The ion source of a thermal ionization mass spectrometer (TIMS) has been designed to enhance the source sensitivity by modifying the geometry of extraction lens. Computer simulation studies on different combinations of the electro-static lens components and experimental validations were carried out. This also involved designing a robust triple filament assembly in a home-built TIMS. A good agreement between the simulated and experimental results, led to overall improvements in terms of the consistency of the source performance, a higher through put and an overall sensitivity enhancement by a factor of two.