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Construction and optimization of low energy (< 240J) miniature repetitive plasma focus neutron source
Author
Rishi Verma
Supervisor
Rawat, Rajdeep Singh
Lee, Paul Choon Keat
Abstract
In the recent years, the upcoming diversified applications of neutron sources have compelled thrust towards the research and development of compact pulsed neutron generators. Amongst various options, miniature plasma focus (MPF) device based compact neutron generators have evolved as simple and economical alternative but yet from the application point of view, the neutron yield is far behind the threshold requirement. The potential limitation of MPF devices for practical applications comes from the fact that the neutron output tends to scale roughly as I4 (where I is the peak discharge current) or as E2 (where E is the energy stored in the system) and since MPF devices typically operate in sub-kilojoule range, their neutron output is in the range of 103 – 106 neutrons/shot (with peak discharge current in the range of 40 – 150kA) depending upon device energy, efficiency and design. In order to make the MPF device useful as compact source of fast neutrons for the widest range of applications, the typical requirement for time averaged neutron output is in the order of 107 – 1010 neutrons/second. To achieve such a time averaged neutron output in MPF devices, it is necessary to operate them in repetitive mode at repetition rate of more than 1Hz.
The work presented in this thesis is mainly based on the design, construction and optimization of indigenously developed three different versions of sub-kilojoule range fast miniature plasma focus devices – FMPF-1, FMPF-2 and FMPF-3 operating at <240J. The designations of the devices have been made on the basis of their pulsed power system configuration and electrical characteristics.
The first version of fast miniature plasma focus device FMPF-1 was primarily made for investigating the performance of the newly designed device at such low energy and to optimize plasma focus tube parameters. After going through several stages of design and experimental iterations of coaxial electrode assembly and other operating parameters, the ‘FMPF-1’ device was successfully demonstrated as a portable neutron source producing maximum average neutron yield of (1.15±0.2)×106 neutrons/shot at 230J/80kA/5.5mbar Deuterium (D2) filling gas pressure. The conceived model of ‘FMPF-1’ is extremely compact and aids the feature of portability/table top model. The overall dimensions of the apparatus, which includes capacitor bank, sparkgap switch and the focus chamber is 0.2m × 0.2m × 0.5m and the total weight of the system is ~25kgs.
The second version of fast miniature plasma focus device FMPF-2 was specifically designed to operate in repetitive mode ranging from 0.2 – 10Hz using newly developed high wattage pulsed power system. Many technological challenges were confronted and resolved for making the successful operation of the device in repetitive mode up to 10Hz. Using the repetitive miniature plasma focus device FMPF-2, for the first time, the enhancement in time averaged neutron yield has been successfully demonstrated by an order of magnitude, realizing the long stated concept of 'enhancement by repetition’. While operating at 228J/80kA/6mbar euterium (D2) filling gas pressure, the time averaged neutron yield of (6.2±4)×105 neutrons/second at 1Hz operation was enhanced to (6.5±0.6)×106 neutrons/second at 10Hz operation for the burst length of 30 consecutive shots.
In the third version of fast miniature plasma focus device FMPF-3 (a four module upgraded version of FMPF-2), significant improvements were made to enhance the efficiency of high wattage pulsed power system used in FMPF-2 device. As an outcome of rigorous transformations made, the FMPF-3 pulsed power system delivered ~20% higher peak discharge current than FMPF-2 pulsed power system while operating at similar energy level. In FMPF-3 device, at 200J/90kA/5.5mbar Deuterium (D2) filling gas pressure, the time averaged neutron yield of (1.4±0.6)×106 neutrons/second at 1Hz operation was enhanced to record yield of (1.4±0.2)×107 neutrons/second at 10Hz operation for the improvised burst length of 50 consecutive shots.
In the thesis, along with exclusive description on design methodology, construction and optimization exercises of all versions of newly developed miniature plasma focus devices (FMPF-1, FMPF-2 and FMPF-3), the time integrated and time resolved investigations on their neutron and hard X-ray emission characteristics have also been dealt in detail.
The work presented in this thesis is mainly based on the design, construction and optimization of indigenously developed three different versions of sub-kilojoule range fast miniature plasma focus devices – FMPF-1, FMPF-2 and FMPF-3 operating at <240J. The designations of the devices have been made on the basis of their pulsed power system configuration and electrical characteristics.
The first version of fast miniature plasma focus device FMPF-1 was primarily made for investigating the performance of the newly designed device at such low energy and to optimize plasma focus tube parameters. After going through several stages of design and experimental iterations of coaxial electrode assembly and other operating parameters, the ‘FMPF-1’ device was successfully demonstrated as a portable neutron source producing maximum average neutron yield of (1.15±0.2)×106 neutrons/shot at 230J/80kA/5.5mbar Deuterium (D2) filling gas pressure. The conceived model of ‘FMPF-1’ is extremely compact and aids the feature of portability/table top model. The overall dimensions of the apparatus, which includes capacitor bank, sparkgap switch and the focus chamber is 0.2m × 0.2m × 0.5m and the total weight of the system is ~25kgs.
The second version of fast miniature plasma focus device FMPF-2 was specifically designed to operate in repetitive mode ranging from 0.2 – 10Hz using newly developed high wattage pulsed power system. Many technological challenges were confronted and resolved for making the successful operation of the device in repetitive mode up to 10Hz. Using the repetitive miniature plasma focus device FMPF-2, for the first time, the enhancement in time averaged neutron yield has been successfully demonstrated by an order of magnitude, realizing the long stated concept of 'enhancement by repetition’. While operating at 228J/80kA/6mbar euterium (D2) filling gas pressure, the time averaged neutron yield of (6.2±4)×105 neutrons/second at 1Hz operation was enhanced to (6.5±0.6)×106 neutrons/second at 10Hz operation for the burst length of 30 consecutive shots.
In the third version of fast miniature plasma focus device FMPF-3 (a four module upgraded version of FMPF-2), significant improvements were made to enhance the efficiency of high wattage pulsed power system used in FMPF-2 device. As an outcome of rigorous transformations made, the FMPF-3 pulsed power system delivered ~20% higher peak discharge current than FMPF-2 pulsed power system while operating at similar energy level. In FMPF-3 device, at 200J/90kA/5.5mbar Deuterium (D2) filling gas pressure, the time averaged neutron yield of (1.4±0.6)×106 neutrons/second at 1Hz operation was enhanced to record yield of (1.4±0.2)×107 neutrons/second at 10Hz operation for the improvised burst length of 50 consecutive shots.
In the thesis, along with exclusive description on design methodology, construction and optimization exercises of all versions of newly developed miniature plasma focus devices (FMPF-1, FMPF-2 and FMPF-3), the time integrated and time resolved investigations on their neutron and hard X-ray emission characteristics have also been dealt in detail.
Date Issued
2010
Call Number
QC718.5.D38 Ver
Date Submitted
2010