Relativistic Backward Wave Oscillator Research Articles

Application of mode coupling method in modes purification of overmoded relativistic backward wave oscillators

This paper applies mode coupling method to the design of modes purification device. With this method, the length of the phase adjustment waveguide can be computed precisely, which can increase the efficiency of design. With the new method, we design a purification device for an X-band overmoded relativistic backward wave oscillator and the output mixed modes of 66.6% TM01 and 33.4% TM02 are purified to be 99.47% TM01 mode and 0.53% TM02 mode, respectively. Since mode composition is hard to measure in experiments, analysis and discussion are made regarding the modes purifier, and moreover calculation formula of purification effect of other modes ratio after going through the mode purifier is derived, which can be used in evaluating and forecasting the effects of the mode purifier.

Effective suppression of pulse shortening in a relativistic backward wave oscillator

This paper discusses pulse shortening present in a C-band relativistic backward wave oscillator (RBWO). Effects of the collector plasma are believed to be the main cause. This viewpoint is first verified in numerical simulation. The simulation results show that light charged particles such as hydrogen ions in the collector plasma would axially enter into the beam-microwave interaction region and suppress high-power microwave (HPM) generation. Simultaneously, heavy charged particles such as oxygen or ferric ions in the collector plasma would radially expand out and change the end reflection of the RBWO. All these effects can result in pulse shortening. Simulations also demonstrate that a coaxial collector can effectively suppress plasma effects by retarding their axial and radial expansions. Furthermore, a HPM experiment has confirmed the validity of the coaxial collector. Using this structure, the output power of the RBWO has been increased from 2.5 GW to 3 GW. No pulse shortening has been observed in the HPM experiment.

A relativistic backward-wave oscillator with frequency-selectable across X- and Ku-bands

The mechanism and realization of a relativistic backward-wave oscillator (RBWO) with frequency-selectable across X- and Ku-bands are presented. Both the transverse and the longitudinal mode selections contribute to the frequency-selectable across different bands. The transverse mode selection is achieved using the property of “surface wave” of the slow-wave structures to excite the coaxial TM01 mode (Ku-band) or the hollow TM01 mode (X-band) without exciting the higher TM modes. The longitudinal mode selection is achieved by the introduction of a coaxial extractor, which contributes to the compact structure and the restraints of the longitudinal mode competitions. In simulation, a Ku-band microwave with a frequency of 12.6 GHz and an X-band microwave with a frequency of 10.6 GHz are radiated when the inner-conductor length is 10.2 cm and 0 cm, respectively. The output power of both bands is above 2 GW. This technical route of RBWO with frequency-selectable across different bands is feasible and provide a guide to design other types of frequency tunable high power microwave sources operating at high frequency bands.

Influence of emission threshold and current increase rate on microwave starting time in relativistic backward wave oscillator

Explosive emission cathodes (EECs) are widely used in high power microwave generators. This paper researches the influence of the emission threshold and the current increase rate of annular EECs on the microwave starting time of a relativistic backward wave oscillator (RBWO) when the current amplitude is not affected. The results show that a moderate delay in explosive emission, as long as it's not too long and the current increase rate keeps fast enough, won't bring about a corresponding delay in the starting time of microwave, but inversely, may suppress the mode competition and thus expedite the starting process slightly. The current increase rate, however, has more prominent influence on the starting time of the RBWO. A slower current increase rate will delay the time when the beam current reaches the starting current and lead to a longer starting time.

Multichannel Ka-Band Microwave Oscillator Based on Frequency-Shifted Relativistic Backward-Wave Oscillators

Experiments on summation of linearly polarized Gaussian wave beams in free space from two and four synchronous nanosecond Ka-band relativistic backward wave oscillators (BWOs), each having an output power of about 200 MW with controllably shifted oscillator frequencies have been carried out. An about 0.5% frequency shift in one of the channels was equivalent to a one-period phase delay of the carrier frequency. In a two-channel version of the device, a full beat cycle was obtained for a pulse with quadratic (with respect to the number of sources) enhancement of the power flux density at the interference maximum of the integral directional pattern (DP). In the case of a four-channel device, a diagonal displacement of the DP lobes has been demonstrated.

A Novel <inline-formula> <tex-math notation="LaTeX">$\text{TM}_{0{n}}$</tex-math> </inline-formula> Mode Content and Relative Phase Analysis Technique

Overmoded relativistic backward wave oscillators (RBWOs) are usually difficult to achieve pure mode ouHowever, until now, radiation equipments for high power microwave (HPM) devices are only matched to a single operating mode. So it is unsuitable to utilize the ready-made HPM antenna to radiate mixed $\text{TM}_{0{n}}$ modes with high radiation efficiency, and thus practical applications of the overmoded RBWO device becomes limited. For converting these hybrid $\text{TM}_{0{n}}$ modes into a pure one (e.g., $\text{TM}_{01}$ or TEM mode), diagnostics of the overmoded RBWO's output mode is required. In this letter, a practical measurement technique without requiring high precision calibrations of the power distribution and relative phases between the $\text{TM}_{0{n}}$ modes propagating in overmoded waveguide is described. This method is validated by measuring the output modes of a specially designed $\text{TM}_{01}\!-\!\text{TM}_{0{n}}$ mixed modes exciter.

Research on the effect of cathode plasma expansion on x-band relativistic backward wave oscillator using moving-boundary conformal PIC method

The cathode plasma expansion has been widely investigated and is recognized as impedance collapse in a relativistic backward wave oscillator (RBWO). However, the process of formation and expansion of cathode plasma is very complicated, and the thickness of plasma is only several millimeters, so the simulation of cathode plasma requires high temporal and spatial resolutions. Only the scaled-down diode model and the thin gas layer model are considered in the previous hybrid simulation, and there are few numerical studies on the effect of cathode plasma expansion on the RBWO. In this paper, the moving-boundary conformal particle-in-cell method is proposed; the cathode plasma front is treated in this novel method as the actual cathode surface, and the explosive electron emission boundary moves as the expansion of cathode plasma. Moreover, in order to accurately simulate the electromagnetic field near the cathode surface, the conformal finite-difference time-domain method based on the enlarged cell technique is adopted. The numerical simulation indicates that the diode voltage decreases and the beam current increases as cathode plasma expands; when the cathode plasma velocity is 10 cm/μs, the pulse duration of the generated microwave decreases from 30 ns to 10 ns, the working frequency decreases from 9.83 GHz to 9.64 GHz, and the output power decreases 30% in the course of cathode plasma expansion.

Superradiant Ka-band Cherenkov oscillator with 2-GW peak power

The generation of a 2-GW microwave superradiance (SR) pulses has been demonstrated at 29-GHz using a single-mode relativistic backward-wave oscillator possessing the beam-to-wave power conversion factor no worse than 100%. A record-breaking radiation power density in the slow-wave structure (SWS) of ∼1.5 GW/cm2 required the use of high guiding magnetic field (7 T) decreasing the beam losses to the SWS in strong rf fields. Despite the field strength at the SWS wall of 2 MV/cm, a single-pass transmission mode of a short SR pulse in the SWS allows one to obtain extremely high power density in subnanosecond time scale due to time delay in the development of the breakdown phenomena.

Phase and frequency structure of superradiance pulses generated by relativistic Ka-band backward-wave oscillator

Phase and frequency stability of electromagnetic oscillations in sub-gigawatt superradiance (SR) pulses generated by an extensive slow-wave structure of a relativistic Ka-band backward-wave oscillator were experimentally investigated. Data on the frequency tuning and radiation phase stability of SR pulses with a variation of the energy and current of electron beam were obtained.

Phase Locking and Mode Switching in a Backward-Wave Oscillator With Reflections

For a relativistic backward-wave oscillator (RBWO), various nonstationary phenomena, such as irregular transients, mode competition, self-modulation, and chaos, are typical. Phase locking by injection of the external signal helps to minimize transient times and to stabilize a single-frequency regime of operation. In this paper, phase locking of an RBWO with end reflections is considered. The results of time-domain numerical modeling are presented, which reveal an intricate pattern of the phase-locked operation. In particular, a possibility of rapid frequency switching by RF priming is demonstrated.

Lifetime experimental study of graphite cathode for relativistic backward wave oscillator

Graphite cathodes are widely used due to their good emission properties, especially their long lifetime. Some previous papers have researched their lifetime under certain conditions and uncovered some important phenomena. This paper is dedicated to research the lifetime of the graphite cathode under higher power. In the lifetime test, the voltage and current amplitudes are about 970 kV and 9.7 kA, respectively. The repetition rate is 20 Hz. An X-band relativistic backward wave oscillator is used to generate high power microwave by utilizing the electron beam energy. The experimental results demonstrate that the emission property of the graphite cathode remains quite stable during 105 pulses, despite some slight deteriorations regarding the beam and microwave parameters. The macroscopic morphology change of the cathode blade due to material evaporation is observed by a laser microscope. The mass loss of the graphite cathode is about 60 μg/C. Meanwhile, the observation by a scanning electron microscope unco...

Research on effect of emission uniformity on X-band relativistic backward oscillator using conformal PIC code

Explosive emission cathodes (EECs) are adopted in relativistic backward wave oscillators (RBWOs) to generate intense relativistic electron beam. The emission uniformity of the EEC can render saturation of the power generation unstable and the output mode impure. However, the direct measurement of the plasma parameters on the cathode surface is quite difficult and there are very few related numerical study reports about this issue. In this paper, a self-developed three-dimensional conformal fully electromagnetic particle in cell code is used to study the effect of emission uniformity on the X-band RBWO; the electron explosive emission model and the field emission model are both implemented in the same cathode surface, and the local field enhancement factor is also considered in the field emission model. The RBWO with a random nonuniform EEC is thoroughly studied using this code; the simulation results reveal that when the area ratio of cathode surface for electron explosive emission is 80%, the output power is unstable and the output mode is impure. When the annular EEC does not emit electron in the angle range of 30°, the RBWO can also operate normally.

Analysis of the hot-cavity mode composition of an X-band overmoded relativistic backward wave oscillators

Overmoded RBWO (Relativistic Backward Wave Oscillators) is utilized more and more often for its high power capacity. However, both sides of SWS (Slow Wave Structure) of overmoded RBWO consist multi TM0n modes; in order to achieve the design of reflector, it is essential to make clear of the mode composition of TM0n. NUDT (National University of Defence Technology) had done research of the output mode composition in overmoded O-type Cerenkov HPM (High Power Microwave) Oscillators in detail, but in the area where the electron beam exists, the influence of electron beam must be taken into account. Hot-cavity dispersion equation is figured out in this article first, and then analyzes the hot-cavity mode composition of an X-band overmoded RBWO tentatively. The results show that in collimating hole, the hot-cavity mode analysis is more accurate.

A Ka-band radial relativistic backward wave oscillator with GW-class output power

A novel radial relativistic backward wave oscillator with a reflector is proposed and designed to generate GW-level high power microwaves at Ka-band. The segmented radial slow wave structure and the reflector are matched to enhance interaction efficiency. We choose the volume wave TM01 mode as the working mode due to the volume wave characteristic. The main structural parameters of the novel device are optimized by particle-in-cell simulation. High power microwaves with power of 2 GW and a frequency of 29.4 GHz are generated with 30% efficiency when the electron beam voltage is 383 kV, the beam current is 17 kA, and the guiding magnetic field is only 0.6 T. Simultaneously, the highest electric field in the novel Ka-band device is just about 960 kV/cm in second slow wave structure.

Design of a high efficiency relativistic backward wave oscillator with low guiding magnetic field

A high efficiency relativistic backward wave oscillator working at a low guiding magnetic field is designed and simulated. A trapezoidal resonant reflector is used to reduce the modulation field in the resonant reflector to avoid overmodulation of the electron beam which will lead to a large momentum spread and then low conversion efficiency. The envelope of the inner radius of the slow wave structure (SWS) increases stepwise to keep conformal to the trajectory of the electron beam which will alleviate the bombardment of the electron on the surface of the SWS. The length of period of the SWS is reduced gradually to make a better match between phase velocity and electron beam, which decelerates continually and improves the RF current distribution. Meanwhile the modulation field is reduced by the introduction of nonuniform SWS also. The particle in cell simulation results reveal that a microwave with a power of 1.8 GW and a frequency of 14.7 GHz is generated with an efficiency of 47% when the diode voltage is 620 kV, the beam current 6.1 kA, and the guiding magnetic field 0.95 T.

Axial motion of collector plasma in a relativistic backward wave oscillator

In this paper, it is proposed that plasma formed at the collector may drift back to the cathode and cause pulse shortening of the relativistic backward wave oscillator. Theoretical analysis shows that the axial drift velocity of plasma ions can be up to 5 mm/ns due to the presence of space charge potential provided by an intense relativistic electron beam. Particle-in-cell simulations indicate that the plasma electrons are initially trapped around the collector surface. With the accumulation of the plasma ions, a large electrostatic field forms and drives the plasma electrons to overcome the space charge potential and enter the beam-wave interaction region along the magnetic field lines. As a result, the beam current modulation is disturbed and the output microwave power falls rapidly. The plasma ions move in the beam-wave interaction region with an average axial velocity of 5–8 mm/ns. After the plasma ions reach the diode region, the emitted current at the cathode rises due to the charge neutralizations by the ions. The impedance collapse leads to further decrease of the microwave power. In experiments, when the diode voltage and beam current were 850 kV and 9.2 kA, and the collector radius was 2.15 cm, the output microwave power was 2.4 GW with a pulse width of less than 20 ns. The ion drift velocity was estimated to be about 5 mm/ns. After an improved collector with 3.35 cm radius was adopted, the pulse width was prolonged to more than 30 ns.

Thermal Properties of an Intense Relativistic Electron Beam Collector Under Repetitive Pulse Operation

With the increase in power and miniaturization in a size of repetitively operated high-power microwave (HPM) sources, such as relativistic backward-wave oscillators, the thermal control and cooling methods for a collector have the great effects on efficient operations of these devices. In this paper, the thermal properties of an intense relativistic electron beam collector under repetitive operation were studied. The bombardment area caused by energetic electrons was experimentally quantified, and the pulsed thermal loading for a 700-keV and 7-kA beam current was calculated. With an ANSYS multiphysics package, the typical results of temperature history of the collector under 10–100-Hz operation were presented. The dependence of peak temperatures on forced cooling conditions at different pulse repetition rates was also obtained. Comparison results show that the cooling process is influenced by collector thermophysical parameters, interface thickness, and convective factor of the forced water cooling. Finally, the hydraulic system necessary to achieve appropriate cooling fluid flow was discussed. These results may supply an efficient reference to the optimization and reasonable use of a repetitively operated beam collector in the HPM tubes.

Design and numerical simulation of a high power capacity relativistic backward wave oscillator with an electron collection cavity

A high power capacity relativistic backward wave oscillator with an electron collection cavity (ECC) placed at the downstream of the slow wave structure (SWS) is presented. The breakdown threshold is increased, and the density of seed electron is suppressed by preventing the secondary electron, plasma, and powder generated from the bombardment of spent electron beam on the surface of the collector drifting to the extractor and beam-wave interaction region. The maximum longitudinal electric field in the device is reduced through extension of the span between electron beam and slow wave structure and weakening the Cerenkov radiation. The conversion efficiency reaches up to 52% owing to enhanced transit time radiation taking place at the entrance of the ECC. The maximum longitudinal electric field is 1.1 MV/cm on the surface of SWSs when the output power is 7.3 GW and the power capacity improves significantly.

A Novel -TEM Mixed-Mode Converter

Overmoded relativistic backward wave oscillators cannot preserve output mode purity, generating a number of ${\rm TM}_{0{ n}}$ modes in general. Until now, terminations for high power microwave (HPM) devices are only matched to a single operating mode. It is quite difficult to properly radiate a combination of ${\rm TM}_{0{ n}}$ modes using HPM antennas. In this paper, a novel efficient ${\rm TM}_{0{ n}}$ –TEM-mode converter is proposed. With the method of power dividing and field matching described, a mixture of ${\rm TM}_{01}$ , ${\rm TM}_{02}$ , and ${\rm TM}_{03}$ modes generated by an HPM device can be successfully converted into a pure coaxial circular waveguide TEM mode utilizing the outer and inner coaxial waveguide structure with an efficiency of over 99% at the center frequency. Such a mode transition will greatly improve the radiation efficiency of HPM antennas.

Design and test of permanent magnet used in high power microwave devices

A permanent magnet (PM) is one of the guiding magnet systems of high power microwave (HPM) sources. A relativistic backward wave oscillator (RBWO) packaged by the PM system has the advantages of compact structure, high efficiency and high repetition frequency. On the basis of the Halbach array, the theoretical design and the manufacture structure of the PM are proposed. The simulated magnetic flux density reaches 0.98 T, within a diameter of 50 mm. By employing the PIC simulation, the integrated design is carried out combining the PM and the RBWO. The output power of the RBWO is larger than 1 GW. Furthermore, the PM is manufactured and tested. The uniform length is 110 mm, the weight is 306 kg and the measured magnetic flux density is 0.88 T in center.