Traveling Wave Tube Amplifiers (TWTAs) are essential for high-power RF amplification in applications such as satellite communications, radar systems, and electronic warfare. Key to enhancing the output power of TWTs in the millimeter wave frequency range is the increase of interaction current capacity and interaction impedance. A design novelty involves tilting the vertical section length by a phase angle of 30 degrees, which extends the interaction region within the Slow Wave Structure (SWS) and maximizes power output suitable for radar applications. In the D-band (130–150 GHz), preliminary analysis has shown an achievable bandwidth of 15% in modified Folded waveguide SWS. Utilizing a round electron beam configuration is crucial, as it ensures maximum electric field distribution at the centre, thereby enhancing the gain of the SWS. Within a square beam tunnel, this system employs a beam current of 70 mA with a beam velocity of 0.303 times the velocity of light. An axial magnetic field of 0.3T is applied to confine the electron beam within the beam tunnel. In the particle in cell simulations, with an input power of 62.5 mW, this design yields a minimum gain of 25.5 dB across the band and reaches a maximum gain of 28.9 dB at 141 GHz.