Dense Plasma Focus - Design Parameters

Design Parameters

The fact that the plasma energy density is constant throughout the range of plasma focus devices, from big to small, is related to the value of a design parameter that needs to be kept at a certain value if the plasma focus is to operate efficiently. The critical 'speed' design parameter is, or the current linear density divided by the square root of the mass density of the fill gas.

For example for neutron-optimised operation in deuterium the value of this critical parameter, experimentally observed over a range of machines from kilojoules to hundreds of kilojoules, is: 9 kA/(mm·Torr0.5), or 780 kA/(m·Pa0.5), with a remarkably small deviation of 10% over such a large range of sizes of machines.

Thus if we have a peak current of 180 kA we require an anode radius of 10 mm with a deuterium fill pressure of 4 Torr (530 Pa). The length of the anode has then to be matched to the risetime of the capacitor current in order to allow an average axial transit speed of the current sheath of just over 50 mm/μs. Thus a capacitor risetime of 3 μs requires a matched anode length of 160 mm.

The above example of peak current of 180 kA rising in 3 µs, anode radius and length of respectively 10 and 160 mm are close to the design parameters of the UNU/ICTP PFF (United Nations University/International Centre for Theoretical Physics Plasma Fusion Facility). This small table-top device was designed as a low-cost integrated experimental system for training and transfer to initiate/strengthen experimental plasma research in developing countries.