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Vacuum 77 (2004) 19–26

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The electron beam gun with thermionic hairpin-like cathodefor welding and surface modifications

Munawar Iqbala,�, Mohammad Rafiqa, Sarfraz A. Bhattia, Fazal-e- Aleemb

aElectron Beam Source Laboratory, Applied Physics Division, PINSTECH, P.O., Nilore, Islamabad-45650, PakistanbCenter for High Energy Physics, University of the Punjab, Quaid-e-Azam,Campus, Lahore-54590, Pakistan

Received 4 May 2004; received in revised form 1 July 2004; accepted 18 July 2004

Abstract

An axial thermionic electron beam emitter assembly with a special geometry of the cathode along with particular

spacing of the electrodes has been used to produce a stable, sharp and high power density image at an acceleration

voltage of 10 kV only. A hairpin-like tungsten wire, with diameter of 0.7 mm having semi-spherical emitting area at the

crown with an angle of 45 degree at the vertex was used as a cathode. A direct heating method was used to heat the

cathode. The emission current of the gun is in accordance with the Langmuir relation. An electromagnetic coil was used

for focusing the beam at the target. A two dimensional programmable movement was applied to control the work site in

the x–y direction. Focusing of the beam has been achieved up to 1 mm in diameter at an acceleration voltage of

10 kV.Thermionic efficiency of the gun is 4 mAW�1 and the power density measured is �105 W cm�2.The gun was used

for welding and surface modification of different materials including refractory metals.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Thermionic emission; Hairpin filament; Electron beam welding

1. Introduction

A good thermionic emitter has to have acombination of a low work function and highoperating temperature. However, metals withhigher melting point have a higher work function.The standard material for comparison is a poly-

e front matter r 2004 Elsevier Ltd. All rights reserv

cuum.2004.07.066

ng author. Tel.: +92-429-231-138; fax: +92-

ss: muniqbal@yahoo.com (M. Iqbal).

crystalline tungsten ‘hairpin’ filament with workfunction around 4.5 V, made of drawn wire a fewtenths mm in diameter, bent and situated in adiode structure. Most of electron beam weldershave used tungsten wire hairpin filament as acathode [1,2]. The wire hairpin filament has servedas a fine approximation to a point source for therelatively small amount of beam current. Inaddition, they are easily manufactured, inexpen-sive and have a reasonable life expectancy of10–50 h. The imaging characteristics of the

ed.

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M. Iqbal et al. / Vacuum 77 (2004) 19–2620

electron guns that uses wire hairpin filaments [3–6]are beam current dependent. This implies that thebeam current has to be focused and refocused forany change in beam current if a sharp focus is tobe maintained. As the beam power increases, thetask of obtaining a sharp focus becomes harder. Inour present work, we have improved the electronsource by employing special geometry of thecathode. It has been used for welding withimproved focusing abilities as the system operatingcapabilities were extending into the high powerregime. The filament we fabricated has highemission current densities and lifetime of morethan 100 h at the temperature of 3000 K. Due tothis geometry we have achieved diameter of thefocusing spot �1mm and the power density�105 W/cm2 at an acceleration voltage of 10 kVonly.

2. System description

2.1. Electron gun

The gun used to produce electron beam with itsassociated electromagnetic focusing system isshown schematically in Fig. 1. The electrons areproduced from a cathode consisting of a tungstenwire, diameter 0.7 mm, bent into a hairpin shape

SS Chamber

220 Volt AC

Focussing Coil

Cathode

Projected view of Filament

PC controlledPosition system

45

BeamFormer

10 kV DC

Apperture

ToVacuumpump

Anode

Work station

Fig. 1. Schematic diagram of the Electron Beam Gun.

(inverted V). An apertured anode is positioneddown the column at a distance of 8 mm from thecathode. Diameter of the aperture is 5mm. Theanode is connected to the ground. By energizingwith AC current (220 V), the cathode is heated upto the emission temperature of such a value that aspace charge limited stream of electrons is drawnby a constant acceleration voltage (10 kV). Theelectrons emitted by the cathode are accelerateddown the column and pass through the aperture ofthe anode to form a beam. The acceleratedelectrons are further collimated by passingthrough an additional aperture of diameter 6mmwhich is also at the ground potential. The move-ment of the work piece beneath the beam wasprogrammed using a personnel computer. At aheight of 0.7 mm from the cathode, a focusingelectrode is introduced to shape the electrons in toa beam. Diameter of the focusing electrode’saperture is 4mm with a negative potential of thesame order as of cathode. For improving the beamgenerating reliability, the focusing electrode wasdesigned [7] in such a manner that the potential ofthe focusing electrode is equal to the potential ofthe cathode. Focusing electrode is used to preventany emission at the cathode edge and to improvethe marginal trajectories [8]. This forms one of thetwo elements comprising the electrostatic lens ofthe gun. The anode forms the other. The potentialfield produced by the configuration of the anodeand the focusing electrode is such that during theacceleration electron flow is concentrated intoparaxial flow with a small dispersion angle.Focusing electrode, anode and aperture are madefrom a tantalum sheet of thickness 1mm with flatrectangular geometry. The amount of filamentrecession, its shape and aperture enlargement areimportant in producing a stable image. By con-structing the hairpin like filament from tungstenwire with optimum geometry, having angle at thevertex equal to 451 with reduced radius at theapex, enhanced the filament life and the quality ofthe image of the filament that ultimately increasesthe power density at the work site. The cathodeproduces a Maxwellian distribution in energy[9,10]. Mean initial energy of the electron iseV=kT for a typical emission temperature of2850K; this yields initial electron energy E0.25 eV.

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M. Iqbal et al. / Vacuum 77 (2004) 19–26 21

Filament is maintained at a negative potential(acceleration voltage) of 10 kV with respect toground so that energy of the beam electronsentering interaction region is close to 10 000 eV.Electrons that pass the aperture at this stage areresponsible for the final energy distribution of theelectron beam. No attempt is made to furthermono-chromate the electron beam at this stage.

In general, one needs to make sure that the gunis fairly well aligned with the aperture. Satisfactorymechanical alignment is made by looking througha small hole bored in the aperture plate. It isimportant that the filament be properly centered inrelation to the opening of the focusing electrodeand is at a proper distance from the opening withoptimum opening diameter. This helps in achiev-ing a space charge limited emission that isnecessary for a constant and stationary image ofthe beam at the work site. This ensures long-termstability for the electron beam, which is importantfor our applications. The interaction region (thework site) is maintained at ground potential. Weoperate at a maximum total beam current of300 mA, the practical limit because of space chargeeffects. Focus of the gun is adjusted to give uswhat we believe is a minimum space charge limitedspot size of 1 mm in the interaction region.

2.2. Focusing lens

Electrons are accelerated to their final velocityafter passing through the anode. The divergentbeam they make up still does not have sufficientpower density. To achieve this, electron beam mustbe focused. According to principle of electronoptics, each gun produces an enlarged image of thecathode in the field free region beyond the anode.Because of the low current densities, these cathodeimages are not suitable for the further electronoptical imaging in the gun. It is the focal spot ofthe gun or the smallest beam diameter that servesas the object of further beam guidance in the gun.We used an annular coil to produce magnetic field,which influences the direction of movement of theelectrons. The coil consists of large number (2200)of windings of copper wire shielded on all sides bya high permeability iron casing. A direct currentflows through the windings of the annular coil

producing a magnetic field, which acts inwardsfrom the iron casing on the electron beam. Theelectrons leave the magnetic lens without experi-encing any change in speed, describing a spiralslightly curved path, and meet at a focal point.Coil current is the main factor that determines thefocal length of the magnetic lens, and is referred toas the lens current. The focal length is inverselyproportional to the square of the lens current andis also directly proportional to the accelerationvoltage. In other words, the focal length can alsobe adjusted by changing the acceleration voltage.This however is not generally done in practice. Abeam of highest power density is more effective,that is, a higher power density beam can accom-plish the required work in the shortest possibletime and thus minimize heat conduction tomaterial adjacent the area being welded. The beampower density must be varied in accordance withthe type of operation to be performed and thecharacteristics of the material to be welded. Inorder to obtain high power density, preciseelectron optics must be applied in focusing thebeam.

The electron gun is a device that not onlyextracts the electrons and accelerates them to veryhigh velocities by virtue of the electric potentialapplied to the electrodes, but it also serves as anelectrostatic lens that shapes the electron flow intoa beam and focuses the beam to form an image ofthe electron source. The image formed by the gunis the ‘object’ that is focused or imaged by themagnetic lens onto the work piece to be welded.Thus, the magnetic lens is used to project theimage onto the target.

3. Results and discussions

The ‘Richardson–Dushman’ equation [11], de-scribes the current density emitted by a heatedfilament, as

JeT ¼ AT2 exp ð�f=kTÞ; (1)

where, A is material constant (60 A/cm2 K2 forTungsten), and f is a thermionic work function(about 4.5 eV for Tungsten) A plot of log (JeT/T2)versus 1/T yields a straight line whose negative

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0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300

Em

issi

on c

urre

nt (

mA

)

Langmuircurve, Zka = 8mm

Input power (W)

Richardson curve, Zka = 6mm

Fig. 2. Behavior of Electron Gun at different anode to filament

heights.

M. Iqbal et al. / Vacuum 77 (2004) 19–2622

slope gives the work function f. This value of f isreferred to as the ‘Richardson’ work function. Asmentioned in the above relation, the emissioncurrent density is strongly depended upon thetemperature of the cathode. Thus, low variationsin temperature will have a considerable effect onthe beam current. For electron beam welding withstringent requirements of weld seam reproduci-bility, this type of dependence would be extremelydetrimental. In such a case, it is possible to useanother physical relationship namely Langmuirrelationship [12],

JeR ¼ 2:3 � 10�6KV 3=2; (2)

where K=1/Z2KA, Z is the distance between the

cathode and anode. According to this, a givenaccelerating voltage is a function of geometricalcharacteristics such as the distance of the cathodefrom the anode. This helps us to extract a givenmaximum stable electron current density (spacecharge limited emission current density), JeR fromthe cathode. It is known that a high power electrongun is more efficient when electron beam isgenerated as a result of space charge limitedemission of its cathode. Moreover, with sufficientcathode heating, the power of the electron beamcan be controlled by varying the accelerationvoltage. However, if the acceleration voltage ischanged, all electric and magnetic fields necessaryfor guiding and positioning the electron beammust be readjusted accordingly [13]. Thus asufficiently high cathode temperature must be setby adjusting the heating current so that even at themaximum accelerating voltage sufficient electronsare still available. In other words, JeT must alwaysbe larger than JeR. Under these conditions thecathode will be surrounded by a cloud of ‘super-fluous’ electrons, which has an inherent charge andwill limit any further emission of electrons. In thiscondition, we say that cathode works in spacecharge mode. Fig. 2 shows how; at a highercathode temperature (heating power) the beamcurrent remains unaffected in the space chargemode. In this region, the increase in temperature isconsumed to increase the radiative power of thecathode. Increase in heating power at this stagewould only serve to reduce the cathode life. Inorder to obtain the limit for constant beam

current, we set the parameter ZKA=8 mm tooperate the gun under Langmuir relation. Thisensures optimum emission behavior over the entirelife of the cathode. At ZKA=6mm, the gun obeythe Richsrdson relation and we obtain an expo-nential increase in emission current at highertemperature but the trade off results in anexponential decrease in the lifetime of the cathodematerial. Our observations in Fig. 2 are in goodagreement with similar results given in theliterature [14].

We known from the light or electron optics [15]that focal length of magnetic lens is defined as

1=f ¼ 1=p þ 1=q; (3)

where, p is the distance of the electrostaticallyfocused image of the electron source (filament)from the center of the magnetic lens and q is thedistance of the focused beam image on the worksite from the center of the magnetic lens. For agiven work piece location q, the focal length f willbe constant for a constant object location p. Theelectron gun is an electrostatic lens system that notonly extracts electrons from the filament andforms them into a beam, but also produces an

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M. Iqbal et al. / Vacuum 77 (2004) 19–26 23

image of the source [16]. The image of the filamentproduced by the electron gun is the ‘object’ for themagnetic lens. Therefore, for f, to be constant, theimage produced by the electron gun must bestationary. By operating the electron gun in thespace charge mode we obtain a stable electron gunimage. This is achieved by adjusting the electrodespaces in such away that we get a stable electronbeam. This also improves the focusing stability ofthe magnetic lens due to stable and sharp focusingof the electron gun.

As the beam power increases, the task ofobtaining a sharp focus also increases. Focallength of magnetic lens [17] is determined by themagnetic field strength which itself is dependent onthe lens current driving the magnetic lens and thehigh voltage accelerating potential of the electrongun. This is described as

f ¼ kV=I2L; (4)

where IL and V are lens current and accelerationvoltage, respectively. The parameter k is depen-dent on various geometric factors includingnumber of turns in the coil and lens bore andgap. Fig. 3 shows the lens focusing current versus

0

50

100

150

200

250

300

350

0 4 8 10 12Beam energy (kV)

Foc

usin

g cu

rren

t (m

A)

f = 8cm f = 13cm f = 18cm

62

Fig. 3. Lens focusing current as a function of beam energy for

different coil focal lengths.

beam energy (acceleration voltage). In electronbeam welding, with increasing focal length andworking distances, the focal diameter increaseswhile the maximum depth of fusion decreases. Thisis due to decrease in the power density of thebeam. At a given beam current I, a weld seam canbe made narrow by decreasing the focal distanceresulting in smaller focal spot [13]. By altering thelens current and observing the point of impinge-ment through an optical system, the beamdiameter is noted. We optimized our lens systemat a focal distance of 13 cm to achieve theminimum spot diameter of the order of 1 mm.Diameters of 0.1–1.0mm are typical of electronbeam welding, depending upon the beam powerand the focal distance [14]. This focal diameterachieves the power density of about 105 W/cm2.As a result of the stable focus produced bythe improved filament in the gun design, thebeam quality, judged through different welds, isdefect free; their tensile strength and the penetra-tion is enough even at 10 kV only. We obtainedbeautiful seam geometry as shown in Fig. 4in which different refractory materials are weldedat 10 kV. Surface modification has also been

Fig. 4. Scanned Electron Beam Welded images on refractory

metals.

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0 20 40 60 80 100 120 140 160 180 200

< 45 degree ~ 20 = 45 degree > 45 degree ~ 90

Pow

er D

ensi

ty (

w/c

m2 )

Input Heating Power (W)

6.0E+05

5.0E+05

4.0E+05

3.0E+05

2.0E+05

1.0E+05

0.0E+00

Fig. 5. Power density as a function of heating power of

different bending angle of the filament at the vertex.

M. Iqbal et al. / Vacuum 77 (2004) 19–2624

accomplished by this gun which is reportedseparately [18].

Thus, a focused and high power density image isproduced as a result of suitable electrodes positionor spacing as well as by the suitable constructionof all the electrodes, specially the cathode. Thecathode is a 0.7 mm wire bent at 45 degree toobtain the maximum strength as well as theoperating life at high emission current. We bendthe tip at an angle of 45 degree to avoid anydeformity in the geometry as well as fusion of thefilament. This helps us in obtaining higher thermalefficiency and consequently achieves higher powerdensity at the target. The angle less than 45 degreeat the bend causes the shift of temperature maximaright or left of the immediate vicinity of the crown.This increases temperature in the immediatevicinity of the crown. This is due to addedradiations from inside resulting in melting offilament. Consequently, we obtain a distortedimage on the work site with reduced power densitythat affects all the welding parameters. Similarly atan angle greater than 45 degree at the bend, thetemperature maximum is shifted to the crown witha lower thermal efficiency of the gun that reducesthe power density at the target. Fig. 5 describespower density vs. heating power for differentbending angles at the vertex of the filament.Emitting area is approximately circular thatenhances the lifetime at higher operating tempera-tures. It also has an advantage of producing asymmetric shape of the image produced by thegun. Alternatively, a pointed area at the crowncauses higher evaporation rate at higher tempera-tures, reducing the filament life rapidly. As anexample, at 2880 K a temperature increase of100 K causes an increase of the vaporization rateby approximately 12%.

Temperature rise is now confined to a narrowarea of the tip of the cathode. Reduced crosssectional area of 0.2 mm2 at the outer side of thebend (apex) increases the resistance of the wire,giving a maximum rise in temperature andemission current. Rest of the area that is adjacentto the crown (apex) will be comparatively lessemissive. However, this reduction in area mini-mizes the operating life, which is compensated bychoosing a wire of greater cross section. It

necessitates an increased heating current level[19]. Micrographs of the improved cathode areshown in Fig. 6. It is observed that cathode area atthe tip is evaporated more. Repeated experimentsshow that melting of the cathode occurs at the tipwhile the rest of the area does not pass the meltingpoint, hence giving maximum emission currentwith minimum divergence.

4. Conclusions

Usefulness of an electron beam welding is highlydependent on the availability of a stable electronsource of high power density with long operatinglife. In electron beam welding (EBW) applications,the most important task is to devise a sustainabledefinite geometry of the seam with optimumparameters (voltage, beam current density, etc.).Most important role is played by the geometry orshape of the cathode. This significantly influencesthe electron beam distribution along with manyother parameters [20].

The cathode design, electrode spacing andfocusing system of the gun ensure a higher stability

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Fig. 6. Micrographs of the cathode showing temperature variations: difference shows that the grain size of the material near the tip is

bigger compared to that in the next part; reflecting that tip is at higher temperature from the area next to the tip (a) Image of the

cathode indicating different areas, (b) The arrow heads show the increased size of the grain, (c) The arrow heads show the original size

of the grain.

M. Iqbal et al. / Vacuum 77 (2004) 19–26 25

and sustainability of the electron beam para-meters. The gun provides a stable emission undernormal vacuum conditions (�10�5 mbar) withenhanced operating life of the filament at hightemperature [21,22]. Space charge at high tem-peratures is removed by using a cathode withhairpin like semi sphere at the crown with an angleof 450 at the vertex. This shape produces a high

electric field in front of the cathode. As a result ofthis geometry, we are able to yield a larger thermalefficiency compared to conventional hairpin cath-odes at higher emission current density. Anotheradvantage is that the astigmatism of the conven-tional hairpin cathode is avoided.

Temperature rise is well confined to a narrowarea of the tip. Due to its high perveance

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10�5 A V�3/2 and power density 105 Wcm�2, wewere able to weld refractory materials. By usingspecial geometry of a hairpin filament, we couldweld materials up to the order of 5mm in thicknesswith 10 KeV beam energy only. This filament issimple in design, rugged in construction andeconomical to manufacture.

Acknowledgements

We acknowledge financial assistance from High-er Education Commission Islamabad, Pakistanthrough ‘‘Indigenous Scholarship Scheme’’. Weare also grateful for useful discussion with Mr.Zahid Majed. Thanks are also due to Mr. ShahidHameed Minhas and Mr. Mohammad Arshad fortheir technical support.

References

[1] Pavel J, Bohumila L, Jakub Z. Microsc. Microannal

2003;9(3):22.

[2] Burno JM, Claude RC, Christian B. US Patent 4357517:1,

1982.

[3] Boesten L, Okada K. Meas Sci Technol 2000;11(5):576.

[4] Mark TB, Kin FM, Ara C. Rev Sci Instrum

1988;59(11):2419.

[5] Peter WE, Edward CZ. Rev Sci Instrum 1982;53(2):225.

[6] Crewe AV. Rev Sci Instrum 1968;39(4):576.

[7] Munawar I, Khalid M, Rafiq M, Maqbool AC, Fazal A.

Proceedings of International Conference on High Power

Electron Guns, ‘ebeam 2002’. Vol. 18. USA 2002, p. 1.

[8] Munawar I, Khalid M, Rafiq M, Maqbool AC, Fazal A.

Rev Sci Instrum 2003;74(3):1196.

[9] Schiller S, Heisigand U, Panzer S. Electron beam

technology. New York: Wiley; 1982.

[10] Martin R. Theory and design of charged particle beams.

New York: Wiley; 1994.

[11] Munawar I, Khalid M, Rafiq M, Maqbool AC, Fazal A.

Rev Sci Instrum 2003;74(9):4616.

[12] Munawar I, Khalid M, Maqbool AC, Rafiq M. Nucleus

2001;38:1.

[13] Schiller S, Jaesch G, Ardenne von A, US Patent 4665297:

1, 1987.

[14] Schultz H. Electron beam welding. Cambridge: Wood-

head; 1993.

[15] Hawkes P, Kasper E. Principles of electron optics. New

York: Academic Press; 1994.

[16] Lawrence GS, US. Patent 3869636: 1, 1975.

[17] Kareh El AB, Kareh EL CJ. Electron beams, lenses, and

optics. New York: Academic Press; 1970.

[18] Maqsood A, Javed IA, Mohammad AS, Akthar M, Iqbal

M, Maqbool AC. J Nucl Mater 2002;301:118.

[19] Otto W, Blazers L, US Patent 4899078: 1, 1990.

[20] Koleva E, Vuchkov I, Proceedings of 7th International

Conference on Electron Beam Technologies, Varna. 2003,

p. 221.

[21] Iiyoshi R, Maruse S, Takematsu H. Nucl Instrum Meths

1995;A363:284.

[22] Iihoshi R, Kitamaura T. Proceedings of 7th International

Conference on Electron Beam Technologies (EBT 2003),

Varna. 2003, p. 19.