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Simulation studies of plasma dynamics and radiation yield in plasma focus device
Author
Tan, Li Ching
Supervisor
Rawat, Rajdeep Singh
Springham, Stuart Victor
Abstract
The Lee Code was enhanced to extend its simulation capability to include dual filling gas admixtures with different volumetric ratios (henceforth referred to as Enhanced Lee Model for Admixtures). The code was used to fit two sets of signals from the fast miniature plasma focus FMPF-1 with two different cathode geometries - the squirrel cage and the tubular cathode - to simulate neutron and x-ray yields for admixture operation.
For the first set of signals from FMPF-1 with squirrel cage cathode, a total of 180 current signals at 9 different filling gas pressures (3.0 – 7.0 mbar) and 4 different deuterium-krypton doping levels (0%, 2%, 5% and 10%) were fitted. Results show that the neutron yield trend for pure deuterium and deuterium doped with 5% and 10% krypton were well predicted. The model parameters, radiation yields, pinch characteristics and drive parameters were then collated and analysed to give us more insight into the plasma dynamics. Results are as follows: The fraction of filling gas being swept up by current sheath in axial phase was found to decrease with increasing concentrations of krypton, as the krypton particles are much heavier than the deuterium particle and are hence more likely to be left behind. On the other hand, current factor was also found to increase with filling gas pressure up to a limit of unity when krypton is added to the deuterium filling gas. Since the average temperature of admixture plasma pinch temperature is (1.08±0.29)×103 eV, at which the deuterium will be fully ionised and the krypton particles heavily ionised, this can be attributed to an increase in free electrons becoming available to conduct current as filling gas pressure increases. The x-ray yield is found to be enhanced by up to several orders of magnitude as more and more krypton is added to the deuterium filling gas. This is because line radiation from the krypton particles will overtake Bremsstrahlung radiation as the main source of x-ray. As a result of increased line radiation when krypton is added, the plasma column cools and pinch temperature is found to be reduced by up to 40%. The loss of pinch plasma energy due to cooling by line radiation allows the confining magnetic field to confine the pinch to a smaller radius. Hence, confinement of plasma column is enhanced, and pinch duration increases by up to 25% as compared to pure deuterium. Pinch radius is found to reduce with increasing concentrations of krypton. Last but not least, the drive parameters at neutron optimised conditions at each admixture concentration were found to lie within the range reported for larger plasma focus devices, S = (89±8) kA/cm⋅Torr0.5. The only exception was when 10% of krypton was added. Those operations produced an average of 39% less neutrons as compared to that of pure deuterium and other concentrations of deuterium-krypton admixtures.
For the second set of signals from FMPF-1 with tubular cathode, a total of 110 current signals at 11 different filling gas pressures (0.2 – 2.2 mbar) and 2 different deuterium-krypton doping levels (0% and 10%) were fitted. Results show that x-ray yields simulated for 10% concentration of krypton were found to be of the same order as that measured during experiments and this gives us confidence that the Enhanced Lee Code for Admixtures is able model the plasma processes reasonably well, and the parameters for the x- ray optimised signal is used to simulate plasma dynamics at higher levels of doping concentration. It was found that peak axial, shock front and magnetic piston speed decreases as more and more krypton is added to the admixture. We attribute this to the gas getting heavier with each addition of krypton, hence lowering the various speeds. With lower speeds, less shock heating occurs and the pinch temperature was found to decrease as more and more krypton is added. In addition, temperature fell more quickly at lower concentrations of krypton, as the dramatic increase in x-ray at lower concentrations of krypton doping cools the plasma down in addition to reduced shock heating. X-ray was found to scale with pinch ion density as line radiation is the dominant source of x-ray, and the more krypton particles available for line radiation to occur, the more x-ray is produced. Last but not least, pinch density is found to scale with pinch radius because the variation in the latter is many times that of the pinch height.
Observing that the simulated neutron yield for admixture operations consistently fell below the measured yield, the voltage multiplier factor was recalibrated. Results show that newly calibrated voltage multiplier factor for admixture operations increases the accuracy of the simulated neutron yield.
For the first set of signals from FMPF-1 with squirrel cage cathode, a total of 180 current signals at 9 different filling gas pressures (3.0 – 7.0 mbar) and 4 different deuterium-krypton doping levels (0%, 2%, 5% and 10%) were fitted. Results show that the neutron yield trend for pure deuterium and deuterium doped with 5% and 10% krypton were well predicted. The model parameters, radiation yields, pinch characteristics and drive parameters were then collated and analysed to give us more insight into the plasma dynamics. Results are as follows: The fraction of filling gas being swept up by current sheath in axial phase was found to decrease with increasing concentrations of krypton, as the krypton particles are much heavier than the deuterium particle and are hence more likely to be left behind. On the other hand, current factor was also found to increase with filling gas pressure up to a limit of unity when krypton is added to the deuterium filling gas. Since the average temperature of admixture plasma pinch temperature is (1.08±0.29)×103 eV, at which the deuterium will be fully ionised and the krypton particles heavily ionised, this can be attributed to an increase in free electrons becoming available to conduct current as filling gas pressure increases. The x-ray yield is found to be enhanced by up to several orders of magnitude as more and more krypton is added to the deuterium filling gas. This is because line radiation from the krypton particles will overtake Bremsstrahlung radiation as the main source of x-ray. As a result of increased line radiation when krypton is added, the plasma column cools and pinch temperature is found to be reduced by up to 40%. The loss of pinch plasma energy due to cooling by line radiation allows the confining magnetic field to confine the pinch to a smaller radius. Hence, confinement of plasma column is enhanced, and pinch duration increases by up to 25% as compared to pure deuterium. Pinch radius is found to reduce with increasing concentrations of krypton. Last but not least, the drive parameters at neutron optimised conditions at each admixture concentration were found to lie within the range reported for larger plasma focus devices, S = (89±8) kA/cm⋅Torr0.5. The only exception was when 10% of krypton was added. Those operations produced an average of 39% less neutrons as compared to that of pure deuterium and other concentrations of deuterium-krypton admixtures.
For the second set of signals from FMPF-1 with tubular cathode, a total of 110 current signals at 11 different filling gas pressures (0.2 – 2.2 mbar) and 2 different deuterium-krypton doping levels (0% and 10%) were fitted. Results show that x-ray yields simulated for 10% concentration of krypton were found to be of the same order as that measured during experiments and this gives us confidence that the Enhanced Lee Code for Admixtures is able model the plasma processes reasonably well, and the parameters for the x- ray optimised signal is used to simulate plasma dynamics at higher levels of doping concentration. It was found that peak axial, shock front and magnetic piston speed decreases as more and more krypton is added to the admixture. We attribute this to the gas getting heavier with each addition of krypton, hence lowering the various speeds. With lower speeds, less shock heating occurs and the pinch temperature was found to decrease as more and more krypton is added. In addition, temperature fell more quickly at lower concentrations of krypton, as the dramatic increase in x-ray at lower concentrations of krypton doping cools the plasma down in addition to reduced shock heating. X-ray was found to scale with pinch ion density as line radiation is the dominant source of x-ray, and the more krypton particles available for line radiation to occur, the more x-ray is produced. Last but not least, pinch density is found to scale with pinch radius because the variation in the latter is many times that of the pinch height.
Observing that the simulated neutron yield for admixture operations consistently fell below the measured yield, the voltage multiplier factor was recalibrated. Results show that newly calibrated voltage multiplier factor for admixture operations increases the accuracy of the simulated neutron yield.
Date Issued
2010
Call Number
QC718.5.D9 Tan
Date Submitted
2010