We performed thermal analysis for our previously reported [M. Iqbal, K. Masood, M. Rafiq, M. A. Chaudhry, and F. Aleem, Rev. Sci. Instrum. 74, 4616 (2003)], long linear filament electron gun assembly using ANSYS software. The source was set under a thermal load of 3000 °C, to evaluate temperature distribution, thermal strain, and heat flux at various components of the gun. We calculated the maximum heat flux (9.0 W/mm2) that produced a thermal strain of 0.05 at the focusing electrodes. However, the minimum value of the heat flux (0.3 W/mm2) was at the anode electrodes which correspond to a negligible thermal strain. The gun was validated experimentally showing a uniform cross section of the beam at the molybdenum work plate comparable to the size of the filament. Our experimental and theoretical results are in agreement. The gun had been in continuous operation for several hours at high temperatures without any thermal run-out.