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Anode geometry and focus characteristics
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Type
Thesis
Author
Serban, Adrian
Supervisor
Lee, Sing
Abstract
An analysis of data for plasma focus devices in their wide range of sizes has been made. The data show that for neutron optimized operation all plasma focus devices have the same peak axial sheath velocity of 10 cm/ps or a little less, driven by a standard value of characteristic current per unit anode radius I,/a of 250 kA/cm.
Dur experimental efforts were devoted to study the evolution, dynamics and focussing characteristics of a 3kJ plasma focus device operated at speeds significantly higher than 10 cm/ps. The increase in speed was achieved by increasing the linear current density using a two stage stepped-down composite anode geometry. This method of increasing the speed allows us to keep the voltage and pressure constant. The composite anode also allows us to vary the length of the speed-enhanced region. Our purpose was to investigate the possibility of enhancing the radiation output, i.e. neutrons and X-rays, for a given Input energy.
Several diagnostic techniques were applied simultaneously including the current derivative, current and voltage. The peak axial speed was measured with an optical fibre detector developed during this project. The radial and pinch phases were investigated using the shadow method. Time-resolved neutron and hard X-ray measurements were carried out using plastic scintiliator/photomultiplier systems and the time-offlight method of analysis. A neutron activation technique was used to measure the total neutron yield. Time-resolved soft X-ray measurements were performed with filtered PIN diodes. The electron temperature of the argon and deuterium focus was determined using a filter ratio method. A one-dimensional three-phase computational model was used to describe the gross dynamics of the plasma focus and to design new electrode geometries. Current shedding and mass loss effects were considered. Discharge parameters and radiation outputs were correlated. Experimental data and results of the computation were compared.
Peak axial sheath velocities up to 15 cm/ps were achieved. The composite anode configuration was optimized at 11.2 cm/p for the neutron output, for which an increase of 70% compared to the standard electrode geometry was achieved. At the same time, the soft X-ray outpu~ increased by almost four times. The results showed that if the speed enhanced section was too long the radial compression was not driven properly resulting in poor reproducibility, efficiency and radiation output. This could be ascribed to a significant separation of the plasma layer from the magnetic piston at the end of the axlal phase.
Shadowgraphic investigations showed that as the speed increased the sheath was more canted. This aspect was correlated with the mass loss effect and was taken into account in the computational model.
Two focus evolution regimes were identified. One is a single compression during the pinch phase and leads to high neutron yield. The other regime features multiple compressions of the plasma with high soft X-ray production. Focussing characteristics depend strongly on gas filling pressure and speed. The optimum conditions for neutron yield are different from those for soft X-ray production. The peak value of plasma impedance (typically greater than 0.2 Q) during focusing was proposed as a factor of merit. Electron temperatures of 1 - 2 kev were determined for both the argon and deuterium focus.
Each linear dimension of the pinch and the lifetime of the focus phase were found to scale proportionally to the anode radius. Scaling laws for the non-beam component of the neutron yield and soft X-ray production were proposed based on the experimental results. These show a strong enhancement of the neutron and soft X-ray yields when the focus axial speed is increased.
Dur experimental efforts were devoted to study the evolution, dynamics and focussing characteristics of a 3kJ plasma focus device operated at speeds significantly higher than 10 cm/ps. The increase in speed was achieved by increasing the linear current density using a two stage stepped-down composite anode geometry. This method of increasing the speed allows us to keep the voltage and pressure constant. The composite anode also allows us to vary the length of the speed-enhanced region. Our purpose was to investigate the possibility of enhancing the radiation output, i.e. neutrons and X-rays, for a given Input energy.
Several diagnostic techniques were applied simultaneously including the current derivative, current and voltage. The peak axial speed was measured with an optical fibre detector developed during this project. The radial and pinch phases were investigated using the shadow method. Time-resolved neutron and hard X-ray measurements were carried out using plastic scintiliator/photomultiplier systems and the time-offlight method of analysis. A neutron activation technique was used to measure the total neutron yield. Time-resolved soft X-ray measurements were performed with filtered PIN diodes. The electron temperature of the argon and deuterium focus was determined using a filter ratio method. A one-dimensional three-phase computational model was used to describe the gross dynamics of the plasma focus and to design new electrode geometries. Current shedding and mass loss effects were considered. Discharge parameters and radiation outputs were correlated. Experimental data and results of the computation were compared.
Peak axial sheath velocities up to 15 cm/ps were achieved. The composite anode configuration was optimized at 11.2 cm/p for the neutron output, for which an increase of 70% compared to the standard electrode geometry was achieved. At the same time, the soft X-ray outpu~ increased by almost four times. The results showed that if the speed enhanced section was too long the radial compression was not driven properly resulting in poor reproducibility, efficiency and radiation output. This could be ascribed to a significant separation of the plasma layer from the magnetic piston at the end of the axlal phase.
Shadowgraphic investigations showed that as the speed increased the sheath was more canted. This aspect was correlated with the mass loss effect and was taken into account in the computational model.
Two focus evolution regimes were identified. One is a single compression during the pinch phase and leads to high neutron yield. The other regime features multiple compressions of the plasma with high soft X-ray production. Focussing characteristics depend strongly on gas filling pressure and speed. The optimum conditions for neutron yield are different from those for soft X-ray production. The peak value of plasma impedance (typically greater than 0.2 Q) during focusing was proposed as a factor of merit. Electron temperatures of 1 - 2 kev were determined for both the argon and deuterium focus.
Each linear dimension of the pinch and the lifetime of the focus phase were found to scale proportionally to the anode radius. Scaling laws for the non-beam component of the neutron yield and soft X-ray production were proposed based on the experimental results. These show a strong enhancement of the neutron and soft X-ray yields when the focus axial speed is increased.
Date Issued
1995
Call Number
QC718.4 Ser
Date Submitted
1995