IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983

A COMPACT 6 MeV PULSED ELECTRON ACCELERATOR

R.M. Hutcheon, L.W. Funk, B.A. Gillies*, S.B. Hodge**, P.J. Metivier*** and S.O. Schriber

AcceleratorAtomic Energy of Canada

Chalk River Nuclear Laboratories,

Summary

A compact 6 MeV electron accelerator has beenbuilt and tested. The unit uses an axially coupledS-band standing wave structure driven by a magnetronsource that delivers a 2 MW peak power, 4 ps pulse at300 pps. The structure (including gun and highvoltage feedthrough) is 12 cm diameter and 90 cmlong.

The gun is a novel telefocus design. Theelectron source is a 5.5 mm diameter loop of carbu-rized thoriated tungsten wire in an electrode-focuscoil geometry that produces a 40 keV input beam of s30 Tr mm mrad ecmittance at 25 mrad convergence half-angle. The gun is rugged, insensitive to mechanicalshock, and requires only 7 watts of filament power toachieve 400 mA emission under operating pressures.The whole accelerator-gun package can be baked at350%C, although a base pressure of 3 x 10-7 Pa isachieved with 24 hours of bakeout at only 150'C.

At an output current of 100 mA (peak) in thepulse, the beam produces dose rates of more than 300RMMW on axis when uising a 4 mm copper brensstrahlungtarget. This roughly translates into a clinicalflattened dose rate of 200 RMM over a 35 x 35 cmfield.

The accelerator package is straightforward tomanufacture and could easily be reduced in length forspecific applications.

Introducti on

A 6 MeV pulsed electron accelerator was built totest the feasibility of producing a high current, highefficiency small size unit with a simple mechanicaldesign, rugged physical characteristics and low cost.

A compact, well engineered, 2 MW pulsed S-bandrf source was available as a product of the THERAC 25accelerator development progran1, and was a logicalpower source. An energy of 6 MeV was chosen to maxi-mize the bremsstrahlung transmiission through mostmaterials.

A standi ng-wave accelerator system2 was anatural choice, for it has a long history of develop-ment and application at CRNL, and it provides the mostefficient conversion of rf power to beam energy forthe standard room temperature linac configuration.

The electron gun must deliver high pulsed beamcurrents to provide reasonable dose rates, and underthese operating conditions, the vacuum pressure oftenreaches 10-4 Pa. The guin must also be able to bechanged with relative ease and speed in case offailure. Thus the usual choice of a dispenser cathodebased gun system did not seen practical. Instead, amuch more rugged thoriated tungsten wire cathodemounted on a conflat flange was developed. The giunvoltage was chosen to be 40 kV so that the sante highvoltage pulse could be used to drive both the magne-tron and the electron gun.t 1 RMM = 1 Rad per minute, 1 metre from source.

Physics BranchLimiited, Research CompanyChalk River, Ontario, Canada KOJ lJO

Accelerator Rf and Mechanical Design

The accel erator (Fi g. 1) i s a 9 cel l S-band on-axis coupled standing wave structure, operating in theT/2 mode to improve field stability. The struictureuses field grading in the first few cells to improvebean bunching about the synchronous phase. Theaccelerator segments were assembled as received fromthe manufacturer except for the few graded-B cellswhich required tuning. The shunt impedance of a simi-lar S-band, brazed OFHC copper structure wasmeasured3 to be 82 MO/m, approximately 95% of thetheoretical value. Theoretically, up to 45% of the dcinjected beam should be accelerated through thestructure. After the structure was brazed, the rfnscell to cell field variation was ± 1.1%, the nearestneighbour coupling constant 5.1% and the pass band gap2.0 ± 1.4 MHz. The on-axis coupling scheme wasadopted not only for rf, mechanical and assemblyreasons, but because ultimnately, a device could bemade with an overall cross-sectional diameter of 12cm.

The ceramic rf window is located at the outputend of the structure. If the rf line lengthl is chosencorrectly, the magnetron frequency will self-lock ontothe structure frequency, eliminating the need for anexternal frequency control system. Auxiliary pumpingchannels, also give mechanical support, are providedbetween the gun region and the ion pump, as the con-ductance of the accelerator itself is quite low. Theweight of the structure, including gun, support, pumpsand rf line, is 50 kg. The water cooling is suf-ficient for 0.1% duty factor.

A 24 hour bakeout at 150°C produced a basepressure of 3 x 10-7 Pa after cooldown and pinch-off. Although such a bakeout does not achieve theultimate low outgassing rate, it results in basepressures as low as inexpensive ion pumps can achieve,avoids external surface oxidation, and can be done insitu tising heating tapes. Rapid recovery fron back-filling to air is easily achieved if the ion pump isvalved off and not vented.

Electron Gun Design and Testing

The electron gun design uses a non-interceptingfocusing electrode which is biased negatively relativeto the electron source. This is conmonly used inelectron microscope guns with hair-pin emitters toproduce hi h brightness beams with suitable opticalproperties The extension of the technique to thefocusing of line sources is not so well known.

The properties of a bearl produced by an emittingfilament recessed in a long slot were nmeasured (Fig.2). The cutoff bias is a function of the wire depth.The size and divergence of the bean at the anode platewere measured and compared with the predictions of theHerrmannsfeldt gun code5 (Fig. 3). Beveling theslot edges (see Fig. 4) increased the beam focusingfor the samie emitted current. A suitable combinationof bias, emission per unit length and wqire depth werechosen. The apparent position of the virtual linesource was measured to be , 5.5 cm behind the actualemitting wire.

Present Addresses: * Ontario Cancer FoundationToronto-Bayvi ew Clinic.

** 115 Algonquin *** National Research CouncilDeep River, Ontario, Canada of Canada, Ottawa, Ontario

0018-9499/83/0400-1418$01.004 1983 IEEE

1418

For a wire ring source recessed in a convexfocus electrode (Fig. 4), the virtual source of allelectrons is a point on the beam axis behind the gun.A gap lens immediately after the anode focuses thebeam, and for any required image distance, a focalstrength can be chosen to produce a point image (asopposed to a ring image). Under these conditions thetransverse beam emittance, measured over a range ofbias and filament currents, was smaller than 30 iTmm-mrad (Fig. 5).

The emitting wire is 0.25 mm diameter 1% thori-ated tungsten, with the outer half of the cross-sectional area converted to tungsten carbide byheating in acetylene. The finished filament assemblyis quite rugged and has been shipped via regularsurface mail without damage.

Operating Experience

Initial conditioning of the accelerator with rfwas straightforward and full peak power was quicklyattained with very little arcing. The magnetronfrequency was then locked to the structure with anautomatic rf phase sensing feedback loop. The gun wasconditioned over a period of a few hours at low prf.Finally, the focusing and steering coils were adjustedto provide maximum beam current on the 10 mm thickcopper Faraday cup. The results in Table 1 show that150 mA peak current is readily available, the limitbeing determined, in practice, by the desired gunlifetime - higher emission means shorter life. Theeffect of Wehnelt bias on the electron beam qualitywas demonstrated by varying the bias voltage and fila-ment current so as to maintain constant cathodeemission (Table 2). At high bias voltages, the trans-mission approached the 45% theoretical value.

The on-axis dose rate was measured 1 metre fronthe 10 mm thick (total stopping) copper bremsstrahlungtarget using a 0.6 cc Baldwin Farmer ion chamber in a7.5 cm diameter lucite buildup cap. A dose rateequivalent to 300 RRM at 300 pps was measured for 100mA peak Faraday cup current; the addition of a 2.5 cmlong copper attenuator cone decreased the dose rate to150 RRMi equivalent.

Table 1

Cathode Emission and Output Faraday Cup Current as a Function of FilamentCurrent at -100 Volts Wehnelt Bias

Filament Filament Peak Cathode Peak Output BeamPower Current Emission Current(W) (A) (mA) (mA)4.2 4.25 20 8.45.3 4.50 55 206.7 4.75 150 438.3 5.00 295 769.4 5.25 480 117

10.5 5.50 680 155

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Table 2

The Effect on Transmission of Varying the Bias and Filament Currentso as to Maintain a Constant Cathode Emission of 250 mA Peak

Fi 1 amentCurrent

(A)

4.925.005.105.245.50

Bias Voltage Output BeamCurrent

(V) (mA)

-100-250-450-650-850

7068799696

PercentageTransmission

2424273333

Conclusions

A compact, small diameter 6 MeV electronaccelerator has been built using proven and reliableconstruction techniques6. It is a baked outstructure, and selection and processing of componentsshould assure a lifetime of 10 years. The energy waschosen at the optimum value for penetration in mostmaterials. The system was designed to be energyefficient, requiring , 2 MW of pulsed power. A doserate equivalent to 150 RMM at 300 pps was obtainedwith a 35 mm copper target and flattener.

With the addition of a thin nickel exit window(similar to that of the THERAC 25) a 60 1pA averageelectron beam could be extracted. With an optimizedbremsstrahlung converter, dose rates of 200 RMM over a35 x 35 cm area could be obtained. The design couldbe adapted to C- or X-band operation with a commensu-rate reduction in power and physical size. Such aunit would be light weight, easily portable, have avery simple rf drive package and would be ideal forportable reconnaissance and material testing.

References

1. T. Taylor, G. Van Dyk, L.W. Funk, R.M. Hutcheon and S.O. Schriber,"Therac 25: A New Medical Accelerator Concept", proceedings this con-ference.

2. S.O. Schriber, L.W. Funk and R.M. Hutcheon, "Effective Shunt ImpedanceComparison Between S-Band Standing Wave Accelerators with On-Axis andOff-Axis Couplers', Proceedings of the 1976 Proton Linear AcceleratorConference, Atomic Energy of Canada Limited, Report AECL-5677, 338(1977).

3. E.A. Knapp, B.C. Knapp and J.M. Potter, "Standing Wave High EnergyLinear Accelerator Structures", Rev. Sci. Instr. 39, 979 (1968).

4. M.E. Haine and D. Linder, 'High Brightness ElecFron Guns", Chap. 2.1,Vol. 1, in Focusing of Charged Particles, edited by A. Septier.

S. W.B. Herrmannsfeldt, 'Electron Trajectory Program", SLAC-166 (1973).6. S.B. Hodge, L.W. Funk and S.O. Schriber, "Mechanical Design Consider-

ations of a Standing Wave S-Band Accelerator with On-Axis Couplers,Proceedings of the 1976 Proton Linear Accelerator Conference, AtomicEnergy of Canada Limited, Report AECL-5677, 344 (1976).

Fig. 1 A compact 6 MeV pulsed S-band electron accelerator built byAccelerator Physics Branch, Atomic Energy of Canada Limited.

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