Ankur Patel

Emission properties of explosive field emission cathodes

The research results of the explosive field emission cathode plasma expansion velocity and the initial emission area in the planar diode configuration with cathodes made of graphite, stainless steel, polymer velvet, carbon coated, and carbon fiber (needle type) cathodes are presented. The experiments have been performed at the electron accelerator LIA-200 (200 kV, 100 ns, and 4 kA). The diode voltage has been varied from 28–225 kV, whereas the current density has been varied from 86–928 A/cm2 with 100 ns pulse duration. The experimentally obtained electron beam diode perveance has been compared with the 1 dimensional Child-Langmuir- law. It was found that initially only a part of the cathode take part in the emission process. The plasma expands at 1.7–5.2 cm/μs for 4 mm anode-cathode gap for various cathode materials. It was found that the plasma expansion velocity increases with the decrease in the cathode diameter. At the beginning of the accelerating pulse, the entire cathode area participates in the e...

Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation

This paper presents the design and development of a repetitive Marx generator rated at 1 kJ, 300 kV, 12 kA, and 10 Hz and which is suitable to drive the load directly. Bipolar charging of a Marx generator scheme has been adopted to get the following: 1) faster rise time and 2) relatively low charging voltage. The faster rise time is due to the reduced number of spark gaps down to six for 12 in-series capacitors (0.15 μF and 50 kV) of a six-stage bipolar-charging Marx generator. Interstage inductive charging further enhances the efficiency of the overall system as compared to resistive charging. The generator is characterized by an aqueous resistive load for both polarities at the output voltage. Output voltages of the same magnitude have been achieved for both polarities. The matched impedance is found to be 25 Ω for which a suitable reflex triode is designed. Intense electron beams and high-power microwaves have been generated using this Marx generator and reflex triode system. Experimental results demonstrating the Marx generator and the reflex triode operation are presented and discussed.

Preliminary results of Linear Induction Accelerator LIA-200

Repetitive Pulsed Power Technology is being developed keeping in mind the potential applications of this technology in material modifications, disinfections of water, timber, and food pasteurization etc. BARC has indigenously developed a Linear Induction Accelerator (LIA-200) rated for 200 kV, 4 kA, 100 ns, 10 Hz. The satisfactory performance of all the sub-systems includ- ing solid state power modulator, amorphous core based pulsed transformers, magnetic switches, water capacitors, water pulse- forming line, induction adder and field-emission diode have been demonstrated. This paper presents some design details and operational results of this pulsed power system. It also highlights the need for further research and development to build reliable and eco- nomic high-average power systems for industrial applications.

Experimental investigations of argon spark gap recovery times by developing a high voltage double pulse generator

The voltage recovery in a spark gap for repetitive switching has been a long research interest. A two-pulse technique is used to determine the voltage recovery times of gas spark gap switch with argon gas. First pulse is applied to the spark gap to over-volt the gap and initiate the breakdown and second pulse is used to determine the recovery voltage of the gap. A pulse transformer based double pulse generator capable of generating 40 kV peak pulses with rise time of 300 ns and 1.5 μs FWHM and with a delay of 10 μs-1 s was developed. A matrix transformer topology is used to get fast rise times by reducing L(l)C(d) product in the circuit. Recovery Experiments have been conducted for 2 mm, 3 mm, and 4 mm gap length with 0-2 bars pressure for argon gas. Electrodes of a sparkgap chamber are of rogowsky profile type, made up of stainless steel material, and thickness of 15 mm are used in the recovery study. The variation in the distance and pressure effects the recovery rate of the spark gap. An intermediate plateu is observed in the spark gap recovery curves. Recovery time decreases with increase in pressure and shorter gaps in length are recovering faster than longer gaps.

Explosive Emission Properties of Cathode Materials in Relativistic Electron Beam Generation

Relativistic electron beam generation studies have been carried out in LIA-400 system through explosive electron emission for various cathode materials. This paper presents the emission properties of different cathode materials at peak diode voltages varying from 10 to 220 kV and at peak current levels from 0.5 to 2.2 kA in a single pulse duration of 160-180 ns. The cathode materials used are graphite, stainless steel, and red polymer velvet. The perveance data calculated from experimental waveforms are compared with 1-D Child Langmuir formula to obtain the cathode plasma expansion velocity for various cathode materials. Various diode parameters are subject to shot to shot variation analysis. Velvet cathode proves to be the best electron emitter because of its lower plasma expansion velocity and least shot to shot variability.

Analysis and development of fourth order LCLC resonant based capacitor charging power supply for pulse power applications

A fourth order (LCLC) resonant converter based capacitor charging power supply (CCPS) is designed and developed for pulse power applications. Resonant converters are preferred t utilize soft switching techniques such as zero current switching (ZCS) and zero voltage switching (ZVS). An attempt has been made to overcome the disadvantages in 2nd and 3rd resonant converter topologies; hence a fourth order resonant topology is used in this paper for CCPS application. In this paper a novel fourth order LCLC based resonant converter has been explored and mathematical analysis carried out to calculate load independent constant current. This topology provides load independent constant current at switching frequency (fs) equal to resonant frequency (fr). By changing switching condition (on time and dead time) this topology has both soft switching techniques such as ZCS and ZVS for better switching action to improve the converter efficiency. This novel technique has special features such as low peak current through switches, DC blocking for transformer, utilizing transformer leakage inductance as resonant component. A prototype has been developed and tested successfully to charge a 100 μF capacitor to 200 V.

Development of synchronization system of two spark gaps

In this article a novel way of synchronizing the critical component for pulsed power applications is described: the spark gap switch. The circuit design of a synchronized spark-gap switch circuit is discussed. The circuit uses a combination of one master spark gap, and a set of inductor and capacitors to synchronize two spark gaps. Critical issues for circuit design are presented together with experimental results. It is observed that the synchronization achieved in terms of switch jitter between two spark gaps is of the order of 1-5ns. Measurements were performed with 30- 34 kV average switching voltage with 30-40ns pulse rise.

Voltage recovery characteristics of spark gap using repetitive pulse power system

Fast recovery switches are never ending demand in pulse power applications. This study uses a two-pulse method to determine the voltage recovery times of gas spark gap switch with argon gas. First pulse is applied to the spark gap to over-volt the gap and initiate the breakdown (FPBDV) and second pulse is used to determine the recovery voltage of the gap (SPBDV). A double pulse generator capable of generating 40 kV peak pulses with a repetition rate of 100 kHz was developed. Recovery Experiments has been conducted for 2mm and 3mm gap length between 0-2 bar pressure range with argon gas as dielectric medium. A sparkgap chamber of which electrodes are of rogowsky profile type, made up of Stainless steel material and thickness of 15mm are used in the recovery study. Pressure has significant effect on the recovery of the spark gap. It is observed that time taken for complete recovery without gas flow techniques is around 1s.

Frequency Variation of a Reflex-Triode Virtual Cathode Oscillator

We present a study of the shot-to-shot variation in frequency of a reflex-triode virtual cathode oscillator with respect to peak diode voltage, current, impedance, and perveance. The typical electron-beam parameters were 185 kV, 7 kA, and 300 ns, with a current density of a few hundreds of amperes per square centimeter. The one-way analysis of variance was employed to examine the statistical correlation of the diode voltage, current, impedance, and perveance with the emitted microwave frequency. It was shown that the microwave dominant frequency variation is statistically correlated only with the peak diode current. However, the secondary emitted frequency is not statistically correlated with any of the electron-beam diode parameters.

Effect of cathode diameter on intense relativistic electron beam generation in the presence of prepulse

Intense gigawatt relativistic electron beam generation studies were carried out in the presence of prepulse. Electron beams were generated using planar and annular graphite cathodes of various diameters at a fixed 25mm anode-cathode gap. For the planar cathode, the beam parameters obtained are 340keV, 24kA, and 100ns at a 680A∕cm2 current density. With an annular cathode, 346keV, 10kA, and 100ns electron beam could be generated at a 3.4kA∕cm2 current density. The peak electric field in the diode varies from 58to138kV∕cm. A bipolar prepulse voltage has been recorded at the diode for both the cathodes. The amplitude of the negative prepulse voltage varies with the Marx generator voltage but the time duration remains same. The positive prepulse voltage varies both in amplitude and time duration with the Marx generator voltage. Some shot to shot variation in the diode voltage and current were recorded for the annular cathode due to the nonreproducibility of the prepulse generated plasma. It was found that the...