Study of Plasma focus discharge

Abstract

Dense Plasma focus is a device whose operation is much simpler as compared to other plasma machines. The device consists of an anode rod of bigger diameter surrounded by six cathode rods of relatively smaller diameter forming a co-axial system. These rods are screwed to the anode and cathode base plates called anode and the cathode headers. An insulator sleeve usually a pyrex glass is used to provide the electrical isolation between the electrodes of the device. The operation of the device begins with the application of high voltage pulse (through a charged capacitor or by a capacitor bank) between the co-axial electrodes. As a result dielectric breakdown of the filling gas occurs between the central anode rod and the cathode base plate via insulator sleeve surface. During this process axially symmetric current sheath is developed which expands radially and moves towards the open end of the electrode assembly due to the magnetic pressure behind the current sheath. It is one of the basic requirements of the Plasmas Focus operation that, when the current sheath arrives at the tip of the central electrode the current through the device is maximum, so that the maximum energy stored is in the magnetic field which is indispensable for efficient compression. Experimentally this is achieved by synchronizing the current sheath arrival time from the base plate to the anode tip with the peak current time (rise time) of the capacitor. This in turn strongly depends upon the length of the anode, pressure and nature of the filling gas. Once this synchronization is achieved by adjusting these parameters, causing the radial collapse of the current sheath resulting, in the formation of hot and dense focused plasma just beyond the face of the central electrode. Experimentally this can be conformed by using a high voltage probe which is actually a resistor divider connected to anode and cathode headers. A sharp spike in the signal of the high voltage probe is an indication of strong pinching during the collapse phase of current sheath. Further when the device is operated with D2 2 as the filling gas intense burst of neutrons and x-rays are emitted due to D-D fusion resulting from the pinching. During the entire duration of the project we have performed a number of experiments on the dense plasma focus. Some of the significant findings in our research work on the device are as under : a) In the laboratory, for the first time deterioration of neutron yield in a low energy plasma focus operated by a single 320, 15 kV (3.6 kJ) capacitorlis observed. When sum of the discharged energy across an insulator sleeve approaches 1.6 MJ, the neutron yield from the device starts deteriorating. The insulator sleeve, when examined, is found to have a —3 i.tm thick copper layer evaporated from the electrodes of the device. It is therefore .._ concluded that the degradation of neutron yield in our low-energy device occurs due to Cu deposition on the sleeve surface. b) We are the first in Pakistan who have successfully designed and developed a simple, low-inductance pressurized sparkgap for the plasma focus operation energized by a single 32 [tF, 15 kV (3.6 kJ) capacitor. The sparkgap in capable of handling discharge current upto 200 kA with a rise time of less than 1 pee. c) A low inductance capacitor bank for the plasma focus operation is also designed and developed. The bank comprises three modules, each consisting of two 2 40 kV capacitors alongwith a field-distortion-type pressurized sparkgap. A peak current of about 250 kA has been estimated when the bank is charged at 18 kV d) The behaviour of the current sheath in the presence of a target placed downstream of the anode is studied. The high voltage probe signal and the sequential bursts of the neutrons suggest the possibility of the plasma focus system to be used as a cascading device for the production of sequential bursts of x-rays and neutrons. 3 e) To enhance the efficiency of the plasma focus system we have studied the effects of anode length and insulator sleeve length variation on the pressure range of neutron emission. It is found that the proper choice of the two parameters broadens the pressure range for the high neutron yield and hence improves the shot-to-shot reproducibility of the system.