This paper investigates the effect of a non-uniform magnetic field on beam particle energy exchange with three-dimensional particle-in-cell (3D-PIC) simulations. For this investigation, high current (kA range) planar sheet beam-driven rippled rectangular waveguide (RRWG) has been considered. For PIC simulations, beam current density was varied by changing the cathode cross-section (CS) in terms of thickness (beam thickness (BT)) with a common cathode width of 10 mm. There are four BTs (1 mm, 2 mm, 3 mm and 4 mm) and for each BT, seven types of magnetic field lines with the same magnitude of 0.6 T are used. These include one uniform magnetic field andsix non-uniform magnetic field patterns. In this paper, the complete structural design of RRWG to generate output power up to around 45 MegaWatt (MW) with a particular type of non-uniform magnetic field pattern/periodicity to guide the sheet electron beam-driven RRWG is discussed. The application of a non-uniform magnetic field pattern to a 2D sheet electron beam with spatial distribution or periodicity at minimum gaps (typically ≤ 1 mm) leads to the enhancement in output power at voltages operational in the backward region of the device.