Multimode calculations of frequency tunable gyrotrons for dynamic nuclear polarization applications

Summary form only given. Recently, gyrotron-based spectrometers suitable for nuclear magnetic resonance (NMR) studies have been constructed to study dynamic nuclear polarization which can result in significant enhancement of the NMR signal. In the present paper multimode calculations of a low-power (above 10W) high-frequency (200 GHz -300 GHz) gyrotron for NMR spectroscopy applications have been performed. The co- rotating TE
_-7,2
mode has been chosen to be the main operating cavity mode. Five neighbor modes (TE
_7,2
; TE
_-4,3
; TE
_4,3
; TE
_-2,4
; TE
_2,4
) whose coupling factors are above 30% of the main mode and most probably can be excited in the cavity are included into the calculation using the FZK SELFT code packet designed for time dependent and self-consistent multimode calculations. The frequency fine tuning was obtained via the excitation of a sequence of longitudinal modes of TE
_-7,2,q
by varying the beam voltage from 15 kV upward and the magnetic field from 9.6 T to 9.77 T. The results show that the main mode TE
_-7,2
is quite stable against the possible transverse mode competitors within this magnetic field range and a continuous frequency tuning range more than 800 MHz (263.43 GHz - 264.28 GHz) has been achieved with a velocity ratio of 1.3, the beam radius and current being 1.33 mm and 100 mA respectively.