An experimental study of plasma focus discharge

Abstract

Dense Plasma Focus (DPF) is a simple machine with generates high temperature plasma for investigation of D-D and D-T fusion. During operation, intense- neutron, charged particles and x-ray bursts are emitted, which enrich the scope of DPF as radiation source for material research, x-ray microscopy and lithography. Thirty five years have passed since the first operation of DPF is USA and former USSR, but there still lies some ambiguities about neutron, charged particle beam and x-ray generation mechanisms in DPF devices. The device consists of coaxial electrode system, one electrode in the form a rod at the axis while the other a set of rods (six in our case) which forms a perforated cy 1 inder surrounding the central electrode. At one end, the cathode extends to anode through a copper plate, whereas other end remains open. At close end two electrodes are separated by an insulator tube like Pyrex. A discharge current (few 100 kA to few MA) is passed by applying a high voltage pulse at the close end. In Islamabad, quring last ten years, different, experiments on DPF are

performed on three devices. Two devices are developed and operated at Quaid-i- Azam University, while the third one at Nilore. The results obtained on three

machines are compared and re-analyzed to understand the dependence of different mechanical parameters thereof. It is found that proper tuning of the focus tube inductance with that of the driver enhances the neutron emission from the focus region, increase the neutron pulse width and broadens the deuterium pressure range for high neutron yield. The insulator contamination which occurs due to Cu evaporated from the electrodes drastically changes the device characteristics. The possibility of using the device as a multi-pulse neutron generator is also examined. The solid state nuclear track detectors (SSNTDs) CR-39 are employed to investigate the fluence anisotropy of charged particles (protons, deuterons and

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tritons) emitted from the focus region. The charged particle flux is the highest in the axial direction and decreases towards the radial direction. The radial charged particle flux is six times smaJler than the flux in the axial direction. The x-ray and neutron emission is investigated using time-integrated and time-resolved detectors. The x-ray emission profile has a width (FWHM) of 40-50 11sec. The neutron emission profile is broader compared to the x-ray emission profile and also delayed by 30-40 nsec. To identify different regimes of x-ray emission, an x-ray pinhole camera (locally designed and developed) with absorption filters is employed. While the x-ray emission is high within a narrow pressure range of 2.0-2.5 mbar, the neutron emission is intense for a wider range of 1.0-4.5 mbar. The intense x-ray emission seems to originate from the axially moving shock wave. These results also indicate rather different production mechanisms for x-ray and neutron emission. Also on comparing the x-ray images with Al(2?Lm), Al(5μm) and Al(9μni) filters, we find that the bulk of x-rays from the focus filament have energies less than 2 keV. Generally the anode in DPF devices is of cylindrical shape of uniform diameter. Johnson [J. Appl. Phys. 45(1974)1147] investigated x-ray production in

a 375 kJ plasma focus using a tapered anode. However, no comparison with a flat- tip cylindrical anode was presented. At Quaid-i-Azam University, the effects on

PF dynamics when the anode shape is tapered towards the open end, is investigated in detail. Anodes of three different shapes: cone shape anode (I), tapered anode (II), and cylindrical flat-end anode (III) are employed to investigate their effects on PF dynamics. Computational results predict that the strength of the magnetic flux density at the anode surface increases steadily along axial transit of the tapered anode. To synchronize the axial transit time ta with the current rise

/JI time from the capacitor bank, one needs to increase the ambient gas density p0, r · that is, the gas pressure. Consequently a PF system with a tapered anode will be

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operated at higher gas pressures. The radiation yield and the filling gas pressure for good focus are found to be strongly dependent on the anode shape. An

appropriate tapering of the anode end enhances the emission threefold for both x- rays and ions. The intensities of the signals are also found to be correlated

mutually as well as with the high-voltage probe signal intensity. Furthermore, time-integrated pinhole images reveal that the x-rays originate predominantly from the anode end surface. In other words, an appropriate shaping of the anode end switches the device to a high-emission mode for both x-rays and charged particle beam. The effect of the presence of magnetic probe placed near the electrodes of the device on the current sheath dynamics is also investigated. It is found that the current sheath during the radial collapse phase strongly interacts with the magnetic probe jack?t (Pyrex glass tube of diameter 5.7 mm) and climbs over it up to several centimeters. Xsray images recorded for Argon pressure range of (0.25 - 3.25)mbar clearly indicate that the size as well as the iritensity of the x-ray source is significantly enhanced compared to the case when no probe is inserted in the system. Generally, the hot spots are formed up to about 30 mm above the anodetip and are only rarely observed 75 mm above the anode tip, which is probably due to the refocusing of the current sheath at the height. These preliminary results suggest that using some suitable material, path of the current sheath can be diverted to any desired direction and then guide it to converge at a pre-selected point . It is not so clear at this stage as to what causes the current sheath to climb

up the probe and why pressure range is broadened for the efficient electron and x- ray emission. Perhaps, the polarization of the magnetic probe material generates

electric field which provide an additional force to lift up current sheath. With stainless steel anode, neutron and x-ray emission is investigated by employing time-integrated and time resolved detectors. The neutron yield of 3.5 x 108 is observed, which is almost double the yield when copper anode is used. It is

5 speculated that low sputtering yield of the anode material lowers the impurity concentration in the plasma and thus 'enhances the neutron yield. One may therefore conclude that proper choice of the electrode material is essential to achieve enhanced radiation yield from plasma focus devices. At pressure of 2.0 mbar, neutron yield is found to be the highest and almost isotropic. Further, multiple foci are also observed. The neutron fluence isotropy and the high yield are attributed to trapping of magnetic flux lines and of the energetic deuterons between the two current sheaths. Correlation of ions, electrons and x-ray emission is studied. At low pressure

of 0.25 mbar, the x-ray emitting region is broad and a considerable amount of x- rays originate from the anode surface. With increase in filling gas pressure, the x- ray emission zone squeezes to pinch filament at the axis. The intensity of x-ray,

electron and ion beams signals are found to be correlated mutually as well as with the high voltage probe signal intensity. The average energy of the Ar ion beam is found to be filling pressure dependent, which is about 2.7 MeV at 0.25 mbar Ar and increases to 4.8 MeV at higher pressure. Neutron and x-ray emission is investigated by time integrated and time resolved detectors. The SSNTDs CR-39 are employed for charged particles' angular distribution study. Correlation of charged particles with neutron and x-ray emission is investigated. The neutron emission profile is composed of two pulses, the intensity and anisotropy of which vary with the filling pressure. The charged particle flux is maximum with high fluence anisotropy for pressure range (2.5-3.0 mbar) which is also the optimum pressure for high neutron emission with low fluence anisotropy (,..., 1.5). The latter is attributed to the presence of trapped deuterons in an anomalous magnetic field. The relevant pressure range generates favorable conditions for plasma density and pinch filament diameter. X-ray emissionis generally high at low pressure. However hard x-rays are detected only

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at high pressure (2.5-4.0 mbar). The x-rays ate dominantly Cu Ka radiation which originate from the anode tip due to impact of electrons in the plasma sheath.