Two-dimensional periodic surface lattice high power millimetre wave experiment

  • Alan Richard Phipps

Student thesis: Doctoral Thesis

Abstract

Millimetre wave radiation generation from a free electron maser based on a two-dimensional (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 W-band (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 peak-to-peak 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 peak-to-peak 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 G-band (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 W-band 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 beam-wave 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. Measureme
Date of Award1 Apr 2014
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde & EPSRC (Engineering and Physical Sciences Research Council)
SupervisorAdrian Cross (Supervisor) & (Supervisor)

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