Research On Key Technologies Of High Order Mode Millimeter Wave Extended Interaction Radiation Source

Millimeter waves have the characteristics of short wavelength,narrow beam,wide frequency band and strong penetration,which bring important value to many fields such as scientific research,military applications and industrial production.Vacuum electron radiation sources（VERS）have irreplaceable advantages in generating high-frequency,high-power millimeter waves.Extended interaction radiation sources（EIRS）,as compact VERS,have received extensive research and applications due to their high gain per unit length characteristics.With the increase in operating frequency,the structure dimensions of high-frequency circuits are significantly reduced due to the wavelength and size coherence effect,resulting in smaller electron beam tunnel.As a result,the improvement of device performance is inevitably constrained by the electron beam current,posing a significant challenge to the development of millimeter-wave EIRS.High-frequency structures operating in high-order modes have larger circuit dimensions,which can provide a new approach to addressing the size limitations of millimeter-wave EIRS.This dissertation aims to improve the operating current of millimeter-wave EIRS.The multi-beam TM_n1 type and single-beam TM_0n type high-frequency circuit schemes are designed respectively.For these two types of high-frequency circuits,it further proposes key technologies to address mode competition,multi-beam generation,increased beam tunnel size,mode conversion,and other critical aspects,aiming to break through the difficulties faced by the development of EIRS in the millimeter wave band.Based on the traditional ladder structure,a high-frequency circuit scheme utilizing the TM_n1 mode is proposed.Multi-beam technology is employed to increase the operating current for improving device output power.By analyzing the evolution of the field distribution characteristics from the basic mode to the high-order mode,and aiming at the mode competition problem existing in the ladder structure of multi-beam high-order modes,the key technology of introducing metal partitions at appropriate positions of the cavity structure is proposed.This technology can adjust the field distribution of the competing mode without affecting the operating mode to suppress the ability of the competing mode to interact with the electron beam.Further,based on this technology and through the parallel arrangement of fundamental TM₁₁ mode circuit units,a rapid design method for circuits suitable for TM_n1 mode with n beams is achieved.Particle simulations indicate that circuits designed based on this approach,including TM₂₁ mode,TM₃₁ mode,TM₄₁ mode,and TM₅₁ mode,can operate stably,and the output power will increase proportionally with the increase in available electron beams.In addition,it is verified that the higher-order mode has a lower gap electric field than the fundamental mode under the same cavity energy storage condition.This reduces the risk of high-frequency breakdown and improves the power capacity of the device,which is of great significance to further increasing the output power of the device.To address the issue of multi-beam generation,a key technology is proposed to design a multi-beam electron optical system using nano cold cathodes.Compared with thermionic cathodes,field emission cold cathodes can achieve electron emission without applying any form of additional energy to electrons,thus avoiding many problems caused by high-temperature operations,and having the advantage of handling heat dissipation in a small volume.Based on the study of field emission experimental data,a 5-sheet beam electron optical system based on carbon nanotube film emitters is designed to match the previously designed 5-beam TM₅₁ mode extended interaction circuit.Through particle simulations utilizing field emission,the feasibility of carbon nanotube cathodes for multi-beam high-order mode millimeter-wave extended interaction circuits has been validated.In view of the advantages of arrayed field emission cold cathodes,a design scheme for a multi-beam multi-cavity cascade amplifier is further proposed.By coupling the output cavity of the previous stage extended interaction amplifier with the input cavity of the subsequent stage extended interaction amplifier,it ensures the collaborative operation of various components on the same platform,thus achieving step-by-step amplification of the input signal.Moreover,the shorter electron beam transport distance reduces the demand for focusing magnetic fields,which is of significant importance for realizing new integrated vacuum microelectronic devices.In response to the challenge of smaller cavity structures and beam tunnel sizes when the device frequency increases to the millimeter-wave high-frequency range,a technical approach for high-frequency circuits with large electron beam tunnel in the TM_0n mode is proposed.Combined with the field intensity distribution in the form of Bessel function on the circular waveguide or cylindrical cavity cross-section,four-coupling hole structures based on the TM_0n mode have been designed.As the mode order n increases,its longitudinal electric field becomes stronger on the central axis.This not only allows the diameter of the beam tunnel to increase as the lateral size of the circuit increases,but also allows the circuit to maintain sufficient beam-wave coupling capability.Regarding the mode competition problem that naturally intensifies in higher-order mode circuits,the composition of the competing modes is systematically analyzed and the influence of structural parameters on high-frequency characteristics is studied.An improvement plan is further proposed to enhance the coupling ability of the electron beam to the working mode while weakening the coupling ability to the competing mode.Based on the cylindrical cavity structure formed by disk loading,the designed quasi-TM₀₃ mode 220GHz extended interaction oscillator and quasi-TM₀₄ mode 300 GHz extended interaction oscillator have achieved cavity inner diameters of 4.4 mm and 4.16 mm,respectively.Additionally,the radius of the beam tunnel has increased to twice or even more than that of conventional slow-wave structures in the same frequency band.Particle simulation studies indicate that such highly overmoded structures have the potential to stably generate electromagnetic radiation in the G-band and even higher frequencies under large-scale conditions.To verify and develop TM_0n high order mode circuit,the key technology of mode conversion is studied.In the high-frequency circuit designed based on TM_0n mode,the longitudinal electric field is mainly concentrated near the central axis,so energy can be coupled input and output through the small holes opened at both ends of the circuit axis.On this basis,a circuit scheme is developed that first converts from the conventional rectangular waveguide TE₁₀ mode to the circular waveguide TM₀₁ mode,and then converts to the TM₀₄ mode.The four-coupled hole structure and the four-cavity loading center ring structure designed based on TM₀₄ mode are tested experimentally through the mode conversion circuit.This not only validates the feasibility of the highly overmoded operation scheme but also compares the transmission and oscillation characteristics of these two structures.This provides valuable reference for the future development of high-frequency oscillators and broadband amplifiers.Based on the experimental research on the TM₀₄ mode,an extended interaction amplifier operating in the Ka band is designed,and an output power of 289 k W and a saturation gain of 51.6 d B are obtained through particle simulation.In order to further increase the output power and improve circuit stability,a new method for enhancing beam-wave coupling and suppressing mode competition is proposed.Based on this,an extended interaction amplifier operating in TM₀₂ mode is designed.Through particle simulation,an output power of 1.11 MW and a saturation gain of 57 d B have been obtained.