K.C. Mittal

Development of a 300-kV Marx generator and its application to drive a relativistic electron beam

We have indigenously developed a twenty-stage vertical structure type Marx generator. At a matched load of 90–100 Ω, for 25 kV DC charging, an output voltage pulse of 230 kV, and duration 150 ns is obtained. This voltage pulse is applied to a relativistic electron beam (REB) planar diode. For a cathodeanode gap of 7.5 mm, an REB having beam voltage 160kV and duration 150ns is obtained. Brass as well as aluminum explosive electron emission-type cathodes have been used

A high power UWB system with subnanosecond rise time using balanced TEM horn antenna

High power ultra wideband (UWB) systems can be used for various applications like buried target detection, electrical characterization of materials, space debris detection and as a source for vulnerability studies on electronic circuits. A high power UWB radiator system is developed at BARC, Mumbai. This is a 120 kV, 66 Ohm system which can generate intense electromagnetic fields. Any UWB system consists of pulse power system and antenna. Pulse power system for this radiator consists of a Marx Generator and a pulse forming line which can generate a 110 kV, 5 ns pulse with a rise time of ~ 400 picoseconds. A balanced TEM horn type antenna was used in this system. Impedance of this antenna is matched to feed i.e. 66 Ohms at feed end and then gradually varies to match with free space impedance. This antenna can effectively radiate this kind of sub-nanosecond rise time pulse. Designed antenna has an acceptable performance in terms of VSWR in desired frequency range of 150 MHz to 1 GHz depending on pulse parameters. This paper discusses the overall system performance with an emphasis on antenna design details and results.

Design, fabrication, RF test at 2 K of 1050MHz, β=0.49 single cell large and fine grain niobium cavity

BARC is developing a technology for the accelerator driven subcritical system (ADSS) that will be mainly utilized for the transmutation of nuclear waste and enrichment of U233. Design and prototyping of a superconducting medium velocity cavity has been taken up as a part of the ADSS project. The cavity design for β = 0.49, f = 1050 MHz has been optimized to minimize the peak electric and magnetic fields, with a goal of 5 MV/m of accelerating gradient at a Q > 5 × 109 at 2 K. After the design optimization, two single cell cavities were fabricated from polycrystalline (RRR > 200) and large grain (RRR > 96) Niobium material. The cavities have been tested at 2 K in a vertical cryostat at Jefferson Lab and both achieved the performance specifications.

High power microwave generation by relativistic backward wave oscillator

In this paper, a high power relativistic backward wave oscillator (BWO) experiment is reported. A 230 kV, 2 kA, and 150 ns relativistic electron beam source is developed using a Marx generator. The beam is then injected into a hollow rippled wall metallic cylindrical tube that forms a slow wave structure (SWS). A BWO is a slow wave structure in the form of a rippled wall wave-guide and is used as a source of high power microwave generation when excited by a relativistic electron beam (REB). The beam is guided using an axial pulsed magnetic field having, the duration 1 ms and strength being 1.0 Tesla, respectively. This field is generated by discharge of a capacitor bank into a solenoidal coil. A synchronization circuit ensures the generation of the electron beam at the instant when the axial magnetic field attains its peak value. The electron beam interacts with the SWS waveguide modes and generates microwaves due to Cherenkov interaction. Estimated power of ~2 MW in TM01 mode is observed.

Sub-nanosecond pulse generator and electron beam source for nToF application

This paper describes a repetitive pulsed power source capable of giving 50-100 kV, 2 ns, 10 Hz pulsed output with sub-nanosecond rise time. The main sub-systems of this pulse generator are air core based, epoxy insulated, pulse transformer, high pressure gas filled pulse forming line (PFL), fast spark gap switch and inductively isolated trigger generator. This system is coupled with an explosive field emission cathode. The generated beam pulses are collected by a suitably designed faraday cup with a proper impedance matching. The measured electron beam pulses are in the range of 30-100 A, 2 ns, 10 Hz. The comparative study has been done for the graphite cathode and carbon fiber cathode in this setup in repetitive operation. This source will be integrated with 1 MeV buncher cavity followed by 30 MeV LINAC for neutron time of Flight studies. Therefore consistency of electron beam generation is critical in nanosecond time domain as well as diagnostic of diode voltage and beam current also.

Absolute instability enhancement for a plasma filled rippled wall rectangular waveguide backward wave oscillator driven by sheet electron beam

Summary form only given. A linear theory is presented for generation of high power microwaves (HPM) through backward wave oscillator (BWO) driven by a sheet electron beam in plasma filled rippled wall rectangular waveguide as a slow wave structure. By matching the boundary conditions at the junction of beam edge, plasma, and metallic boundary the dispersion relation and the growth rate (temporal and spatial) have been derived numerically. While analysis, we have used sheet beam of 6 kA at 250 kV with transverse cross section of about 0.5 cm by 2 cm and relativistic factor gamma /spl gamma/=1.5 this beam is currently operational at D.A. University Indore. To see the effect of plasma on the TM/sub 01/ cold wave structure mode and on the generated frequency, the parameters used are average waveguide height a=1.5 cm axial corrugation period z/sub g/=1.67 cm and corrugation amplitude h=0.225 cm. The plasma density is varied from 0.0-8/spl times/10/sup 19/ cm/sup -3/. The presence of plasma tends to raise the TM/sub 01/ mode cutoff frequency (10 GHz at plasma density 8/spl times/10/sup 15/ cm/sup -3/) relative to the vacuum cutoff frequency (5 GHz) while also causing a decrease in the group velocity everywhere, resulting in a flattening of the dispersion relation. With the introduction of plasma an enhancement in spatial and temporal growth was observed.

Dispersion relation of a sheet beam driven backward wave oscillator

Summary form only given, as follows. Backward wave oscillators (BWOs) driven by high current relativistic electron beams produce high power coherent radiations in a wide frequency range. The device consists of a periodic metallic structure into which a high current relativistic electron beam is injected. The slow wave structure reduces the phase velocity of electromagnetic waves below the vacuum speed of light so that Cerenkov synchronism is satisfied. This allows the electrons to give up energy to one of the eigenmodes of the slow wave structure. High power radiation output necessitates high beam power. To obtain this capability without invoking high current density, we propose the use of a sheet or a ribbon electron beam. Such beams are especially suited to stable and well focused transport. We consider a modified version of a relativistic backward wave oscillator whose walls are the two parallel surface of a rectangular waveguide. A sheet beam having thickness t and width w is being injected through the rippled wall waveguide. The dispersion relation of such a configuration is obtained and stability of the system is discussed.