Analysis of temperature-limited flow, space-charge-limited flow, and the transition between them using a simple planar diode with a thermionic cathode, in which the cathode surface has spatially nonuniform emission properties, is presented. Our theoretical results, which are derived from a model based on solutions to the Vlasov and Poisson equations, compare well with the results of particle-in-cell simulations. We find that the location and the shape of the knee in the anode current versus temperature characteristic (Miram or “rollover” curve) are significantly affected by non-uniformities in the space-charge density in the A–K gap, but are relatively unaffected by the electron motion parallel to the electrode surfaces. In particular, emission from an actively emitting region is strongly affected by the forces (or lack thereof) exerted by the space-charge of the electrons emitted by their neighbors. Perhaps, most remarkably, we find that the limiting current reaching the anode is approximately given by the classical 1-D Child–Langmuir law, even if a significant fraction of the cathode surface is non-emitting.

/pdf/effect-of-nonuniform-emission-on-miram-curves-3lifz5fge5.pdf

Effect of Nonuniform Emission on Miram Curves

Recent advancements made in millimeter-wave helix-traveling wave tubes (TWTs) at L-3 Communications Electron Technologies (L-3 ETI), Inc. (formerly Electron Dynamic Devices, Inc. and originally Hughes Electron Dynamics Division), in continuous-wave (CW) output power capability and the overall efficiency of Ka-band and Q-band devices are presented. The 8921HP, L-3 ETI's latest high-power communications Ka-band TWT model, demonstrates 250-300-W CW output power and 47% minimum overall efficiency with a two-stage collector over 27.5-31 GHz. Another Ka-band device, the 8922HP, developed for pulsed radar applications over 33.4-36 GHz, produces 230-W minimum output power over the bandwidth with 49% minimum overall efficiency. This device was developed for pulsed operations but has demonstrated CW power-handling capability up to 270 W. In Q-band, the 8925HP, derived from the current production 120-W Q-band helix-TWT (8905HP), significantly extends the CW output power capability, demonstrating 230-W minimum over 43.5-45.5 GHz. The beam focusing is improved in both the Ka-band and the Q-band TWT models, with saturated radio-frequency beam interception well below 1% of the nominal beam current of 95 mA. The devices can be operated in pulsed mode by using the focus electrode to cut off the beam. The electron gun typically requires a focus electrode voltage of -800 to -900 V with respect to cathode for beam cutoff.

Development of high-power Ka-band and Q-band helix-TWTs

Boeing EDD continues to make advances in its millimeter-wave helix-TWTs by pushing the CW output power capability and increasing the overall efficiency of Ka-band and Q-band devices for communications. The 8921HP, EDD's latest high power Ka-band TWT model, demonstrates 250 W to 300 W CW output power and 47 % minimum overall efficiency with a two-stage collector over 27.5 GHz-31 GHz. In Q-band, the 8925HP, derived from the current production 120 W Q-Band helix-TWT (8905HP), significantly extends the CW output power capability demonstrating 230 W minimum over 43.5 GHz-45.5 GHz. The beam focusing is improved in both the Ka-band and the Q-band TWT models, with RF beam interception well below 1% of the nominal beam current of 95 mA. The above devices are primarily designed for CW operation but can also be operated in pulsed mode by using the focus electrode to cut-off the beam. The electron gun typically requires a focus electrode voltage of -800 V with respect to the cathode for beam cut-off.

Development of high power Ka-band and Q-band helix-TWTs

This thesis reports on the theory, design, and experimental investigation of a gyrotron travelling-wave-tube (TWT) amplifier at 140 GHz. The gyro-TWT uses the HE06 mode of a confocal geometry as its operating mode. The linear and nonlinear theory of the confocal waveguide is presented, along with a quasi-optical approach for describing the modes of a confocal waveguide. Both the equations of motion and the mode excitation equation are derived in detail. A beamlet code is introduced as a tool for calculating the linear and nonlinear gain of the azimuthally asymmetric confocal modes that interact with the electron beam with an annular distribution of electron guiding centers. This code has been successfully benchmarked against the code MAGY for azimuthally symmetric cases, and extends the capabilities of nonlinear gyroamplifier theory to configurations that lack azimuthal symmetry. The gyro-TWT experimental setup is presented in detail. Quasi-optical mode converters were designed, fabricated, and tested, ultimately achieving a coupling of -3 dB into the HE06 mode. The source of competing, parasitic oscillations was identified and addressed by the addition of dielectric loading attached to the side of the open geometry of the confocal waveguide. The improvements to the gyro-TWT system allowed for zero-drive stable operation at currents up to 3 A. The design frequency of 140.0 GHz was successfully amplified with 35 dB circuit gain and a -3 dB bandwidth of 1.2 GHz from a 48 kV, 3 A electron beam with a beam pitch factor of 0.64 and a perpendicular velocity spread of 6%. The gyro-TWT produced 550 W at 140.0 GHz under the same operating conditions. During experimental testing, the pulse length used was 2 microseconds. The performance of the confocal gyro-TWT suggests that it may be a candidate for application to Dynamic Nuclear Polarization (DNP) Nuclear Magnetic Resonance (NMR) experiments. Thesis Supervisor: Richard J. Temkin Title: Senior Research Scientist, Department of Physics

A 140 GHz gyro-amplifier using a confocal waveguide : theory and experimental results

A fully two-dimensional (2-D) dc space charge model has been implemented in a large-signal traveling-wave amplifier code. The simulation algorithm takes an iterative approach by alternately solving the Poisson equation and the beam trajectory equations to converge toward a self-consistent steady-state solution. This approach is similar to that employed in steady-state gun codes. However, it is well known from gun simulations that the iterative algorithm can be slow to converge. We have found the slow convergence is due to a convective numerical instability. To speed up convergence, we implemented and tested stabilization schemes based on mixing one-dimensional and 2-D Poisson potentials during the iteration cycles. These schemes are shown to accelerate convergence considerably. The fully 2-D dc space-charge model permits accurate treatment of the axial dc space-charge field in the computation of the large signal gain and efficiency, taking into account the fast variation of beam parameters along the device axis. Therefore, it can be applied to a mismatched beam with large scalloping motion. The methodology of incorporating dc space charge is general and could be incorporated in other large signal codes.

Calculation of DC space-charge fields in a traveling-wave amplifier in the large signal regime

The MICHELLE code is a new Finite-Element (FE) two-dimensional (2D) and three-dimensional (3D) electrostatic particle-in-cell (PIC) code that has been designed to address the recent beam optics modeling and simulation requirements for vacuum electron devices, ion sources, and charged-particle transport. Problem classes specifically targeted include depressed collectors, gridded-guns, multi-beam guns and sheet-beam guns. The focus of the development program is to combine modern finite-element techniques with improved physics models. The code employs a conformal mesh, including both structured and unstructured mesh architectures for meshing flexibility, along with a new method for accurate, efficient particle tracking. New particle emission models for thermionic beam representation are included that support primary emission, and secondary emission is handled with an advanced model. This presentation focuses on two significant advances to MICHELLE over the past year; hybrid structured/unstructured mesh support, and a time-domain electrostatic algorithm.

Recent advances in the MICHELLE 2D/3D electron gun and collector modeling code

The focus of the development program is to combine modern finite element techniques with improved physics models. The code employs a conformal mesh, including both structured and unstructured mesh architectures for meshing flexibility, along with a new method for accurate, efficient particle tracking. New particle emission models for thermionic beam representation are included that support primary emission, and secondary emission is handled with an advanced model.

Application of the 2D/3D MICHELLE code to electron gun and collector modeling

Summary form only given Three-dimensional (3D) electron guns, focusing channels, and collectors are the trend to improved device performance, and accurate modeling of these devices leads to reductions in development time and costs This includes gridded guns, multibeam guns, sheet beam guns, quadrupole focusing, multibeam collectors, and asymmetrical collectors The new MICHELLE1 2D/3D steady-state and time-domain particle-in-cell (PIC) code that employs electrostatic and now magnetostatic finite-element field solvers In recent years the code has been employed successfully to design and analyze a wide variety of devices that include the above and also ion thrusters In this presentation we will present how MICHELLE has been used to model a variety of devices Two designs in particular are a MM/SMM wave sheet beam gun and a Ka band gun with quadrupole (strong) focusing The design of a sheet beam electron gun is underway at NRL for application to a MM/SMM wave orotron The gun design parameters are for a beam of 3 mm by 40 mum (aspect ratio of 75:1), and a total current of 50-100 mA The smaller of these dimensions is set by the requirement that the beam thickness be smaller than the transverse scale length of Lperp = gammabetalambda/2pi The preliminary gun design utilizes an immersed flow design with a flat rectangular cathode Further results from this design effort will be presented At mm-wavelengths, a key limitation to the peak and average power of vacuum electronics devices stems from the difficulty of control and confinement of high-current density beams In this paper we summarize an ongoing design study of a strong-focusing lattice employing permanent magnet quadrupoles (PMQs) capable of transporting a sub-mm radius electron beam for vacuum electronics applications

Applications of the michelle 2d/3d electron gun and collector code

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Recent advances in the MICHELLE 2D/3D electron gun and collector modeling code

Application of the 2D/3D MICHELLE code to electron gun and collector modeling

Applications of the michelle 2d/3d electron gun and collector code