| In the electromagnetic wave spectrum,terahertz（THz）wave is located between microwave and infrared light,and its development lags behind other wave bands.However,due to the special position of terahertz wave in the electromagnetic spectrum,terahertz wave has many excellent characteristics and has great academic and application value,THz science and technology has got widespread attention in the academic circles.High-power terahertz radiation source is an important development direction of terahertz science and technology.The gyrotron based on the electron cyclotron maser instability can effectively radiate terahertz waves from watt to megawatt,and has great research value.Frequency-tunable terahertz gyrotron is one of the research hotspots.Frequency-tunable gyrotron has potential applications in many fields,especially in dynamic nuclear polarization enhanced nuclear magnetic resonance（DNP-NMR）driven by terahertz wave.Nuclear magnetic resonance（NMR）is favored by researchers in various fields because of its accuracy and non-destructiveness in detecting the internal structure of substances.However,its low sensitivity limits its further development.Dynamic nuclear polarization is an effective means to enhance the sensitivity of NMR.With the development of nuclear magnetic resonance towards high field,the frequency of the radiating source required for dynamic nuclear polarization falls into the terahertz band,and the output power of the radiating source is preferably in the range of several watts to hundreds of watts.DNP-NMR also requires the radiating source to have a long service life,good power and frequency stability,and the terahertz radiation source with a certain frequency-tuning range can better meet the requirements of DNP-NMR system.Therefore,frequency-tunable gyrotron is the most suitable terahertz radiation source for DNP-NMR system at present.With the support of the project of the research group,frequency-tunable gyrotrons were investigated in this thesis,and several typical issues were studied.A multi-mode frequency-tunable gyrotron for the DNP-NMR system was designed and optimized.In addition,the multi-mode frequency-tunable gyrotron has been manufactured and investigated experimentally.The detailed research contents are as follows:1.Based on Maxwell’s equations and the coupled wave theory,the active transmission line equations of TE and TM waves in the circular waveguide with angular uniform but axial gradient were derived in detail.At the same time,the electron motion equation under the action of high-frequency and DC field was also derived.The active transmission line equation and the electron motion equation together constitute the self-consistent nonlinear theory of gyrotrons.Based on this theory,the frequency-domain steady-state code of the beam-wave interaction of gyrotrons was developed.2.Based on the frequency-domain steady-state code,the characteristics of the forward-wave（gyromonotron）and backward-wave operation（reflected-type gyro-BWO）of frequency-tunable gyrotrons were analyzed,including the interaction efficiency,the frequency-tuning characteristics,and the axial field distribution.When gyrotrons operate at the forward-wave state,the beam-wave interaction efficiency is high,but the frequency-tunable range is narrow;When gyrotrons operate at the backward-wave state,the frequency-tunable range is wide,however,the beam-wave interaction efficiency is low.The nonstationary phenomenon in a frequency-tunable gyrotron was also investigated.The gyrotron transitions from the steady state to the nonstationary state with the increase of the beam current.In this thesis,the ratio of the threshold current of the nonstationary state to the starting current was adopted to measure the ability of gyrotrons to maintain steady-state operation.Simulation results demonstrate that among the three regimes,gyromonotron is of the highest capability to keep stationary and the transition regime is the most likely to become nonstationary.3.A multi-mode frequency-tunable gyrotron has been designed and optimized,the operating modes were TE02,TE43,and TE24,and the operating frequencies were 140GHz,250GHz,and 263GHz,respectively.Simulation results showed that the operating modes are excited successively with the increase of the magnetic field,and a certain frequency-tuning range can be realized at each frequency band by means of exciting a series of high-order axial modes.4.A magnetron injection gun（MIG）for the multi-mode frequency-tunable gyrotron was designed and optimized.Within the corresponding operating range of each mode,the electron gun can emit high-quality electron beams（low velocity spread,high pitch factor）.In addition,the influence of the non-ideal electron beam on the beam-wave interaction was analyzed.5.The multi-mode frequency-tunable gyrotron has been experimentally investigated ed in detail.All components were fabricated and the welded,and the gyrotron was assembled and exhausted.The power and frequency measurement system were built.The output power was measured and calibrated by a pyroelectric detector,and the power of each operating mode was in the range of several watts to hundreds of watts.The operating frequency was measured by the frequency mixer and the F-P（Fabry Perot）cavity.The operating frequency of TE43 and TE24 mode was measured by mixer.By adjusting the operating magnetic field,the frequency tuning range of 1.46GHz and 1.11GHz was obtained for TE43 and TE24 modes,respectively;the frequency-tuning bandwidth of0.5GHz and 1.12GHz was obtained for TE43 and TE24 modes through changing the beam voltage,respectively.Since the operating frequency of TE02 mode was not within the operating range of the existing mixer in the laboratory,a frequency measuring system based on F-P cavity has been built.Compared to the measurement results of the mixer,the accuracy of F-P cavity measurement was completely within an acceptable range.The preliminary measurement results of F-P cavity show that the TE02 mode also has a certain bandwidth by means of adjusting the operating magnetic field. |
