Title:

Twodimensional periodic surface lattice high power millimetre wave experiment

Millimetre wave radiation generation from a free electron maser based on a twodimensional (2D) periodic surface lattice has been demonstrated. Second harmonic Gyrotron Backward Wave Oscillator (BWO) interaction with a two dimensional periodic surface lattice (PSL) has been observed. The major achievements in the thesis are the 1) design, 2) simulation, 3) construction and 4) operation of the experiment. Two different methods were used to manufacture two different types of 2D periodic surface lattice. The first method used 1) electrochemcial deposition of copper on an aluminium former with the alumium subsequently removed by dissolving in strong alkali solution. The second method used a 2) 3D additive manufacturing technique resulting in a silver 2D PSL. For construction method 1): four different copper 2D PSLs with the same inner diameter of 20 mm and the same number of longitudinal corrugations (7), period (3.0 mm) and azimuthal variations (20) but with different peak to peak amplitudes of 0.5 mm; 0.6 mm; 1 mm and 1.6 mm were manufactured. A Wband (75GHz to 110GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the 20 mm inner diameter copper 2D PSLs in order to investigate the effect of the amplitude of the perturbations (0.5 mm; 0.6 mm; 1 mm; 1.6 mm) has on the coupling of the surface and volume fields. By comparing the measured transmission of millimetre waves through the four copper 2D PSLs with different amplitude of corrugations it was shown that an increase in the peaktopeak perturbation amplitude resulted in excitation of surface fields by an obliquely incident wave and the excitation of an eigenmode of the PSL. For the 3D additive construction method 2): a silver high contrast structure defined as having a peaktopeak amplitude of corrugation of 1.6 mm (from top to bottom of the grating) that is larger than a quarter of the operating wavelength was manufactured. A Gband (140GHz to 220GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the silver 2D PSL of 7.2 mm inner diameter having 16 longitudinal periods each period of length 1.6 mm having a perturbation amplitude of 1.6 mm (peak to peak) and an azimuthal period of 3.5mm. In VNA millimetre wave measurements a resonance of the 1st harmonic at 171.1 GHz due to electromagnetic wave interaction with the 3.5 mm azimuthal period which corresponded to a Wband resonance at zero axial wavenumbers of ~85 GHz was observed. Theoretical, numerical and experimental investigation of a proof of principle 2nd harmonic gyrotron BWO based on the silver 2D PSL was carried out. An electron gun that used a velvet cathode was designed and constructed. Experiments were conducted using the velvet cathode electron gun with the electron accelerating voltage produced by a cable Blumlein generator. The electron beam formed and transported through the 7.2 mm inner diameter silver 2D PSL beamwave interaction region within an 18 mm bore 1.8 T solenoid was measured. An 80 kV, 100 A electron beam with a beam outer diameter of 4 mm and inner diameter of 2mm which was approximately 1.8 mm away from the inner surface of the 2D PSL corrugation was measured. Numerical simulations predicted an electron beam of longitudinal velocity of 0.46c which excited an electromagnetic wave on the 2D PSL with a longitudinal (period 1.6 mm) and azimuthal (3.5 mm period) corrugations. Propagating the electron beam through the 2D PSL a possible 2nd harmonic gyrotron BWO was identified at a frequency of ~80GHz from measurements of the frequency using high pass cut off filters and the mode pattern as compared to numerical simulation and the electron beam wave dispersion calculations. Millimetre wave radiation at a frequency of ~80GHz at an output power of 134 ± 5 kW corresponding to an operating efficiency of ~1.7 % was measured. Measurements of the frequency and mode pattern indicate that individual scatterers from the 2D PSL may have been synchronized by the lattice resulting in different parts of the electron beam interacting with the coherent Surface Field (SF) via the 2nd harmonic gyrotron Backward Wave Oscillator instability. Evidence indicates possible electron beam excitation of a cavity eigenmode consisting of the superposition between a TM0,2 incident Volume Field (VF) and a structurally induced TE5,1 Surface Field (SF). Alternative interactions include a 2nd harmonic gyrotron Backward Wave Oscillator with a pure TE5.1 mode, 2nd harmonic gyrotron Backward Wave Oscillator with a pure TM0,2 mode (~75GHz) and a BWO interaction operating with the first spatial harmonic of the TM0,2 volume mode (~75GHz) or a combination of all four. For for a high contrast periodic surface lattice (corrugation depth of 1.6 mm is greater than λ/4) a hybrid EH type mode can be excited within the structure. To conclusively prove experimentally which interaction is dominating a more accurate 1) frequency measuremnet using a heterodyne frequency diagnostic and 2) mode pattern measurement achieved by automating the scan are required and is part of the future work. The results of the experiment have been used to inform the design of a Cherenkov Maser which couples a TM0 ,6 volume field with a HE16,1 surface field on a 20 mm diameter 2D PSL structure with 20 longitudinal corrugations of period 3.2 mm and 16 azimuthal variations of peak to peak amplitude of 1 mm. The beamwave interaction in the 20 mm inner diameter 2D PSL was modelled using the particle in cell code Magic 3D. In the simulations an axial magnetic field of 6 T was used to guide a 1.6 kA, 300 kV annular electron beam with a mean radius of 8.5 mm and beam thickness of 1 mm through the 2D PSL where the beam was located 0.5 mm away from the inner diameter of the 2DPSL. Numerical simulations predict excitation of the 2D PSL by this electron beam will generate 150MW of power at an operating frequency of 97 GHz corresponding to an electronic efficiency of 30 %.
