Cecilia Möller

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.

Study of a Compact HPM System With a Reflex Triode and a Marx Generator

To study the performance of compact systems for microwave generation, a series of experiments have been performed with a microwave source powered directly by a Marx generator. The system consists of a 20-stage 400-kV/400-J Marx generator, a powerful 40-kV charger, a reflex triode, and a vircator-type microwave source. Different parametric studies were performed such as variation of the anode-cathode distances and the emitting area of the cathode. The results have been analyzed and compared to an equivalent electric circuit model of the system. The experiments, generating microwave frequencies between 3 and 5 GHz, can be fairly accurately reproduced by the model both in terms of discharge currents and microwave frequencies

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 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.

MAGNETIC FIELD MEASUREMENT SYSTEM FOR HPM RESEARCH

One method to characterize the radiated microwave field from a high‐power microwave (HPM) source is to measure the radiated high‐level electromagnetic field in several locations at a high sampling rate registering the frequency time dependence, thus being able to determine the radiated pattern and mode. A complete free‐field measurement system for measuring the magnetic field component in high‐level electromagnetic fields has been developed at FOI.The system consists of a B‐dot sensor and a balun, both designed and constructed at FOI. The B‐dot sensor is designed as two cylindrical loop sensors with differential output. The balun is a microstrip design etched on a dual sided PTFE circuit board. Complete systems have been calibrated at SP Technical Research Institute of Sweden. A method to analyze the data from the free‐field systems has been developed.

Numerical simulations of the influence of a reflector in a coaxial vircator

Particle-in-cell simulations of the influence of the position of a waveguide reflector have been made for the traditional coaxial vircator and a coaxial vircator with a sectioned emitter. The results show a great impact on the radiated power due to the reflector position.

Experimental studies on a coaxial vircator, designed for operation in TE 11 mode

Experiments on a coaxial vircator have been performed. A vircator is a narrow band high power microwave source without any external generated magnetic field [1]. A coaxial vircator is an advantageous design of a microwave radiation source for a compact HPM-system. With a limited size and outer diameter it is possible to use a larger emitting area compared to an axial design. A conventional coaxial vircator will generate the radiation in TM01 mode, due to its geometrical properties. For a compact HPM-system, radiation in TE11 mode is preferred when the radiated energy needs to be focused on a specific target. For operation in TE11 mode a sectioned emitter can be used rather than a circumcircular.

Experimental study of a vircator with premodulated electron beam

An initial experimental study of a vircator where a feedback mechanism is used to premodulate the electron beam has been performed. The anode-cathode gap distance and the applied voltage were varied and their influence on the frequency content and field strength of the generated microwave pulse are investigated and compared with particle in cell simulations. The frequency content of the microwave radiation was in good agreement between experiments and simulations. The vircator was very narrow banded at 2 GHz.

Optimization of excitation point in a coaxial TE11 vircator

The vircator is a compact and robust narrow band High-Power Microwave (HPM) source with a simple geometry. In the most basic version of the vircator, the operating frequency depends only on the distance between the anode and cathode and on the applied voltage. In a compact system the vircator is often powered by a Marx generator resulting in a frequency chirp caused by the double exponential voltage pulse generated by the Marx generator. For a coaxial vircator, the anode/cathode structure can be utilized as a resonant cavity to achieve higher output power and monochromatic radiation from the vircator.An experimental campaign utilizing a coaxial TE11 vircator has been performed, in which the excitation point and the applied voltage have been varied. Experimental evidence is presented showing that the frequency shift during the pulse can be avoided by choosing the proper excitation point. It was found that the excitation point has more influence on the operating frequency than the applied voltage. When the highest radiated fields were recorded the microwave radiation was monochromatic and only the fundamental mode was present. Both far field and waveguide measurements were in agreement with the TE11 mode. At the optimum positions the impedance of the vircator was lower, indicating improved coupling between electron beam and microwave field. To verify the experimental findings on optimum excitation point, electromagnetic simulations were performed. Even though the simulation model was greatly simplified by replacing the electron beams with current sheaths at the position of the emitters, the results were in good agreement with experiments.

Optimization study with respect to the point of excitation in coaxial TE 11 vircator

The vircator is a compact and robust narrow band HPM source with a simple geometry. In the most basic version of the vircator, the operating frequency depends only on the distance between the anode and cathode and the applied voltage. In a compact system the vircator is often powered by a marx generator resulting in a frequency chirp caused by the double exponential voltage pulse generated by the marx. An experimental series with a coaxial TE11 vircator has been performed in which the excitation point and the applied voltage have been varied. Experimental evidence is presented showing that the frequency shift during the pulse can be avoided by choosing the proper excitation point. The highest radiated fields were recorded when the bandwidth of the microwave radiation was narrow and only one mode was present. Both far field and waveguide measurements were in agreement with the TE11 mode. At the optimum positions the input impedance of the vircator was lower, indicating improved coupling between electron beam and microwave field. To verify the experimental findings on optimum excitation point, electromagnetic simulations were performed. Even though the simulation model was greatly simplified by replacing the electron beam with a current sheath at the position of the emitters, the results were in good agreement with experiments.