R. Shukla

Radial shearing interferometers using a laser source.

Radial shearing interferometers have been devised by several workers earlier. All of them were based on conventional light sources, and consequently full white light compensation was required. It is possible to devise several schemes of radial shearing interferometers ignoring the condition of white light compensation when a laser source is used in place of a conventional source. Some of these arrangements are described in the present paper.

Note: Compact helical pulse forming line for the generation of longer duration rectangular pulse

The helical pulsed forming line (PFL) can generate longer duration rectangular pulse in a smaller length. A compact PFL using helical water line is designed and experimentally investigated. The impedance of the helical PFL is 22 [ohm sign]. The compactness is achieved in terms of reduction in length of the PFL by a factor of 5.5 using helical water PFL as compared to coaxial water PFL of same length. The helical PFL was pulsed charged to 200 kV using a high voltage pulse transformer in 4.5 μs and discharged into the matched 22 Ω resistive load through a self-breakdown pressurized spark gap switch. The rectangular voltage pulse of 100 kV, 260 ns (FWHM) is measured across the load. The effect of reduction in water temperature on the pulse width is also studied experimentally. The increase in pulse width up to 7% more is observed by reducing the temperature of the deionized water to 5 °C. It will further reduce the length of the PFL and make the system small for compact pulsed power drivers.

Compact pulse forming line using barium titanate ceramic material

Ceramic material has very high relative permittivity, so compact pulse forming line can be made using these materials. Barium titanate (BaTiO(3)) has a relative permittivity of 1200 so it is used for making compact pulse forming line (PFL). Barium titanate also has piezoelectric effects so it cracks during high voltages discharges due to stresses developed in it. Barium titanate is mixed with rubber which absorbs the piezoelectric stresses when the PFL is charged and regain its original shape after the discharge. A composite mixture of barium titanate with the neoprene rubber is prepared. The relative permittivity of the composite mixture is measured to be 85. A coaxial pulse forming line of inner diameter 120 mm, outer diameter 240 mm, and length 350 mm is made and the composite mixture of barium titanate and neoprene rubber is filled between the inner and outer cylinders. The PFL is charged up to 120 kV and discharged into 5 Ω load. The voltage pulse of 70 kV, 21 ns is measured across the load. The conventional PFL is made up of oil or plastics dielectrics with the relative permittivity of 2-10 [D. R. Linde, CRC Handbook of Chemistry and Physics, 90th ed. (CRC, 2009); Xia et al., Rev. Sci. Instrum. 79, 086113 (2008); Yang et al., Rev. Sci. Instrum. 81, 43303 (2010)], which increases the length of PFL. We have reported the compactness in length achieved due to increase in relative permittivity of composite mixture by adding barium titanate in neoprene rubber.

Low voltage operation of plasma focus

Plasma foci of compact sizes and operating with low energies (from tens of joules to few hundred joules) have found application in recent years and have attracted plasma-physics scientists and engineers for research in this direction. We are presenting a low energy and miniature plasma focus which operates from a capacitor bank of 8.4 muF capacity, charged at 4.2-4.3 kV and delivering approximately 52 kA peak current at approximately 60 nH calculated circuit inductance. The total circuit inductance includes the plasma focus inductance. The reported plasma focus operates at the lowest voltage among all reported plasma foci so far. Moreover the cost of capacitor bank used for plasma focus is nearly 20 U.S. dollars making it very cheap. At low voltage operation of plasma focus, the initial breakdown mechanism becomes important for operation of plasma focus. The quartz glass tube is used as insulator and breakdown initiation is done on its surface. The total energy of the plasma focus is approximately 75 J. The plasma focus system is made compact and the switching of capacitor bank energy is done by manual operating switch. The focus is operated with hydrogen and deuterium filled at 1-2 mbar.

Nanosecond rise time air-core current transformer for long-pulse current measurement in pulsed power

A slow-wave delay line type air-core (nonmagnetic Nylon former) current transformer fabricated using silver epoxy for the measurement of currents of long pulse durations and few nanoseconds rise times is reported in this article. The advantage of using silver epoxy is that it fills all the voids between coil and shield and enhances the proximity of the coil to the shield, leading to a high value of distributed capacitance. Thus the transit time of the coil increases and it can measure fast current pulses of longer durations. Increasing the inductance of the coil can compensate for the resulting reduction in the sensitivity of the coil for matched termination. An easy experimental technique to find the value of the matched terminating resistor is also reported in this article. We have also done simulations of the slow wave current transformer using PSPICE.

Note: Repetitive operation of the capacitor bank of the low-voltage miniature plasma focus at 50 Hz.

We have already reported the low-voltage operation of a plasma focus describing the operation of plasma focus at 4.2 kV which proposes possibility of making a repetitive system using compact driving source. Another recent article describes that the same capacitor-bank can drive the plasma focus for a measured ~5 × 10(4) neutrons per shot at 5 kV and 59 kA current. In the present work, repetitive operation of the capacitor-bank of plasma focus is done and that too is being reported at a very high repetition rate of 50 Hz using very simple scheme of charging and triggering the bank. The bank is continuously discharged to burst duration of 20 s in this configuration admeasuring a thousand shots.

Characterization of High Power Microwave Radiation by an Axially Extracted Vircator

Characterization results of high-power microwave radiation, from an axial vircator driven by pulsed electron beam accelerator AMBICA-600 are reported in this paper. We present a study on variation in pulsed microwave power output and dominant frequency by discretely varying anode-cathode (A-C) gap. While keeping the cathode diameter fixed at 40 mm, for the A-C gap distance in the range 5-9 mm, dominant frequencies have been measured to be lying in the range 4.7-9.8 GHz. The trend of a subsequent increase in the dominant frequency at lower A-C gap distances (and vice versa) revealed that center frequency is mainly governed by the longitudinal size of the potential well. The highest microwave power of ~ 14 MW for ~ 75-ns pulse duration was obtained at A-C gap of 7 mm having the dominant frequency in C-band at ~ 6.9 GHz. The beam-to-microwave power conversion efficiency of ~ 1.2% has been demonstrated in our experiments. On the basis of power distribution pattern obtained by the gas breakdown technique, the dominant mode of emission is believed to be transverse magnetic mode. Relative analysis of frequency spectrums obtained for various A-C gap distances evidenced experimental recognition of optimum A-C spacing as a generation of narrowband distinct frequency peak of large magnitude with minimal mode hopping.

Portable & Low Cost Giga-Watt Pulsed Power Source for Intense Electron Beam Generation

An alternative approach for applications requiring production of intense electron beam without using conventional configuration of Marx/Tesla accompanying Pulse Forming Line has been explored. The developed portable system utilizes four stage Blumlein pulse forming network made from inexpensive commercially available URM-67 coaxial cables having characteristic impedance of 50Omega. All stages are charged in parallel and then synchronously discharged through single low inductance railgap switch. Use of low jitter (<5 ns) railgap switch allows synchronization with other events and improves reproducibility of the system. Each stage of Blumlein is configured as eight parallel pulse forming network, with a resultant output impedance of 6.25 Omega per stage. For four stages the output impedance is therefore 25 Omega. A 24 V battery driven 50 kV DC to DC Converter has been used for charging the system. The generator is capable of delivering power up to 200 kV, 4 kA across matched load of 50 Omega. The voltage pulse duration and rise time are 50 ns and 8 ns respectively. In the presented paper, generator construction has been described and performance of the system is evaluated to realize adverse effect of parasitic impedance on the voltage gain and pulse shape. Also its operation has been simulated by PSPICE circuit simulator program and good agreement has been obtained between simulated and experimental results. The entire cost of the generator including raw material and labor is under US

Impulse electromagnetic interference generator

2500. Other than low cost of the generator, added advantage of cable based system is that - the slow DC charging of transmission line to a known voltage eliminates the possibility of diode voltage prepulse in electron beam generation experiments. Applications of this pulse generator also include flash X- ray generation, breakdown tests, ion implantation, streamer discharge studies, ultra wideband generation etc.

Experimental study of exploding wire method for production of metal nanoparticles

Electromagnetic interference (EMI) related malfunctioning of equipment containing electronic control devices has become a serious problem these days, as it couples to wires and PCBs in the equipment, reflecting and resonating and then being amplified by an IC finally resulting in malfunction. In order to test and simulate the immunity of electronic circuits to an EMI environment, an electrical fast transient/burst generator has been developed which generates broadband interference spectrum in the range of 20 MHz to 600 MHz. The burst generator is mainly comprised of coaxial cables, pressurized spark gaps, a HV Power Supply, a terminating resistor and a TEM horn which acts as load. For pulse sharpening, switching has been done at two stages. Depending upon the delay time of coaxial line, the width of the square wave pulse has been kept at 50 ns. The total energy of the portable system is 34 Joules, and it weighs less than 50 kg. In the presented paper we describe the system design, results of measurements and areas of further improvements in energy transfer efficiency.