Purushottam Shrivastava

Development of a 33 kV, 20 A long pulse converter modulator for high average power klystron

Research, design, and development of high average power, long pulse modulators for the proposed Indian Spallation Neutron Source are underway at Raja Ramanna Centre for Advanced Technology. With this objective, a prototype of long pulse modulator capable of delivering 33 kV, 20 A at 5 Hz repetition rate has been designed and developed. Three Insulated Gate Bipolar Transistors (IGBT) based switching modules driving high frequency, high voltage transformers have been used to generate high voltage output. The IGBT based switching modules are shifted in phase by 120° with respect to each other. The switching frequency is 25 kHz. Pulses of 1.6 ms pulse width, 80 μs rise time, and 70 μs fall time have been achieved at the modulator output. A droop of ±0.6% is achieved using a simple segmented digital droop correction technique. The total fault energy transferred to the load during fault has been measured by conducting wire burn tests and is found to be within 3.5 J.

Improved Expression for Estimation of Leakage Inductance in E Core Transformer Using Energy Method

This paper proposes a simpler and more accurate expression for estimation of leakage inductance in E core transformer, which is the most widely used transformer structure. The derived expression for leakage inductance accounts for the flux extending into air. The finite element method (FEM) analysis is made on the secondary shorted transformer to observe the H-field pattern. The results obtained from FEM analysis are used for approximating the field that is extending into air to derive an expression for leakage inductance. This expression is experimentally validated on prototype transformers of different core dimensions.

Design and development of a prototype 25 kV, 10 A long pulse Marx modulator for high power klystron

Research, design, and development of high average power modulators for the proposed Indian Spallation Neutron Source are in progress at Raja Ramanna Centre for Advanced Technology. With this objective, a prototype of 25 kV, 10 A, 1 ms Marx modulator at repetition rate of 1 Hz has been designed and developed which serves as a proof of principle and technology assessment stage for further development of high repetition rate high voltage high average power modulators. Insulated Gate Bipolar Transistor (IGBT) based modules of 2.8 kV switching capability have been used as main modules. The modulator had 8.2% droop in output voltage pulse without any droop compensation circuit. A droop compensation involving 15 corrector modules has been used to reduce the droop up to 1%. We have used IGBT based 250 V switches to realize the corrector module. A microcontroller based control unit was designed and developed for triggering the main and corrector modules. With this control unit, programmable output pulse has been achieved. Electrical isolation between high voltage circuits and control circuit has been achieved by the use of fiber optic based control signal transmission. Output pulses of 1 ms pulse width, 800 ns rise time, and 5 μs fall time have been achieved. The modulator has advantages of modular design, adjustable pulse width, adjustable rise time, and fall time.

Research, design and development activities on high power pulsed RF/Microwave systems and test facilities for particle accelerators

Several high power S Band microwave systems in the peak power range from 2 MW to 6.5 MW and mean power upto 25kW along with associated waveguide components, pulse power technology were developed in house at RRCAT. These systems have been used extensively in the development of particle accelerators in the country. Several test facilities were developed in house and supplied to tube R & D labs for development, conditioning and evaluation of microwave tubes indigenously under collaborative efforts. RRCAT has also taken up establishing high power test facilities for advanced accelerators. For such use design and development of 45 MW peak power S Band test facility as well as energy doubler scheme is underway. A 1.3 MW pulsed test stand at 352.2 MHz was also successfully designed and developed to qualify devices, subsystems and components developed in-house. The present paper will describe the research, design and development efforts towards the high power pulsed RF/Microwave systems, test facilities and components. The results and performance of these developments are reported.

A compact 10 kW, 476 MHz solid state radio frequency amplifier for pre-buncher cavity of free electron laser injector linear accelerator

A 10 kW, 476 MHz, 0.1% duty cycle solid state RF amplifier system for driving sub-harmonic, pre-buncher cavity of IR-FEL injector LINAC, has been developed at RRCAT. The 10 kW power is achieved by combining output of eight 1400 W amplifier modules using 8-way planar corporate combiner. The solid state amplifier modules have been developed using 50 V RF LDMOS transistors which although meant for push-pull operation are being used in single ended configuration with matching circuit developed on a thin (25 mils), high dielectric constant (9.7), low loss microwave laminate with an aim to have a compact structure. Ease of fabrication, modularity, small size, and low cost are the important features of this design which could be used as a template for low duty cycle medium to high pulsed power UHF amplifier system.

A double output pulsed high current thyratron driver

The design and development of a double output pulsed high current driver for high power multi-grid thyratron is reported. The driver generates a 100 A current pulse of 2 μs duration with a compliance voltage of 1000 V for driving grid-1. A voltage pulse of 1200 V and 1.2 μs duration, superimposed with -150 V bias has been generated for driving grid-2. A delay of 1 μs between the two drive pulses is achieved with the use of a simple circuit. The rate of rise of voltage better than 10 kV/μs and jitter of ±3 ns is achieved for grid-2 pulse. This driver module has been successfully used in a 50 kV, CX1575C thyratron switched pulsed power supply to drive a multi-joule transversely excited atmospheric CO(2) laser at 100 Hz. The grid driver module can also be used for driving any high current thyratrons with minor external changes.

Compact high voltage, high peak power, high frequency transformer for converter type modulator applications

The design and development of a compact high voltage, high peak power, high frequency transformer for a converter type modulator of klystron amplifiers is presented. The transformer has been designed to operate at a frequency of 20 kHz and at a flux swing of ±0.6 T. Iron (Fe) based nanocrystalline material has been selected as a core for the construction of the transformer. The transformer employs a specially designed solid Teflon bobbin having 120 kV insulation for winding the high voltage secondary windings. The flux swing of the core has been experimentally found by plotting the hysteresis loop at actual operating conditions. Based on the design, a prototype transformer has been built which is per se a unique combination of high voltage, high frequency, and peak power specifications. The transformer was able to provide 58 kV (pk-pk) at the secondary with a peak power handling capability of 700 kVA. The transformation ratio was 1:17. The performance of the transformer is also presented and discussed.

A high voltage test stand for electron gun qualification for LINACs

An electron gun test stand has been developed at RRCAT. The test stand consists of a high voltage pulsed power supply, electron gun filament supply, grid supply, UHV system and electron gun current measurement system. Several electron guns developed indigenously were evaluated on this test stand. The shielding is provided for the electron gun set up. Electron gun tests can be tested upto 55kV with pulse width of 15 microsecs and pulse repetition rates up to 200 Hz. The technical details of the subsystems are furnished and results of performance of the test stand have been reported in this paper.