Nuclear Instruments and Methods in Physics Research A 641 (2011) 1–4

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

Univers

E-m

journal homepage: www.elsevier.com/locate/nima

Electrostatic focusing of directly heated linear filament gun using EGUN

Munawar Iqbal a,n, M.A.K. Lodhi a, Zahid Majeed b, Dimitri Batani c

a Department of Physics, Texas Tech University, Lubbock, TX 79409, USAb National Engineering and Science Commission, Islamabad, Pakistanc Dipartimento di Fisica, Universit�a degli Studi di Milano Bicocca, Italy

a r t i c l e i n f o

Article history:

Received 29 November 2010

Received in revised form

17 February 2011

Accepted 19 February 2011Available online 29 March 2011

Keywords:

Line source

EGUN

Minimum beam spread

Axial focusing distance

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.02.081

esponding author. Present address: Centre

ity of the Punjab, Quaid-e-Azam Campus, Lah

ail addresses: muniqbal@yahoo.com, muniqba

a b s t r a c t

This paper presents the optimization of a line source rectangular electron gun using electrostatic

focusing. We optimized the gun by shaping the configuration of its electrodes in order to achieve the

desired focusing characteristics, namely maximum focusing distance and minimum beam spread. The

optimization has been carried out using the software EGUN. We have also simplified the gun design

using only one focusing electrode at the same potential as that of the cathode and by avoiding magnetic

focusing field, separate focusing electrodes and additional power supply, thus minimizing the cost

without any loss in its accuracy and efficient performance. This gun with the optimum configuration

was used in actual experiment and the results of the simulation were compared with the experimental

measurements.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Thermionic electron beam guns are widely used in metallurgical,accelerator and optical applications [1–4]. In fact, the line beamsource electron guns are the best suited equipment for heattreatment of large surface areas and coating of large substrates atvery fast evaporation rates. They are also used in large scale vacuummetallurgy plants for melting, welding and physical vapor deposi-tion on large area surfaces [5–8]. In actual applications of electronbeam for melting and evaporation processes, it is necessary tocorrectly simulate the trajectory of the electrons emitted from theelectron gun and transported to the work site [9].

The present study is based on electron-optics calculationsusing the computer code EGUN, developed at SLAC [10]. Theprogram EGUN is written particularly for the measurement oftrajectories of electrons in electrostatic and magnetostatic fieldswhere space charge effects are also present. The program can beused either in rectangular co-ordinates or in cylindrical co-ordinates determined by the sign of the potential. This workinvolves the analysis and optimization of a line source rectangularelectron gun using electrostatic focusing, in which a pre-proces-sor named POLYGON is used to define the starting and boundaryconditions for the main program. The output of the program wasviewed by a post-processor named EPLOT.

The goal of these calculations is to find a suitable configurationand electrode potentials for the gun producing a line-shaped

ll rights reserved.

for High Energy Physics,

ore, Pakistan.

l@chep.pu.edu.pk (M. Iqbal).

electron beam that is as parallel as possible in the interactionregion and whose energy spread is as small as possible. Indeed,the trajectories of an electrically guided beam do not dependupon absolute potentials at electrodes but on the shape ofelectron-optics whenever the thermal energy spread of the beamparticles is negligibly small.

2. Line beam source electron gun

Design and construction of the line source electron gun used inthis experiment have been reported earlier [11], in which themagnetic field has been removed from the original gun. The line-shaped cathode is made of 140-mm long Tungsten wire of0.9-mm diameter. The anode and focusing electrode (FE) aremade of 1-mm thick Tantalum. A schematic diagram of the gunis given in Fig. 1.

The whole gun assembly was placed in a water-cooled copperhearth keeping the assembly cool during the operation inside thestainless steel chamber. Operational vacuum inside the chamberwas 1.4�10�5 mbar. We used a 1-mm thick stainless steel plateas a target, exposed to beam for 30 s only. The target plate wasfixed in a copper jig to avoid any non-uniformity due to anythermal load on it.

3. Results and discussions

The calculations have been done for a two-dimensional slice ofthe gun in the beam direction, assuming that the third dimensionextends to infinity. The parameters that affect the shape, focusing

Fig. 1. Assembly diagram of line beam source electron gun [11].

Table 1Input parameters for simulation of line source gun.

Sl.# Parameter Value

1 Cathode–anode distane (mm) 7.0

2 Anode slit spacing (mm) 7.5

3 Focusing electrodes gap (mm) 6.0

4 Anode–focusing electrodes distance (mm) 4.0

5 Distance between cathode and edge of focusing electrodes (mm) 3.0

6 Distance between anode and target (mm) 45

7 Anode potential (kV) 10

8 Cathode potential (kV) 0

9 Focusing electrodes potentials (kV) 0

10 Target potential (kV)

Fig. 2. Ebeam simulation of line source before optimization.

Table 2Input parameters fixed for optimization of line source gun.

M. Iqbal et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 1–42

of electron trajectories and the thermal load on the electrodes dueto beam are:

Sl.# Parameter Value

1. cathode–anode distance,

1 Cathode–anode distance (mm) 9.0

2. cathode–focusing electrode distance, 2 Anode slit spacing (mm) 3.5 3. spacing between the focusing electrodes, 3 Focusing electrode gap (mm) 7.0 4. spacing and thickness of anode slit, 4 Anode–focusing electrode distance (mm) 4.0

5 Distance between cathode and edge of focusing electrodes 5.0

5. radius and emission surface area of cathode.

6 Distance between anode and target (mm) 45

7 Anode potential (kV) 10

8 Cathode potential (kV) 0

9 Focusing electrode potential (kV) 0

10 Target potential (kV) 10

The initial configuration parameters for line beam electron gunare listed in Table 1.

The result of simulated beam has been shown in Fig. 2. Theminimum beam spread has been calculated as 2.3 mm at the

Fig. 3. Beam focusing distance as a function of cathode position.

Fig. 4. Minimum beam spread profiles as a function of cathode position.

Fig. 5. Ebeam simulation of line

M. Iqbal et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 1–4 3

position 0 mm from anode, which may not be achieved practicallydue to non-workability at this position.

The gun was optimized for the following objectives:

1.

sou

maximum focusing distance in post anode region and

2. minimum beam spread.

The parameters that were fixed for the optimization to achieveaforementioned goals are listed in Table 2.

The gun was simulated for different positions of cathode withrespect to the edge of FE. The simulation was repeated for differentvalues of focusing electrodes gap, ranging from 5 to 8 mm. Thefocusing distance in post-anode region was calculated for eachconfiguration. Their profiles have been shown in Fig. 3.

The beam focusing distance is significant for the physicalworkability of the electron gun. The optimum workability of thebeam has been obtained with 7 mm, focusing electrode gap.The value of beam focus distance achieved is 7.4 mm from theanode surface, with the cathode position at 5 mm from the edgeof the focusing electrode.

The profiles of minimum beam spread as a function of cathodeposition have been shown in Fig. 4. The minimum beam spreaddecreases with increase of the cathode distance from FE, regard-less of the focusing distance. Although a 0.37-mm minimumbeam spread of a beam also does appear in the plot, it is notworkable as the focus is found in the pre-anode region. The beamspread for the optimized workable configuration is 1.2 mm, whichis quite reasonable for material processing purpose. The simu-lated output for this optimized configuration has been shown inFig. 5.

The optimum configuration was applied experimentally and apicture of the target plate, placed at distance 7.4 mm from theanode, is shown in Fig. 6.

The experimentally obtained minimum beam spread is1.6 mm, which is nearly 33% higher than the simulated result.This difference may be due to following reasons:

A.

geometric imperfections in the shape of electrodes and B. broadening of melting zone due to heat conduction.

4. Conclusions

A well focused beam with minimum beam spread 1.6 mm hasbeen obtained experimentally by optimization with the help of

rce after optimization.

Fig. 6. Impression of minimum beam diameter on target plate.

M. Iqbal et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 1–44

EGUN simulation, which was initially 2.3 mm prior to optimiza-tion. The minimum beam spread has been obtained at 7.4 mmfrom anode surface, which was initially at 0 mm. This is areasonably workable and quite focused beam for all sorts ofmetallurgical applications. For the optimum configuration nothermal load signs have been observed on the FE and anodesurfaces. Moreover the simplest design approach has been used tobuild this gun without the use of extra electrostatic and magneticoptics. The experimental results have been verified by thesimulated one and are in good agreement with each other.

Acknowledgments

We are especially grateful to Professor Williams B. Hermanns-feldt for providing the EGUN software and valuable suggestionsduring this work. Specials thanks are also due to Department ofPhysics, Texas Tech University, Lubbock, TX, USA, for providingthe research facilities to one of us (M.I.) to accomplish this work.

The work was performed partially under a joint grant of the USDepartment of State and the Higher Education Commission,Pakistan.

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