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Vacuum 85 (2011) 654e656

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Vacuum

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Optimal welding parameters with 10 keV point source electron gun

Munawar Iqbal a,b,d,*, M. Ayub a, Zahid Majeed c, H.M. Akramd

aCHEP, University of the Punjab, Quaid-e-Azam Campus, Lahore, PakistanbNational Institute of Lasers & Optronics, Islamabad, PakistancNational Engineering and Science Commission, Islamabad, PakistandNINVAST, NCP Complex, Quaid-e-Azam University, Islamabad, Pakistan

a r t i c l e i n f o

Article history:Received 4 May 2010Received in revised form12 August 2010Accepted 30 August 2010

Keywords:Thermionic electron beam gunEmission characteristicsWelding

* Corresponding author. CHEP, University of the PuLahore, Pakistan.

E-mail address: muniqbal@yahoo.com (M. Iqbal).

0042-207X/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.vacuum.2010.08.027

a b s t r a c t

We present the update on the emission characteristics and results from previously reported (Vacuum 77(2004) 19e26), experimental study of axial point source thermionic electron beam emitter assembly.Emission characteristics for this gun have been presented. Optimal conditions to obtain the requisitebeam characteristics for welding of refractory metals at 10 keV energy are reported.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Thermionic electron beam guns have become the basic and keyinstruments for the latest and the state of the art technologies likeFree Electron Lasers (FELs) [1e3] and Linear Accelerators (LINACs)[4]. Electron beam guns used in LINAC and for many technologicalapplications (surface modifications, welding, etc.) have a number ofsimilar requirements and characteristics. As a rule, thermionic hotcathode emitters are used in both applications. The most widelyused thermionic electron guns incorporate tungsten hairpin whichcan operate under somewhat bad vacuum conditions [5]. The maingoal of this paper is to find the optimal parameter between thecathode and focusing electrode of our axial thermionic electronbeam gun [6] for EB welding applications of refractory metals. Inthe following section, the design and experiment on an axialthermionic electron beam emitter assembly are reported and alsothe results are presented.

2. Design & specifications

Electrons are emitted from the heated filament (which is a hotcathode perpendicular to the surface) as a result of the heatingcurrent applied by a 220 V AC power supply. A �10 kV power

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supply is connected to the filament as well to give a kick to theseelectrons liberated from the filament. An electrode made fromtantalum (a refractory metal which also minimizes the amount ofthe molten material in the region close to the heat source) platehaving a hole in the center for the beam exit is placed 1 mm awayfrom the filament. This beam former actually shapes the electronsinto a regular beam. The potential of this electrode is same as thatof filament (�10 kV). Actually one end of it is connected with�10 kV supply and the other end is kept insulated. Due to that theonly electrons that see the potential of the anode through thehole of this electrode will pass through and will be acceleratedwith energy of 10 keV towards the anode. The negative chargewill accumulate by time at the focusing electrode that raises thepotential of this electrode. Due to accumulated charge the elec-trons coming from filament will be reflected back. Only thoseelectrons will pass through the hole that see the positivepotential at the anode with a kick towards the optical center ofthe hole due to same charge repulsion effect that pinches theelectrons and to form a well collimated and parallel beam ofelectrons. The saturation limit of this electrode cannot exceed toits maximum value due to the leakage of the electrons throughhole inside. Therefore, it is pertinent to mention that the shape ofthe beam depends upon the size, shape and particularly the exitof the electrode.

Now the parallel stream of electrons approaches to the anodewith a solid angle at the focusing electrode, although the wholeelectrons received at the anode would be registered by the powersupply. However, again here the amount of charge/current received

Fig. 1. Schematic of the electron beam gun assembly.

Table 1Values of the potentials applied to different electrodes.

Electrodes Max. potentials

Filament/cathode �10 kV DC & 220 kV ACBeam shaper �10 kV DCAnode 0 V DCFocusing coil 1e15 V DCWork plate 0 V DCWork chamber 0 V DC

filament heating power 150W

filament heating power 100W

filament heating power 50W

0 2000 4000 6000 8000 10 0000

100

200

300

400

Acceleration voltage (V)

Emis

sion

cur

rent

(m

A)

Fig. 3. Emission current variation as a function of acceleration voltage at differentheating powers.

Fig. 4. Beam profile on SS Sheet.

M. Iqbal et al. / Vacuum 85 (2011) 654e656 655

at the work plate/piece will depend on the size, shape and natureof the anode and aperture both. It does not depend upon thepotential of the anode. The work plate is at the same potential(ground as of anode). Almost 20e30% of charge will be blocked bythe anode. Only those electrons will reach at the work plates whichare in front of the hole of the anode. Rest of the electrons will beblocked by the anode surface although they would also be regis-tered by the power supply. The total current will be the sum of theboth at work plate and anode surface electrons. Therefore, it willreceive almost 70% of the charge due to the focusing current to thelens placed in-between the anode and work plate. The rest of 30%is absorbed by the anode. The schematic of the assembly is shownin Fig. 1 and Table 1 describes the overall potentials applied to theelectrodes.

Fig. 2. Emission current versus distance between the filament and focusing electrodeat different input voltages.

3. Emission characteristics

Beam current is dependent on the position, geometry andnature of the orifice of the focusing electrode through which thebeam is passing. It is also very much dependent on the potential ofthis electrode as well. This potential may vary from a minimum toa maximum value of the cathode potential. In our case, we use thesame potential as of cathode with particular geometry of rectan-gular shape, and spherical opening, we optimized our emission by

J max

0.8J max

J

50 100 150 200

50

100

150

200

Fig. 5. Emission current density variation across the cathode.

Fig. 6. (a) Aluminum joining, (b) stainless steel joining.

M. Iqbal et al. / Vacuum 85 (2011) 654e656656

varying the electrode at different positions as can be seen in Fig. 2.Having same potential has the facility to use the same power supplyas of cathode to energize the electrons. We can also achieve evenhigher beam current by increasing the vacuum to higher valuesbecause besides anti-scattering of electrons, the higher vacuumprovides a good insulation as required by the electrodes. However,the beam current of 130 mA at a distance of 1 mm (Fig. 2), isachieved for an input voltage of 30 V, at a vacuum of the order of2.4 � 10�4 mbar. This is the minimum value of vacuum duringoperation while in the beginning this value was 1 �10�5 mbar. Forwelding purposes this range is good enough, however, for evapo-ration (coating purposes) higher vacuum (>10�6 mbar) is needed.At higher distances, increased input voltages to the filament cannotenhance the emission current although it increases the inputheating power to the filament that makes available more electronsfrom the cathode. We set cathode to anode distance at 6 mm for allvalues of cathode to focusing electrode. The optimization of thisdistance is reported in Ref. [6]. At this stage only the accelerationvoltages can enhance the emission from the cathode. The beamemission remains stable throughout the gun operation. Therefore,an emission current of 400 mA is achieved at an input heatingpower of 150 W with acceleration voltage of 10 kV to the cathodethat corresponds to a maximum 4 kW power. This variation can beseen in Fig. 3.

4. Results & discussion

The beam profile of the source at theworksite was found to havethe Gaussian distribution. The physical picture of the profile isrecorded on SS Sheet as shown in Fig. 4, and the emission currentvariation across the cathode is shown in Fig. 5. The minimumdiameter of the beam achieved at the worksite is 2 mm. The spotsizewas adjustedwith the help of focusing lens. For a fixed focusingcurrent to the electromagnetic lens, the beam diameter remainsunchanged. Of course plasma emission is produced by the impact ofthe beam with the target that could ruin the weld piece due todefocusing of the beam. This emission is controlled by beamcurrent that could not be exceeded by the melting temperature ofthe work piece. However, by efficient vacuum environment weovercome this problem as well.

With the help of the optimized updated design of the emitterassembly we welded both the similar and dissimilar metals. InFig. 6(a), the Aluminum metal is welded with beautiful seamgeometry. The penetration depth of the beam is up to 3 mm with10 kV of acceleration voltage and the beam diameter is 2 mm. Thebeam constancy is obvious throughout from the weld seamgeometry. An electronic positioning systemwas used to control the

seam welding in XeY directions. Radiograph analysis reveals thatthere is no crack developed in the weld. The tensile strength of theweld (not shown here) is one order higher than the as-receivedsample. The same is also true for the SS samples as shown inFig. 6(b).

5. Conclusions

Besides other parameters, high emission current is a function ofdistance between the electrodes (cathode, focusing and anode).This position may produce both the space and temperature limitedmodes of operation of the gun. Even higher input voltages cannotenhance the emission from the gun unless the distances betweenthe electrodes are not optimal. These distances have upper andlower bound limits. In our gun design, the upper limits for focusingto anode electrode lie from 6 to 10 mm and the lower limit liesbetween 1 and 2 mm. However, the optimal distance betweencathode and focusing electrode is 1 mm while between focusingand anode is 6 mm. Fixing these optimal distances, we obtaineda focused and constant beam due to that we welded the refractorymaterials with acceleration voltage of 10 kV with a penetrationdepth of 3e5 mm. The stability of the energy was better than 95%per hour of the gun operation. Power density was reported of theorder of 105 W/cm2 [6] which is the maximum value achieved forthe said gun. High power density permits to carry out such tech-nological processes as electron beam welding of thick metalcomponents.

Acknowledgments

We are very grateful to the NILOP, Islamabad for the experi-mental phase of this work. One of us (MI) also acknowledges therole of NCP, Islamabad, for providing the research facility for thetheoretical stage of the work.

References

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