B. Adhikary

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.

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.

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

A Portable Electron Beam Generator for Relativistic Electron Beam Characterization Experiments

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.

An 1.2 MJ Capacitor Bank “RUDRA” for MTF & Plasma Focus Experiments

A compact pulsed power system, which is reported in this paper, is designed for conducting relativistic-electron-beam characterization experiments in the space-charge limited flow. The experiments are conducted for various cathode materials like graphite and velvet and also for various anode transparencies and geometries by making them of thin foil as well as of mesh. The reported Pulsed power system is made very compact as well as portable by using solid dielectric pulsed forming line. The system consists of a tesla transformer, which is of helical secondary and cylindrical-sheet single-turn primary. Tesla charges a pulse forming line made of cascade of 50 ohm transmission lines, which are of high wattage as well as high voltage ratings under pulsed operation. The net impedance of this cable cascade is such that it is matched for a designed diode as load. The impedance of designed diode is 13.5 ohm and is designed to operate at 250 kV and hence almost 18.5 kA current levels for 100 ns pulse duration. Related design details as well as all the diagnostics, used for measurement of current, voltage and other parameters of interest, are reported and finally the results are compiled in this paper.

Design and development of compact pulsed power driver for electron beam experiments

Pinch experiments like Magnetized Target Fusion (MTF) and Dense Plasma Focus require for their operation fast rising high power and high energy pulses. To perform such experiments, an 1.2MJ Capacitor Bank capable of delivering 3.6MA of peak current with 5 to 7μs rise time has been designed and commissioned. The major application of bank is focused on fast discharge applications where large peak currents are required.

A Low Impedance Marx Generator as a Test bed for Vacuum Diodes

Pulsed electron beam generation requires high power pulses of fast rise, short duration pulse with flat top. With this objective we have designed a low cost compact pulsed power driver based on water dielectric transmission line. The paper describes the design aspects and construction of the pulse power driver and its experimental results. The pulsed power driver consist of a capacitor bank and its charging power supply, high voltage generator, high voltage switch and pulse compression system.

Compact pulsed electron beam system for microwave generation

A low impedance Marx generator was developed, which will serve as a test bed for Vacuum diodes of various electrode materials and geometries. The vacuum diodes will be used for high power microwave generation. The generator is capable to supply ~3GW of pulsed power to the vacuum diodes which is sufficient enough to produce plasma within the diode for electron beam generation. A vacuum of 10−5Torr is required for virtual cathode formation within the diode, when the beam current exceeds the space charge limiting current. A vacuum diode of reflex triode geometry has been designed and vacuum of 10−5 Torr has been achieved. The repetitive operation of the vacuum diode depends upon the recovery of the diode, the importance of the vacuum system on the recovery of the diode will be explained. A vacuum system with high voltage isolator has been installed for getting the desired vacuum within the diode. The design criterion of the vacuum system will be discussed. The 300kV/1.8kJ Marx generator which will power the vacuum diode has six stages with stage capacitance and voltage of 240nF and 50kV respectively. It has an impedance of ~7 ohm and can deliver 200kV voltage across the diode in critically damped load condition. The generator has a very fast rise time of 200ns.The operational characteristics of the Marx generator are determined experimentally. The results have been analyzed and compared to an equivalent circuit model of the system.