethe dielectric properties of the plasma vary with the rfpower, the gas feed rate and the magnetic-field config-uration.

The microwave power is introduced to the plasmachamber through a two-layer window. The first layer, a10 mm thick quartz plate terminating the waveguide,creates a vacuum seal . The second layer, a 4.0 mmthick plate of boron nitride adjacent to the plasma,conducts away the heat generated by electrons back-streaming from the extraction column . The watercooled oxygen-free high-conductivity (OFHC) copperplasma chamber is 90 mm in diameter and 100 mmlong . An axial magnetic induction of up to 100 mT isgenerated by two solenoids. The solenoids can betranslated axially to vary the distribution of the mag-netic induction in the plasma chamber or to gain accessto the plasma chamber and the extraction column .

The extraction column is a multi-aperture triode .The diameter of the apertures in the plasma electrodeand the deceleration electrode is 4.0 mm while theapertures in the acceleration electrode are 3 .5 mm in

DECEL ELECTRODE -~

..wOV.OApQO~ 9.

_

~, !HN

Y

SOLENOID

Fig. 1 . Schematic of high-current low-emittance do ECR proton source .

PLASMA CHAMBER

diameter . The apertures are shaped to maximizebrightness [5] . A close-packed hexagonal array of up toseven apertures can be accommodated, however, themeasurements reported here were made either with asingle aperture or with three collinear apertures tofacilitate diagnosis . The centre-to-centre spacing of theapertures is 6.5 mm . The acceleration gap is 6.0 mmwhile the deceleration gap is 2.0 mm. The electrodeswere fabricated from OFHC copper faced with TZMmolybdenum alloy. The facing improves resistance tosparking initiated by backstreaming electrons .

3. Performance

Prior to the installation of the extraction column,the plasma generator was studied with a double cylin-drical Langmuir probe [6] . The saturation ion currentdensity and the electron temperature were measuredas a function of the peak magnetic induction, the axialposition of the solenoids, the microwave power and thehydrogen feed rate . The probe could be translatedboth axially and radially via a slot in the plasma elec-trode.

The maximum ion current density exceeded 300mA/em z. The electron temperature was typically 20eV . The density declined rapidly with increasing radialdisplacement . At half of the radius of the chamber, thedensity was invariably less than half of the maximum.On the other hand, the density varied relatively littlewith axial position .

Following the Langmuir probe studies, the extrac-tion column was installed for the measurement ofbeam currents, mass spectra and emittances .

The current density of an ion beam extracted fromthree apertures and collected in a Faraday cup isshown in fig . 2 as a function of the magnetic inductionat the microwave window . The microwave power was400 W and the hydrogen mass flow rate was 3.5 seem(5 .3 l.Lg/s) . The three stub tuner was adjusted formatched impedance at each value of the magneticinduction. At the maximum beam current density, themagnetic induction adjacent to the microwave windowwas 87 .5 mT, satisfying the ECR condition for 2.45GHz microwaves . The linear background is probablyattributable to the excitation of left-hand circularlypolarized electromagnetic waves that travel parallel tothe magnetic field [7] . (The critical plasma density forthese L-waves propagating in a modest magnetic fieldis considerably higher than the critical plasma densityfor the incident microwaves .)

Measurements similar to those shown in fig . 2 weremade for various solenoid positions with either one orboth of the solenoids energized. The optimum mag-netic induction at the microwave window invariablysatisfied the ECR condition. Keeping in mind that the

T. Taylor, J S.C. Wills / A de ECR proton source 3 9

Fig . 2 . Beam current density vs axial magnetic induction at themicrowave window with matched impedance.

two solenoids were energized by a single power supplyand that the solenoids were fastened together, themagnetic field configuration that generated the highestion current density is shown in fig. 3 . The magneticinduction is reasonably uniform over the length of theplasma chamber. Thus, the plasma is confined radiallyso that the density is essentially constant along the axis .

The beam current density is plotted as a function ofthe microwave power at a fixed hydrogen mass flowrate of 1 .0 seem (1 .5 lkg/s) in fig . 4. Fig. 5 shows the

0-50 -25 0 25 50 75 100 125 150Axial Displacement (mm)

Fig . 3 . Magnetic induction on the axis of the solenoids as afunction of axial displacement from the microwave window.The dashed vertical lines define the axial extent of the plasma

chamber.

40

Fig . 4 . Beam current density vs microwave power with BNlining (solid) and without (open) .

beam current density as a function of the flow rate at afixed power of 1000 W. The ion current density in-creased continuously with increasing microwave powerand with decreasing hydrogen mass flow rate . The ionsource was unstable at hydrogen mass flow rates of lessthan 1 .0 sccm (1 .5 Fig/s) . It is worth noting that thecurrent density was almost unchanged when the plasmaelectrode was lined with boron nitride . Although themeasurements displayed here were generated with asingle extraction aperture, similar results were ob-tained with three collinear apertures, except that thehydrogen mass flow rate was, of course, tripled . The

Fig. 5 . Beam current density vs hydrogen mass flow rate withBN lining (solid) and without(open) .

T. Taylor, J.S .C. Wills / A dc ECR proton source

1000

Q)

có

Q)C).,

Fig . 6. Typical mass spectrum for three collinear extractionapertures. Contributions of species in parentheses are rela-

tively insignificant .

intensity varied by only 12% from beamlet to beamletwhen three apertures were used . During these meas-urements, the magnetic induction adjacent to the mi-crowave window was only about 80 mT, significantlylower than the 87.5 mT that corresponds to the elec-tron cyclotron resonance. Current densities as high as350 mA/cmz could be obtained by operating on theECR resonance at a microwave power of 1000 W.However, the ion source usually performed more stablyoff resonance, and, in any case, the beam currentdensity was substantial even below the resonance.

~08

0

10

00

100

10

100

N� ` N�'

H2; ( Nie~) H2

H,'(HS')H

50 100 150 200 250

Displacement (mm)

200 400 600 800 1000 1200

Microwave Power (W)Fig . 7. Species fractions vs microwave power with BN lining

(solid) and without (open).

X 08

0

UcÓ06

204U

10

02

00

Hz+

Flow Rate (seem)Fig. 8. Species fractions vs hydrogen mass flow rate with BN

lining (solid) and without (open) .

A typical mass spectrum for three collinear extrac-tion apertures is shown in fig . 6. The measurementswere made with a system described elsewhere [8]. Thespecies fractions calculated from similar spectra areplotted against the microwave power and the hydrogen

mass flow rate in figs . 7 and 8. The proton fraction, like

the beam current density, increased continuously withincreasing microwave power and with decreasing hy-drogen mass flow rate . However, the addition of aboron nitride plasma electrode liner changed the massspectra dramatically . Indeed, the maximum protonfraction increased from 55% to 90%. Similar results

were obtained with liners of quartz or alumina,The proton fraction enhancement observed here

was foreseen by Chan et al . [9] using a comprehensiveplasma generator model that simultaneously solved a

set of particle balance equations based on 11 atomicand molecular reactions. The model predicted that the

proton fraction would increase significantly if the hy-

drogen atom recombination coefficient of the wall ma-terial were reduced. Quartz has a much smaller hydro-gen atom recombination coefficient than copper [10].

The emittance was measured for a single extractionaperture and for a linear array of three extractionapertures with a two slit emittance measuring system[8]. A typical phase-space plot for three collinear aper-tures is shown in fig. 9. The rms divergence of thebeamlets from each of three apertures as well as therms divergence of the entire beam are plotted as afunction of the extraction voltage in fig. 10 . The mini-mum normalized rms emittance for a single aperturewas 0.07 rr mm mrad corresponding to an rms diver-gence of 15 mrad and an rms radius of 0.5 mm at the

T. Taylor, J S. C. Wills / A de ECR proton source

60 ~

40

X -20

-40

-60-40 -20 0 20 40

x (mm)

Extraction Voltage (kV)

41

Fig. 9. Typical phase space diagram for three collinear extraction apertures. Contours are at 2% (dashed) and 5% (solid) of

maximum intensity .

waist . In this case, the beam current was 25 mA peraperture so that the brightness was 5 A/( ,rr mm mrad)Z.The emittance of the beam from a closely packed

array of N extraction apertures can be inferred fromthe one- and three-aperture measurements because the

radius of the waist is determined almost entirely by the

number of apertures and the aperture separation . As-suming that the centre-to-centre spacing of the aper-tures is fixed at 6.5 mm and that the rms divergence is

o H3

Fig. 10 . Rms divergence vs extraction voltage for each of threebeamlets and for entire beam .

42

a constant 15 mrad, the normalized rms emittance isgiven by

E � = 0.2 vfN(,rr mm mrad) .

4. Discussion

A simple ECR plasma generator has proved to be apractical device for generating dc proton beams withhigh current densities and low emittances . Hydrogenion beams of up to 130 mA have been extracted fromthree apertures having a total area of only 0.38 cm2.

Proton fractions as high as 90% have been achieved bylining the plasma electrode with suitable materials .Because the rms divergence is only 15 mrad, exception-ally bright multi-beamlet proton beams can be ob-tained . For example, a hexagonal array of seven 4 mmdiameter apertures would yield a 280 mA proton beamwith a normalized rms emittance of just 0.5 rr mmmrad .

The success of the present ECR proton source hasencouraged the development of an improved design . Adc waveguide break has been developed so that themicrowave power supply can be at ground potential.An automatic impedance matching network will re-place the three stub tuner. A single-layer aluminumnitride window, that dissipates the energy of the back-streaming electrons as well as providing a vacuum seal,is being evaluated. The solenoids will be isolated fromthe plasma chamber so that the associated dc powersupplies can be at ground potential . The extractionsystem has been completely redesigned to facilitateinstallation and maintenance and to ensure that theelectrostatic field enhancement in the extraction gap isminimized. The improved design is expected to operatemore reliably at the extraction voltages required for ahigh-current cw RFQ injector.

T. Taylor, J.S.C. Wills / A dc ECR proton source

A cw RFQ has recently accelerated more than 75mA of 50 keV protons from the original ion source toan energy of 600 keV [111 .

Acknowledgements

The authors are indebted to E.C . Douglas and D.G .Hewitt for invaluable contributions to the design andthe fabrication of the ion source, to G.F. Morin forindispensable assistance in the maintenance of the ionsource test stand and to M.S . de Jong and N.A .Ebrahim for helpful comments on the manuscript .

References

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and P. Wucherer, Nucl . Instr . and Meth . A268 (1988) 5 .[51 W.S . Cooper, K.H . Berkner and R.V. Pyle, Nucl . Fusion

12 (1972) 263.[6] E.O . Johnson and L. Malter, Phys . Rev. 80 (1950) 58 .[71 T.H. Stix, The Theory of Plasma Waves (McGraw-Hill,

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[10] B.J . Wood and H. Wise, J. Phys . Chem . 65 (1961) 1976 .[111 G.M . Arbique, T. Taylor, M.H . Thrasher and J.S .C .

Wills, Proc . 1991 IEEE Particle Accelerator Conference,to be published .