Axial Virtual Cathode Oscillator Research Articles

ROS production in response to high-power microwave pulses induces p53 activation and DNA damage in brain cells: Radiosensitivity and biological dosimetry evaluation.

Background: Pulsed high-power microwave (HPM) has many applications and is constantly being researched to expand its uses in the future. As the number of applications grows, the biological effects and safety level of pulsed HPM become a serious issue, requiring further research. Objective: The brain is regarded as the most vulnerable organ to radiation, raising concerns about determining an acceptable level of exposure. The effect of nanosecond pulses and the mechanisms underlying HPM on the brain has not been studied. For the first time, we observed the effect of pulsed 3.5GHz HPM on brain normal astrocytes and cancer U87 MG cells, as well as the likely mechanisms involved. Methods: To generate 3.5GHz HPM, an axial virtual cathode oscillator was constructed on pulsed power generator "Chundoong". The cells were directly exposed to HPM (10, 25, 40, and 60) pulses (1mJ/pulse), with each pulse delivered after 1min of charging time to evaluate the dose dependent effects. Results: A strong electric field (∼23kV/cm) of HPM irradiation primarily causes the production of reactive oxygen species (ROS), altering cell viability, mitochondrial activity, and cell death rates in U87 and astrocytes at certain dosages. The ROS generation in response to HPM exposure was primarily responsible for DNA damage and p53 activation. The hazardous dosage of 60 pulses is acknowledged as having damaging effects on brain normal cells. Interestingly, the particular 25 pulses exhibited therapeutic effects on U87 cells via p53, Bax, and Caspase-3 activation. Conclusion: HPM pulses induced apoptosis-related events such as ROS burst and increased oxidative DNA damage at higher dosages in normal cells and specific 25 pulses in cancer U87. These findings are useful to understand the physiological mechanisms driving HPM-induced cell death, as well as the safety threshold range for HPM exposure on normal cells and therapeutic effects on cancer U87. As HPM technology advances, we believe this study is timely and will benefit humanity and future research.

Novel Configuration for a C-Band Axial Vircator With High Output Power

We present a novel configuration for an axial virtual cathode oscillator (Vircator) operating in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> -band and designed for high output power applications. In order to enhance the efficiency, we have employed a geometry with up to seven reflectors along the drift space. While in previous research works this optimization process has been performed only varying the reflectors radii while using a fixed distance between reflectors, in our work we propose an optimization in terms of both reflectors radii and distances. Our optimized structure for the axial Vircator is able to provide an efficiency up to 15.6% and an output power of 1.6 GW in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> -band, with a working voltage of 510 kV and a cathodic current of 24 kA.

Investigation of the Optimum Diameter of the Ring Reflector for an Axial Virtual Cathode Oscillator

The optimum hole diameter of a ring reflector was experimentally investigated using an axial virtual cathode oscillator (vircator) to enhance its microwave power. The ring reflector enhances the microwave power from the axial vircator by forming a cavity. The ring reflector was installed 9 mm behind the anode. The optimum hole diameter of the ring reflector was analyzed through simulations and experiments by changing the diameter from 60 mm to 160. PIC simulations show that the maximum peak microwave power was generated when the hole diameter was 116 mm and enhanced by 210%. The experiments show similar results to the simulations. The largest peak maximum power was 23.67 MW when the hole diameter was 120 mm. The simulations show that the dominant microwave frequency was formed between 5.33 GHz and 6.7 GHz. The experiments show that the dominant microwave frequency was formed between 5.3 GHz and 5.8 GHz. The frequency trend was approximately similar to that of the simulation results. However, the trend depending on the hole diameter was not as obvious as in the simulations. Although the frequency change was not as clear as in the simulations, experiments show that the hole diameter of the ring reflector affects the vircator operation.

Investigation of an Axial Virtual Cathode Oscillator with an Open-Ended Coaxial Cathode

A cathode with an open-ended coaxial structure is experimentally investigated using an axial virtual cathode oscillator (vircator). To enhance the microwave power output, an open-ended coaxial cathode is installed in the axial vircator. The proposed cathode is designed based on the reciprocating frequency of the vircator. The operation features of an axial vircator with a solid cathode, an annular cathode, and an open-ended coaxial cathode are comparatively analyzed through simulations and experiments. Three cathodes are machined using graphite. A stainless steel mesh with a transparency of 70% is used as an anode. The anode-to-cathode gap is fixed to 6 mm. The vircator is driven using a 10-stage PFN-Marx generator with a characteristic impedance of 31 Ω. The PFN-Marx generator applies -150 kV voltage pulses with a 170–200 ns pulse width into the vircator. The microwave power from the solid and annular cathodes is 11.22 MW and 11.27 MW, respectively. The proposed cathode generates a microwave with a power of 12.65 MW while enhancing the microwave power by 13% compared with the solid and annular cathodes. The proposed cathode shows a frequency shift to 3.4 GHz, which is a much lower frequency than that of the solid cathode at 6.34 GHz.

Experimental Investigation into the Optimum Position of a Ring Reflector for an Axial Virtual Cathode Oscillator

A ring reflector was experimentally investigated using an axial virtual cathode oscillator (vircator). The ring reflector was installed behind the mesh anode of the axial vircator to enhance the microwave power output by forming a resonant cavity and increasing the electron beam to microwave energy conversion efficiency. The optimum position of the ring reflector is analyzed through simulations and experiments by varying the anode to reflector distance from 6 mm to 24 mm in 3 mm steps. PIC simulations show that the ring reflector enhances the microwave power of the axial vircator up to 220%. Experiments show that the microwave power from the axial vircator without the ring reflector is 11.22 MW. The maximum average peak microwave power of the axial vircator with the ring reflector is 25.82 MW when the anode to ring reflector distance is 18 mm. From the simulations and experiments, it can be seen that the ring reflector yields decaying enhancement that is inversely proportional to the anode to ring reflector distance and there is no noticeable microwave enhancement after 24 mm. The frequency range attained from the simulations and experiments is 5.8 to 6.7 GHz and 5.16 to 5.8 GHz, respectively. The difference between the simulation and experimental results is due to the error in the anode to cathode gap distance. Although the frequency is slightly changed, the ring reflector seems to have no influence on the frequency of the generated microwave.

Focus of high-power microwaves with positive and negative zone plate to increase the receiving power in axial virtual cathode oscillator

We investigated an axial vircator, constructed in a pulsed power generator, called Chundoong, and simulated by a three-dimensional particle-in-cell simulation code, MAGIC. We attempted to increase the receiving power of generated microwaves by directing them at a desired focal point. To do so, a positive and negative zone plates (ZPs) are constructed at main frequency 3.5 GHz, and its focal length is 18.8 cm. The power generated from the virtual cathode is 0.72 GW, from which 24.6 kW is received by receiving antenna without a ZP. The receiving power increased significantly to 45.4 kW (92.6%) and 47.5 kW (84.5%) with positive and negative ZPs, respectively. In addition, both positive and negative ZPs have similar focusing properties for the generated microwaves. The ZP contributes significantly to increasing the receiving power. These findings might be useful for real applications to increase the receiving power at a desired focal point.

Experimental studies on DC axial virtual cathode electric discharge

A new modification is carried out on axial virtual cathode oscillator to improve the device performance. The device operates by virtual cathode theory to generate high power microwave from the interaction between energetic electron beam and virtual cathode field. The electrical parameters are measured to understand the behavior of the discharge by using nitrogen gas. The breakdown potential Vb against Pd is investigated. A single electric probe is used to measure and estimate the plasma parameters. The virtual cathode position is defined from the axial distribution of electron density, axial distribution of electric field and axial distribution of plasma potential. The position of virtual cathode has been estimated to be at about 2cm from the mesh anode which can be confirmed from the electric field changing its direction and from the maximum value of plasma density at that position. The exposed samples to microwave radiation ensure the effects of the microwave radiation on them.

High-efficiency virtual cathode oscillator with photonic crystal

We study the properties of microwave generation in a virtual cathode oscillator with a photonic crystal composed of metal grids. Our simulation results show the high efficiency of photonic crystal structure utilization in comparison with the standard scheme of an axial virtual cathode oscillator: operation efficiency reaches 20% at the optimal parameters. The obtained results demonstrate that the virtual cathode oscillator with a photonic crystal can be considered as a prospective high-power microwave source where the vircator operation mechanism and photonic crystal properties complement each other to produce high-power electromagnetic radiation.

High power microwaves generation from intense relativistic electron beam using Kali-1000 pulse power system

Intense relativistic electron beam (IREB) is generated using Kali-1000 (Kilo Ampere Linear Injector) pulse power system, providing input electric power of 4 GW and stored energy of 1000 J. IREB is used for generating Non Ionizing (High Power Microwaves, HPM) and Ionizing (Flash X-rays) radiation. The present manuscript discusses generation of HPM by Kali-1000 system using an axial Virtual cathode oscillator. Kali-1000 pulse power system is a low impedance (~4 Ω) high voltage generator, consisting of Radial Tesla transformer to step up the voltage from 28 to 300 kV in 5 μs, which energizes the line, pulse discharged by a self triggered pressurized SF6 closing switch across a Relativistic Electron Beam (REB) diode load. The typical electron beam parameters are 300 kV, 15 kA and 100 ns with a current density of 3–4 hundreds of amperes per square centimeter. Experiments are carried with a REB diode consisting of graphite cathode and SS anode mesh. Measured pulse duration of HPM pulse (FWHM) varies from 70 to 110 ns on shot-to-shot basis. The major operating output frequencies have been measured to be 5.8 and 6.2 GHz within a range of 4.8–6.7 GHz using B-dot magnetic field sensor coupled to high frequency digital oscilloscope. Microwave output power from the virtual cathode oscillator has been measured to be 50 MW using double ridged waveguide horn antenna coupled with Schottky detector diode. The estimated beam to microwave power conversion efficiency is thereby found to be 1.1%. Particle-in-Cell (PIC) simulation has been performed for a set of Vircator experiments being driven by the Kali-1000 system. Experimentally measured cathode current, dominant output frequency, radiation pattern, etc., have been compared with simulation results and are in good agreement. The dominant emission mode from the virtual cathode oscillator is shown to be the TE01 mode based on the emission pattern of microwave induced air breakdown in this experiment.

Improved Design of a Multistage Axial Vircator With Reflectors for Enhanced Performances

The basic design of an axial virtual cathode oscillator (vircator) with axial extraction operating in TM01 mode is modified by introducing thin conducting disks, also called reflectors, into the cylindrical waveguide. The operation principal of this novel type of device relies on the formation of a series of virtual cathodes, located at the center of adjacent quasi-cavities. The behavior of this new type of multistage vircator is numerically investigated using CST Particle Studio 3-D particle-in-cell code. Progressively decreasing the radii of the reflectors installed upstream in the tube allows the mitigation of spurious modes. Tapering the radii of the reflectors turns out to be crucial in focusing the electron beam on axis in the downstream region and maximizing the TM01 power conversion efficiency. This novel architecture enables a five-reflector vircator operating with an injected electron beam of 508-kV mean voltage and 19-kA mean current to deliver up to 2-GW mean power sustained only by the TM01 mode in the S-band with a power conversion efficiency close to 21%.

3-D PIC Numerical Investigations of a Novel Concept of Multistage Axial Vircator for Enhanced Microwave Generation

The enhancement of power conversion efficiency of a classical axial VIRtual CAthode oscillaTOR (vircator) by introducing one or more reflectors beyond the anode in the cylindrical waveguide is numerically investigated. The targeted microwave (MW) output frequency lies in the S-band at around 3 GHz for an operation in TM01 mode. Powered by a 511-kV voltage signal for a duration of 45 ns, the design under consideration operates with an injected electron beam of mean voltage and mean current of around 508 kV and 19 kA, respectively. Full-wave 3-D modeling is performed using well-tested electromagnetic particle-in-cell codes such as Computer Simulation Technology Particle Studio and Magic. Simple rules for designing and installing the reflectors are given. The number of reflectors required to maximize the efficiency is discussed. The power conversion efficiency is shown to be improved over a classical axial vircator design by a factor of 12.8. A maximum mean output power of about 1.26 GW is delivered off-axis in the S-band at around 3 GHz, with an efficiency of nearly 13%. Besides, it is also shown that increasing the number of reflectors allows switching the operation mode from TM01 to TE11 along with a shift of MWfrequency from the S- to the L-band. A five- or six-reflector configuration is predicted to generate MW at both 2.86 and 1.4 GHz with conversion efficiencies ranging from 3.5% to 6.6%. A vircator including seven reflectors is expected to operate in TE11 mode at 1.4 GHz with an efficiency of about 8%.

Electrical Circuit Modeling for Efficiency Improvement of Axial Virtual Cathode Oscillator

The virtual cathode oscillator (vircator), which is one of the microwave sources, has been widely studied for about 40 years, since it has advantages, such as its simple structure and high-power radiation. To implement the behavior of the vircator, a simulation model of the axial vircator using circuit elements is studied in this paper. There are two steps in simulation of this paper: 1) obtaining the optimized case in the fixed input power that has a maximum efficiency and 2) at the case, conducting the circuit modeling. Passive elements and switches are used to model the power losses, emission threshold, etc. This circuit analysis is simulated by the Electro Magnetic Transient Program. Through the simulation model of the axial vircator using circuit elements, the high-power electromagnetic system can be handled as an integrated system, which contains a prime power part, a pulsed power part, a microwave source part, etc. This means that the circuit analysis can be carried out from the prime power to the vircator.

Pulsewidth Variation of an Axial Vircator

The KALI-1000 pulse power system has been used to generate single-pulse nanosecond-duration high-power microwaves from an axial virtual cathode oscillator. The typical electron beam parameters were 250 kV, 15 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. The shot-to-shot variation of the microwave pulsewidth was studied for various anode-cathode gaps and two different cathode materials (velvet and graphite). Results indicated that the average microwave pulsewidth for a diode with a graphite cathode increased as the anode-cathode gap was increased. The one-way analysis of variance was employed to examine the statistical correlation between the diode voltage, current, perveance, and the microwave pulsewidth for various diode gaps and for two different cathode materials. It was shown that the microwave pulsewidth variations are not statistically correlated with the diode voltage, current, and perveance.

Experimental Studies of the Influence of a Resonance Cavity in an Axial Vircator

Experiments on an axial virtual-cathode oscillator (vircator) with a resonance cavity enclosing the virtual cathode are reported. The vircator is driven by a repetitive Marx generator operating in a single-shot mode. To be able to separate different radiation mechanisms, the design of the vircator allows adjustment of the cavity depth as well as the way microwave radiation is extracted. The microwave radiation is measured with a pair of free-field B-dot sensors. The maximum field strengths were registered when the bandwidth was very narrow.

Experimental Studies of Anode and Cathode Materials in a Repetitive Driven Axial Vircator

Repetitive use of a high-power-microwave radiation source implies strong erosion on cathode and anode materials. Electrode-material endurance has been studied in a series of experiments with an axial virtual cathode oscillator powered by a compact Marx generator. The Marx generator is operated in a 10-Hz repetitive mode with a burst of ten pulses. Velvet and graphite was used as electron-emitting materials, and they showed markedly different pulse characteristics. The following three different anode materials were used: stainless-steel mesh, stainless-steel wires, and molybdenum wires, which all had different influence on the pulse characteristics.

Efficiency Enhancement of the Axial VIRCATOR

We present in this paper the particle-in-cell simulation results of a modified axial virtual cathode oscillator having a metallic drum of radius R1 placed coaxially inside the drift tube at a distance D1 from the foil anode. The enlarged parameter space permits control of the current transmitted to the collection plate, the net diode current, and the size of the electromagnetic emission and extraction regions. The geometry allows low-frequency radiation as well as high-frequency microwaves to leak out from the extraction region. The microwave content in the total electromagnetic power is found to be maximum when the drum is sufficiently close to the anode but far enough to allow the formation of a virtual cathode and permit a large emission region. The microwave power and efficiency also depend sensitively on the radius of the metallic drum. As the drum radius increases, the diode current and cutoff-frequency for transverse-magnetic (TM) modes increase while the extraction window shrinks in size. For parameter values studied, a peak locally time-averaged (over several periods) microwave power of 210 MW was obtained using a square voltage pulse of 250 kV at an efficiency of 7% while the peak-period averaged power was found to be 275 MW. In the absence of the drum, the peak locally time-averaged microwave power obtained was 93 MW at an efficiency of 3.3%