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Vacuum 81 (2006) 499–501

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Design optimization of a hairpin electron sourcefor electron beam welding

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

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 31 August 2005; received in revised form 24 May 2006; accepted 9 July 2006

Abstract

We report the optimization of the design and performance of recently reported hairpin tungsten electron source as a cathode in diode

type geometry of the gun. The temperature maximum has been shifted close to the crown of the source. Focusing of the beam has been

achieved up to 1mm in diameter with Gaussian profile of the beam at the target. The perveance and power density measured are

10�5AV�3/2 and 106Wcm�2, respectively.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Hairpin filament; Electron source; Electron beam welding; Power density

1. Introduction

Presently two types of electron sources are used toproduce intense electron beams: a diode and a triode. Theemission current density in the triode source is affected notonly by the additional space charge of the reflectedelectrons in the source region, but also by the current,because part of it is absorbed at the grid electrode. Theextra current to the grid causes a decrease in the gridpotential. The decrease in the grid voltage leads to asmaller emission current density at the cathode [1]. Thesimplest version is the diode source with plane electrodeswhere the beam current can be varied by a change in eitherthe cathode temperature or the acceleration voltage. Incontinuation to our work [2–6] on electron guns, wedesigned and experimentally verified the filament geometryof two hairpin sources in a diode configuration to producean intense beam and compared the performance of thesesources. One of the fabricated filaments has hairpingeometry while the other one is close to V-shape. In a

ee front matter r 2006 Elsevier Ltd. All rights reserved.

cuum.2006.07.007

ing author. Center for High Energy Physics, University of

aid-e-Azam, Campus, Lahore 54590, Pakistan.

31138; fax: +92 42 9231253.

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

wider sense, both can be called hairpin sources. The wirehairpin filament is a fine approximation to a point sourcefor the relatively small amount of beam current.

2. Design and principles

Fabrication of thermionic cathodes is a highly specia-lized art where the choice and composition of materials isguided by the requirements of low work function W, longlifetime (at high temperature), smoothness of the emittingsurface, and the other factors [7]. Our choice of material istungsten for a number of its suitable properties [8].Pure tungsten has a work function of W ¼ 4.5 eV, andtungsten cathodes operate at a temperature of 2500K(kB T�0.2 eV). The source of electrons comprises afilament, which is made up of tungsten wire of 0.7mm indiameter heated with AC current of 220V.Energy distribution of conduction electrons in metals

obeys the Fermi–Dirac statistics. ‘‘The electrons emittedfrom the thermionic cathode belongs to the Maxwellian tailof Fermi–Dirac distribution and the current density Jth isgiven by the Richardsons–Dushman equation

Jth ¼ AT2e�W=k2B

T . (1)

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600

500

400space chargeregion

t (m

A)

hairpin sourceV-shaped source

M. Iqbal et al. / Vacuum 81 (2006) 499–501500

Here T is the cathode temperature, W the work function ofthe cathode material, and kB is the Boltzmann’s constant(8.6175� 10�5) eV/K. The theoretical value for the con-stant A is

A ¼ 4pemk2B=h3, (2)

where e is electron charge, m electron mass and h the Plankconstant.

The basic design concept of charged particle sourcessuffices the simple diode configurations. In such sources,the space-charge electric field limits the amount of thecurrent that can be accelerated by the given voltage V. Forplanner electrode geometry with a gap spacing d betweenthe two plates, the limiting current density J (in the nonrelativistic limit and in MKS units) is given by the formula

J ¼ 1:67� 10�3ðq=mc2Þ1=2V3=2=d2ðA=m2Þ, (3)

where q and m are the charge and mass, respectively, and c

the speed of light. The relation is called the Child orChild–Langmuir Law. If we apply this result to a uniformbeam emitted from a circular area with radius r, the beamcurrent will be obtained as

I ¼ 1:67p� 10�3ðq=mc2Þ1=2V 3=2ðr=dÞ2 ðAÞ. (4)

The ratio I/V3/2 is a constant known as the perveance of thebeam, which is a function of the geometry of the system’’[9]. However, if the voltage becomes sufficiently high, theRichardson limit for current is reached when the emissionbecomes temperature limited. As mentioned earlier, weused the hairpin filament to produce a stable and highpower electron beam keeping all the parameters constantexcept the geometry of the source. The acceleration voltagewe used is �10 kV with filament to anode gap of 6mm.

3. Performance

We tried different geometry for electron sources relatedwith hairpin as shown in Fig. 1 and with the help ofrepeated experiments optimized the shape of the source forour gun. The source shown in Fig. 1a (hairpin) has highemission current in the regime of low input powercompared to the source (V-shape) shown in Fig. 1b. Thisis due to effect of high temperature gain because of thecrossed radiations produced by the closer legs near the

Fig. 1. Electron sources of different geometry.

crown area of the source. When the heating power isincreased further, the temperature of this source gets closeto its melting point and saturates the emission current. Thisresults in melting of the filament at low emission current byfurther increase in heating power compared to the othersource in discussion. According to Eq. (3) this saturationcan be changed either by applying greater accelerationvoltage or by reducing the filament to anode gap. Theemission characteristics of the electron sources used for thebeam production in a diode are shown in Fig. 2. Theexperimental values of emission characteristics are in goodagreement with theoretical values [10]. In the space-chargelimited mode, electron source as of Fig. 1b produced astable and good quality image compared to the other whilein temperature-limited mode it behaves oppositely.Next, we noted the temperature of the sources at the

crown with the help of a pyrometer. We observed that thetemperature maximum is shifted closer to the crown of thefilament in case of V-shaped source. It is for this reasonthat the beam diameter of the V-shaped source producesthe Gaussian distribution of the current or power. Theprofile of the beam thus obtained at the target is shown inFig. 3. The distorted image produced by the hairpin sourceis due to the shifting of temperature maximum furtheraway from the crown. The current density distribution inthe focal spot, which corresponds to a Gaussian distribu-tion because of the Maxwellian electron velocity distribu-tion, is described by

J ¼ Jð0Þ expð�r2=r2BÞ, (5)

where r is the radial distance and rB is the radius of animaginary focal spot having a uniform current density, J.The bounds of diameter of focal point is the radius whereintensity becomes Maximum Intensity/e or J ¼ Jmax/2.71828183. In a beam with such diameter there exists aflow that amount to 63.8% of the beam current. Beampatterns are likely to occur at the transition from the space-

300

200

100

00 20 40 60 80 100 120 140 160

Input heating power (W)

temperaturelimited region

Em

issi

on c

urre

n

Fig. 2. Emission characteristics of the sources.

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Fig. 3. Profile of the beam at SS target sheets.

Fig. 4. Scanned electron beam welded specimen of SS metals from V-

shaped source.

M. Iqbal et al. / Vacuum 81 (2006) 499–501 501

charge regime to the saturation regime of a cathode. Thesebeam patterns enlarge the focal spot diameter [11].

In the post anode region, electron beam radii of thesesources practically remain same. In the pre anode region,when beam enters in the field free region, these radii thenincrease due to space charge repulsion. In order to preventdivergence due to space-charge forces or transverse velocityspread, the beam has to be focused with appropriatemagnetic lenses, as the effect of temperature on the beamradius is negligible compared to the space-charge forces. Itis desirable to compress the beam radially. The compres-sion is achieved by shaping the electrode and by utilizingthe focusing action of the magnetic field. We used anannular coil enclosed on three sides in a magnetic ironcasing. By choosing the appropriate electrode angles, onecan obtain a transverse focusing force at the beam edgethat either exactly balances the space-charge force bykeeping the radius constant or exceeds the space charge toobtain a converging beam (for radial compression) in thegun region. Welding the SS sheets as shown in Fig. 4checked the constancy of the beam.

4. Conclusions

Most of electron beam welders have used tungstenhairpin filament as a cathode [12–14]. But still there was aneed to further optimize its geometry for high powerapplications like welding, surface modification and physi-

cal vapor deposition. The geometry used in Fig. 1a is usefulfor the field emission type of sources due to the geometryclose to the point cathode types of filaments [8]. Electronbeam source as in Fig. 1a has the disadvantage, as it needsvacuum of the order of 10�9mbar. In our geometry ofbeam source, however, with thermioinic emission pro-cesses, can be operated even in hard vacuum that is10�4mbar. This filament geometry is sustainable for veryhigh emission current and long duration. This hairpinfilament has served as a fine approximation to a pointsource for the relatively small amount of beam current. Ithas been used for welding with improved focusing abilitiesas the system operating capabilities were extending into thehigh-power regime. Due to its high perveance, 10�5AV�3/2

and power density 106W cm�2, we were able to weldrefractory materials up to the order of 2mm in thicknesswith 10KeV beam energy only. This filament is simple indesign, rugged in construction and economical to manu-facture.

Acknowledgements

We acknowledge the enable role of the Higher EducationCommission Islamabad, Pakistan and appreciate itsfinancial support through ‘‘Indigenous Scholarship Schemefor PhD Studies in Science & Technology’’. Thanks arealso due Mr. Shahid Hameed Minhas and Mr. MohammadArshad for their technical support during the experiment.

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