GW Microwave Research Articles

A compact repetitive high energy-density accelerator HEART-20 based on propylene carbonate pulse forming line.

The development of pulsed power technology requires an electron beam accelerator with high output power and repetitive operation. A compact repetitive electron beam accelerator based on a pulse transformer and a pulse forming line of high permittivity liquid, as an essential type of one, has attracted extensive attention at the present time. In this paper, the development of a compact high energy-density electron beam accelerator, viz., HEART-20, based on a propylene carbonate (PC) forming line is presented. The accelerator HEART-20 consists of a primary energy source, a pulse transformer, a PC pulse forming line, a gas spark gap switch, and a vacuum diode. First, the operation principle of the accelerator is described. Second, the design of the accelerator's parameters is presented. A pulse transformer is developed for rapid charging of the PC-filled pulse forming line. The coupling coefficient is above 0.9, the voltage ratio is about 200, and the operation voltage is about 800kV. Third, the energy storage characteristics of PC are investigated. The insulation characteristics of PC under positive charging voltage are found to perform better than those under negative charging voltage. The insulating strength of PC can be improved by pressurization. Finally, the development of the accelerator HEART-20 is presented. Across a vacuum diode load, it can steadily operate at a 20 GW output power in 5Hz rep-rate. Moreover, it can drive a magnetically insulated line oscillator to produce about 2.0 GW microwave. These findings provide a good foundation for the development of a rep-rate intensive electron beam accelerator with promising applications for the future.

Over-Sized Mode-Selective Relativistic Backward Wave Oscillator

An oversized mode-selective relativistic backward wave oscillator is proposed. This device operates using the pure TM02 mode in the beam-wave interaction area and transmits a pure TM01 mode in the output waveguide. In the beam-wave interaction area, mode selection is well realized through oversized slow-wave structures with a coaxial inner conductor that provides perfect boundary conditions for the TM02 mode and simultaneously eliminates other modes. A large proportion of the TM02 mode field is eventually exported into the output waveguide in the form of a pure TM01 mode. With the large diameter and high mode purity, the device can be expected to have high power capacity. With a 770 kV, 11.0 kA beam guided by a 3.5 T magnetic field, a 3.2 GW microwave at 9.42 GHz has been obtained in our simulation. The device efficiency is 38%.

An improved Ku-band TTO with compact solenoid and better plasma-suppressing collector

A Ku-band transit-time oscillator (TTO) with compact magnetic coil and a novel collector structure is proposed. By utilizing the natural bending of the magnetic field, this TTO could decrease over 25% length of uniform magnetic field and the required energy of the solenoid. Besides, the larger radius and decreased magnetic field at the collector lead to larger collection area and less current density in collector surface, simulation shows that the energy deposition peak is 2.5 times less in this novel collector. That this novel collector also has evident advantages in suppressing the adverse effect of plasmas than traditional one in simulation, when hydrogen gas is preset on the collector surface. Particle-in-cell simulation shows that with a 2.4GW beam guided by a decreasing magnetic field of about 0.7T, 0.85GW microwave power pulse at 13.23GHz is generated in this TTO, yielding a conversion efficiency of about 35.4%.

A Ku-band coaxial transit-time oscillator with Pierce-like cathode under permanent magnet packaging

A novel Ku-band coaxial transit-time oscillator (TTO) with Pierce-like cathode is proposed in this paper. TTO has the characteristics of low space-charge effect and short interaction distance. Combining with these features, we design a novel Pierce-like cathode to lower the demand for the guiding magnetic field. The guiding magnetic field intensity is reduced to about 0.32T by loading the Pierce-like cathode, but the magnetic field intensity is not a stable value, it fluctuates at 0.32T because of the properties of permanent magnet. To achieve the light and small goals of TTO, we design a novel permanent magnet structure with NdFeB. Based on the research, the typical simulation result is that when the diode voltage is 440kV, the beam current is 11kA, the guiding magnetic field is about 0.32T, the device can output 1.46GW microwave pulse at 14.2GHz. The power conversion efficiency is about 30%.

A novel Ku-band relativistic transit-time oscillator with three-cavity extractor and distance-tunable reflector

A novel Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector is proposed. The physical mechanism of the three-cavity extractor and distance-tunable reflector is investigated by theory and simulation. It is shown that by introducing a three-cavity extractor instead of the dual-cavity one, the novel device has higher power capacity. Meanwhile, the three-cavity extractor is more favorable for increasing the beam-wave conversion efficiency of the device. In addition, analysis indicates that the distance-tunable reflector employed in the device plays important roles in the enhancement of conversion efficiency. Particle-in-cell simulations show that with the three-cavity extractor and distance-tunable reflector, this proposed device generates a 12.4 GHz, 2.2 GW microwave, while the diode voltage is 570 kV, the beam current is 10.5 kA, and the guiding magnetic field is 0.7 T. The corresponding power efficiency reaches 37%.

Study on a discal TEM-TE11 mode converter loaded high-efficiency magnetically insulated transmission line oscillator.

An integrative high power microwave device is proposed, which consists of a high-efficiency L-band Magnetically Insulated Transmission Line Oscillator (MILO) and a discal TEM-TE11 mode converter. The MILO with a shrunken load composed of a step-like cathode and a ladder-like beam collector can generate a 1.58 GHz, 5.7 GW microwave with the efficiency of 20.8% at the diode voltage of 560 kV in simulation. The discal converter utilizes a pair of sectorial two-double radial waveguides and a pair of sectorial cross section waveguides to adjust the phase-difference and realizes the mode conversion in a length of less than half wavelength at 1.58 GHz. In the preliminary experiment, the proposed device generates over 2 GW, 1.575 GHz microwave with the pulse duration of over 50 ns in a 420 kV diode voltage; the corresponding efficiency is 14.9%; the radiation pattern is the ideal TE11 mode.

Preliminary experimental investigation of a complex dual-band high power microwave source

In order to promote the power conversion efficiency of a magnetically insulated transmission line oscillator (MILO) and obtain microwaves in dual bands, an axially extracted C-band virtual cathode oscillator (VCO) with multiple resonant cavities is introduced to partially utilize the load current of an S-band MILO. The formed novel dual-band high power microwave source called MILO and VCO is investigated with simulation and experimentally. A dual-band radiation antenna is designed to effectively radiate microwaves generated by the MILO and the VCO, respectively, while avoiding them being influenced by the microwave reflection and diffraction. The preliminary experimental results measured by the dual-band diagnostic system show that both the MILO and the VCO operate normally under repeated shots. A microwave of 2.1 GHz, 1.70 GW is generated from the MILO and a 0.37 GW microwave at frequencies of 4.1 GHz and 3.8 GHz is generated from the VCO under the condition of about 440 kV and 35 kA. Compared with a single MILO (10.6%), a MILO and VCO achieves higher total power and efficiency (13.4%) in both S and C bands, indicating that the load current of the MILO partially couples into the beam-wave interaction in the VCO and then contributes to the output microwaves. However, more works are needed regarding the spectrum purification of the VCO and promotion of the output power of both the MILO and the VCO.

Power combiner with high power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators

To realize power combination of two phase-locked relativistic backward wave oscillators (RBWOs), a compact power combiner is designed and investigated by 3-D particle-in-cell (PIC) simulation and experiment. The power combiner consists of two TM01-TE11 serpentine mode converters with a common output. When the two incident ports are fed with TM01 modes with a relative phase of 180° and power of 2.5 GW at each port, the conversion efficiency from the incident TM01 modes to the combined TE11 mode is 95.2% at 9.3 GHz, and the maximum electric field in the combiner is 714 kV/cm. The PIC simulation shows that the output power from the common port is 4.2 GW when the power combiner is connected to the two RBWOs with input signals, both producing 2.2 GW microwave, corresponding to a combination efficiency of 95.4%. In the high power microwave test, a method is proposed to obtain the combination efficiency without breaking the vacuum, which is 94.1% when the two phase-locked RBWOs output 1.8 GW and 2.2 GW. The power capacity of multi-gigawatts has been demonstrated.

Improved foilless Ku-band transit-time oscillator for generating gigawatt level microwave with low guiding magnetic field

An improved foilless Ku-band transit-time oscillator with low guiding magnetic field is proposed and investigated in this paper. With a non-uniform buncher and a coaxial TM02 mode dual-resonant reflector, this improved device can output gigawatt level Ku-band microwave with relatively compact radial dimensions. Besides the above virtue, this novel reflector also has the merits of high TEM reflectance, being more suitable for pre-modulating the electron beam and enhancing the conversion efficiency. Moreover, in order to further increase the conversion efficiency and lower the power saturation time, a depth-tunable coaxial collector and a resonant cavity located before the extractor are employed in our device. Main structure parameters of the device are optimized by particle in cell simulations. The typical simulation result is that, with a 380 kV, 8.2 kA beam guided by a magnetic field of about 0.6 T, 1.15 GW microwave pulse at 14.25 GHz is generated, yielding a conversion efficiency of about 37%.

A novel coaxial Ku-band transit radiation oscillator without external guiding magnetic field

A novel coaxial transit radiation oscillator without external guiding magnetic field is designed to generate high power microwave at Ku-band. By using a coaxial structure, the space-charge potential energy is suppressed significantly, that is good for enhancing efficient beam-wave interaction. In order to improve the transmission stability of the unmagnetized intense relativistic electron beam, a Pierce-like cathode is employed in the novel device. By contrast with conventional relativistic microwave generators, this kind of device has the advantages of high stability, non-guiding magnetic field, and high efficiency. Moreover, with the coaxial design, it is possible to improve the power-handing capacity by increasing the radial dimension of the Ku-band device. With a 550 keV and 7.5 kA electron beam, a 1.25 GW microwave pulse at 12.08 GHz has been obtained in the simulation. The power conversion efficiency is about 30%.

Role of dc space charge field in the optimization of microwave conversion efficiency from a modulated intense relativistic electron beam

We demonstrate an efficiency of 70% with 5.1 GW microwave power for a diode voltage of 770 kV and a current modulation coefficient of 1.67 in a klystron-like relativistic backward wave oscillator. The device combines the advantages of reducing electron beam radius, adopting dual-cavity extractor, and introducing two pre-modulation cavities. A large dc space charge field is present due to the conversion of considerable potential energy to kinetic energy at the end of beam-wave interaction region. A nonlinear theory is developed to show that the dc space charge field can increase the current modulation coefficient and microwave conversion efficiency significantly.

Numerical Simulation Study and Preliminary Experiments of a Coaxial Vircator With Radial Dual-Cavity Premodulation

Numerical and experimental studies on a coaxial vircator with radial dual-cavity premodulation are presented. The physics of mechanisms on microwave frequency determination and effects on microwave generating efficiency are discussed. It is proved that the microwave frequency is mainly determined by the modulation frequency, while the optimum operating frequency is determined by the intrinsic frequency of the beam itself. When the two frequencies match, the maximum efficiency is yielded. A structure capable of generating 6.0 GW microwave, 17% efficiency, and 50% frequency tuning bandwidth has been obtained by numerical simulations. The preliminary experiments were carried out on the MC55 accelerator. A 2.0 GW output microwave was experimentally obtained and the corresponding device efficiency is about 7%.

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structure and depth-tunable extractor

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structures (SWSs) and depth-tunable extractor is presented. The physical mechanism to increase the power efficiency is investigated theoretically and experimentally. It is shown that the nonuniform SWSs, the guiding magnetic field distribution, and the coaxial extractor depth play key roles in the enhancement of the beam-wave power conversion efficiency. The experimental results show that a 1.609 GHz, 2.3 GW microwave can be generated when the diode voltage is 890 kV and the beam current is 7.7 kA. The corresponding power efficiency reaches 33.6%.

An oversized X-band transit radiation oscillator

An oversized transit radiation oscillator is designed to generate high power microwave at X-band. By using a coaxial structure, the power capacity of the device is improved significantly. In order to realize long-pulse operation, the cathode-anode gap, the collector location, and the surface field strength have been emphatically considered in our design. With a 710 kV, 14.5 kA beam guided by a 0.8 T magnetic field, a 2.7 GW microwave at 9.38 GHz has been obtained in the simulation. The power conversion efficiency is 26.2%. The simulation also indicates that the highest axial electric field strength on the surface of electrodynamic structure is only 590 kV/cm, which could be further decreased by increasing the radial dimension of the X-band device.

Experimental study of a compact P-band coaxial relativistic backward wave oscillator with three periods slow wave structure

A compact P-band coaxial relativistic backward wave oscillator with three periods slow wave structure was investigated experimentally. The experimental results show that the frequency of the P-band coaxial relativistic backward wave oscillator is 897 MHz and the microwave power is 1.47 GW with an efficiency of about 32% in the case in which the diode voltage is 572 kV, the beam current is 8.0 kA, and the guide magnetic field is about 0.86 T. In addition, the device can generate a 3.14 GW microwave radiation as the guide magnetic field increases to 1.2 T at the diode voltage of 997 kV and the beam current of 15.3 kA. The experimental results are in good agreement with those obtained earlier by numerical simulations.

Recent Advance in Long-Pulse HPM Sources With Repetitive Operation in S-, C-, and X-Bands

Recent experimental results of three kinds of long-pulse high-power microwave (HPM) sources operating in S-, C-, and X-bands are reported. The difficulties in producing a long-pulse HPM for the O-type Cerenkov HPM source were analyzed theoretically. In S- and C-bands, single-mode relativistic backward-wave oscillators were designed to achieve long-pulse HPM outputs; in X-band, because of its shorter wavelength, an O-type Cerenkov HPM source with overmoded slow-wave systems was designed to increase power capacity. In experiments, driven by a repetitive long-pulse accelerator, both S- and C-band sources generated HPMs with power of about 2 GW and pulse duration of about 100 ns in single-shot mode, and the S-band source operated stably with output power of 1.2 GW in 20-Hz repetition mode. The X-band source generated 2 GW microwaves power with pulse duration of 80 ns in the single-shot mode and 1.2 GW microwave power with pulse duration of about 100 ns in the 20-Hz repetition mode. The experiments show good performances of the O-type Cerenkov HPM source in generating repetitive long-pulse HPMs, especially in S- and C-bands. It was suggested that explosive emissions on surfaces of designed eletrodynamic structures restrained pulse duration and operation stability.

Generation of 3 GW microwave pulses in X-band from a combination of a relativistic backward-wave oscillator and a helical-waveguide compressor

The phenomenon of passive compression of frequency-modulated (FM) pulses in a dispersive media (DM) was used to increase the peak microwave power up to the multigigawatt level. A helically corrugated waveguide was used as the DM, while a relativistic X-band backward-wave oscillator (RBWO) with a descending-during-the-pulse accelerating voltage served as a source of FM pulses. Compression of pulses down to a halfwidth of 2.2 ns accompanied by a 4.5-fold power increase up to a value of about 3.2 GW has been demonstrated.

Experimental investigation of backward wave oscillator with low magnetic field

A backward wave oscillator (BWO) is introduced in the paper. On the accelerator of Sinus-700, it is experimentally investigated. Under the condition that the electron energy is 740 keV, the beam current is 7 kA and the guiding magnetic field is at 0.68 T, the performance of 1.15 GW microwave output power at 9.1 GHz microwave frequency with 22 ns pulse width and 22% conversion efficiency are reached.

A high power microwave vircator with an enhanced efficiency

A radially extracted vircator (virtual-cathode oscillator) with two identical opposite output ports is presented. The microwave modes propagating inside the WR650 output rectangular waveguide are determined by the calibrated E-probes lined up in the direction perpendicular to the axis of the WR650 waveguide. Velvet-covered cathodes are used to emit electrons. Several diameters of velvet are chosen to see how the emitting area effects microwave generation. Evidence of virtual cathode formation, onset of electron reflexing, and simultaneous microwave emission are discussed. This vircator is found to have the capability of generating a maximum 1.4 GW microwave pulse with the dominant frequency at 8.2 GHz. The radiation pattern is dominated by the TE/sub 10/ mode and the instantaneous efficiency for producing this microwave pulse is around 6+or-0.8%. Simple E-probes are applied to pick up the microwave signals inside the WR650 output rectangular waveguide. >

Starwisp - An ultra-light interstellar probe

Starwisp is an interstellar flyby probe of wire mesh sail with microcircuits at each intersection. It is pushed by beamed microwaves at high acceleration and reaches a coast velocity near light speed while still close to the transmilting antenna. Upon arrival at the target star, the transmitter floods the star system with microwave energy. Using the wires as microwave antennas, the microcircuits collect energy to power their optical detectors and logic circuits to form images of the planets in the system. The phase of the microwaves at each point of the mesh is sensed and used to form a retrodirective phased array that beams a signal back to Earth. A minimal Starwisp would be a I km mesh sail weighing 16 g and carrying 4 g of microcircuits. It would be accelerated at 115 g by a 10 GW microwave beam, reaching one-fifth of the speed of light in one week. Upon arrival at Alpha Centauri 21 years later, Starwisp would collect sufficient microwave power to send 'back high-resolution television pictures during its fly through of the system.