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The electromagnetic characteristics of the input cavity with electron beams loaded are measured successfully in an X-band triaxial relativistic klystron amplifier (TKA) with a high frequency electrical probe. To solve the frequency response issue in the X band, a coaxial electrical probe is optimized and inserted obliquely into the input cavity at the designed angle so that the microwaves in the cavity can be sampled and extracted with the required coupling coefficient while the probe does not interfere with the solenoid coil, which is tightly integrated outside the TKA. Besides, the TKA is redesigned as a structure integrally sealed by the solenoid coil, and the secondary sealing induced by probe insertion is implemented on the flange of the solenoid coils rather than on the wall of the TKA, which is conducive to simplifying the coupling structure while maintaining the high vacuum level. The design is examined in a GW level TKA experiment, indicating that microwaves with a power of about 670 MW are generated in the input cavity at a frequency of 8.40 GHz, which is completely consistent with the injected and output microwave frequencies. [ABSTRACT FROM AUTHOR]

The frequency hopping technology is one of the most significant research directions for high-power microwave (HPM) devices. This paper presented a novel HPM oscillator with frequency hopping across C, X, and Ku bands based on magnetic field tuning. A coaxial transit time oscillator (TTO) is nested onto the outer conductor of the hollow relativistic Cherenkov microwave oscillator, which forms a dual electromagnetic structure with a single-annular cathode. When the electron beam is guided by gradient magnetic fields, it interacts with TTO to produce Ku-band HPMs. If the gradient magnetic field changes into the uniform magnetic field, the electron beam would enter the relativistic Cherenkov microwave oscillator, and the frequency of generated microwaves decreases, which are decided by the strength of the magnetic fields according to the cyclotron resonance absorption theory. In the particle-in-cell simulation, when the diode voltage and gradient magnetic field are 580 kV and 0.5 T, respectively, a Ku-band HPM output with a frequency of 13.9 GHz and a power of 2.09 GW is obtained, corresponding to power efficiency of 42%. When the magnetic field transforms into uniform, the device produces an X-band HPM output with a frequency of 9 GHz and a power of 2.4 GW at a diode voltage of 683 kV and a magnetic field of 0.7 T. When the voltage and magnetic field strength are increased to 699 kV and 1.5 T, respectively, the device generates a C-band HPM output with a frequency of 4.5 GHz and a power of 2.1 GW. The corresponding conversion efficiency of the X-band and C-band Cherenkov microwave oscillators is 35% and 30.7%, respectively. [ABSTRACT FROM AUTHOR]

Because of the scaling invariance, the over-mode ratio of the coaxial resonator can be increased to increase the power handling capability. However, as the over-mode ratio increases, the characteristic impedance and external quality factor decrease, which causes the modulation of the electron beam to be weakened. Moreover, when the output microwave power increases, the double-gap output cavity will suffer from severe radio frequency breakdown. Therefore, an X-band high-power and high-efficiency coaxial relativistic klystron oscillator with a four-gap modulation cavity and a three-gap extraction cavity is proposed. First, a four-gap modulation cavity can increase the modulation depth of the electron beam to improve the beam-wave conversion efficiency. The operating mode of the modulation cavity is the 3π/4 mode of the coaxial TM01 mode. Second, a three-gap extraction cavity is adopted to enhance the microwave extraction energy and reduce the RF field strength. The simulation results show that when the diode voltage is 650 kV, the beam current is 15.4 kA, and the guiding magnetic field is 0.48 T, the device outputs a microwave power of 4.2 GW, a frequency of 8.4 GHz, and an efficiency of 42%. [ABSTRACT FROM AUTHOR]

An improved X-band coaxial transit time oscillator is proposed in this paper. First, the device uses a non-uniform three-gap modulation cavity to improve the clustering of electron beams under a low magnetic field and to increase the depth of the fundamental current modulation. The operating mode of the modulation cavity is the 2π/3 mode of TM01. Second, the dual-cavity extraction structure works in π mode, which can continuously and intensively extract the electron beam energy. The physical characteristics of the device are studied through simulation and experiment. The experimental results indicate that the high power microwave with a frequency of 8.38 GHz and a power of 1.78 GW is generated when the diode voltage is 520 kV and the guiding magnetic field is 0.65 T, yielding a pulse width of 27 ns and an efficiency of 32%. [ABSTRACT FROM AUTHOR]

In recent years, high-power microwave (HPM) generator with a low working magnetic field has caught more attention as this device has the merit of low energy consumption and the potential of using a permanent magnet package in the HPM system. However, the conversion efficiency of this device is generally low because the electron beam will fluctuate transversely and the modulation depth will decline when the guiding magnetic field is low. To obtain concurrently high-power, high-efficiency, and low operating magnetic field, a transit-time oscillator (TTO) with two bunchers is designed and fabricated. Two bunchers are adopted in the TTO to modulate the electron beam twice. By introducing the second buncher, the modulation depth of the electron beam increases by 23%, and the conversion efficiency increases from 25% to 40.5% in simulations. A microwave with a power of 3.65 GW and a frequency of 4.31 GHz is obtained with a 600-kV, 15-kA electron beam under the guidance of a 0.5-T magnetic field in simulations. Furthermore, the corresponding experiments are carried out to verify the two-buncher TTO, and a microwave with a power of 3.3 GW and a frequency of 4.31 GHz is obtained. The experimental efficiency is 32% with an external magnetic field of 0.48 T. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of relativistic klystron amplifiers (RKAs) is severely limited by the strong space-charge repulsion force caused by the intense electron beam. A radial-line RKA (RL-RKA) driven by the disk beam may provide great potentials to alleviate this issue due to its radially propagating characteristic of the electron beam. However, such potential advantage of high efficiency has not been fully exploited in previous research due to the insufficient beam modulation and relatively low beam-wave coupling strength. In this article, an initial RL-RKA is improved significantly by utilizing a three-stage cascaded bunching link, a four-gap output cavity, and a lengthened drift tube. The three-stage cascaded bunching link plays a role in increasing the fundamental harmonic current and decreasing the velocity spread of the electron beam simultaneously. The four-gap output cavity is designed to improve the coupling impedance and strengthen the beam-wave power conversion. Moreover, a drift tube is lengthened suitably to boost the fundamental harmonic current further. Eventually, the power conversion efficiency of the RL-RKA is enhanced up to 67% by these measures, which indicates a significant advance over the initial state of 38%. [ABSTRACT FROM AUTHOR]

A novel dual-band nested transit time oscillator (TTO) is proposed in this paper, which consists of an outer X-band coaxial TTO and an inner Ka-band coaxial TTO. It is capable of generating dual-band microwaves simultaneously with three distinctive features. First, the emission processes of inner and outer annular electron beams are independent. Second, the inner and outer high frequency structures are spatially isolated. Third, the inner and outer annular electron beams share the guiding magnetic field. Particle-in-cell simulation is performed where a voltage of 450 kV and a guiding magnetic field of 0.6 T are used. Simulation results demonstrate that microwave output with a frequency of 8.4 GHz and a power of 1.72 GW is obtained for the X-band, corresponding to a conversion efficiency of 42% and a microwave saturation time of 30 ns. Meanwhile, microwave output with a frequency of 31.3 GHz and a power of 1.18 GW is achieved for the Ka-band, corresponding to a conversion efficiency of 40% and a microwave saturation time of 15 ns. [ABSTRACT FROM AUTHOR]

The energy consumption of high-power microwave sources has attracted much attention in the research field of high-power microwaves. One way of decreasing the energy consumption is to reduce the guiding magnetic field. In this work, a high-efficiency transit-time oscillator is proposed with a low guiding magnetic field. In order to enhance the modulation depth of electrons and increase the conversion efficiency under a low guiding magnetic field, two bunchers are adopted to modulate the electron beam. 2.5D simulation results show that the proposed device generates microwaves with a power of 3.6 GW and a frequency of 4.31 GHz. The power conversion efficiency is 40% and the modulation depth is 125% when the guiding magnetic field is only 0.5 T. In addition, a 3D simulation verifies the results. [ABSTRACT FROM AUTHOR]

High efficiency and high frequency are key research topics for the development of high power microwave devices. There are, however, serious issues with mode competition in relativistic Cerenkov devices based on a single electron beam, which can lead to low efficiency in the beam-wave interaction process. In this paper, a high-efficiency relativistic Cerenkov oscillator based on a dual electron beam is proposed. The hollow slow wave structure is embedded in the inner conductor of the coaxial waveguide to obtain a double coaxial electromagnetic structure, which can suppress mode competition and improve the beam-wave interaction efficiency of the dual-band radiation. Moreover, each band of radiation is independent of the other, such that high frequency output radiation is generated. Using the particle-in-cell method, the physical processes involving the beam-wave interaction in the oscillator are studied with the aim of designing a high-efficiency double-electromagnetic structure. Simulation results indicated that the output power levels for the X-band and Ka-band were 1.50 GW and 710 MW, respectively, and the efficiencies were 34.7% and 30.7%, respectively, for a diode voltage of 550 kV and a guiding magnetic field of 0.85 T. [ABSTRACT FROM AUTHOR]

The mechanism and realization of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment are presented. The device adopts coaxial dual-annular cathode to emit the inner and the outer electron beams, independently. When the inner annular cathode is extended and the outer annular cathode is retracted, the electron beams are emitted from the inner cathode and interact with the internal electromagnetic structure to generate Ka-band microwaves. On the contrary, the outer electron beams interact with the external electromagnetic structure to generate X-band microwaves. Thus, the cross-band microwave generation can be achieved only by adjusting the protrusion and retraction of dual-annular cathode. By the particle-in-cell (PIC) method, the physical process of beam-wave interaction in the device is studied, and the high efficiency internal-external double electromagnetic structures are designed. Simulation results show that the X-band microwave with a power of 1.50 GW and a frequency of 8.55 GHz is generated, corresponding to an efficiency of 32%. Meanwhile, the Ka-band microwave with power of 650 MW and frequency of 31.80 GHz is generated, corresponding to efficiency of 28%. [ABSTRACT FROM AUTHOR]

The physical mechanism, simulation results, and main experimental results of an S-band long-pulse relativistic backward-wave oscillator (RBWO) with a coaxial extractor are presented. The central ideas of the device include: 1) dual resonant cavities replacing the cutoff neck are used to enhance the efficiency and to decrease the electric field; 2) nonuniform slow-wave structures (SWSs) are introduced in the device to improve the efficiency; 3) a coaxial extractor is well designed, which is beneficial to the extraction of generated microwaves; and 4) the diode and the collector are reasonably designed to reduce the influence of cathode and collector plasmas. In simulation, microwaves centered at 3.77 GHz are generated, with the power of 3.3 GW and efficiency of 43%. However, the pulse duration of the device does not exceed 70 ns owing to the pulse-shortening phenomenon in the initial experiment. It was found that the plasma caused by an intense radio frequency electric field on the surface of the coaxial extractor played a main role in pulse shortening. To suppress the pulse shortening, we focus on two aspects. One was to optimize the electrodynamic structures to reduce the electric field on the surfaces of the coaxial extractor while maintaining relatively high efficiency. The other was to improve the surface roughness of SWSs thereby increasing the breakdown threshold of the RF field. In the experiment after these measures were taken, microwaves centered at 3.752 GHz are generated, with the power of 2.3 GW, efficiency of 35%, repetition rate of 20 Hz, and pulse duration above 110 ns. [ABSTRACT FROM AUTHOR]

High power coherent electromagnetic radiation of a millimeter waveband is a tough research topic of plasma physics. In axial millimeter wave electromagnetic radiation devices, low power capacity and mode competition limited the output power. The radial structure is a promising candidate to realize high power millimeter wave radiation. Structural characteristics of a radial Ka-band high power millimeter wave transit time oscillator (TTO) are studied in this paper. Analysis without an electron beam indicates that if the average radius of radial cavities is increased, the mode of the stimulated field will not change, the power capacity will be improved, and the quality factor due to ohmic loss will remain unchanged. Particle-in-cell simulation proves that, for radial millimeter wave TTO, increasing the radius of the cathode and keeping sizes of the other parts unchanged, the output power will be increased, the efficiency will remain the same, and electric field growth will be slower than that in axial devices. Besides, taking ohmic loss into consideration, the efficiency will increase to a stable value. Therefore, by this method, high power coherent electromagnetic radiation in a millimeter wave can be realized. [ABSTRACT FROM AUTHOR]

This paper presents a large signal theory of multiple cascaded bunching cavities for the design of high-efficiency triaxial klystron amplifiers (TKAs). The theoretical analysis of multiple cascaded bunching cavities is presented, focusing on the relationship between gap voltage and first harmonic current and velocity dispersion, which can exactly describe the clustering state of intense relativistic electron beams. The theoretical results of the first harmonic current and velocity dispersion are basically consistent with its simulation results, which can justify a high degree of confidence in the validity of that theory. This theory can predict the possibility of deep modulation of intense relativistic electron beams when the depth of the first harmonic current is about 150% by multiple cascaded bunching cavities. By properly accounting for this theory, we can design a Ku-band TKA with nearly 60% microwave conversion efficiency, which can provide theoretical and simulation guidance for the design of high-efficiency TKAs. More importantly, when we increase the electron beam voltage from 300 kV to 600 kV and keep the relativistic perveance constant, this device also can obtain more than 50% efficiency and 40 dB gain. As a result, we can design a Ku-band TKA with high average output power of about 1.5 GW, 52% efficiency and 46 dB gain. [ABSTRACT FROM AUTHOR]