Design Modifications in RF Interaction Cavity of a 140 GHZ Gyrotron to Achieve Wide Tunable Bandwidth for DNP NMR Applications

The tapered RF interaction cavity of 140 GHz tunable continuous wave (CW) gyrotron operating in TE0,3,q mode has been tailored with the aim of providing RF power over a tunable bandwidth for 212 MHz DNP NMR spectroscopy applications. Gyrotron device RF interaction cavity design, its beam absent RF characteristics as well as electron beam and RF wave interaction behavior, both analytical and Particle-in-Cell (PIC) simulation studies have been presented. Using linear analysis, the start oscillation currents and the RF field profiles for the various axial operating modes indices q = 1, 2, 3 have been determined. Suitable modifications in the interaction cavity have been incorporated for the enhancement of device tunable bandwidth by operating the gyrotron in the high order axial indices via magnetic tuning. Gyrotron beam-wave interaction behaviour explored using time dependent non-linear multi-mode analysis for various beam currents and magnetic fields and more than >15W of RF power over a tunable bandwidth of 400MHz has been achieved through magnetic field tuning. This tunable bandwidth gyrotron design will of immense use for enhancement of sensitivity of the DNP NMR spectroscopy.

Thermo-Mechanical Analysis and Its Effect on Rf Behaviour of A Tapered Cavity of the W-Band Gyrotron Oscillator

Thermo-mechanical analysis of a tapered cylindrical RF interaction cavity of a TE6,2, 95 GHz, 100 kW gyrotron has been carried out to study the effect of ohmic loss generated due to radiated microwave energy. For stable device operation, with the help of design relationships, describing the approach, an optimum thermal system design has been presented and performances got analyzed using a commercial simulation code “COMSOL Multiphysics”. For various cavity thicknesses, and convective heat transfer coefficient values under without fins and with radial fins conditions system performance have been investigated. Taking water as coolant at ~293 K, hydraulic diameter and flow rate range has been determined for the optimum convective heat transfer coefficient values. An optimized simple cooling system thus designed keeps the maximum RF cavity radius deformation (increase) ~ 3.2 µm maintaining the average cavity outer surface temperature of 308 K. Further, using nonlinear time-dependent multimode analysis, a decrease of 48 MHz in resonance and a decrement of 20 in diffractive quality factor of the interaction cavity along with reduction of 2 kW of the device output power have been observed in the case for the deformed cavity from those of the initial cavity gyrotron device, which are found within the tolerance limit of such devices.