Electron beam focussing and transmission in the klystron tube is guided by the magnetic field and it has implications on the output power, bandwidth, gain, volume, and mass of the vacuum tube. High frequency Klystrons with high pulse repetition factor and high efficiency are operated in immersive flow beam confinement mode. In this mode electron beam has strong beam stiffness in the beam–wave interaction region but it necessitates at least 2–4 times the Brillouin magnetic field strength thus necessitating a stronger magnetic field for beam focusing. Waveguide and tuner aperture ports introduce asymmetries in the magnet design which can interfere with the uniform solenoidal coil configuration. Spatial constraints introducing asymmetric field configuration is more severe in the output cavity region where the magnetic field reduces due to presence of output waveguide port. On the contrary, magnetic field requirement in the output cavity beam-wave interaction region is higher than the first feeding cavity, buncher cavities and penultimate cavity to reduce beam perturbations of current modulated bunched beam. A water cooled tunable multi-coil electromagnet assembly has been designed for 6-MW peak power S-Band Klystron tube in order to meet 0.11T field requirement in the output cavity region. Electromagnetic analysis and simulation studies were done to optimize the axial magnetic flux density in the good field region to meet beam optics requirements with given spatial constraints using three dimensional Finite element method (FEM). Due to the multi-coil design of the magnetic system, easy tuning is possible to tune the beam filling ratio within a reasonable range by adjusting the coil currents to evaluate tube performance during RF conditioning and trials. This paper discusses the electromagnetic design and optimization studies for tuning the magnetic field profile [1] and the peak field location in the vicinity of output cavity region of the S-Band Klystron tube.