Vishnu Srivastava

Analysis of Sheet Electron Beam Transport Under Uniform Magnetic Field

The transport of sheet electron beam through the drift tube tunnel under uniform magnetic field has been analyzed. The edge curling due to the sharp increase of horizontal component of the space charge field has been explained by a windlike shear model, and an expression for vertical displacement of the beam edge from the top or bottom surface of the beam is derived on the basis of the aforementioned model. The effect of the height fill factor of the beam on the vertical displacement of the beam edge is analyzed analytically and then validated with numerical results by CST Particle Studio and OPERA 3-D software tools by considering some useful beam parameters. The vertical displacement of the beam edge enhances, and the higher portion of the beam is affected when it transports through short periodic vane-loaded interaction structures. For the sheet beam transport through planar vane-loaded structures suitable for subterahertz traveling-wave tube, the sum of the drift tube tunnel height and twice of the vane height should be considered as the effective tunnel height to calculate the required magnetic field in order to keep the vertical displacement maximum up to some desired value so that the beam edges must not strike on the vanes.

Sheet Electron-Beam Transport Analysis Through Closed Short PCM for Vacuum Subterahertz Devices

Closed short periodic cusped magnets consisting of miniature permanent magnets of material NdFeB 35H have been designed for transporting the sheet electron beam suitable for vacuum subterahertz devices. The aspect ratio of the magnetic tunnel is selected as 2 to provide proper focusing force in both horizontal and vertical directions. Numerical analysis performed by CST Particle Studio shows that both the peak value of the magnetic field and the nature of variation of the transverse components of the magnetic field along their respective transverse directions depend on the transverse thickness of the magnets. The peak value of the magnetic field increases by increasing the transverse thickness of the magnets. When the magnetic period, axial thickness, and transverse thickness of the magnets are 3.4, 1.3, and 8 mm, respectively, the simulated peak value of the magnetic field is 0.131 T. Under the tunnel of the aforementioned structure, the sheet electron beam of size 3 mm (width) × 0.15 mm (height), current density 100 A/cm2, and kinetic energy 20 keV transports up to a 60-mm distance without any significant instability through the drift tube of tunnel size 5 mm (width) × 0.3 mm (height). When the magnetic period, axial thickness, and transverse thickness of the magnets are 2.8, 1, and 8 mm, respectively, the simulated peak value of the magnetic field is 0.181 T. Under the tunnel of this structure, the sheet electron beam of size 2 mm (width) × 0.1 mm (height), current density 120 A/cm2 , and kinetic energy 20 keV transports up to a 60-mm distance without any significant instability through the drift tube of tunnel size 3 mm (width) × 0.2 mm (height).

Design and Development of Helix Slow-Wave Structure for Ku-Band TWT

A helix slow-wave structure (SWS) for a high-efficiency (electronic efficiency >; 25%) Ku-band 140-W space traveling-wave tube (TWT) has been designed. The cold circuit parameters of helix SWS, like propagation constant and on-axis interaction impedance, were determined using 3-D electromagnetic field simulators ANSOFT-HFSS and CST-MWS, and those were validated with the experimental results. The in-house developed large-signal code SUNRAY-1D was used to design a complete helix SWS in two sections with sever and taper to achieve desired output power, gain, efficiency, and other linearity parameters like phase shift, AM/PM factor, and I/M components. The simulated RF performance of the Ku-band 140-W helix TWT was validated with the experimental results. A close agreement between the simulated and experimental results has been found.

20.6: Design and development of 100 GHz folded waveguide TWT

In the recent decade there has been a growing interest of the researchers all over the world in the design and development of the high frequency (in terahertz range), moderate power and efficient devices based on MEMS (semiconductor) and vacuum electronics technologies. Development of so-called vacuum microelectronic devices (VMDs) for the mm and sub-mm wave devices surmount the limitations of conventional microwave tubes (MWTs) as well as of solid-state devices. CEERI has started work towards the design and development of Folded wave guide TWT (FWTWT) initially for 100 GHz. This paper would report design and development work done for electron gun and RF structure of 100 GHz FWTWT.

Spectroscopic Diagnostic of Volume Discharge Arrangement of a DBD Source and Comparison With PIC Simulation Code

This paper reports the spectroscopic analysis in volume discharge arrangement of a dielectric barrier discharge source in parallel plate geometry of a width of 2 mm. Helium is used as a working gas. The investigations are carried out using sinusoidal supply for the generation of discharges where two current pulses have been observed with different polarities in one period. The electron plasma density and temperature during the discharge have been estimated using the line-ratio technique from the observed visible neutral helium lines. To validate the results, a commercial particle-in-cell simulation code, OOPIC-Pro, has been used, which confirms filamentary as well as diffused discharges observed in the experiment. This code analysis also validates the estimated electron plasma density and temperature measurements at two different working pressures and at a fixed operating frequency.

Experimental Investigation of Pseudospark generated electron beam

The pseudospark (PS) discharge is, however, more recently recognized as a different type of discharge which is capable of generating electron beams with the highest combined current density and brightness of any known type of electron source. PS discharge is a specific type of gas discharge, which operates on the left-hand side of the hollow cathode analogy to the Paschen curve with axially symmetric parallel electrodes and central holes on the electrodes. The PS discharge generated electron beam has tremendous applications in plasma filled microwave sources where normal material cathode cannot be used. Analysis of the electron beam profile has been carried out experimentally for different applied voltages. The investigation has been done at different axial and radial location inside the drift tube in argon atmosphere. This paper represents experimentally derived axial and radial variation of the beam current inside the plasma filled drift tube of PS discharge based plasma cathode electron (PCE) gun. With the help of current density estimation the focusing and defocusing point of electron beam in axial direction can be analyzed. It has been further confirmed the successful propagation of electron beam in confined manner without any assistance of external magnetic field.

Development of low-pressure high-current plasma cathode electron gun and use of associated techniques

A plasma cathode electron (PCE) gun has capabilities for generating high-current, broad, and focused beams for plasma-assisted microwave sources. A pseudospark-based hollow cathode PCE gun has been designed and developed for microwave generation which is operated in argon atmosphere. An analysis of the electron beam profile inside the drift space at different operating conditions has been carried out. This has been performed at several axial and radial locations inside the drift space which shows coherent phases of beam profiles in radial direction. The focusing and defocusing points in axial direction are also obtained. The beam current at different axial location for different applied voltages has been estimated. The obtained beam current is in close agreement with the beam current estimated by the particle-in-cell simulation code for the same geometry.

DESIGN AND CHARACTERIZATION OF HELIX SLOW WAVE STRUCTURE FOR KU-BAND SPACE TWT

A helix slow-wave structure (SWS) for a high e-ciency Ku-band 140W space TWT has been designed. Cold circuit parameters of helix SWS like propagation constant and on axis interaction impedance were determined using 3-D electromagnetic fleld simulators ANSOFT-HFSS and CST-MWS and validated with the experimental results. In-house developed large-signal code SUNRAY- 1D was used to design the complete helix SWS in 2-section with sever and taper to achieve desired output power, gain, e-ciency, and other linearity parameters such as phase shift, AM/PM factor, I/M components. Simulated RF performance of the Ku-band 140W helix TWT was validated with the experimental results. A close agreement between the simulated and experimental results has been found.

Analysis of Helix Slow-wave Structure for Space TWT using Ansoft HFSS

Analysis of helix slow-wave structure (SWS) for a high efficiency space TWT is carried out using Ansoft HFSS code, which is a 3D electromagnetic field simulator. Two approaches of computing the dispersion characteristics and impedance characteristics of the helix SWS have been discussed and compared with analytical method

Analysis of discharge parameters and optimization study of coaxial DBDs for efficient excimer light sources

In this work, a xenon-filled quartz coaxial dielectric barrier discharge (DBD) tube (ID 6 mm, OD 12 mm) at 400-mbar pressure has been studied at different operating conditions. High-frequency sinusoidal and unipolar pulse-like voltages are applied at the discharge electrodes for the generation of micro-discharge plasma. Visual images of the discharge and the electrical waveform confirm the diffused-type discharge. The mechanism that is involved in the ignition, development and extinction of DBDs is quantitatively explained by dynamic processes in the discharge. An equivalent electrical model representing the DBD phenomenon has also been used to validate the characteristic discharge parameters. The relative intensity analysis of the Xe continuum peak at wavelength 172 nm in the optical emission spectra of the vacuum ultraviolet region has been carried out for different operating conditions. Approximately three times increment in the radiation is observed in pulse excitation over sinusoidal excitation. It infers that the pulsed excitation of DBD sources is advantageous for excimer light sources.