High power RF sources are used in many applications including Satellite communications, Radar, Electronic Counter-Measures (ECM). Most of them consist of Klystrons, Magnetrons or Gyrotrons which are narrow band, bulky structures. Very few TWTs compatible with very high power (above 100KW) in S-band have been reported in literature. A challenge in designing small, high power TWTs relates to over-heating that counter the principles for designing slow-wave structures. Other difficulties lie in the formulating very high beam velocity using very small electrical lengths. More specifically, small electrical lengths do not interact efficiently with the incoming electron beam. Also, for large phase velocity waves, the interaction impedance decreases, causing lower gain and output power. With these challenges in mind, and with goal of reducing the size of TWTs, in this paper we investigate (1) novel slow-wave structures to achieve large RF phase velocity with moderate interaction impedance, (2) materials for structural stability to counter heating effects.