Construction adsorption and photocatalytic interfaces between C, O co-doped BN and Pd-Cu alloy nanocrystals for effective conversion of CO2 to CO.

Photocatalytic reduction of carbon dioxide (CO2) into fuels is an auspicious route to alleviate the energy and environmental crisis brought by the continuous depletion of fossil fuels. The CO2 adsorption state on the surface of photocatalytic materials plays a significant role in its efficient conversion. The limited CO2 adsorption capacity of conventional semiconductor materials inhibit their photocatalytic performances. In this work, a bifunctional material for CO2 capture and photocatalytic reduction was fabricated by introducing palladium (Pd)-copper (Cu) alloy nanocrystals onto the surface of carbon, oxygen co-doped boron nitride (BN). The elemental doped BN with abundant ultra-micropores had high CO2 capture ability, and CO2 was adsorbed in the form of bicarbonate on its surface with the presence of water vapor. The Pd/Cu molar ratio had great impact on the grain size of Pd-Cu alloy and their distribution on BN. The CO2 molecules tended to be converted to carbon monoxide (CO) at interfaces of BN and Pd-Cu alloys due to their bidirectional interactions to the adsorbed intermediate species while methane (CH4) evolution might occur on the surface of Pd-Cu alloys. Owing to the uniform distribution of smaller Pd-Cu nanocrystals on BN, more effective interfaces were created in the Pd5Cu1/BN sample and it gave a CO production rate of 7.74 µmolg-1h-1 under simulated solar light irradiation, higher than the other PdCu/BN composites. This work can pave a new way for constructing effective bifunctional photo-catalysts with high selectivity to convert CO2 to CO.

A novel compact solid-state high power pulse generator based on magnetic switch and square waveform pulse transformer.

The high power pulse generators have been widely used in high power microwave generation and plasma physics research. In this paper, a novel compact solid-state high power pulse generator is studied, numerically and experimentally. The generator is mainly composed of the primary energy supply, the magnetic pulse compressor, the Blumlein type low-impedance pulse forming network, and the square waveform pulse transformer. Especially, design considerations for a solid-state high power pulse generator are proposed. Experimental results show that pulses with a peak power of 2 GW, a duration of 150 ns, and a repetitive rate of 10 Hz are continuously achieved on a dummy load. The dimension is Φ60 × 210 cm2, and the average power density reaches ∼5 W/L. Experimental results show reasonable agreement with numerical analysis.

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 800 kV. 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 5 Hz 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.

The design of a high-voltage, long-pulse width, flat-top compensation pulse generator based on metal oxide varistors.

In recent years, the pulse forming technology based on metal oxide varistors (MOVs) has been verified to be an effective way to generate high-voltage quasi-square pulses. Due to the limited varistor voltage of a single MOV brick, multiple MOV bricks connected in series are required to stabilize a pulse with high amplitude (larger than hundreds of kV), which leads to the rise of the series inductance of the MOV branch and the flat-top droop in the output waveform. This paper provides two solutions to reduce the influence of the MOV branch inductance on output waveforms. One is that a coaxial evolute structure of the MOV bricks connected in series is designed, which can not only improve the insulation capacity but also reduce the branch inductance. Another is that a flat-top compensation scheme named "PFN-MOV" (Pulse Forming Network) is proposed, which adds an LC filtering branch to shape the signal into a flat-top rising wave with ripple and then offsets the flat-top droop caused by the inductance of the MOV branch. Based on the above ideas, a high-voltage, long-pulse width, flat-top compensation pulse generator is designed and tested, and a quasi-square pulse with voltage amplitude of more than 500 kV, pulse width greater than 800 ns, rise time of less than 50 ns, and flat top of about 600 ns is obtained experimentally. This MOV based generator has the advantage of simple design, compact construction, and better flat top, which is promising to be used as a compact long-pulse driver in many fields, such as high-current accelerator, industrial dedusting, medical sterilization, and cancer treatment.

A new solid-state LC-Marx generator based on saturable pulse transformer.

The saturable pulse transformer (SPT) and six stage LC-Marx generator are proposed in this solid-state system. In the experiments, the output voltage of 14.5 kV and the rise time of 720 ns are achieved when the isolation inductance is 35 µH and the primary capacitor is charged to 350 V. The output voltage is 4.1 times larger than the charging voltage on the single capacitor. For larger output voltage and shorter rise time, isolation inductance is changed with the high-voltage silicon stack, which is a kind of rectifier diode consisting of several diodes enclosed in the resin with larger inverse voltage. In the same experiment condition, the output voltage is increased to 17.5 kV, which is 5.0 times larger than the voltage on the charging capacitor. The rise time gets down to 500 ns. The results show that the high-voltage silicon stack can further enhance the working performance of the SPT LC-Marx generator on the output voltage and rise time. Finally, experiments are carried out to test the working performance of the SPT LC-Marx generator at a repetitive frequency. The results show that the output voltage reaches 12 kV, which proves that the generator can work stably at 1 Hz.

A compact low impedance angular distribution Blumlein-type pulse forming network.

A pulse forming network (PFN) has the advantages of compactness and long pulse achievability, which could meet the requirements of military and industrial applications of pulsed power technology well. In this paper, a compact low impedance Blumlein-type PFN based on ceramic capacitors is investigated numerically and experimentally. Generally, in order to increase the compactness of the PFN, an angular distribution and an axially parallel connected structure with a theoretical peak energy density of up to 5.8 J/L are employed. The dimensions of the PFN are Φ 560 × 345 mm2. A sharpening switch, which can efficiently reduce rise-time of the output pulses, is utilized to improve the performance of the PFN. The compact low impedance Blumlein-type PFN was assembled in our laboratory. The results of low voltage experiments show that the PFN could generate quasi-square pulses with an output power of 50 MW and a peak voltage of approximately 13.2 kV on a matched dummy load. Impedance and output pulse duration of the PFN are 3 Ω and 135 ns, respectively. The results of high voltage experiments show that pulses with a power of about 1 GW and an energy density of about 2.5 J/L were obtained. Experiments show reasonable agreement with numerical analysis.

A high-efficiency L-band coaxial three-period relativistic Cherenkov oscillator.

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.

Numerical simulation and experiment of pulsed switching of a ferromagnetic core.

Ferromagnetic core based magnetic switches are widely used in various pulsed power facilities. The dynamic characteristics of high-power magnetic switches, which have important impacts on the pulse modulation process, are analyzed via an improved numerical model in this paper. The model is established by simultaneously solving the circuit equations and the magnetic field diffusion equations. An implicit finite difference method is used in solving the diffusion equations, which has no numerical convergence problems, and the Jiles-Atherton model is used to obtain an accurate hysteresis loop of the core. The improved model predicts the performance of the magnetic switch quite well. It is then used to analyze the detailed dynamic saturation process of a core, and the core's saturation time predicted by the model is consistent with the experimental data, the error being less than 5%. Furthermore, the interlamination electric field is calculated and analyzed, and it is predicted that breakdown is most likely to occur at the inner side of the core and at the edge of the lamination.

A compact solid-state high voltage pulse generator.

In this paper, a compact solid-state high voltage pulse generator, composed of a pulse transformer and a magnetic pulse compressor, is investigated numerically and experimentally. The generator can achieve pulses with a peak voltage over several tens of kilovolts and a rise-time in the microsecond level, which can be widely used in plasma physics research, high power microwave generation, and material treatment. Specifically, PSpice software is used to analyze the performance of the generator. Then, the generator was constructed in our laboratory. The experimental results illustrate that when the charging voltage of the generator was changing from 10 kV to 14 kV, typical pulses with a peak voltage ranging from 67 kV to 95 kV and rise-time between 10 µs and 12 µs were obtained on a dummy load. The generator can continually work over 10 min with a repetitive rate of 20 Hz, and until now, it has been successfully achieving over 1 × 106 high voltage pulses. Experiments show reasonable agreement with numerical analysis.

Investigation on a fast rise time high voltage pulse transformer.

A pulse transformer has advantages of compactness and long lifetime achievability, and is considered an important candidate for the development of high voltage pulse generators. In this paper, a fast rise time high voltage pulse transformer with a compact structure is studied numerically and experimentally. Specifically, the structure of the pulse transformer was designed with paralleled winding groups. The response and insulation characteristics of the pulse transformer are investigated numerically. After that, a fast rise time high voltage pulse transformer was built, and scaling experiments were performed to analyze the influence of the feed structure of the pulse transformer. Importantly, the pulse transformer was successfully used in a low impedance generator, and high voltage pulses with a peak voltage of about 260 kV, a pulse duration of 170 ns, and a rise time of 65 ns were achieved on a resistive load of 32 Ω. The ratio of the transformer is approximately 1:3. Experiments show reasonable agreement with the numerical analysis.

Experiments of a 100 kV-level pulse generator based on metal-oxide varistor.

This paper introduces the development and experiments of a 100 kV-level pulse generator based on a metal-oxide varistor (MOV). MOV has a high energy handling capacity and nonlinear voltage-current (V-I) characteristics, which makes it useful for high voltage pulse shaping. Circuit simulations based on the measured voltage-current characteristics of MOV verified the shaping concept and showed that a circuit containing a two-section pulse forming network (PFN) will result in better defined square pulse than a simple L-C discharging circuit. A reduced-scale experiment was carried out and the result agreed well with simulation prediction. Then a 100 kV-level pulse generator with multiple MOVs in a stack and a two-section pulse forming network (PFN) was experimented. A pulse with a voltage amplitude of 90 kV, rise time of about 50 ns, pulse width of 500 ns, and flat top of about 400 ns was obtained with a water dummy load of 50 Ω. The results reveal that the combination of PFN and MOV is a practical way to generate high voltage pulses with better flat top waveforms, and the load voltage is stable even if the load's impedance varies. Such pulse generator can be applied in many fields such as surface treatment, corona plasma generation, industrial dedusting, and medical disinfection.

A gigawatt level repetitive rate adjustable magnetic pulse compressor.

In this paper, a gigawatt level repetitive rate adjustable magnetic pulse compressor is investigated both numerically and experimentally. The device has advantages of high power level, high repetitive rate achievability, and long lifetime reliability. Importantly, dominate parameters including the saturation time, the peak voltage, and even the compression ratio can be potentially adjusted continuously and reliably, which significantly expands the applicable area of the device and generators based on it. Specifically, a two-stage adjustable magnetic pulse compressor, utilized for charging the pulse forming network of a high power pulse generator, is designed with different compression ratios of 25 and 18 through an optimized design process. Equivalent circuit analysis shows that the modification of compression ratio can be achieved by just changing the turn number of the winding. At the same time, increasing inductance of the grounded inductor will decrease the peak voltage and delay the charging process. Based on these analyses, an adjustable compressor was built and studied experimentally in both the single shot mode and repetitive rate mode. Pulses with peak voltage of 60 kV and energy per pulse of 360 J were obtained in the experiment. The rise times of the pulses were compressed from 25 µs to 1 µs and from 18 µs to 1 µs, respectively, at repetitive rate of 20 Hz with good repeatability. Experimental results show reasonable agreement with analyses.

Influence of different illumination profiles on the on-state resistances of silicon carbide photoconductive semiconductor switches.

Characteristics of a silicon-carbide (SiC) photoconductive switch under different illumination profiles are presented. We triggered a V-doped semi-insulated 6H-SiC switch with lateral geometry using a laser beam of 532-nm wavelength. Photoconductivity tests for different spot profiles and locations show that such switches achieve a minimum on-state resistance when the switching gap is illuminated. The differences between on-state resistances are small for various partial illuminations of the switching gap. Semiconductor modeling is used to simulate the electric field and current profiles for different partial illuminations. The simulation results show poor on-state switch performance when partially illuminated. Based on these results, a more revealing circuit model for the switch matches well with experimental results for partial illuminations.

An improved rolled strip pulse forming line.

The rolled strip pulse forming line (RSPFL) has advantages of compactness, portability, and long pulse achievability which could well meet the requirements of industrial application of the pulse power technology. In this paper, an improved RSPFL with an additional insulator between the grounded conductors is investigated numerically and experimentally. Results demonstrate that the jitter on the flat-top of the output voltage waveform is reduced to 3.8% due to the improved structure. Theoretical analysis shows that the electromagnetic coupling between the conductors of the RSPFL strongly influences the output voltage waveform. Therefore, the new structure was designed to minimize the detrimental effect of the electromagnetic coupling. Simulation results show that the electromagnetic coupling can be efficiently reduced in the improved RSPFL. Experimental results illustrate that the improved RSPFL, with dimensions and weight of Φ 290 × 250 mm and 16 kg, when used as a simple pulse forming line, could generate a well shaped quasi-square pulse with output power of hundreds of MW and pulse duration of 250 ns. Importantly, the improved RSPFL was successfully used as a Blumlein pulse forming line, and a 10.8 kV, 260 ns quasi-square pulse was obtained on a 2 Ω dummy load. Experiments show reasonable agreement with numerical analysis.

Note: a 3-stage stacked Blumlein using ceramic for energy storage.

We have developed a novel stacked Blumlein with high compactness by using ceramic for energy storage. The total volume of this stacked Blumlein is only 320 × 100 × 185 mm(3). By triggering 3 spark gaps simultaneously, the developed stacked Blumlein is capable of producing a rectangular pulse with a voltage multiplication. A 32 ns quasi-rectangular pulse of 11.4 kV is measured across a 10 Ω dummy load when the 3-stage stacked Blumlein is DC charged up to 4 kV. The voltage multiplication is about 2.9, and the energy efficiency is about 96%. Simulation results indicate that vacuum or transformer oil is appropriate to be the insulation medium for the stacked Blumlein.

A modularized pulse forming line using glass-ceramic slabs.

In our lab, a kind of glass-ceramic slab has been chosen to study the issues of applying solid-state dielectrics to pulse forming lines (PFLs). Limited by the manufacture of the glass-ceramic bulk with large sizes, a single ceramic slab is hard to store sufficient power for the PFL. Therefore, a modularized PFL design concept is proposed in this paper. We regard a single ceramic slab as a module to form each single Blumlein PFL. We connect ceramic slabs in series to enlarge pulse width, and stack the ceramic Blumlein PFLs in parallel to increase the output voltage amplitude. Testing results of a single Blumlein PFL indicate that one ceramic slab contributes about 11 ns to the total pulse width which has a linear relation to the number of the ceramic slabs. We have developed a prototype facility of the 2-stage stacked Blumlein PFL with a length of 2 ceramic slabs. The PFL is dc charged up to 5 kV, and the output voltage pulse of 10 kV, 22 ns is measured across an 8 Ω load. Simulation and experiment results in good agreement demonstrate that the modularized design is reasonable.

Effects of the transparent cathode on the performance of a relativistic magnetron with axial radiation.

Experimental investigation of the transparent cathode used in a relativistic magnetron with axial radiation is reported in this paper. The transparent cathode is composed of six separate stalks with the diameter of 6 mm. Under the working condition of 549 kV and ∼0.38 T, the relativistic magnetron with the transparent cathode experimentally produces a 550 MW microwave. The radiation mode is TE(11) at the frequency of 2.35 GHz. The total efficiency is 16.7%. The variations of the relative positions between the separate stalks and the anode blocks can perform the maximum difference of 4 ns in microwave duration. Compared with the conventional solid cathode, the transparent cathode provides faster startup time of 12 ns, relatively wider pulse duration of 35% and relatively higher efficiency of 10.6%.

An all solid-state, rolled strip pulse forming line with low impedance and compact structure.

An all solid-state and compact pulsed strip pulse forming line (PFL) is investigated both theoretically and experimentally. The electromagnetic field distribution and the pulse formation in the strip PFL are analyzed numerically. Based on the theoretical analysis and numerical results, a rolled strip PFL with output voltage of 20 kV, pulse duration of 230 ns, and characteristic impedance of 0.5 Omega was designed and manufactured. We use the Mylar film and copper as the dielectric and conductor of the strip PFL. The dimension of the strip line is 23,000 x 400 x 1.6 mm(3) in the case in which the strip line is unrolled, and the strip line is finally rolled into a cylinder of diameter of 311 mm for the experiment. The dimension and weight are about ten times smaller than those of traditional dielectric (oil or pure water) PFL with the same electrical parameters. Two experiments were performed using the strip line. One was for a transmission line experiment, and the other was for a PFL experiment. In the experiment of transmission line, the transmission time of the voltage signal was 115 ns, and the signal had almost no distortion, which verified the design. In the PFL experiment, results gave a 17.8 kV, 270 ns (full width at half maximum) voltage pulse which was a quasisquare wave on the water load of 0.5 Omega. The current going through the load is about 35.6 kA.

A ceramic radial insulation structure for a relativistic electron beam vacuum diode.

For one kind of a high current diode composed of a small disk-type alumina ceramic insulator water/vacuum interface, the insulation structure was designed and experimentally investigated. According to the theories of vacuum flashover and the rules for radial insulators, a "cone-column" anode outline and the cathode shielding rings were adopted. The electrostatic field along the insulator surface was obtained by finite element analysis simulating. By adjusting the outline of the anode and reshaping the shielding rings, the electric fields were well distributed and the field around the cathode triple junction was effectively controlled. Area weighted statistical method was applied to estimate the surface breakdown field. In addition, the operating process of an accelerator based on a spiral pulse forming line (PFL) was simulated through the PSPICE software to get the waveform of charging and diode voltage. The high voltage test was carried out on a water dielectric spiral PFL accelerator with long pulse duration, and results show that the diode can work stably in 420 kV, 200 ns conditions. The experimental results agree with the theoretical and simulated results.