- Email: [email protected]
The electron beam gun with thermionic hairpin-like cathode for welding and surface modifications
ARTICLE IN PRESS
Vacuum 77 (2004) 19–26 www.elsevier.com/locate/vacuum
The electron beam gun with thermionic hairpin-like cathode for welding and surface modifications Munawar Iqbala,, Mohammad Rafiqa, Sarfraz A. Bhattia, Fazal-e- Aleemb a
Electron Beam Source Laboratory, Applied Physics Division, PINSTECH, P.O., Nilore, Islamabad-45650, Pakistan b Center 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 mA W1 and the power density measured is 105 W cm2.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 a combination of a low work function and high operating temperature. However, metals with higher melting point have a higher work function. The standard material for comparison is a polyCorresponding author. Tel.: +92-429-231-138; fax: +92-
429-231-253 E-mail address: [email protected] (M. Iqbal).
crystalline tungsten ‘hairpin’ filament with work function around 4.5 V, made of drawn wire a few tenths mm in diameter, bent and situated in a diode structure. Most of electron beam welders have used tungsten wire hairpin filament as a cathode [1,2]. The wire hairpin filament has served as a fine approximation to a point source for the relatively small amount of beam current. In addition, they are easily manufactured, inexpensive and have a reasonable life expectancy of 10–50 h. The imaging characteristics of the
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.066
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
20
electron guns that uses wire hairpin filaments [3–6] are beam current dependent. This implies that the beam current has to be focused and refocused for any change in beam current if a sharp focus is to be maintained. As the beam power increases, the task of obtaining a sharp focus becomes harder. In our present work, we have improved the electron source by employing special geometry of the cathode. It has been used for welding with improved focusing abilities as the system operating capabilities were extending into the high power regime. The filament we fabricated has high emission current densities and lifetime of more than 100 h at the temperature of 3000 K. Due to this geometry we have achieved diameter of the focusing spot 1 mm and the power density 105 W/cm2 at an acceleration voltage of 10 kV only.
2. System description 2.1. Electron gun The gun used to produce electron beam with its associated electromagnetic focusing system is shown schematically in Fig. 1. The electrons are produced from a cathode consisting of a tungsten wire, diameter 0.7 mm, bent into a hairpin shape
10 kV DC
220 Volt AC Cathode
Beam Former Anode Apperture
Focussing Coil
PC controlled Position system
45
Work station SS Chamber
To Vacuum pump
Projected view of Filament
Fig. 1. Schematic diagram of the Electron Beam Gun.
(inverted V). An apertured anode is positioned down the column at a distance of 8 mm from the cathode. Diameter of the aperture is 5 mm. The anode is connected to the ground. By energizing with AC current (220 V), the cathode is heated up to the emission temperature of such a value that a space charge limited stream of electrons is drawn by a constant acceleration voltage (10 kV). The electrons emitted by the cathode are accelerated down the column and pass through the aperture of the anode to form a beam. The accelerated electrons are further collimated by passing through an additional aperture of diameter 6 mm which is also at the ground potential. The movement of the work piece beneath the beam was programmed using a personnel computer. At a height of 0.7 mm from the cathode, a focusing electrode is introduced to shape the electrons in to a beam. Diameter of the focusing electrode’s aperture is 4 mm with a negative potential of the same order as of cathode. For improving the beam generating reliability, the focusing electrode was designed [7] in such a manner that the potential of the focusing electrode is equal to the potential of the cathode. Focusing electrode is used to prevent any emission at the cathode edge and to improve the marginal trajectories [8]. This forms one of the two elements comprising the electrostatic lens of the gun. The anode forms the other. The potential field produced by the configuration of the anode and the focusing electrode is such that during the acceleration electron flow is concentrated into paraxial flow with a small dispersion angle. Focusing electrode, anode and aperture are made from a tantalum sheet of thickness 1 mm with flat rectangular geometry. The amount of filament recession, its shape and aperture enlargement are important in producing a stable image. By constructing the hairpin like filament from tungsten wire with optimum geometry, having angle at the vertex equal to 451 with reduced radius at the apex, enhanced the filament life and the quality of the image of the filament that ultimately increases the power density at the work site. The cathode produces a Maxwellian distribution in energy [9,10]. Mean initial energy of the electron is eV=kT for a typical emission temperature of 2850K; this yields initial electron energy E0.25 eV.
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
Filament is maintained at a negative potential (acceleration voltage) of 10 kV with respect to ground so that energy of the beam electrons entering interaction region is close to 10 000 eV. Electrons that pass the aperture at this stage are responsible for the final energy distribution of the electron beam. No attempt is made to further mono-chromate the electron beam at this stage. In general, one needs to make sure that the gun is fairly well aligned with the aperture. Satisfactory mechanical alignment is made by looking through a small hole bored in the aperture plate. It is important that the filament be properly centered in relation to the opening of the focusing electrode and is at a proper distance from the opening with optimum opening diameter. This helps in achieving a space charge limited emission that is necessary for a constant and stationary image of the beam at the work site. This ensures long-term stability for the electron beam, which is important for our applications. The interaction region (the work site) is maintained at ground potential. We operate at a maximum total beam current of 300 mA, the practical limit because of space charge effects. Focus of the gun is adjusted to give us what we believe is a minimum space charge limited spot size of 1 mm in the interaction region. 2.2. Focusing lens Electrons are accelerated to their final velocity after passing through the anode. The divergent beam they make up still does not have sufficient power density. To achieve this, electron beam must be focused. According to principle of electron optics, each gun produces an enlarged image of the cathode in the field free region beyond the anode. Because of the low current densities, these cathode images are not suitable for the further electron optical imaging in the gun. It is the focal spot of the gun or the smallest beam diameter that serves as 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 the electrons. The coil consists of large number (2200) of windings of copper wire shielded on all sides by a high permeability iron casing. A direct current flows through the windings of the annular coil
21
producing a magnetic field, which acts inwards from the iron casing on the electron beam. The electrons leave the magnetic lens without experiencing any change in speed, describing a spiral slightly curved path, and meet at a focal point. Coil current is the main factor that determines the focal length of the magnetic lens, and is referred to as the lens current. The focal length is inversely proportional to the square of the lens current and is also directly proportional to the acceleration voltage. In other words, the focal length can also be adjusted by changing the acceleration voltage. This however is not generally done in practice. A beam of highest power density is more effective, that is, a higher power density beam can accomplish the required work in the shortest possible time and thus minimize heat conduction to material adjacent the area being welded. The beam power density must be varied in accordance with the type of operation to be performed and the characteristics of the material to be welded. In order to obtain high power density, precise electron optics must be applied in focusing the beam. The electron gun is a device that not only extracts the electrons and accelerates them to very high velocities by virtue of the electric potential applied to the electrodes, but it also serves as an electrostatic lens that shapes the electron flow into a beam and focuses the beam to form an image of the electron source. The image formed by the gun is the ‘object’ that is focused or imaged by the magnetic lens onto the work piece to be welded. Thus, the magnetic lens is used to project the image onto the target.
3. Results and discussions The ‘Richardson–Dushman’ equation [11], describes the current density emitted by a heated filament, as J eT ¼ AT 2 exp ðf=kTÞ;
(1) 2
2
where, A is material constant (60 A/cm K for Tungsten), 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
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
slope gives the work function f. This value of f is referred to as the ‘Richardson’ work function. As mentioned in the above relation, the emission current density is strongly depended upon the temperature of the cathode. Thus, low variations in temperature will have a considerable effect on the beam current. For electron beam welding with stringent requirements of weld seam reproducibility, this type of dependence would be extremely detrimental. In such a case, it is possible to use another physical relationship namely Langmuir relationship [12], J eR ¼ 2:3 106 KV 3=2 ;
(2)
where K=1/Z2KA, Z is the distance between the cathode and anode. According to this, a given accelerating voltage is a function of geometrical characteristics such as the distance of the cathode from the anode. This helps us to extract a given maximum stable electron current density (space charge limited emission current density), JeR from the cathode. It is known that a high power electron gun is more efficient when electron beam is generated as a result of space charge limited emission of its cathode. Moreover, with sufficient cathode heating, the power of the electron beam can be controlled by varying the acceleration voltage. However, if the acceleration voltage is changed, all electric and magnetic fields necessary for guiding and positioning the electron beam must be readjusted accordingly [13]. Thus a sufficiently high cathode temperature must be set by adjusting the heating current so that even at the maximum accelerating voltage sufficient electrons are still available. In other words, JeT must always be larger than JeR. Under these conditions the cathode will be surrounded by a cloud of ‘superfluous’ electrons, which has an inherent charge and will limit any further emission of electrons. In this condition, we say that cathode works in space charge mode. Fig. 2 shows how; at a higher cathode temperature (heating power) the beam current remains unaffected in the space charge mode. In this region, the increase in temperature is consumed to increase the radiative power of the cathode. Increase in heating power at this stage would only serve to reduce the cathode life. In order to obtain the limit for constant beam
Richardson curve, Zka = 6mm
Langmuircurve, Zka = 8mm
450 400 350 Emission current (mA)
22
300 250 200 150 100 50 0 0
50
100 150 200 Input power (W)
250
300
Fig. 2. Behavior of Electron Gun at different anode to filament heights.
current, we set the parameter ZKA=8 mm to operate the gun under Langmuir relation. This ensures optimum emission behavior over the entire life of the cathode. At ZKA=6 mm, the gun obey the Richsrdson relation and we obtain an exponential increase in emission current at higher temperature but the trade off results in an exponential decrease in the lifetime of the cathode material. Our observations in Fig. 2 are in good agreement with similar results given in the literature [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 electrostatically focused image of the electron source (filament) from the center of the magnetic lens and q is the distance of the focused beam image on the work site from the center of the magnetic lens. For a given work piece location q, the focal length f will be constant for a constant object location p. The electron gun is an electrostatic lens system that not only extracts electrons from the filament and forms them into a beam, but also produces an
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image of the source [16]. The image of the filament produced by the electron gun is the ‘object’ for the magnetic lens. Therefore, for f, to be constant, the image produced by the electron gun must be stationary. By operating the electron gun in the space charge mode we obtain a stable electron gun image. This is achieved by adjusting the electrode spaces in such away that we get a stable electron beam. This also improves the focusing stability of the magnetic lens due to stable and sharp focusing of the electron gun. As the beam power increases, the task of obtaining a sharp focus also increases. Focal length of magnetic lens [17] is determined by the magnetic field strength which itself is dependent on the lens current driving the magnetic lens and the high voltage accelerating potential of the electron gun. This is described as f ¼ kV =I 2L ;
(4)
where IL and V are lens current and acceleration voltage, respectively. The parameter k is dependent on various geometric factors including number of turns in the coil and lens bore and gap. Fig. 3 shows the lens focusing current versus f = 8cm
f = 13cm
23
beam energy (acceleration voltage). In electron beam welding, with increasing focal length and working distances, the focal diameter increases while the maximum depth of fusion decreases. This is due to decrease in the power density of the beam. At a given beam current I, a weld seam can be made narrow by decreasing the focal distance resulting in smaller focal spot [13]. By altering the lens current and observing the point of impingement through an optical system, the beam diameter is noted. We optimized our lens system at a focal distance of 13 cm to achieve the minimum spot diameter of the order of 1 mm. Diameters of 0.1–1.0 mm are typical of electron beam welding, depending upon the beam power and the focal distance [14]. This focal diameter achieves the power density of about 105 W/cm2. As a result of the stable focus produced by the improved filament in the gun design, the beam quality, judged through different welds, is defect free; their tensile strength and the penetration is enough even at 10 kV only. We obtained beautiful seam geometry as shown in Fig. 4 in which different refractory materials are welded at 10 kV. Surface modification has also been
f = 18cm
350
300
Focusing current (mA)
250
200
150
100
50
0 0
2
4 8 6 Beam energy (kV)
10
12
Fig. 3. Lens focusing current as a function of beam energy for different coil focal lengths.
Fig. 4. Scanned Electron Beam Welded images on refractory metals.
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accomplished by this gun which is reported separately [18]. Thus, a focused and high power density image is produced as a result of suitable electrodes position or spacing as well as by the suitable construction of all the electrodes, specially the cathode. The cathode is a 0.7 mm wire bent at 45 degree to obtain the maximum strength as well as the operating life at high emission current. We bend the tip at an angle of 45 degree to avoid any deformity in the geometry as well as fusion of the filament. This helps us in obtaining higher thermal efficiency and consequently achieves higher power density at the target. The angle less than 45 degree at the bend causes the shift of temperature maxima right or left of the immediate vicinity of the crown. This increases temperature in the immediate vicinity of the crown. This is due to added radiations from inside resulting in melting of filament. Consequently, we obtain a distorted image on the work site with reduced power density that affects all the welding parameters. Similarly at an angle greater than 45 degree at the bend, the temperature maximum is shifted to the crown with a lower thermal efficiency of the gun that reduces the power density at the target. Fig. 5 describes power density vs. heating power for different bending angles at the vertex of the filament. Emitting area is approximately circular that enhances the lifetime at higher operating temperatures. It also has an advantage of producing a symmetric shape of the image produced by the gun. Alternatively, a pointed area at the crown causes higher evaporation rate at higher temperatures, reducing the filament life rapidly. As an example, at 2880 K a temperature increase of 100 K causes an increase of the vaporization rate by approximately 12%. Temperature rise is now confined to a narrow area of the tip of the cathode. Reduced cross sectional area of 0.2 mm2 at the outer side of the bend (apex) increases the resistance of the wire, giving a maximum rise in temperature and emission current. Rest of the area that is adjacent to the crown (apex) will be comparatively less emissive. However, this reduction in area minimizes the operating life, which is compensated by choosing a wire of greater cross section. It
< 45 degree ~ 20
= 45 degree
> 45 degree ~ 90
6.0E+05
5.0E+05
Power Density (w/cm2)
24
4.0E+05
3.0E+05
2.0E+05
1.0E+05
0.0E+00 0
20
40
60
80
100 120 140 160 180 200
Input Heating Power (W)
Fig. 5. Power density as a function of heating power of different bending angle of the filament at the vertex.
necessitates an increased heating current level [19]. Micrographs of the improved cathode are shown in Fig. 6. It is observed that cathode area at the tip is evaporated more. Repeated experiments show that melting of the cathode occurs at the tip while the rest of the area does not pass the melting point, hence giving maximum emission current with minimum divergence.
4. Conclusions Usefulness of an electron beam welding is highly dependent on the availability of a stable electron source of high power density with long operating life. In electron beam welding (EBW) applications, the most important task is to devise a sustainable definite geometry of the seam with optimum parameters (voltage, beam current density, etc.). Most important role is played by the geometry or shape of the cathode. This significantly influences the electron beam distribution along with many other parameters [20]. The cathode design, electrode spacing and focusing 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.
and sustainability of the electron beam parameters. The gun provides a stable emission under normal vacuum conditions (105 mbar) with enhanced operating life of the filament at high temperature [21,22]. Space charge at high temperatures is removed by using a cathode with hairpin like semi sphere at the crown with an angle of 450 at the vertex. This shape produces a high
electric field in front of the cathode. As a result of this geometry, we are able to yield a larger thermal efficiency compared to conventional hairpin cathodes at higher emission current density. Another advantage is that the astigmatism of the conventional hairpin cathode is avoided. Temperature rise is well confined to a narrow area of the tip. Due to its high perveance
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105 A V3/2 and power density 105 W cm2, we were able to weld refractory materials. By using special geometry of a hairpin filament, we could weld materials up to the order of 5 mm in thickness with 10 KeV beam energy only. This filament is simple in design, rugged in construction and economical to manufacture.
Acknowledgements We acknowledge financial assistance from Higher Education Commission Islamabad, Pakistan through ‘‘Indigenous Scholarship Scheme’’. We are also grateful for useful discussion with Mr. Zahid Majed. Thanks are also due to Mr. Shahid Hameed Minhas and Mr. Mohammad Arshad for their 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: Woodhead; 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.
Vacuum 77 (2004) 19–26 www.elsevier.com/locate/vacuum
The electron beam gun with thermionic hairpin-like cathode for welding and surface modifications Munawar Iqbala,, Mohammad Rafiqa, Sarfraz A. Bhattia, Fazal-e- Aleemb a
Electron Beam Source Laboratory, Applied Physics Division, PINSTECH, P.O., Nilore, Islamabad-45650, Pakistan b Center 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 mA W1 and the power density measured is 105 W cm2.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 a combination of a low work function and high operating temperature. However, metals with higher melting point have a higher work function. The standard material for comparison is a polyCorresponding author. Tel.: +92-429-231-138; fax: +92-
429-231-253 E-mail address: [email protected] (M. Iqbal).
crystalline tungsten ‘hairpin’ filament with work function around 4.5 V, made of drawn wire a few tenths mm in diameter, bent and situated in a diode structure. Most of electron beam welders have used tungsten wire hairpin filament as a cathode [1,2]. The wire hairpin filament has served as a fine approximation to a point source for the relatively small amount of beam current. In addition, they are easily manufactured, inexpensive and have a reasonable life expectancy of 10–50 h. The imaging characteristics of the
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.066
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
20
electron guns that uses wire hairpin filaments [3–6] are beam current dependent. This implies that the beam current has to be focused and refocused for any change in beam current if a sharp focus is to be maintained. As the beam power increases, the task of obtaining a sharp focus becomes harder. In our present work, we have improved the electron source by employing special geometry of the cathode. It has been used for welding with improved focusing abilities as the system operating capabilities were extending into the high power regime. The filament we fabricated has high emission current densities and lifetime of more than 100 h at the temperature of 3000 K. Due to this geometry we have achieved diameter of the focusing spot 1 mm and the power density 105 W/cm2 at an acceleration voltage of 10 kV only.
2. System description 2.1. Electron gun The gun used to produce electron beam with its associated electromagnetic focusing system is shown schematically in Fig. 1. The electrons are produced from a cathode consisting of a tungsten wire, diameter 0.7 mm, bent into a hairpin shape
10 kV DC
220 Volt AC Cathode
Beam Former Anode Apperture
Focussing Coil
PC controlled Position system
45
Work station SS Chamber
To Vacuum pump
Projected view of Filament
Fig. 1. Schematic diagram of the Electron Beam Gun.
(inverted V). An apertured anode is positioned down the column at a distance of 8 mm from the cathode. Diameter of the aperture is 5 mm. The anode is connected to the ground. By energizing with AC current (220 V), the cathode is heated up to the emission temperature of such a value that a space charge limited stream of electrons is drawn by a constant acceleration voltage (10 kV). The electrons emitted by the cathode are accelerated down the column and pass through the aperture of the anode to form a beam. The accelerated electrons are further collimated by passing through an additional aperture of diameter 6 mm which is also at the ground potential. The movement of the work piece beneath the beam was programmed using a personnel computer. At a height of 0.7 mm from the cathode, a focusing electrode is introduced to shape the electrons in to a beam. Diameter of the focusing electrode’s aperture is 4 mm with a negative potential of the same order as of cathode. For improving the beam generating reliability, the focusing electrode was designed [7] in such a manner that the potential of the focusing electrode is equal to the potential of the cathode. Focusing electrode is used to prevent any emission at the cathode edge and to improve the marginal trajectories [8]. This forms one of the two elements comprising the electrostatic lens of the gun. The anode forms the other. The potential field produced by the configuration of the anode and the focusing electrode is such that during the acceleration electron flow is concentrated into paraxial flow with a small dispersion angle. Focusing electrode, anode and aperture are made from a tantalum sheet of thickness 1 mm with flat rectangular geometry. The amount of filament recession, its shape and aperture enlargement are important in producing a stable image. By constructing the hairpin like filament from tungsten wire with optimum geometry, having angle at the vertex equal to 451 with reduced radius at the apex, enhanced the filament life and the quality of the image of the filament that ultimately increases the power density at the work site. The cathode produces a Maxwellian distribution in energy [9,10]. Mean initial energy of the electron is eV=kT for a typical emission temperature of 2850K; this yields initial electron energy E0.25 eV.
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
Filament is maintained at a negative potential (acceleration voltage) of 10 kV with respect to ground so that energy of the beam electrons entering interaction region is close to 10 000 eV. Electrons that pass the aperture at this stage are responsible for the final energy distribution of the electron beam. No attempt is made to further mono-chromate the electron beam at this stage. In general, one needs to make sure that the gun is fairly well aligned with the aperture. Satisfactory mechanical alignment is made by looking through a small hole bored in the aperture plate. It is important that the filament be properly centered in relation to the opening of the focusing electrode and is at a proper distance from the opening with optimum opening diameter. This helps in achieving a space charge limited emission that is necessary for a constant and stationary image of the beam at the work site. This ensures long-term stability for the electron beam, which is important for our applications. The interaction region (the work site) is maintained at ground potential. We operate at a maximum total beam current of 300 mA, the practical limit because of space charge effects. Focus of the gun is adjusted to give us what we believe is a minimum space charge limited spot size of 1 mm in the interaction region. 2.2. Focusing lens Electrons are accelerated to their final velocity after passing through the anode. The divergent beam they make up still does not have sufficient power density. To achieve this, electron beam must be focused. According to principle of electron optics, each gun produces an enlarged image of the cathode in the field free region beyond the anode. Because of the low current densities, these cathode images are not suitable for the further electron optical imaging in the gun. It is the focal spot of the gun or the smallest beam diameter that serves as 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 the electrons. The coil consists of large number (2200) of windings of copper wire shielded on all sides by a high permeability iron casing. A direct current flows through the windings of the annular coil
21
producing a magnetic field, which acts inwards from the iron casing on the electron beam. The electrons leave the magnetic lens without experiencing any change in speed, describing a spiral slightly curved path, and meet at a focal point. Coil current is the main factor that determines the focal length of the magnetic lens, and is referred to as the lens current. The focal length is inversely proportional to the square of the lens current and is also directly proportional to the acceleration voltage. In other words, the focal length can also be adjusted by changing the acceleration voltage. This however is not generally done in practice. A beam of highest power density is more effective, that is, a higher power density beam can accomplish the required work in the shortest possible time and thus minimize heat conduction to material adjacent the area being welded. The beam power density must be varied in accordance with the type of operation to be performed and the characteristics of the material to be welded. In order to obtain high power density, precise electron optics must be applied in focusing the beam. The electron gun is a device that not only extracts the electrons and accelerates them to very high velocities by virtue of the electric potential applied to the electrodes, but it also serves as an electrostatic lens that shapes the electron flow into a beam and focuses the beam to form an image of the electron source. The image formed by the gun is the ‘object’ that is focused or imaged by the magnetic lens onto the work piece to be welded. Thus, the magnetic lens is used to project the image onto the target.
3. Results and discussions The ‘Richardson–Dushman’ equation [11], describes the current density emitted by a heated filament, as J eT ¼ AT 2 exp ðf=kTÞ;
(1) 2
2
where, A is material constant (60 A/cm K for Tungsten), 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
ARTICLE IN PRESS M. Iqbal et al. / Vacuum 77 (2004) 19–26
slope gives the work function f. This value of f is referred to as the ‘Richardson’ work function. As mentioned in the above relation, the emission current density is strongly depended upon the temperature of the cathode. Thus, low variations in temperature will have a considerable effect on the beam current. For electron beam welding with stringent requirements of weld seam reproducibility, this type of dependence would be extremely detrimental. In such a case, it is possible to use another physical relationship namely Langmuir relationship [12], J eR ¼ 2:3 106 KV 3=2 ;
(2)
where K=1/Z2KA, Z is the distance between the cathode and anode. According to this, a given accelerating voltage is a function of geometrical characteristics such as the distance of the cathode from the anode. This helps us to extract a given maximum stable electron current density (space charge limited emission current density), JeR from the cathode. It is known that a high power electron gun is more efficient when electron beam is generated as a result of space charge limited emission of its cathode. Moreover, with sufficient cathode heating, the power of the electron beam can be controlled by varying the acceleration voltage. However, if the acceleration voltage is changed, all electric and magnetic fields necessary for guiding and positioning the electron beam must be readjusted accordingly [13]. Thus a sufficiently high cathode temperature must be set by adjusting the heating current so that even at the maximum accelerating voltage sufficient electrons are still available. In other words, JeT must always be larger than JeR. Under these conditions the cathode will be surrounded by a cloud of ‘superfluous’ electrons, which has an inherent charge and will limit any further emission of electrons. In this condition, we say that cathode works in space charge mode. Fig. 2 shows how; at a higher cathode temperature (heating power) the beam current remains unaffected in the space charge mode. In this region, the increase in temperature is consumed to increase the radiative power of the cathode. Increase in heating power at this stage would only serve to reduce the cathode life. In order to obtain the limit for constant beam
Richardson curve, Zka = 6mm
Langmuircurve, Zka = 8mm
450 400 350 Emission current (mA)
22
300 250 200 150 100 50 0 0
50
100 150 200 Input power (W)
250
300
Fig. 2. Behavior of Electron Gun at different anode to filament heights.
current, we set the parameter ZKA=8 mm to operate the gun under Langmuir relation. This ensures optimum emission behavior over the entire life of the cathode. At ZKA=6 mm, the gun obey the Richsrdson relation and we obtain an exponential increase in emission current at higher temperature but the trade off results in an exponential decrease in the lifetime of the cathode material. Our observations in Fig. 2 are in good agreement with similar results given in the literature [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 electrostatically focused image of the electron source (filament) from the center of the magnetic lens and q is the distance of the focused beam image on the work site from the center of the magnetic lens. For a given work piece location q, the focal length f will be constant for a constant object location p. The electron gun is an electrostatic lens system that not only extracts electrons from the filament and forms them into a beam, but also produces an
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image of the source [16]. The image of the filament produced by the electron gun is the ‘object’ for the magnetic lens. Therefore, for f, to be constant, the image produced by the electron gun must be stationary. By operating the electron gun in the space charge mode we obtain a stable electron gun image. This is achieved by adjusting the electrode spaces in such away that we get a stable electron beam. This also improves the focusing stability of the magnetic lens due to stable and sharp focusing of the electron gun. As the beam power increases, the task of obtaining a sharp focus also increases. Focal length of magnetic lens [17] is determined by the magnetic field strength which itself is dependent on the lens current driving the magnetic lens and the high voltage accelerating potential of the electron gun. This is described as f ¼ kV =I 2L ;
(4)
where IL and V are lens current and acceleration voltage, respectively. The parameter k is dependent on various geometric factors including number of turns in the coil and lens bore and gap. Fig. 3 shows the lens focusing current versus f = 8cm
f = 13cm
23
beam energy (acceleration voltage). In electron beam welding, with increasing focal length and working distances, the focal diameter increases while the maximum depth of fusion decreases. This is due to decrease in the power density of the beam. At a given beam current I, a weld seam can be made narrow by decreasing the focal distance resulting in smaller focal spot [13]. By altering the lens current and observing the point of impingement through an optical system, the beam diameter is noted. We optimized our lens system at a focal distance of 13 cm to achieve the minimum spot diameter of the order of 1 mm. Diameters of 0.1–1.0 mm are typical of electron beam welding, depending upon the beam power and the focal distance [14]. This focal diameter achieves the power density of about 105 W/cm2. As a result of the stable focus produced by the improved filament in the gun design, the beam quality, judged through different welds, is defect free; their tensile strength and the penetration is enough even at 10 kV only. We obtained beautiful seam geometry as shown in Fig. 4 in which different refractory materials are welded at 10 kV. Surface modification has also been
f = 18cm
350
300
Focusing current (mA)
250
200
150
100
50
0 0
2
4 8 6 Beam energy (kV)
10
12
Fig. 3. Lens focusing current as a function of beam energy for different coil focal lengths.
Fig. 4. Scanned Electron Beam Welded images on refractory metals.
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accomplished by this gun which is reported separately [18]. Thus, a focused and high power density image is produced as a result of suitable electrodes position or spacing as well as by the suitable construction of all the electrodes, specially the cathode. The cathode is a 0.7 mm wire bent at 45 degree to obtain the maximum strength as well as the operating life at high emission current. We bend the tip at an angle of 45 degree to avoid any deformity in the geometry as well as fusion of the filament. This helps us in obtaining higher thermal efficiency and consequently achieves higher power density at the target. The angle less than 45 degree at the bend causes the shift of temperature maxima right or left of the immediate vicinity of the crown. This increases temperature in the immediate vicinity of the crown. This is due to added radiations from inside resulting in melting of filament. Consequently, we obtain a distorted image on the work site with reduced power density that affects all the welding parameters. Similarly at an angle greater than 45 degree at the bend, the temperature maximum is shifted to the crown with a lower thermal efficiency of the gun that reduces the power density at the target. Fig. 5 describes power density vs. heating power for different bending angles at the vertex of the filament. Emitting area is approximately circular that enhances the lifetime at higher operating temperatures. It also has an advantage of producing a symmetric shape of the image produced by the gun. Alternatively, a pointed area at the crown causes higher evaporation rate at higher temperatures, reducing the filament life rapidly. As an example, at 2880 K a temperature increase of 100 K causes an increase of the vaporization rate by approximately 12%. Temperature rise is now confined to a narrow area of the tip of the cathode. Reduced cross sectional area of 0.2 mm2 at the outer side of the bend (apex) increases the resistance of the wire, giving a maximum rise in temperature and emission current. Rest of the area that is adjacent to the crown (apex) will be comparatively less emissive. However, this reduction in area minimizes the operating life, which is compensated by choosing a wire of greater cross section. It
< 45 degree ~ 20
= 45 degree
> 45 degree ~ 90
6.0E+05
5.0E+05
Power Density (w/cm2)
24
4.0E+05
3.0E+05
2.0E+05
1.0E+05
0.0E+00 0
20
40
60
80
100 120 140 160 180 200
Input Heating Power (W)
Fig. 5. Power density as a function of heating power of different bending angle of the filament at the vertex.
necessitates an increased heating current level [19]. Micrographs of the improved cathode are shown in Fig. 6. It is observed that cathode area at the tip is evaporated more. Repeated experiments show that melting of the cathode occurs at the tip while the rest of the area does not pass the melting point, hence giving maximum emission current with minimum divergence.
4. Conclusions Usefulness of an electron beam welding is highly dependent on the availability of a stable electron source of high power density with long operating life. In electron beam welding (EBW) applications, the most important task is to devise a sustainable definite geometry of the seam with optimum parameters (voltage, beam current density, etc.). Most important role is played by the geometry or shape of the cathode. This significantly influences the electron beam distribution along with many other parameters [20]. The cathode design, electrode spacing and focusing 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.
and sustainability of the electron beam parameters. The gun provides a stable emission under normal vacuum conditions (105 mbar) with enhanced operating life of the filament at high temperature [21,22]. Space charge at high temperatures is removed by using a cathode with hairpin like semi sphere at the crown with an angle of 450 at the vertex. This shape produces a high
electric field in front of the cathode. As a result of this geometry, we are able to yield a larger thermal efficiency compared to conventional hairpin cathodes at higher emission current density. Another advantage is that the astigmatism of the conventional hairpin cathode is avoided. Temperature rise is well confined to a narrow area of the tip. Due to its high perveance
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105 A V3/2 and power density 105 W cm2, we were able to weld refractory materials. By using special geometry of a hairpin filament, we could weld materials up to the order of 5 mm in thickness with 10 KeV beam energy only. This filament is simple in design, rugged in construction and economical to manufacture.
Acknowledgements We acknowledge financial assistance from Higher Education Commission Islamabad, Pakistan through ‘‘Indigenous Scholarship Scheme’’. We are also grateful for useful discussion with Mr. Zahid Majed. Thanks are also due to Mr. Shahid Hameed Minhas and Mr. Mohammad Arshad for their 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: Woodhead; 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.