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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.

An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.

Compact is a significant research direction of low-frequency high-power microwave devices. In this paper, a compact S-band relativistic backward wave oscillator is presented. The radial dimension of the device is reduced by utilizing the characteristic that the cutoff frequency of the fundamental mode (quasi-TEM mode) of the coaxial Slow-Wave Structure (SWS) is zero. Moreover, a coaxial extraction structure is designed to enhance its resonance characteristics to reduce the number of slow-wave structures. In addition, a reflective structure with the phase difference between inner and outer conductors is used, which replaces the pre-reflection cavity and shortens the axial size of the device. A relativistic backward oscillator with a 2.5-period coaxial slow-wave structure is proposed through theoretical analysis, simulation calculation, and experiment. Its radial dimension is 160 mm (∼1.28λ), and its axial dimension is 320 mm (∼2.56λ). In the experiment, the results show that the output microwave has a center frequency of 2.41 GHz, a power of 1.5 GW, a pulse width of 92 ns, and continuous operation for 20 s at a repetition of 10 Hz.

During hypersonic vehicle flight at high speed, plasma sheath on the vehicle surface will attenuate or even interrupt the communication signal, leading to the “communication blackout” problem. The vehicle probably moves a long distance during the communication blackout due to its high speed, which is a serious threat to the safety of the vehicle. This paper proposes a method to solve the communication blackout problem using high-power microwave (HPM) irradiation. The multicomponent compressible model, finite difference time domain algorithm, and multi-fluid model are used to simulate the interaction between HPM and plasma sheath. The results show that after HPM irradiation, the electromagnetic (EM) wave transmissivity of the plasma sheath will change, and the electric field (E-field) amplitude and irradiation time of HPM significantly influence the change of transmissivity. Thereafter, analyses of the changes of the collision and plasma frequencies of the plasma sheath after HPM irradiation showed the transmissivity of the plasma sheath to low-frequency EM waves is improved by optimizing E-field amplitude and irradiation time of HPM. Therefore, HPM irradiation can be performed to enhance the transmissivity of the plasma sheath to the communication signal, thus alleviating the communication blackout problem.

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%.

Self-oscillation of three cascaded reflectors easily occurs in a triaxial klystron amplifier (TKA) since the TEM mode leakage cannot be cut off by a coaxial waveguide. To solve this issue, we propose a non-uniform reflector to obtain an optimal external quality factor, which indicates its minimum TEM mode leakage. The self-oscillation of three non-uniform reflectors is then suppressed by selecting a specific drift tube length between them. Moreover, different eigenfrequencies are chosen for the three improved reflectors to enhance the tolerance of the drift tube length between them. With these methods, the self-oscillation of three cascaded reflectors can be successfully eliminated, and a TKA with high output efficiency is capable of achieving a long pulse output.

A compact coaxial relativistic klystron amplifier (RKA) with three cascaded bunching cavities is investigated to obtain high efficiency output in this paper. When the injection power and drift tube length are both decreased for the compactness of the initial coaxial RKA, the conversion efficiency of this device decreases to only 20%, which is mainly due to the insufficient modulation of intense relativistic electron beam (IREB). To solve this issue, three cascaded bunching cavities are designed to strengthen the modulation of IREB, and each bunching cavity is designed to a non-uniform single-gap cavity with a high external quality factor. Moreover, a specific drift tube length is chosen for minimizing the TEM mode leakage between every two adjacent cavities. With these methods, the self-oscillation between input and three cascaded bunching cavities can be successfully suppressed without loading any reflectors. The proposed three cascaded single-gap bunching cavities are further examined in a Ku-band coaxial RKA, and the output efficiency of this device increases to 44%, which is more than twice that of the initial coaxial RKA. Furthermore, the injection power and axial length of the propagating IREB in the improved coaxial RKA are reduced to only 5 kW and 10 wavelengths, respectively.

Amorphous magnetic core has been widely used in pulsed power technology with applications including plasma science research, high-power microwave generation, and material surface treatment. For different applications, dynamic properties of the magnetic core will vary with stimulated magnetic field which determines by magnetization time and waveform of the driving voltage. In this paper, toroidal amorphous magnetic core ( $\Phi ~540$ – $740\times25$ mm) was investigated theoretically, numerically, and experimentally. Specifically, the circuit is analyzed. A high-voltage test stand, which can generate different driving voltages with same components, was designed and developed in our laboratory. Dynamic properties, including magnetic induction change, coercive force and losses, were studied under three different typical waveforms with the same magnetic flux. Driving voltages of the pulses were 12, 12, and 6 kV and the magnetization rates were 0.6, 1.2, and 0.6 T/ $\mu \text{s}$ , respectively. Experimental results show reasonable agreement with the numerical analysis.

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.

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.

A novel V-band coaxial transit-time oscillator (TTO) with high power handling capacity is presented. The interaction region is designed to operate in TM02 mode to increase power handling capacity of the device. By using a traveling wave output structure, the saturation time of the device is improved, and the maximum surface electric field is decreased. The output structure is also good for improving beam-wave interaction. To avoid mode competition, deep corrugation is applied to the slow wave structure, so the Rayleigh hypothesis is not satisfied. Hence the dispersion relation of the SWS in output structure is analyzed by mode matching method instead of Floquet harmonic expansion method. In numerical simulation, a 642MW microwave pulse at 60.0GHz is obtained with a diode voltage of 400kV and beam current of 5kA. The power conversion efficiency is about 32.1%.

A novel Millimeter-wave oversized coaxial relativistic oscillator with low guiding magnetic field and high power capacity is proposed. The physical mechanism of the proposed oscillator is investigated by theory and simulation. It is shown that by introducing a coaxial structure and quasi body wave adopted as the operating mode, the oscillator can run stably under low guiding magnetic. Meanwhile, with an oversized structure and extended interaction extractor operating at higher order mode, the proposed oscillator has the potential of obtaining high output power close to Giggawatt-class. Main structure parameters of the device are optimized by particle in cell simulations. Particle-in-cell simulations show that with the oversized coaxial structure, quasi body wave and extended interaction extractor operating at higher order mode, this proposed oscillator generates a 31 GHz, 840 MW microwave, while the diode voltage is 400 kV, the beam current is 6.1 kA, and the guiding magnetic f is 0.6 T. The corresponding power efficiency reaches 35%.

Relativistic backward wave oscillators (RBWOs) are one type of the promising devices that can produce high power microwaves (HPMs) and is extensively investigated because of its merits such as compact structure, high efficiency, and good stability. In L-, S-, C-, and X-bands, RBWOs had achieved output power of 2-6.5 GW, efficiency of 20%-36%, and repetition rate of 20-100 Hz. However, the pulse duration of RBWOs usually does not exceed 20-30 ns owing to the “pulse shortening” phenomenon. Recently, many studies on measures restraining pulse shortening are taken to prolong the microwave pulse duration. In 2008, the Institute of High Current Electronics, Russian Academy of Sciences reported the experimental results that they had gotten a microwave with power of 3 GW and pulse duration of 90 ns in an S-band RBWO. Compared with the diode voltage pulse width (300 ns), there occurred serious pulse shortening phenomenon. And there are no repetition rate experimental reports. In 2011, the National University of Defense Technology in China reported recent advances in long-pulse HPM sources with repetitive operation in S-, C-, and X-Bands. In experiments, driven by a repetitive long-pulse accelerator (the diode voltage pulse width is about 160 ns), the S-band source operated stably with output power of 1.2 GW with pulse duration of about 100 ns in 20 Hz repetition mode. The X-band source generated 1.2 GW microwave power with pulse duration of about 100 ns in the 20 Hz repetition mode. The C-band source generated HPMs with power of about 2 GW and pulse duration of about 100 ns in single-shot mode. It was suggested that explosive emissions on surfaces of designed eletrodynamic structures restrained pulse duration and operation stability. But the beam-wave conversion efficiencies of the above HPM sources are all below 25%.

Miniaturization is one of the important research directions of low frequency high power microwave sources. This paper presents a three-period coaxial slow-wave structure L-band high-power microwave source. Because the coaxial Quasi-TEM mode has no cut-off frequency, the radial size of the device can be reduced. At the same time, in order to reduce the transverse dimension, the coaxial extractor structure is introduced to realize the longitudinal mode selection and improve the conversion efficiency of the device. In simulation, the device obtains the microwave output with the central frequency of 1.53 GHz, the average power of 3.3 GW and the efficiency of 40%. By optimizing the scheme of electron beam collection, the phenomenon of pulse shortening is effectively suppressed. In the experiment, the device obtains the microwave output with the central frequency of 1.52 GHz, the average power of 3 GW, the efficiency of 33% and the pulse width of 40 ns.

High efficiency and short saturation time have great significance on the practical work of high power microwave devices. A dual-band Cerenkov microwave generator with high-efficiency based on electrical tuning is presented in this paper. Two-segment slow wave structures for the C-band and X-band separately are adopted, which are isolated by a drift cavity. The collimating hole is replaced by a resonant reflector, which can effectively shorten the saturation time. Moreover, in order to realize dual-band microwave output, the electrical tuning method is used in this paper. In particle-in-cell simulation, when the guiding magnetic field is 0.7 T, the output microwaves at the X-band can be obtained with an efficiency of 38% and saturation time for nearly 20 ns; when the guiding magnetic field is 1.5 T, the output microwaves at the C-band can be obtained with an efficiency of 32% and saturation time for nearly 20 ns. Thus, microwaves at the X-band and C-band can be generated separately by modulating the guiding magnetic field, which can be realized just by altering the current in the magnetic coil. It should be mentioned that such a modulating method is much more convenient than mechanical modulation.

Research progresses on Cherenkov and transit-time high-power microwave (HPM) sources in National University of Defense Technology (NUDT) of China are presented. The research issues are focused on the following aspects. The pulse-shortening phenomenon in O-type Cerenkov HPM devices is suppressed. The compact coaxial relativistic backward-wave oscillators (RBWOs) at low bands are developed. The power efficiency in M-Type HPM tubes without guiding magnetic field increased. The power capacities and power efficiencies in the triaxial klystron amplifier (TKA) and relativistic transit-time oscillator (TTO) at higher frequencies increased. In experiments, some exciting results were obtained. The X-band source generated 2 GW microwave power with a pulse duration of 110 ns in 30 Hz repetition mode. Both L- and P-band compact RBWOs generated over 2 GW microwave power with a power efficiency of over 30%. There is approximately a 75% decline of the volume compared with that of conventional RBWO under the same power capacity conditions. A 1.755 GHz MILO produced 3.1 GW microwave power with power efficiency of 10.4%. A 9.37 GHz TKA produced the 240 MW microwave power with the gain of 34 dB. A 14.3 GHz TTO produced 1 GW microwave power with power efficiency of 20%.

Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.

Radial transit time oscillator (RTTO) can be relied on to generate effective coherent electromagnetic radiation in the millimeter wave domain. In this paper, the preliminary experimental research on a Ka-band RTTO is reported for the first time. In experiments, the radial electron beam emitted from the knife edge of the disk cathode is guided by the radial magnetic field. The bombardment trace on the nylon target verifies the emission and transportation uniformity of the radial e-beam. When the diode voltage is 410 kV, the beam current is 7.8 kA, and the magnetic field is 0.6 T, the RTTO can output microwaves with the power of 320 MW and the frequency of 31.35 GHz. As the diode voltage increases from 350 kV to 430 kV, the output microwave power will grow accordingly. Taking the ohmic loss into consideration, the experimental results are in agreement with the simulation ones.

Coaxial slow wave structures have always been used to enhance the power handling capacity in the relativistic O-type Cherenkov oscillators generally. However, little attention has been paid to the regularity and the problem it may bring by using large average radius. The aim of this study was to investigate the high-frequency characteristics of coaxial slow wave structures in order to evaluate the influence of the radial dimension comprehensively. The results indicate that a large overmoded ratio can make the power handling capacity increase significantly, but as a result, it will bring some problems such as low efficiency, long starting time, and the difficulty in mode control. In addition, this paper also gives some suggestions and precautions for using large radial dimensions. The conclusion of this paper has strong applicability for the design of coaxial slow wave structures.

The radial transit time oscillator (RTTO) is promising to realize high power output of millimeter-waves. Although the radial structure can enhance the power capacity, less cavities and small radial dimension make it difficult to improve the power capacity in RTTOs, especially in the extraction cavity. A one-gap extraction cavity in the Ka-band RTTO is proposed in this paper to improve the power capacity. Without electrons, taking the TM011 cavity as an example, the radial reversal resonant electric field can intersect with radial electrons. By choosing the sizes of the cavity, the synchronization of the electrons and the electric field can be realized to achieve effective energy exchange. In particle-in-cell simulation, the RTTO with the TM011 extraction cavity can output 1.0 GW high power microwaves (HPMs) at 31.2 GHz, and the beam-wave conversion efficiency is 31.6%. The maximum electric field in the TM011 cavity is only 800 kV/cm, which is less than one third that in the TM010 extraction cavities. In addition, the TM012 extraction cavity is employed to improve the efficiency to 35.4%. At the same time, because of the increase in the output power, the maximum radial electric field in the TM012 cavity increases to 850 kV/cm. Therefore, the one-gap extraction cavity can realize multiple energy exchanges to get high beam-wave conversion efficiency and enhance the power capacity in the extraction cavity significantly.

Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.

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.

A novel transit-time oscillator (TTO) is proposed in this paper. An axial cathode which has been widely used in high power microwave (HPM) source and an extractor with radial feature are adopted. In this way, the inherent advantages of axial and radial TTO, both of which can be utilized as high-quality intense relativistic electron beam (IREB), can be generated and the power capacity is also increased. The working mode is π/2 mode of TM01 based on small-signal theory, and under the same energy storage, the maximum electric field in extractor decreases 16.3%. Besides, by utilizing the natural bending of the solenoid, this TTO saves over 60% of the length required by the uniform magnetic field, and consequently reduces the energy consumed by solenoid. The PIC simulation shows that by using 1.0-T decreasing magnetic field generated by the shorter solenoid, 3.37-GW microwave at 12.43 GHz is generated with 620-kV and 13.27-kA input, and the overall conversion efficiency is 41%.

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%.

This paper, based on a relativistic electron-beam accelerator with inductive energy accumulation, investigates the layer structure, plasma jet, and thermal dynamics of Cu target under the irradiation of pulsed electron beam (~350 kV, ~4 kA, ~300 ns). A description of a relativistic electron beam source with a carbon fiber cathode is presented. After the electron-beam irradiation at ~13 J/cm2energy density, microcraters with 0.5–1 µm diameter appeared on the target surface, and the target cross section is characterized by multilayer structures with a ~20 µm thickness melting layer and a cellular layer. Further, it was found that the carbon content increased significantly not only on the target surface but also on the cross section. The gas liberation per pulse induced by electron beam is analyzed. A good agreement between the experimental and calculated perveances was observed, with the exception at the end of the accelerating pulse possibly due to the participation of ion flow from the anode target. In the pulsed emission, there existed material transfer from anode to cathode, which is observed by the identification of elemental compositions on cathode surface. As the beam energy is deposited on target surface, the anode plasma jet is generated, and expands toward the cathode at a velocity of ~3 cm/μs. By solving the one-dimensional heat equation, 109K/s heating rate and 107K/m temperature gradient can be obtained. After the heating of pulsed electron beam, the thermal conduction is dominant, with a cooling rate on the order of 107 K/s. The relativistic electron beam sources may provide a potential development for target experiments and high energy density physics.

Research activities on High-power microwave (HPM) sources in National University of Defense Technology (NUDT) of China are presented. The research issues are focused on following aspects. (1) Suppression of pulse-shortening phenomenon in O-type Cerenkov HPM devices. (2) Enhancement of the power efficiency in M-Type HPM tubes without guiding magnetic field. (3) Enhancement of the power efficiency in Space-charge device with multi-cavity. (4) Development of the compact coaxial relativistic backward-wave oscillator (RBWO) at low bands. (5) Enhancement of the power capacity and power efficiency in triaxial klystron amplifier (TKA) at higher frequencies. (6) Enhancement of the power capacity and power efficiency in relativistic transit-time oscillator (TTO) at higher frequencies. In experiments, some exciting results were obtained. The X-band source generated 2GW microwave power with pulse duration of 110ns in 30Hz repetition mode. A 1.755GHz MILO produced 3.1GW microwave power with power efficiency of 10.4%. Furthermore, the experimental waveforms for a 10 pulse, 2Hz shot series are obtained. A 4.1GHz multi-cavity VCO produced 1GW microwave power with power efficiency of 6.6%. Both L- and P-band compact RBWOs generated over 2GW microwave power with power efficiency of over 30%. There is approximately a 75% decline of the volume compared with that of conventional RBWO under the same power level. A 9.37GHz RKA produced the 240MW microwave power with the gain of 34dB. A 14.3GHz TTO produced 1GW microwave power with power efficiency of 20%.