The development of a new three-dimensional electron gun and collector design tool is reported. This new simulation code has been designed to address the shortcomings of current beam optics simulation and modeling tools used for vacuum electron devices, ion sources, and charged-particle transport. The design tool specifically targets problem classes including gridded-guns, sheet-beam guns, multibeam devices, and anisotropic collectors, with a focus on improved physics models. The code includes both structured and unstructured grid systems for meshing flexibility. A new method for accurate particle tracking through the mesh is discussed. In the area of particle emission, new models for thermionic beam representation are included that support primary emission and secondary emission. Also discussed are new methods for temperature-limited and space-charge-limited (Child's law) emission, including the Longo-Vaughn formulation. A new secondary emission model is presented that captures true secondaries and the full range rediffused electrons. A description of the MICHELLE code is presented.

The MICHELLE three-dimensional electron gun and collector modeling tool: theory and design

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the ...

100 years of the physics of diodes

Recent developments to the MICHELLE electron gun and collector design tool are reported in this paper. The MICHELLE code is a new finite-element (FE) two-dimensional and three-dimensional electrostatic particle-in-cell 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, multibeam guns, sheet-beam guns, and ion thrusters. The focus of the development program is to combine modern FE 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, with an advanced secondary emission model. This paper reports on three significant advances to MICHELLE over the past year; hybrid structured/unstructured mesh support, a time-domain electrostatic algorithm, and an ion plasma model with charge exchange.

Recent developments to the MICHELLE 2-D/3-D electron gun and collector modeling code

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ${\sim}{17\%}$ has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: 1) tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and 2) the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

The development of microwave-tube simulator suite (MTSS) is reported. MTSS is an integrated full-featured software package for microwave-tube analysis and design. It includes a friendly user-interface module and three physics simulators. Microwave-tube design environment is the user-interface module, which provides a powerful solid-modeling front end and graphical and computational postprocessing functionality. Electron optics simulator (EOS) is a fully 3-D finite-element design tool for microwave-tube electron guns and collectors. An EOS is used to design gridded guns, multibeam guns, and anisotropic collectors. A high-frequency circuit simulator is an advanced 3-D electromagnetic (EM) simulation tool for RF components, which has two solution methods based on a finite-integration technique and a finite-element method. Beam-wave interaction simulator (BWIS) is a design tool for electron beam and EM-wave interaction in microwave tubes. The 1-D, 2-D, and 3-D multifrequency nonlinear beam-wave interactions for helix traveling-wave tubes have been implemented in BWIS. The architecture, features, theory, and design of MTSS are presented.

Theory and Design of Microwave-Tube Simulator Suite

We develop a methodology for the design of multiple-cavity klystron interaction circuits. We demonstrate our approach with the detailed design of a collector and a four-cavity circuit for a multiple-beam klystron (MBK) operating in the fundamental mode at a center frequency of 3.27 GHz (S-band). These elements are designed to be used with a 32-A 45-kV magnetically shielded eight-beam electron gun currently under fabrication . Upon integration of the gun, circuit, and collector, the MBK will be used for beam transport and beam-wave interaction studies and to validate developmental design codes and design methodologies. The device has a predicted gain of 33 dB at a peak pulsed output power of 750 kW with a corresponding electronic efficiency of 52%. For the present design, broad bandwidth is not a design objective, and the 3-dB bandwidth is 2.5%. Downstream of the output cavity, the magnetic field profile and the interior surface profile of the collector are carefully shaped to minimize the space-charge potential depression at the entrance to the collector, minimizing reflected electrons. The maximum calculated instantaneous power density on the walls of the collector is approximately 55 kW/cm/sup 2/; at low duty cycles (<1.8%), the average power density is well within the limits for liquid cooling for pulse lengths up to 1.3 ms.

High-power four-cavity S-band multiple-beam klystron design

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High-power four-cavity S-band multiple-beam klystron design