Baruch Levush

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

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 (Childs 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.

A gyrotron-traveling-wave tube amplifier experiment with a ceramic loaded interaction region

The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

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

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.

A quarter century of gyrotron research and development

The development of small-orbit gyrotrons operating at voltages /spl les/100 kV is reviewed. Gyrotron oscillators have been developed to produce unprecedented 200-kW average power levels at frequencies spanning the range of 28-140 GHz with current work aimed at achieving 1-MW average power. They are widely used in plasma-heating studies and are the natural choice for material processing in the millimeter-wave region. Gyrotron amplifiers have exceeded the peak power limits of more conventional amplifiers at both 35 and 94 GHz, and have been used in a few radars. Gyro-amplifiers under development have been designed to surpass both the peak power and the average power limits of conventional amplifiers, and are anticipated to be widely accepted in millimeter-wave radar systems. Gyrotron amplifiers operating at voltages /spl sim/0.5 MV that are being evaluated for accelerator applications were reviewed in this journal in 1996 and are not included in this review paper,.

Advances in modeling and simulation of vacuum electronic devices

Recent advances in the modeling and simulation of vacuum electronic devices are reviewed. Design of these devices makes use of a variety of physical models and numerical code types. Progress in the development of these models and codes is outlined and illustrated with specific examples. The state of the art in device simulation is evolving to the point such that devices can be designed on the computer, thereby eliminating many trial and error fabrication and test steps. The role of numerical simulation in the design places can be expected to grow further in the future.

A TE/sub 11/ K/sub a/-band gyro-TWT amplifier with high-average power compatible distributed loss

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain /spl sim/60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in K/sub a/-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE/sub 11/ mode and for stabilization employs a high-average power compatible diffractive loading technique.

Vacuum tube amplifiers

This article gives a brief overview of the common vacuum electronic tube amplifiers used in high-power transmitters. Only three types of devices [travelling wave tubes (TWTs) including helix and coupled-cavity types, microwave power modules (MPMs), and klystrons] are covered, with emphasis on the recent advance in the millimeter-wave band.

Intense Sheet Electron Beam Transport in a Uniform Solenoidal Magnetic Field

In this paper, the transport of intense sheet electron beams in a uniform solenoidal magnetic field in high-power vacuum electronic devices is theoretically examined with the 3-D beam optics code MICHELLE. It is shown that a solenoidal magnetic field can be an effective transport mechanism for sheet electron beams, provided the beam tunnel is matched to the beam shape, and vice versa. The advantage of solenoidal magnetic field transport relative to periodic magnetic transport resides in the feasibility of transporting higher current density beams due to the higher average field strength achievable in practice and the lower susceptibility to field errors from mechanical misalignments. In addition, a solenoidally transported electron beam is not susceptible to voltage cutoff as in a periodic magnetic focusing system; hence, device efficiency is potentially higher.

Experimental investigation of W-band (93 GHz) gyroklystron amplifiers

The results from W-band gyroklystron amplifier experiments are presented. Two circuit configurations, WGKL1 and WGKL2, have been demonstrated. The WGKL1 circuit achieved 67 kW peak output power, corresponding to 28% efficiency, for a 55-kV, 4.3-A electron beam. The full width half maximum (FWHM) bandwidth was measured to be 460 MHz, a significant increase over the bandwidth demonstrated in previous W-band gyroklystron experiments. The amplifier was unconditionally stable at the operating point. The WGKL2 circuit, which was designed to have a broader bandwidth, produced 60 kW peak output power at 25% efficiency with a 640 MHz FWHM bandwidth for a 58 kV, 4.2 A electron beam. The results from both experiments are compared with theory and good agreement is obtained.

Simulation of microwave devices with external cavities using MAGY

A self-consistent large-signal beam-field interaction model for vacuum electronic microwave sources with external cavities is described. The model includes a self-consistent solution of the three-dimensional equations of electron motion and the time-dependent field equations. The RF fields are decomposed into the fields inside the beam region and the fields inside outer resonators. The RF fields inside the beam region are represented as a superposition of local waveguide modes. The RF fields inside resonators are represented as a sum over resonator modes. The various modes are coupled together due to gaps connecting cavities with each other and with the beam region. The numerical implementation of the model requires additional analytical steps to obtain an effective, convergent, and stable numerical solution. The modified version of the code MAGY has been tested by a comparison with known results and also with measured data.