R. K. Rout

Development of Multi Faraday Cup Assembly for Ion Beam Measurements from a Low Energy Plasma Focus Device

A multiple Faraday cup assembly has been developed for measuring pulsed ion beam of a low energy plasma focus device. The Faraday cups operating in biased ion collector mode have nanosecond response and these have been used to determine the energy spectrum and flux of fast nitrogen ion beam emerging out of the pinched plasma column. The design feature that makes our Faraday cups unique is that they can register ion energy of higher kinetic value (~hundreds of keV) as well as lower kinetic value (~keV). It has been possible to register the ion energy upto a lower kinetic energy threshold of ~5 keV which is a value much lower than that obtained in any previous works. The correlation of the ion beam flux with filling gas pressure is also reported. Angular distribution of ion measurement reveals a highly anisotropic emission indicating an ion dip at the electrode axis.

Comparative study of soft x-ray emission characteristics in a low energy dense plasma focus device

An investigation on the soft x rays emitted in a 2.2 kJ Mather-type dense plasma focus device using a multichannel diode spectrometer and a simple pinhole camera is reported. Emitted x rays associated with different shapes (hollow, solid, and hemispherical) of anode and in hydrogen/nitrogen gas medium are compared. The structure of x-ray emitting sites as well as x-ray yields were found to be strongly influenced by the shape of the anode and the filling gas pressure. The maximum yield of 2.2 J into 4π sr was obtained in the case of hemispherical anode in hydrogen gas medium. The x-ray pinhole images of the collapsed plasma with the hemispherical anode indicated spot-like structure having 500–800 μm in diameter. On the contrary, other anode shapes showed columnar pinched structure of 8–10 mm in length and 1–2 mm in diameter. Results indicated that an appropriate design of the anode could enhance the x-ray yield by more than tenfold in a conventional low energy dense plasma focus device.

Effect of Anode Designs on Ion Emission Characteristics of a Plasma Focus Device

A comparative study on the ion emission characteristics such as flux and energy, and their variation in angular positions and operating gas pressures has been carried out in a nitrogen-filling plasma focus device. Three different designs of cylindrical anode (central electrode) having hollow, solid and hemispherical tip have been tested for this study. The ion emission characteristics were investigated by employing three Faraday cups at various angular positions. The ion flux depends on the operating gas pressure irrespective of the anode designs and the maximum ion flux is found to be in the pressure range 0.3 to 0.5 Torr for all the anode designs. The hemispherical anode yields highest ion flux while the hollow anode emits lowest ion flux. The angular variation of ion flux is seen to be anisotropic irrespective of the anode designs with an ion dip at 0° (axis of the device) and maximal at 5° angular positions. The anisotropic character of ion emission is less in the case of the hemispherical anode than the hollow anode. The ion energy, measured by the time of flight method, shows its dependence on the anode designs. The maximum ion energy is found to be around 830 keV at an angular position 5° in the case of the hemispherical anode design. The most probable ions are found to be with energy less than 100 keV irrespective of the anode designs and the angular positions. This study indicates that the plasma focus device could be optimized to a great extent for optimal ions yield by using an appropriate anode design.

Influence of electrode and insulator materials on the neutron emission in a low energy plasma focus device

Insertion of internal impurities is one of the factors for the degradation of neutron output in plasma focus devices. In this context, neutron emission measurements were carried out in a 2.2 kJ (7.2 /spl mu/F, 25 kV) Mather type squirrel cage plasma focus device with various combinations of electrode and insulator materials. The neutron yield was measured by silver activation detector. The results indicate that a central electrode of low erosion rate material produces maximum neutron yield with the highest anisotropy factor. A time integrated X-ray pinhole image shows that focus corresponding to the highest neutron yield material is the most compressed. The dielectric constant of the ceramic insulators has, at best, a very weak correlation with neutron output. The nonceramic insulators like perspex, nylon, or teflon neither produce focus nor neutrons.

Magnetic probe measurements of current?sheet dynamics in a coaxial plasma accelerator

A high-frequency multiple magnetic probe assembly has been specifically fabricated for the study of current sheet dynamics in the axial acceleration phase of a low-energy dense plasma focus (DPF) device operated in a nitrogen gas medium. The response time of each probe is of the order of 1 ns and the tiny structure of the probe is well suited to sense the magnetic field associated with a pulsed plasma without perturbing the plasma unduly. The magnetic probes were calibrated using a simple, novel and reliable calibration technique and the calibration factor is found to be 0.34 ± 0.028 T V−1. Our study reveals that the parabolic current sheet accelerates as it propagates through the electrode assembly, reaching a rundown velocity of ~6.1 cm µs−1. The average current sheet thickness in the axial acceleration phase is found to be ~3 cm. In our case, the current shedding and mass loss factors are estimated to be 32% and 40% respectively. Our approach of using a high-frequency multiple magnetic probe assembly for the study of current sheet dynamics in a DPF device is highly effective in obtaining precise and accurate measurements.

Palm top plasma focus device as a portable pulsed neutron source

Development of a palm top plasma focus device generating (5.2 ± 0.8) × 10(4) neutrons∕pulse into 4π steradians with a pulse width of 15 ± 3 ns is reported for the first time. The weight of the system is less than 1.5 kg. The system comprises a compact capacitor bank, a triggered open air spark gap switch, and a sealed type miniature plasma focus tube. The setup is around 14 cm in diameter and 12.5 cm in length. The energy driver for the unit is a capacitor bank of four cylindrical commercially available electrolytic capacitors. Each capacitor is of 2 μF capacity, 4.5 cm in diameter, and 9.8 cm in length. The cost of each capacitor is less than US

Line broadening studies in low energy plasma focus

10. The internal diameter and the effective length of the plasma focus unit are 2.9 cm and 5 cm, respectively. A DC to DC converter power supply powered by two rechargeable batteries charges the capacitor bank to the desired voltage and also provides a trigger pulse of -15 kV to the spark gap. The maximum energy of operation of the device is 100 J (8 μF, 5 kV, 59 kA) with deuterium gas filling pressure of 3 mbar. The neutrons have also been produced at energy as low as 36 J (3 kV) of operation. The neutron diagnostics are carried out with a bank of (3)He detectors and with a plastic scintillator detector. The device is portable, reusable, and can be operated for multiple shots with a single gas filling.

REPRODUCIBLE, ANOMALOUS EMISSIONS FROM PALLADIUM DEUTERIDE/HYDRIDE

Optical emission spectroscopic studies were carried out to characterise the plasma leading to the estimation of two plasma parameters, electron density and temperature. These experiments were conducted on a 2 kJ plasma device which is equipped with squirrel cage electrode configuration enclosed in a glass vacuum chamber filled with hydrogen at a pressure of 5 mbar. Spectral emissions obtained from each flash were photographed in the region of 4000–6000 Å using one metre Czerny-Turner spectrograph cum monochromator. Detailed examination of the observed features showed that theHβ andHλ lines of hydrogen showed significant broadening of the order of 35 Å FWHM which is due to Stark effect expected in high density plasmas. Further several atomic lines of Cu and Zn from the electrode material (brass) showed broadening which was due to quadratic Stark effect. A comparative study of the broadening of lines obtained in DC arc, hollow cathode and plasma focus was made. Electron density from Stark broadened hydrogen lines and quadratic Stark Coefficient C4 for the CuI and ZnI lines were evaluated. The excitation temperature was determined from the line intensity ratio method using CuI lines.

Delayed hot spots in a low energy plasma focus

Each and every palladium sample loaded/reloaded either with hydrogen or deuterium was observed to fog radiographic films kept in its close proximity in air. Strangely, even with ten layers of black paper (thickness ≃63 mg/cm 2 ) as a filter between film and sample, fogging was observed. On the other hand, no fogging could be observed even when thin beryllium foil (≃1.4 mg/ cm 2 ), three layers of transparent polyester foils (≃10 mg/ cm 2 ), or thin aluminized polycarbonate (0.3 mg/cm 2 ) were employed as filters. Several experiments have been performed to identify the phenomenon responsible for fogging. These experiments appear to rule out any of the known mechanisms, suggesting a new, strange, and unknown phenomena.

Detection of High Tritium Activity on the Central Titanium Electrode of a Plasma Focus Device

In a low energy Mather-type plasma focus device, hot spots having temperature in the range of few KeV have been observed even 1 µs after the pinch disintegration and in regions away from the pinch area. These hot spots are perhaps created by the thermal runway due to temperature fluctuations in the background gas.