Abstract:CVD diamond material, applying in broad band millimeter wave helical Traveling Wave Tube (TWT), is discussed. Two kinds of novel helical slow wave circuit, supported by CVD diamond, are presented. Dispersion, interaction impedance and attenuation of these circuits are simulated by MAFIA, compared with circuit supported by BeO rods. Nonlinear beam and wave interaction of the TWTs, replaced slow wave circuit supported by BeO rods with new circuit, are analyzed using BWI module in TWTCAD Integral Framework, which is a suite of helix TWT design code. The results show that Stud-Diamond helical slow wave circuit has broader band and excellent cooling capability is implied, it should has important apply to broad band millimeter wave power helical TWT.