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In this paper, a 220GHz broadband high-power interleaved double-blade traveling wave tube is designed and verified.First, a planar double-beam staggered double-blade slow-wave structure is proposed.By using a dual-mode operation scheme, the transmission performance and bandwidth are nearly double that of single-mode.Secondly, in order to meet the requirements of high output power and improve the stability of the traveling wave tube, a double pencil-shaped electronic optical system is designed, the driving voltage is 20~21 kV, and the current is 2 × 80 mA.Design goals.By using the mask part and control electrode in the double beam gun, the two pencil beams can be focused along their respective centers with a compression ratio of 7, the focusing distance is about 0.18mm, and the stability is good.The uniform magnetic focusing system has also been optimized.The stable transmission distance of the planar double electron beam can reach 45 mm, and the focusing magnetic field is 0.6 T, which is sufficient to cover the entire high frequency system (HFS).Then, to verify the usability of the electron-optical system and the performance of the slow-wave structure, particle cell (PIC) simulations were also performed on the entire HFS.The results show that the beam-interaction system can achieve a peak output power of nearly 310 W at 220 GHz, the optimized beam voltage is 20.6 kV, the beam current is 2 × 80 mA, the gain is 38 dB, and the 3-dB bandwidth exceeds 35 dB about 70 GHz.Finally, high-precision microstructure fabrication is performed to verify the performance of the HFS, and the results show that the bandwidth and transmission characteristics are in good agreement with the simulation results.Therefore, the scheme proposed in this paper is expected to develop high-power, ultra-broadband terahertz-band radiation sources with potential for future applications.
As a traditional vacuum electronic device, traveling wave tube (TWT) plays an irreplaceable role in many applications such as high-resolution radar, satellite communication systems, and space exploration1,2,3.However, as the operating frequency enters the terahertz band, the traditional coupled-cavity TWT and helical TWT have been unable to meet people’s needs due to relatively low output power, narrow bandwidth, and difficult manufacturing processes.Therefore, how to comprehensively improve the performance of the THz band has become a very concerned issue for many scientific research institutions.In recent years, novel slow-wave structures (SWSs), such as staggered dual-blade (SDV) structures and folded waveguide (FW) structures, have received extensive attention due to their natural planar structures, especially the novel SDV-SWSs with promising potential.This structure was proposed by UC-Davis in 20084.The planar structure can be easily fabricated by micro-nano processing techniques such as computer numerical control (CNC) and UV-LIGA, the all-metal package structure can provide larger thermal capacity with higher output power and gain, and the waveguide-like structure can also provide a wider working bandwidth.Currently, UC Davis demonstrated for the first time in 2017 that SDV-TWT can generate high-power outputs in excess of 100 W and nearly 14 GHz bandwidth signals in the G-band5.However, these results still have gaps that cannot meet the related requirements of high power and wide bandwidth in the terahertz band.For UC-Davis’s G-band SDV-TWT, sheet electron beams have been used.Although this scheme can significantly improve the current-carrying capacity of the beam, it is difficult to maintain a long transmission distance due to the instability of the sheet beam electron optical system (EOS), and there is an over-mode beam tunnel, which may also cause the beam to self-regulate. – Excitation and oscillation 6,7.In order to meet the requirements of high output power, wide bandwidth and good stability of THz TWT, a dual-beam SDV-SWS with dual-mode operation is proposed in this paper.That is, in order to increase the operating bandwidth, dual-mode operation is proposed and introduced in this structure.And, in order to increase the output power, a planar distribution of double pencil beams is also used.Single pencil beam radios are relatively small due to vertical size constraints.If the current density is too high, the beam current must be reduced, resulting in a relatively low output power.To improve the beam current, planar distributed multibeam EOS has emerged, which exploits the lateral size of the SWS.Due to the independent beam tunneling, the planar distributed multi-beam can achieve high output power by maintaining a high total beam current and a small current per beam, which can avoid overmode beam tunneling compared to sheet-beam devices.Therefore, it is beneficial to maintain the stability of the traveling wave tube.On the basis of previous work8,9, this paper proposes a G-band uniform magnetic field focusing double pencil beam EOS, which can greatly improve the stable transmission distance of the beam and further increase the beam interaction area, thereby greatly improving the output power.
The structure of this paper is as follows.First, the SWS cell design with parameters, dispersion characteristics analysis and high frequency simulation results are described.Then, according to the structure of the unit cell, a double pencil beam EOS and beam interaction system are designed in this paper.Intracellular particle simulation results are also presented to verify the usability of EOS and the performance of SDV-TWT.In addition, the paper briefly presents the fabrication and cold test results to verify the correctness of the entire HFS.Finally make a summary.
As one of the most important components of the TWT, the dispersive properties of the slow-wave structure indicate whether the electron velocity matches the phase velocity of the SWS, and thus has a great influence on the beam-wave interaction.To improve the performance of the whole TWT, an improved interaction structure is designed.The structure of the unit cell is shown in Figure 1.Considering the instability of the sheet beam and the power limitation of the single pen beam, the structure adopts a double pen beam to further improve the output power and operation stability. Meanwhile, in order to increase the working bandwidth, a dual mode has been proposed to SWS operate.Due to the symmetry of the SDV structure, the solution of the electromagnetic field dispersion equation can be divided into odd and even modes.At the same time, the fundamental odd mode of the low frequency band and the fundamental even mode of the high frequency band are used to realize the broadband synchronization of the beam interaction, thereby further improving the working bandwidth.
According to the power requirements, the whole tube is designed with a driving voltage of 20 kV and a double beam current of 2 × 80 mA.In order to match the voltage as closely as possible to the operating bandwidth of the SDV-SWS, we need to calculate the length of the period p.The relationship between beam voltage and period is shown in equation (1)10:
By setting the phase shift to 2.5π at the center frequency of 220 GHz, the period p can be calculated to be 0.46 mm.Figure 2a shows the dispersion properties of the SWS unit cell.The 20 kV beamline matches the bimodal curve very well.Matching frequency bands can reach around 70 GHz in the 210–265.3 GHz (odd mode) and 265.4–280 GHz (even mode) ranges.Figure 2b shows the average coupling impedance, which is greater than 0.6 Ω from 210 to 290 GHz, indicating that strong interactions may occur in the operating bandwidth.
(a) Dispersion characteristics of a dual-mode SDV-SWS with a 20 kV electron beamline.(b) Interaction impedance of the SDV slow-wave circuit.
However, it is important to note that there is a band gap between the odd and even modes, and we usually refer to this band gap as the stop band, as shown in Figure 2a.If the TWT is operated near this frequency band, strong beam coupling strength may occur, which will lead to unwanted oscillations.In practical applications, we generally avoid using TWT near the stopband.However, it can be seen that the band gap of this slow-wave structure is only 0.1 GHz.It is difficult to determine whether this small band gap causes oscillations.Therefore, the stability of operation around the stop band will be investigated in the following PIC simulation section to analyze whether unwanted oscillations may occur.
The model of the entire HFS is shown in Figure 3.It consists of two stages of SDV-SWS, connected by Bragg reflectors.The function of the reflector is to cut off the signal transmission between the two stages, suppress the oscillation and reflection of non-working modes such as high-order modes generated between the upper and lower blades, thereby greatly improving the stability of the entire tube.For connection to the external environment, a linear tapered coupler is also used to connect the SWS to a WR-4 standard waveguide.The transmission coefficient of the two-level structure is measured by a time domain solver in the 3D simulation software.Considering the actual effect of the terahertz band on the material, the material of the vacuum envelope is initially set to copper, and the conductivity is reduced to 2.25×107 S/m12.
Figure 4 shows the transmission results for HFS with and without linear tapered couplers.The results show that the coupler has little effect on the transmission performance of the entire HFS.The return loss (S11 < − 10 dB) and insertion loss (S21 > − 5 dB) of the whole system in the 207~280 GHz broadband show that HFS has good transmission characteristics.
As the power supply of vacuum electronic devices, the electron gun directly determines whether the device can generate enough output power.Combined with the analysis of HFS in Section II, a dual-beam EOS needs to be designed to provide sufficient power.In this part, based on previous work in W-band8,9, a double pencil electron gun is designed using a planar mask part and control electrodes.First, according to the design requirements of SWS in Sect.As shown in FIG. 2 , the driving voltage Ua of the electron beams is initially set to 20 kV, the currents I of the two electron beams are both 80 mA, and the beam diameter dw of the electron beams is 0.13 mm.At the same time, in order to ensure that the current density of the electron beam and the cathode can be achieved, the compression ratio of the electron beam is set to 7, so the current density of the electron beam is 603 A/cm2, and the current density of the cathode is 86 A/cm2, which can be achieved by This is achieved using new cathode materials.According to design theory 14, 15, 16, 17, a typical Pierce electron gun can be uniquely identified.
Figure 5 shows the horizontal and vertical schematic diagrams of the gun, respectively.It can be seen that the profile of the electron gun in the x-direction is almost identical to that of a typical sheet-like electron gun, while in the y-direction the two electron beams are partially separated by the mask.The positions of the two cathodes are at x = – 0.155 mm, y = 0 mm and x = 0.155 mm, y = 0 mm, respectively.According to the design requirements of compression ratio and electron injection size, the dimensions of the two cathode surfaces are determined to be 0.91 mm × 0.13 mm.
In order to make the focused electric field received by each electron beam in the x-direction symmetrical about its own center, this paper applies a control electrode to the electron gun.By setting the voltage of the focusing electrode and the control electrode to −20 kV, and the voltage of the anode to 0 V, we can obtain the trajectory distribution of the dual beam gun, as shown in Fig. 6.It can be seen that the emitted electrons have good compressibility in the y-direction, and each electron beam converges toward the x-direction along its own center of symmetry, which indicates that the control electrode balances the unequal electric field generated by the focusing electrode.
Figure 7 shows the beam envelope in the x and y directions.The results show that the projection distance of the electron beam in the x-direction is different from that in the y-direction.The throw distance in the x direction is about 4mm, and the throw distance in the y direction is close to 7mm.Therefore, the actual throw distance should be chosen between 4 and 7 mm.Figure 8 shows the cross-section of the electron beam at 4.6 mm from the cathode surface.We can see that the shape of the cross section is closest to a standard circular electron beam.The distance between the two electron beams is close to the designed 0.31 mm, and the radius is about 0.13 mm, which meets the design requirements.Figure 9 shows the simulation results of the beam current.It can be seen that the two beam currents are 76mA, which is in good agreement with the designed 80mA.
Considering the fluctuation of driving voltage in practical applications, it is necessary to study the voltage sensitivity of this model.In the voltage range of 19.8 ~ 20.6 kV, the current and beam current envelopes are obtained, as shown in Figure 1 and Figure 1.10 and 11.From the results, it can be seen that the change of driving voltage has no effect on the electron beam envelope, and the electron beam current only changes from 0.74 to 0.78 A.Therefore, it can be considered that the electron gun designed in this paper has a good sensitivity to voltage.
The effect of driving voltage fluctuations on the x- and y-direction beam envelopes.
A uniform magnetic focusing field is a common permanent magnet focusing system.Due to the uniform magnetic field distribution throughout the beam channel, it is very suitable for axisymmetric electron beams.In this section, a uniform magnetic focusing system for maintaining the long-distance transmission of double pencil beams is proposed.By analyzing the generated magnetic field and beam envelope, the design scheme of the focusing system is proposed, and the sensitivity problem is studied.According to the stable transmission theory of a single pencil beam18,19, the Brillouin magnetic field value can be calculated by equation (2).In this paper, we also use this equivalence to estimate the magnetic field of a laterally distributed double pencil beam.Combined with the electron gun designed in this paper, the calculated magnetic field value is about 4000 Gs.According to Ref. 20, 1.5-2 times the calculated value is usually chosen in practical designs.
Figure 12 shows the structure of a uniform magnetic field focusing field system.The blue part is the permanent magnet magnetized in the axial direction.Material selection is NdFeB or FeCoNi.The remanence Br set in the simulation model is 1.3 T and the permeability is 1.05.In order to ensure the stable transmission of the beam in the whole circuit, the length of the magnet is initially set to 70 mm.In addition, the size of the magnet in the x direction determines whether the transverse magnetic field in the beam channel is uniform, which requires that the size in the x direction cannot be too small.At the same time, considering the cost and the weight of the whole tube, the size of the magnet should not be too large.Therefore, the magnets are initially set to 150 mm × 150 mm × 70 mm.Meanwhile, to ensure that the entire slow-wave circuit can be placed in the focusing system, the distance between the magnets is set to 20mm.
In 2015, Purna Chandra Panda21 proposed a pole piece with a new stepped hole in a uniform magnetic focusing system, which can further reduce the magnitude of flux leakage to the cathode and the transverse magnetic field generated at the pole piece hole.In this paper, we add a stepped structure to the pole piece of the focusing system.The thickness of the pole piece is initially set to 1.5mm, the height and width of the three steps are 0.5mm, and the distance between the pole piece holes is 2mm, as shown in Figure 13.
Figure 14a shows the axial magnetic field distribution along the centerlines of the two electron beams.It can be seen that the magnetic field forces along the two electron beams are equal.The magnetic field value is about 6000 Gs, which is 1.5 times the theoretical Brillouin field to increase transmission and focusing performance.At the same time, the magnetic field at the cathode is almost 0, indicating that the pole piece has a good effect on preventing magnetic flux leakage.Figure 14b shows the transverse magnetic field distribution By in the z direction at the upper edge of the two electron beams.It can be seen that the transverse magnetic field is less than 200 Gs only at the pole piece hole, while in the slow-wave circuit, the transverse magnetic field is almost zero, which proves that the influence of the transverse magnetic field on the electron beam is negligible.To prevent magnetic saturation of the pole pieces, it is necessary to study the magnetic field strength inside the pole pieces.Figure 14c shows the absolute value of the magnetic field distribution inside the pole piece.It can be seen that the absolute value of the magnetic field strength is less than 1.2T, indicating that the magnetic saturation of the pole piece will not occur.
Magnetic field strength distribution for Br = 1.3 T.(a) Axial field distribution.(b) Lateral field distribution By in the z direction.(c) Absolute value of field distribution within the pole piece.
Based on the CST PS module, the axial relative position of the dual beam gun and the focusing system is optimized.According to Ref. 9 and simulations, the optimal location is where the anode piece overlaps the pole piece away from the magnet.However, it was found that if the remanence was set to 1.3T, the transmittance of the electron beam could not reach 99%.By increasing the remanence to 1.4 T, the focusing magnetic field will be increased to 6500 Gs.The beam trajectories on the xoz and yoz planes are shown in Figure 15. It can be seen that the beam has good transmission, small fluctuation, and a transmission distance greater than 45mm.
Trajectories of double pencil beams under a homogeneous magnetic system with Br = 1.4 T.(a) xoz plane.(b) yoz aircraft.
Figure 16 shows the cross-section of the beam at different positions away from the cathode.It can be seen that the shape of the beam section in the focusing system is well maintained, and the section diameter does not change much.Figure 17 shows the beam envelopes in the x and y directions, respectively.It can be seen that the fluctuation of the beam in both directions is very small.Figure 18 shows the simulation results of the beam current.The results show that the current is about 2 × 80 mA, which is consistent with the calculated value in the electron gun design.
Electron beam cross section (with focusing system) at different positions away from the cathode.
Considering a series of problems such as assembly errors, voltage fluctuations, and changes in magnetic field strength in practical processing applications, it is necessary to analyze the sensitivity of the focusing system.Because there is a gap between the anode piece and the pole piece in actual processing, this gap needs to be set in the simulation.The gap value was set to 0.2 mm and Figure 19a shows the beam envelope and beam current in the y direction.This result shows that the change in the beam envelope is not significant and the beam current hardly changes.Therefore, the system is insensitive to assembly errors.For the fluctuation of the driving voltage, the error range is set to ±0.5 kV.Figure 19b shows the comparison results.It can be seen that the voltage change has little effect on the beam envelope.The error range is set from -0.02 to +0.03 T for changes in magnetic field strength.The comparison results are shown in Figure 20.It can be seen that the beam envelope hardly changes, which means that the entire EOS is insensitive to changes in the magnetic field strength.
Beam envelope and current results under a uniform magnetic focusing system.(a) Assembly tolerance is 0.2 mm.(b) The driving voltage fluctuation is ±0.5 kV.
Beam envelope under a uniform magnetic focusing system with axial magnetic field strength fluctuations ranging from 0.63 to 0.68 T.
In order to ensure that the focusing system designed in this paper can match with HFS, it is necessary to combine the focusing system and HFS for research.Figure 21 shows a comparison of beam envelopes with and without HFS loaded.The results show that the beam envelope does not change much when the entire HFS is loaded.Therefore, the focusing system is suitable for the traveling wave tube HFS of the above design.
To verify the correctness of the EOS proposed in Section III and investigate the performance of the 220 GHz SDV-TWT, a 3D-PIC simulation of beam-wave interaction is performed.Due to simulation software limitations, we were unable to add the entire EOS to HFS.Therefore, the electron gun was replaced with an equivalent emitting surface with a diameter of 0.13mm and a distance between the two surfaces of 0.31mm, the same parameters as the electron gun designed above.Due to the insensitivity and good stability of EOS, the driving voltage can be properly optimized to achieve the best output power in the PIC simulation.The simulation results show that the saturated output power and gain can be obtained at a driving voltage of 20.6 kV, a beam current of 2 × 80 mA (603 A/cm2), and an input power of 0.05 W.
In order to obtain the best output signal, the number of cycles also needs to be optimized.The best output power is obtained when the number of two stages is 42 + 48 cycles, as shown in Figure 22a.A 0.05 W input signal is amplified to 314 W with a gain of 38 dB.The output power spectrum obtained by Fast Fourier Transform (FFT) is pure, peaking at 220 GHz.Figure 22b shows the axial position distribution of electron energy in the SWS, with most of the electrons losing energy.This result indicates that the SDV-SWS can convert the kinetic energy of electrons into RF signals, thereby realizing signal amplification.
SDV-SWS output signal at 220 GHz.(a) Output power with included spectrum.(b) Energy distribution of electrons with the electron beam at the end of the SWS inset.
Figure 23 shows the output power bandwidth and gain of a dual-mode dual-beam SDV-TWT.Output performance can be further improved by sweeping frequencies from 200 to 275 GHz and optimizing the drive voltage.This result shows that the 3-dB bandwidth can cover 205 to 275 GHz, which means that dual-mode operation can greatly broaden the operating bandwidth.
However, according to Fig. 2a, we know that there is a stop band between the odd and even modes, which may lead to unwanted oscillations.Therefore, work stability around the stops needs to be studied.Figures 24a-c are the 20 ns simulation results at 265.3 GHz, 265.35 GHz, and 265.4 GHz, respectively.It can be seen that although the simulation results have some fluctuations, the output power is relatively stable.The spectrum is also shown in Figure 24 respectively, the spectrum is pure.These results indicate that there is no self-oscillation near the stopband.
Fabrication and measurement are necessary to verify the correctness of the entire HFS.In this part, the HFS is fabricated using computer numerical control (CNC) technology with a tool diameter of 0.1 mm and a machining accuracy of 10 μm.The material for the high-frequency structure is provided by oxygen-free high-conductivity (OFHC) copper.Figure 25a shows the fabricated structure.The entire structure has a length of 66.00 mm, a width of 20.00 mm and a height of 8.66 mm.Eight pin holes are distributed around the structure.Figure 25b shows the structure by scanning electron microscopy (SEM).The blades of this structure are uniformly produced and have good surface roughness.After precise measurement, the overall machining error is less than 5%, and the surface roughness is about 0.4μm.The machining structure meets the design and precision requirements.
Figure 26 shows the comparison between actual test results and simulations of transmission performance.Port 1 and Port 2 in Figure 26a correspond to the input and output ports of the HFS, respectively, and are equivalent to Port 1 and Port 4 in Figure 3.The actual measurement results of S11 are slightly better than the simulation results.At the same time, the measured results of the S21 are slightly worse.The reason may be that the material conductivity set in the simulation is too high and the surface roughness after actual machining is poor.Overall, the measured results are in good agreement with the simulation results, and the transmission bandwidth meets the requirement of 70 GHz, which verifies the feasibility and correctness of the proposed dual-mode SDV-TWT.Therefore, combined with the actual fabrication process and test results, the ultra-broadband dual-beam SDV-TWT design proposed in this paper can be used for subsequent fabrication and applications.
In this paper, a detailed design of a planar distribution 220 GHz dual-beam SDV-TWT is presented.The combination of dual-mode operation and dual-beam excitation further increases the operating bandwidth and output power.The fabrication and cold test are also carried out to verify the correctness of the entire HFS. The actual measurement results are in good agreement with the simulation results.For the designed two-beam EOS, a mask section and control electrodes have been used together to produce a two-pencil beam.Under the designed uniform focusing magnetic field, the electron beam can be stably transmitted over long distances with good shape.In the future, the production and testing of EOS will be carried out, and the thermal test of the entire TWT will also be carried out.This SDV-TWT design scheme proposed in this paper fully combines the current mature plane processing technology, and shows great potential in performance indicators and processing and assembly.Therefore, this paper believes that the planar structure is most likely to become the development trend of vacuum electronic devices in the terahertz band.
Most of the raw data and analytical models in this study have been included in this paper.Further relevant information may be obtained from the corresponding author upon reasonable request.
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