Research On Extended Interaction Oscillator Based On Pseudospark Cathode

Millimeter wave and terahertz wave technologies are widely used in radars,electronic countermeasures,5G communications,meteorological monitoring,and security inspections.Vacuum electronic devices are the main millimeter wave and terahertz wave radiation sources.In the high-frequency bands,vacuum electronic devices have the advantages of high efficiency and high power.The extended interaction devices combine the advantages of the traveling wave tube and the klystron,not only having the high gain and high efficiency of the klystron but also the wide bandwidth of the traveling wave tube,which stands out among many vacuum electronic devices.As the operating frequency of the extended interaction device gradually approaches the higher frequency band of millimeter wave and terahertz wave,the size of the device becomes very small.The small size brings a series of problems,such as a decrease in input current,a decrease in power capacity,an increase in manufacturing and assembly difficulty,and an increase in ohmic loss.This thesis aims to solve the above problems from two aspects:improving the input current of the device and optimizing the device structure to increase power capacity and reduce manufacturing difficulty.The commonly used electronic source for the extended interaction oscillator is a thermionic cathode,but the current emitted by the thermionic cathode electron gun decreases significantly as the operating frequency increases.Replacing the thermionic cathode with a higher current density can solve this problem to some extent.Pseudospark discharge belongs to a type of gas discharge,and the current density of the electron beam it produces is high,reaching 10⁴ A/cm².In addition,the pseudospark cathode has the advantage that no additional magnetic system is needed to focus the electron beam.During the discharge process,it generates an ion channel that constrains the movement of the electron beam,keeping it in a certain shape during transmission.Combining the pseudospark cathode with the extended interaction oscillator provides a new idea for a high-power and compact millimeter wave and terahertz wave radiation source.The electron beam generated by pseudospark discharge belongs to a pulse signal with a pulse width of several tens of nanoseconds.It imposes a requirement on the extended interaction oscillator based on a pseudospark cathode:the oscillation startup time（the time required for the device’s output power to reach 90%of its maximum value）must be short enough so that the device can effectively utilize the pseudospark electron beam to oscillate.It is found that the gap length of the extended interaction oscillator is the key dimension that affects the oscillation startup time.The gap length affects the distribution of the electric field in the electron beam tunnel.A strong electric field is favorable for the interaction between the electron beam and the high-frequency electromagnetic wave,and a stronger interaction promotes rapid oscillation.For the extended interaction oscillator based on the pseudospark cathode,the oscillation startup time can be effectively reduced by adjusting the gap length.The use of a pseudospark cathode instead of a thermionic cathode can solve the problem of low input current.As for the issues of low power capacity and difficult manufacturing and assembly,they can be addressed by optimizing the structure of the device.This thesis proposes the use of a coaxial structure multiple-beam extended interaction oscillator operating in the TM₃₁-3πmode to replace the traditional rectangular single-beam extended interaction oscillator.The multiple-beam device can increase the total cross-sectional area of the electron beam tunnels,further increasing the input current.For the coaxial extended interaction oscillator,the commonly used operating mode is the TM₀₁-2πmode.In the selection of the transverse mode,the advantage of the TM₃₁ mode over the fundamental TM₀₁ mode is that the resonant cavity volume of high-order modes is larger at the same operating frequency,thereby increasing the power capacity of the device.As for the longitudinal mode,according to the synchronization condition formula of the interaction circuit,the device operating in the 3πmode has a larger period length than the device operating in the commonly used 2πmode at the same electron beam voltage and operating frequency.For devices operating in high frequency,the small interaction gap length in the longitudinal direction has always been a problem in mechanical manufacturing.The extended interaction oscillator operating in the 3πmode can alleviate this problem.The electronic optical system of a multiple-beam thermionic cathode electron gun is more complex than that of a single-beam electron gun.However,the pseudospark cathode avoids the problem of electronic optical systems because it can self-focus.By changing the number and shape of the holes on the collimator,the pseudospark cathode can form the desired type of electron beam.Based on this principle,a multiple-beam electron source can be easily obtained.To verify the above idea,this thesis designs a G-band multiple-beam extended interaction oscillator based on the pseudospark cathode.Six pencil beam tunnels are distributed symmetrically on the coaxial structure.Considering the background material’s effective conductivity of 2×10⁷ S/m,when the electron beam voltage is 43 k V and the current of each electron beam is 2.5 A,the output power of the device is 39.2 k W and the operating frequency is 217 GHz,with a corresponding efficiency of 6.1%.When the frequency reaches the terahertz band,the series of problems brought by small size become more prominent.Therefore,a terahertz-band multiple-beam extended interaction oscillator driven by a pseudospark cathode is designed based on the above structure.In order to further increase the input current,the six electron beam tunnels are changed from pencil beams to sheet beams.In the simulations,the effective conductivity of the background material is set to 1.1×10⁷ S/m;the electron beam voltage is 41 k V;and the current of each electron beam is 1.4 A.The simulation results show that the output power of the device can reach 4.9 k W,the operating frequency is 0.35 THz,and the efficiency is 1.42%.Compared to the previously designed extended interaction oscillator operating at 0.35 THz,this device showed significant improvements in both output power and efficiency.The problem of small size has been solved by using a multiple beam extended interaction oscillator operating in the TM₃₁-3πmode.However,the use of multiple beams also introduces a new problem:the non-uniformity of the electron beam currents.During the discharge process,the pseudospark cathode generates plasma,but the distribution of the plasma inside the hollow cathode is not uniform.Therefore,for a multiple-beam pseudospark electron gun,it is difficult to achieve the same number of electrons passing through each emitting surface per unit time,resulting in different currents in multiple electron beam tunnels.This thesis analyzes the performance of the multiple-beam extended interaction oscillator based on the pseudospark cathode under different electron beam currents,and it is found that the non-uniformity of the electron beam current has little effect on the device.When the beam voltage is constant,the output power of the EIO is positively correlated with the total input current,and the device can operate normally under non-uniform current conditions.