A novel Lagrangian code for fast simulation of non-rotationally symmetric slow wave structure for millimetre wave TWTs is proposed. Novel slow wave structures suitable for fabrication at millimetre wave frequencies often have non-rotationally symmetric, differently from the helix that has a cylindrical symmetry. This prevents the use of conventional Lagrangian simulators valid for symmetric cylindrical symmetry. A modified Lagrangian simulator to simulate arbitrary symmetry SWS is proposed. The proposed simulator permits a very fast simulation of the large signal performance of the novel slow wave structures, as the Double Corrugated Waveguide.