Note: Charateristic beam parameter for the line electron gun M. Iqbal1, 2a), G. U. Islam1, Z. Zhou2 and Y. Chi2 1Centre for High Energy Physics, University of the Punjab, Lahore 45590, Pakistan of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 2Institute (Received XXXXX; accepted XXXXX; published online XXXXX) (Dates appearing here are provided by the Editorial Office) We have optimized the beam parameters of line source electron gun using Stanford Linear Accelerator Centre electron beam trajectory program (EGUN), utilizing electrostatic focusing only. We measured minimum beam diameter as 0.5 mm that corresponds to power density of 68.9 kW/cm2 at 13.5 mm in the post anode region which is more than two-fold (33 kW/cm2), of the previously reported results. The gun was operated for the validation of the theoretical results and found in good agreement. The gun is now without any magnetic and electrostatic focusing thus much simpler and more powerful. An electron gun with a long filament (line source) is indispensable for processing of large area surfaces. It provides a homogenous emission over a long period of time, across a length of several centimeters which causes the uniform evaporation. It produces a higher emission beam current and adequate resolution in the narrow direction1-2 which is termed as characteristic parameter that determines the quality of the source in use. High emission current density is responsible for producing high beam current that eventually produced high power density at the target. In this work, we optimized the beam and then calculated the characteristic parameter for the line gun using EGUN software3 followed by experimental validation of the results. We have removed the magnetic focusing from the actual assembly and utilized the self focusing of the gun. A detailed description of the design, performance and thermal analysis of the actual gun has already been reported earlier 4-6. The gun was simulated with parameters listed in table 1, and the applied potentials to different electrodes are given in table 2. TABLE 2. Potential applied to electrodes for the two configurations. Electrodes Optimized gun Actual gun Cathode (kV) Focusing Electrodes (kV) Anode (kV) Target (kV) -10 -10 -10 -10 0 0 0 0 The obtained electron beam trajectories for the actual configuration using electrostatic focusing only are shown in figure 1. The figure shows the beam diameter at 13.5 mm form anode surface is 5.8 mm. TABLE 1. Parameters fixed for beam optimization. Parameters Cathode to anode distance (mm) Anode slit spacing (mm) Focusing electrode spacing (mm) Anode to focusing electrodes distance (mm) Cathode to focusing electrodes edge distance (mm) a) Actual gun configuration parameters 8 Optimized gun configuration parameters 6 7.5 2.5 6 4.3 4 3 4 3 Author to whom correspondence should be addressed. Electronic mail: muniqbal.chep@pu.edu.pk. muniqbal@yahoo.com FIG 1. Electron beam trajectories of the actual gun. Then, we calculated electron beams trajectories for the optimized configuration and presented in figure 2. The beam focuses in the post anode region at 13.5 mm from upper side of the anode surface where beam diameter was measured to be 0.50 mm. Finally, the gun was installed in a vacuum chamber under pressure of 10-6 mbar for experimental validation of the theoretical results. Using the optimized parameters and acceleration potential of 10 kV, a uniform beam spot of diameter 0.62 mm was observed at the target that can be seen in figure 6. The measured beam current was 5 A. FIG 4: Beam profile of line beam electron gun at the target. FIG 2. Electron beam trajectories of the optimized gun. Emission current densities as a function of beam radius for the both configurations are shown in figure 3. The profiles are plot normalized to 1.0 with the peaks, as shown at the top of both plots, with values of 0.000689 A/(mesh unit)2 and 0.0001 A/(mesh unit)2 respectively. Our mesh unit (mu) is equal to 0.1 mm. So, by applying the mesh scale to put the plots in real dimensions, we obtained maximum emission current density equal to 6.89 A/cm2 for optimized and 1 A/cm2 for the actual configuration. The average values for these emission current densities are equal to 0.5 A/cm2 and 0.37 A/cm2 respectively. The two peaks in the actual configuration are due to the electron trajectories concentrations which are condensed at the edges and weak in the center of the beam profile. However, in the optimized configuration; these trajectories are concentrated at the center of the beam profile reducing its size from 5.8 mm to 0.5 mm. Consequently, the calculated maximum power density was 68.9 kW/cm2 at 13.5 mm away from anode, where the beam diameter was 0.5 mm. FIG 3: Current densities profile for the two configurations as a function of beam radius. Therefore, the measured value of power density was 67.2 kW/cm2. This value is close to our theoretical value of 68.9 kW/cm2 and is more than two times of the previous reference5. Thus, with the help of simulation by EGUN software, a sharp focused beam is obtained that enhanced the power of the gun more than two times. The obtained beam profile shows the maximum focusibilty, constancy and uniformity through-out the emission surface of the source. The gun is without any electrostatic and magnetic focussing that requires extra power supplies, hence much simpler and powerful now. Besides metallurgical applications, the gun is useful for thermal barrier coatings and electron accelerators for fundamental and applied research. 1 K.B. Thakur, G.K.Sahu, R.V.Tamhankar, and P.Kartik, Rev. Sci. Instrum. 72, 207 (2001). 2 A. D. Brodie and W. C Nixon, Microelectronics Engineering, 6, 111 (1987). 3 W. B. Herrmannsfeldt, SLAC-331-UC-28, 1 (1988). 4 M. Iqbal, K. Masood, M. Rafiq, M.A. Chaudhry, and F. Aleem, Rev Sci. Instrum.74, 4616 (2003). 5 M. Iqbal and F. Aleem, Rev. Sci. Instrum.77, 106101 (2006). M. Iqbal, A.Wasy, M. A. K. Lodhi, Rev. Sci. Instrum. 84, 056113 (2013). 6