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This thesis is based on the research project of a W-band gyrotron backward wave oscillator (gyro-BWO) using a helically corrugated waveguide which is currently being built and upgraded in the University of Strathclyde. The gyro-BWO was optimally designed through numerical simulations to achieve an output maximum power of ~ 10 kW with a -3 dB frequency tuning range of 84 - 104 GHz. To increase the overall efficiency of the W-band gyro-BWO, an energy recovery system of four-stage depressed collector was designed, numerically optimized and fabricated on the gyro-BWO. Microwave components including the Bragg reflectors, the side-wall coupler, the three-layer microwave window and the pillbox window were designed, simulated and measured to facilitate the practical use of the energy recovery system. This thesis includes the analytically calculated results, the numerical simulations as well as the experimental results of the said components and system. A 14-section Bragg reflector together with the side-wall coupler located at the upstream of the helically corrugated interaction cavity was used to couple the microwave radiation out. This allowed the installation of the depressed collector at the downstream side of the gyro-BWO. The transmission coefficient of the coupler was numerically optimized to achieve -1.0 dB over the frequency tuning range, from 84 - 104 GHz. The Bragg reflector measurement agrees well with the simulation. The input coupler achieves an average -13 dB reflection over the frequency in the measurement. Theoretical analysis of the pillbox type window and multi-layer window based on mode-matching method was carried out. The simulation and optimization of the pillbox window achieved a reflection of less than -15 dB in the whole operating frequency range of 84 - 104 GHz. The three-layer window can achieve less than 30 dB reflection in the frequency range of 84 - 104 GHz in the simulation. A three-layer window and a pillbox window which particularly optimized in frequency range of 90 - 100 GHz (the operating frequency range of the gyro- TW A that shares the same experimental setup as the gyro-BWO) were fabricated. With manufacturing constraints the design of the three-layer window achieved an average -10 dB measured reflection in 84 - 104 GHz and better than -15 dB in 90 - 100 GHz. In the downstream side of the gyro-BWO, another 18-section Bragg reflector was used to reflect the radiation back into the upstream interaction cavity. And the transmission coefficient of -30 dB was obtained in the microwave measurements using a VNA, which means the microwave power leakage was less than 1%. The measurement results agreed well with the simulations. A four-stage depressed collector was designed to recover the energy from the spent electrons. The 3D PlC code MAGIC and a genetic algorithm were used to simulate and optimize the geometry of the electrodes. Secondary electron emissions were simulated and a few emission models were compared to investigate their effects on the overall recovery efficiency and the backstreaming rate for the multistage collector. The optimization of the shape and dimensions of each stage of the collector using a genetic algorithm achieved an overall recovery efficiency of about 70%, with a minimized backstreaming rate of 4.9%. The heat distribution on the collector was calculated and the maximum heat density on the electrodes was 240W/cm2 and the generation of "hot spots" could be avoided. The electric field distribution inside the depressed collector was calculated and the geometries of these electrodes were properly shaped to avoid the voltage breakdown in vacuum.

This thesis investigates the behaviour of light in waveguides with cores composed of a negative refractive index material. The ability of these structures to drastically slow and even halt light pulses is studied. The Finite-Difference Time-Domain computational simulation method is used to simulate these structures and the results produced are compared against those from analytic calculations. It is observed that pulses in waveguides with thinner cores have smaller group ve- locities. By recording the change of the pulse's magnetic field with distance the pulse's effective refractive index is extracted and is found to match that calculated analytically. Reducing the magnitude of the core's intrinsic refractive index is also found to reduce the pulse's group velocity. Due to the loss present in current negative refractive index materials, a method of supplying gain to the propagating pulse was investigated. This was achieved by placing a strip of gain material within the waveguide cladding. The gain supplied is found to increase when the separation between the strip and core is decreased, when the width of the strip is increased and when the width of the waveguide's core is reduced. Including the gain material creates a frequency region where the pulse experiences gain and the real part of its effective index is negative. Through recording the change in the pulse's magnetic field over time its complex frequency was extracted. Using the pulse's energy velocity it was possible to convert between the complex frequency and complex wavevector. The use of a prism to couple to the waveguide was also examined. This allowed exceedingly slow modes, of the order of 0.00002c to be excited. It was discovered that keeping the prism in place once the light had coupled into the waveguide caused the value of the pulse's maximum wavevector component to shift.