Nuclear Instruments and Methods in Physics Research A 336 (1993) 1-5North-Holland

An advanced high-current low-emittance dc microwave proton source

Terence Taylor and Jozef F. MourisChalk Ruer Laboratories, AECL Research, Chalk River, Ontario, Canada KOJ IJO

Received 5 July 1993

The high-current low-emittance microwave proton source introduced two years ago act the Chalk River Laboratories has beenextensively redesigned . The 2.45 GHz microwave line was modified to incorporate a 50 kV do break and a rectangular-to-ridgedwaveguide transition . The layered microwave window has been replaced by a single plate of aluminum nitride . The solenoids wereelectrically isolated from the plasma chamber. Only the plasma chamber remains at the extraction potential. The total beamcurrent and the proton beam current have been measured as a function of the microwave power, the hydrogen mass flow and thesolenoid positions and currents . More than 95 mA do of hydrogen ions with a proton fraction in excess of 85% were extracted froma single 2.5 mm radius aperture with only 500 Wof microwave power and 2.0 seem (3 Wg/s) of hydrogen mass flow . The perveanceat minimum divergence and the minimum emittance have been determined for various extraction geometries . The variation of theperveance with the aspect ratio of the extraction system was found to be virtually identical for microwave and arc discharge ionsources . The normalized rms emittance was less than 0.13 pmmmrad whatever the extraction geometry.

1 . Introduction

The advantages of using a microwave ion source togenerate a high-current low-emittance do proton beamwere demonstrated two years ago [1]. The introductionof dielectric plasma chamber liners increased the pro-ton fraction of the hydrogen ion beam to as high as90%. Limiting the axial magnetic induction to less than95 mT maintained a normalized rms emittance as lowas 0.07 ,rr mm mrad . The absence of a cathode gave riseto a plasma generator lifetime of hundreds of hours.The technology has advanced significantly in the mean-time . The present paper describes the design changesintroduced at the Chalk River Laboratories and thecorresponding performance improvements . The suc-cessful use of 165 and 90 mA microwave proton sourceswith, respectively, 600 keV and 1.25 MeV cw RFQaccelerators has been described elsewhere [2,3] .

2. Design changes

The basic principles of high-current microwave ionsources are summarized in ref. [4] . In general, a mi-crowave proton source consists of a hydrogen-filledchamber with a superimposed magnetic field and ameans of introducing microwaves, all mated to a set ofextraction electrodes .

The latest version of the Chalk River microwave ionsource system is shown in fig . 1 . The 2.45 GHz mi-

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crowave line is changed somewhat from ref. [1]. Themechanical design of the plasma generator and theextraction system shown in fig . 2 has been extensivelyrevised to improve reliability and to simplify mainte-nance.

The 1 kW cw magnetron is still coupled to theplasma generator via a circulator, a directional couplerand a three-stub tuner but a 50 kV do waveguide breakhas been introduced so that all of the active elementsare at ground potential. The break consists of a 3 mmthick sheet of Teflon clamped between a choke flangeand a standard flange by two acrylic plates . Stubslocated a quarter of a wavelength on either side of thebreak are adjusted to tune out any impedance mis-match introduced by the break.A rectangular-to-ridged waveguide transition has

been inserted immediately ahead of the microwavewindow to enhance microwave efficiency . The dimen-sions of the four quarter-wavelength long ridges werechosen to optimize the impedance match between theWR284 microwave line and the "nominal" plasma .The ridges also concentrate the electric field at thecentre of the plasma chamber and redistribute thethermal load on the O-ring seal .

The microwave window is now a single, 3 mm thick,plate of aluminum nitride that combines the mechani-cal properties of the quartz plate that earlier providedthe vacuum seal with the thermal properties of theboron nitride plate that previously dissipated the en-ergy of the electrons backstreaming from the extractioncolumn .

2 T. Taylor, TF. Mourns / High-current low-enuttance nucrowave proton source

DUMMY

L DIRECTIONAL

LWAVEGUIDE

`-SOLENOIDSLOAD

COUPLER BREAK

Fig. l Schematic of complete microwave proton source system Only components enclosed by dashed line are at extractionpotential .

The 100 mm long plasma chamber was retained,although comparable performance has been obtainedwith a plasma chamber as short as 50 mm. Only thecomponents of the plasma chamber that are adjacentto the axis of the ion source are still lined with boronnitride to enhance the proton fraction . Lining theremainder proved to be minimally effective becausethe plasma density is insignificant at the full radius ofthe plasma chamber [1] .

The variability of the magnetic field configurationhas been increased significantly by allowing each of the

IOOmm

two solenoids to be independently positioned and ener-gized. The inside diameter of the solenoids has beenincreased to 185 mm to accommodate an acrylic insula-tor so that the solenoids and the de power supplies canoperate at ground potential. Each of the solenoids isnow 50 mm long with an outside diameter of 290 mm.

The triode extraction system was designed for apotential of up to 50 kV . The clamped column wasabandoned in favour of an assembly of alumina insula-tors and stainless steel electrode holders glued withpolyvinylacetate. The copper accel and decel elec-

RIDGED WAVEGUIDE

WINDOW

Fig . 2 High-current low-emittance microwave proton source .

trodes are faced with TZM molybdenum alloy. Thenominal 5 mm extraction gap is easily increased so thatreduced perveance can be traded for increased reliabil-ity . Multiple extraction apertures can be accommo-dated as in the original design but, because of the highproton fraction produced by the lined plasma genera-tor, a single recessed extraction aperture has provedsufficient, even for high current cw RFQ accelerators[2,3] .

3. Performance improvements

The total beam current and the proton beam cur-rent extracted from an aperture with a radius of 2.5mm were measured for various combinations of mi-crowave power and hydrogen mass flow . The centre-to-centre separation of the solenoids was fixed at 104mm. The solenoids were centred on the plasma cham-ber and wired in series so that the magnetic fieldprofile was symmetrical . The three-stub tuner was re-optimized for each set of operating parameters . Figs . 3and 4 represent typical data . The total beam currentand the proton beam current are plotted against themicrowave power for a hydrogen mass flow of 1.0 seem(1 .5 wg/s) in fig . 3 and against the hydrogen mass flowfor a microwave power of 600 W in fig . 4. The beamcurrent density for a given power and mass flow isthree to four times higher than that reported earlier[1] . The improvement is primarily attributable to therectangular-to-ridged waveguide transition .

An extensive study of the influence of the magneticfield profile on beam current and proton fraction wasundertaken for a 2.5 mm radius extraction aperture

T Taylor, J.F. Mourcs /High-current low-emutance microwave proton source

400 600 800 1000Microwave Power (W)

Fig 3. Total beam current (open symbols) and proton beamcurrent (closed symbols) as a function of microwave power.

d

S~Vs..

U

cdN

100

90

80

70

60

50

40

30

20

10

00 1 2 3 4 5

1100

Û 105

U 95

Mass Flow (stem)

3

Fig . 4. Total beam current (open symbols) and proton beamcurrent (closed symbols) as a function of hydrogen mass flow .

with a constant microwave power of 500 W and a fixedhydrogen mass flow of 2 seem (3 IWg/s) . The current tothe solenoid adjacent to the microwave window wasoptimized with respect to the total beam current foreach of a series of solenoid positions, solenoid spacingsand currents to the solenoid neighbouring the extrac-tion system . Examples of the resultant field profiles areplotted in fig . 5. The variation of the magnetic induc-tion on the axis of the plasma chamber with the solenoidspacing, assuming that the current to both of the

------ 79 mm- - - 104 mm

130 mm

80,50 -40 -30 -20 -10 0

10 20 30 40 50Displacement (mm)

Fig . 5 . Magnetic induction on the axis of the plasma chamberfor various centre-to-centre solenoid spacings with thesolenoids centred on the plasma chamber. (The plasma elec-trode is at 50 mm and the microwave window is at -50 mm .)

solenoids is the same and that the solenoids are cen-tred on the plasma chamber, is shown. It was foundthat the optimum current for the window solenoidinvariably corresponded to a magnetic induction of 92to 93 mT at the microwave window . The magneticinduction that satisfies the electron cyclotron reso-nance (ECR) condition is only 87 .5 mT . Popov hasspeculated that, in an overdense plasma, the highermagnetic field may be more effective because the mi-crowaves are more efficiently absorbed via Landaudamping of "whistler waves" rather than by ECR [5].

The maximum total beam current and the commcn-suratc proton beam current are plotted against thesolenoid spacing in fig . 6. The more linear magneticfield profiles generated by the larger solenoid spacingsappear to yield the higher beam currents . In fig. 7, themaximum total beam current and the correspondingproton beam current are shown as a function of themagnetic induction at the plasma electrode. The dataseem to suggest that the optimum may be slightlyhigher than the 92 to 93 mT observed at the microwavewindow . The ideal value is probably defined by thetrade-off between the tendencies of higher fields eitherto focus the ions down onto the extraction aperture orto reflect the ions back into the plasma chamber. Theoptimum magnetic induction at the plasma electrodewould then depend on the extraction geometry.

The beam quality was examined for various extrac-tion geometries using the two-slit emittance measure-ment system described in ref . 6. The radius of theextraction aperture was increased from 2.0 to 3.5 mmto 0.5 mm steps . Measurements were made with an

T. Taylor, J.p. Moues / High-current low-emtllance microwave proton source

Fig. 6 . Total beam current (open symbols) and proton beamcurrent (closed symbols) as a function of centre-to-centre

solenoid spacing

100

90

80

70

60

50

40

30

20

10

50 60 70 80 90 100 110 120 130 140 150Magnetic Induction (mT)

Fig . 7. Total beam current (open symbols) and proton beamcurrent (closed symbols) as a function of magnetic induction

at extraction aperture

extraction gap of 5.0, 6.0 or 7.0 mm. The beam currentdensity was fixed at a modest 250 mA/cm2 that gaverise to a proton fraction of only 70% and an effectivemass of almost 1.25 amu. The divergence and, to alesser degree, the emittance depended on the degreeof space charge neutralization which, in turn, is influ-enced by the residual gas pressure in the beamline . Apressure of 4 x 10-5 Torr was maintained throughoutthe measurements by bleeding in the appropriateamount of hydrogen gas. The normalized rms emit-tance,

was measured as a function of perveance,

p= JU -312(Alq) 1 /2 ,

(2)for each geometry . (Here x and x' are transversedisplacement and divergence while I and U are ioncurrent and extraction voltage and A and q are atomicmass number and charge state.)

In fig . 8, the perveance corresponding to the mini-mum emittance is plotted against the aspect ratio,S= r/d, where r is the radius of the extraction aper-ture and d is the width of the extraction gap. Thecurve conforms to the expression,p,xn =IIS 2/(1 +aS2),

identified by Coupland et al . [7] using a duoPIGatronion source . The empirical constants, lI and a, were setto 1.9 mA/kV 3/2 and 1 .7 respectively, the values rec-ommended by Keller [8] for a "sophisticated" extrac-tion system . The correlation is remarkably good .

Aspect RatioFig. 8. Perveance at minimum divergence as a function of

extraction system aspect ratio .

The minimum normalized rms emittance for eachextraction geometry is shown as a function of theaperture radius in fig. 9. The three data points for aradius of 3.5 mm correspond to the various widths ofthe extraction gap. The emittance is virtually indepen-dent of the extraction geometry . The line is solely aleast-squares best fit . The contribution of the relativelylow magnetic induction to the emittance is negligible[9] . The ion temperature cannot account for a normal-ized rms emittance of 0.1-rr mm mrad at extraction volt-

"0cd

02

K 01

UU

«0

W00

T. Taylor, J.F. Mourns / High-current low-enuttance nucrowave proton source

1 .5 2.0 2.5 3.0 3.5 4.0Aperture Radius (mm)

Fig . 9 . Minimum normalized mis emittance as a function ofextraction aperture radius .

ages in excess of a few tens of kV . The emittanceappears to be dominated by aberrations .

4. Discussion

5

The Chalk River do microwave ion source systemhas been modified so that only the plasma chamberremains at the extraction potential. The isolation trans-formers for the microwave and the do power supplieshave been eliminated and the control system has beengreatly simplified .

More than 475 mA/cmz of hydrogen ions with aproton fraction of greater than 85% were generated byonly 500 W of microwave power. The number of ex-traction apertures required for high-current applica-tions has been reduced with a commensurate improve-ment in emittance. The hydrogen gas consumptionefficiency was over 50% at the maximum beam currentand was even higher when the ion source was "starved".The capacity of the vacuum system was reduced ac-cordingly.

The advantages of microwave ion sources are cur-rently so significant that many designers of high-cur-rent cw accelerator injectors are rejecting all otheralternatives .

Acknowledgements

The authors are indebted to E.C . Douglas for con-tributing to the mechanical design, to D.G . Hewitt forfabricating most of the components, to J .S.C. Wills foracquiring some of the data and to J .D . Sherman of theLos Alamos National Laboratory for reviewing themanuscript .

References

[1] T. Taylor and J.S .C . Wills, Nucl . Instr . and Meth . A 309(1991) 37 .

[2] G.M . Arbique, T. Taylor, M.H. Thrasher and J.S .C . Wills,Proc . 1991 IEEE Part . Accel. Conf., IEEE Conf . Record91CH3038-7, p. 842.

[3] G.M . Arbique, T. Taylor, A.D Davidson and J.S.C . Wills,Proc . 1992 Linear Accel . Conf., Atomic Energy of CanadaLtd. Report AECL-10278, p 52 .

[4] T. Taylor, Rev. Sci . Instr . 63 (1992) 2507 .[5] O.A . Popov, J. Vac. Sci. Technol. A 9 (1991) 711 .[6] T. Taylor, M.S . de Jong and WL Michel, 1988 Linear

Accel . Conf . Proc ., Continuous Electron Beam Accelera-tor Facility Report CEBAF 89-001, June 1989, p. 100

[7] J.R . Coupland, T.S . Green, D P

Hammond and A.C.Riviere, Rev. Sci . Instr . 44 (1973) 1258 .

[8] R. Keller, Nucl . Instr . and Meth . A 298 (1990) 247.[9] W. Kraus-Vogt, H. Beuscher, H.L . Hagedoorn, J Reich

and P Wucherer, Nucl . Instr. and Meth . A 268 (1988) 5 .