Studies on planar helical slow-wave structures for traveling-wave tube applications

This thesis presents extensive studies on a new planar helix slow-wave structure (SWS) which consists of a planar helix with straight-edge connections (PH-SEC).The PH-SEC has been studied in the context of traveling-wave tube (TWT) applications.Unlike the conventional circular helix, the PH-SEC can be fabricated easily using printed circuit techniques or microfabrication techniques.In addition, by changing the aspect ratio, the PH-SEC can become suitable for interaction with a sheet electron beam which can offer many advantages for high frequency TWTs.To avoid oscillations at high power levels, the PH-SEC has also been modified into a rectangular ring-bar slow-wave structure with straight-edge connections (RRB-SEC); this modification is similar to that of the circular helix into the circular ringbar SWS.For the PH-SEC, dispersion characteristics for several practical modifications to the basic structure have been examined.These modifications comprise a vacuum tunnel, metal shield and multilayer dielectric substrates.A modified effective dielectric constant (MEDC) method has been proposed to obtain the dispersion characteristics for the different possible configurations.Further, coupling impedance for the different configurations has been calculated using the corresponding two-dimensional approximations.Effects of variations in the aspect ratio, metal shield distance and dielectric constant of the substrates on the phase velocity and the coupling impedance have been studied.The PH-SEC structure incorporating coplanar waveguide (CPW) feed has been designed and fabricated for printed circuit fabrication and microfabrication.Effects of dimensional parameters have been studied.Several PH-SECs with band edge frequency less than 10 GHz have been fabricated using printed circuit techniques.The measured results for these structures validate the analytical results obtained using the MEDC method.A microfabrication process, involving several UV-LIGA steps, has been proposed and demonstrated to produce high-aspect-ratio PH-SEC structures at W-band (75 -110 GHz).On-wafer measurements have been carried out on a number of microfabricated SWSs.The cold-test parameters