Experimental and theoretical investigations of rectangular grating slow wave structure for low-voltage travelling wave tube amplifiers

A rectangular grating periodic structure is being investigated for Ku-Band forward and backward wave low-voltage ({dollar}le{dollar}10 kV) millimeter wavelength Travelling Wave Tube (TWT) amplifiers. A complex dispersion relation for the hybrid modes in the rectangular grating waveguide which includes electron beam effects is derived and solved with the GRating AMPlifier (GRAMP) computer code. Briggs' criteria is utilized to analyze the instabilities in a rectangular waveguide grating TWT amplifier in three different regimes which are the forward wave mode, backward wave mode and transition regimes.; For forward wave operation at low voltage, it is necessary that the ratio of groove depth d to grating period p be large (i.e. {dollar}d/p > 5{dollar}) while small d/p allows backward wave operation. For large d/p, it is important to maintain good contact resistance between the grating slots and sidewalls to obtain experimental dispersion relations close to the theoretical ones and allow forward wave interaction with the beam. We have carried out phase and amplitude measurements of several grating structures utilizing a slotted line and Fast Fourier Transform (FFT) analysis as well as network analyzer measurements. The results show the shallow groove gratings (d/p = 0.446) and deep groove gratings (d/p = 7.43) dispersion relations agree very well with the theoretical predictions. However, substantial skin effect absorption can occur at frequencies close to the stop band for a deep groove grating which can eliminate the net gain for forward wave operation. For amplifier experiments, a round "probe" beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solenoidal magnetic field. Backward wave, shallow groove tuning curves and amplifier growth rate experimental results are presented. The experimental growth rate is compared to theoretical growth rates.