Analysis of Dielectric Cerenkov Masers and Wiggler Free-Electron Lasers in the Millimeter Wavelength Range.

A rectangular dielectric Cerenkov Maser amplifier, an input/output hybrid mode coupler, and three different Free-Electron Laser amplifiers are analyzed with the objective of optimizing their performance in the millimeter wavelength range. A dispersion relation for the charged particle and hybrid mode interaction of a dielectric Cerenkov maser in a rectangular geometry is derived. For a tenuous electron beam, explicit expressions for hybrid mode growth rate and extraction efficiency are obtained for amplifier operation. For a high-dielectric constant material (varepsilon _{r} >= 10), a low-voltage (<60 kV) beam is suitable for a hybrid mode growth rate of 0.8-2 dB/cm at 35 GHz. A dielectric-lined mode converter with a tapered dielectric thickness is examined for conversion of the vacuum waveguide TE_{10} mode to the desired hybrid mode EH_{10} for slow-wave Cerenkov amplification. General coupled mode equations are obtained from the Maxwell equations. An explicit analytic solution for two-mode forward coupling is obtained. It is shown that a linear taper profile efficiently converts the conventional TE_{10} fast mode to the desired EH_{10 } slow mode with the total undesired mode power 13 dB lower than the desired mode. A piecewise linear profile is able to reduce the undesired mode power to a level of -27 dB. Free-Electron Laser amplifiers with linear dipole, helical dipole and helical quadrupole wigglers are analyzed in the Compton regime by using an approach which includes nonlinear particle-wave interactions, waveguide mode competition, and the effects of beam quality. A set of governing equations is derived and solved by a 3 -D simulation code for each case. An FEL amplifier with uniform linear dipole or helical dipole wigglers shows a high gain (>60 dB) and modest efficiency (~5%) in the 300 GHz range. The wiggler amplitude for the linear and helical dipole cases is optimized by using the maximum phase flux principle. The efficiency is increased to 50% with a concomitant reduction of higher-order-mode content to 5% and an increase in the bandwidth to 48%. A comparison of these two configurations shows that the helical dipole wiggler has a larger growth rate per unit length, broader bandwidth, and lower undesired mode content. The quadrupole wiggler configuration is found to have comparable performance with the helical dipole wiggler in the 35 GHz range. Due to the substantial radial variation of the quadrupole wiggler field, it is shown to have superior beam confinement, which allows the use of higher current and lower voltage beams. However, this feature is offset by the fabrication complexity and the very high beam quality required due to the rapid radial variation of the wiggler magnetic field.