Metamaterial-enhanced traveling wave tubes

We present the design of a folded waveguide traveling wave tube (TWT) periodically loaded by epsilon negative (ENG) metamaterial slabs. When a given folded waveguide slow-wave structure (SWS) is loaded with ENG metamaterials, its band diagram is shifted to higher frequencies. To shift the band diagram back to the desired (lower) frequency of operation, we scale up the structure´s dimensions. This enlargement of the SWS, however, allows for increasing the beam tunnel diameter of the SWS for a given frequency of operation and also increases the interaction impedance of the TWT. Both of these physical phenomena can be exploited to increase the gain of the TWT for a given number of periods in the SWS and achieve a higher output power level. This concept is expected to be particularly useful at millimeter-wave frequencies where the beam diameter and current are usually limited by the small dimensions of the SWS thereby limiting the gain and total output power of millimeter-wave TWTs.