Multiple Beam Devices Research Articles

Advances in the Theory and Modeling of Linear Beam VE Amplifiers

We present an overview of the progress made by our Naval Research Laboratory (NRL)-based group over the last 25 years in the development of computational tools used in the large-signal modeling and design of linear-beam vacuum electronic (VE) amplifiers, klystrons, and traveling-wave tubes (TWTs). These tools belong to a class of codes that we call parametric beam–wave interaction (PBWI) codes, which includes family of the codes CHRISTINE and TESLA. Notable progress includes the formulation and application of an impedance matrix description of a general class of slow wave circuits, the tracking of slow or reversed particles in klystrons, parallel modeling of multiple-beam devices, linear stability analysis based on Nyquist theory, and geometrical and electrical tolerance analysis and optimization based on solutions to the adjoint equations that govern the beam–wave interaction. The impedance matrix description, in particular, has allowed the detailed effects of circuit geometry, including complex terminations, matching sections, and severs, to be included in PBWI simulations for the first time.

Analysis of the Resonator Part of a Ka-Band Multiple-Beam Extended-Interaction Oscillator through Electric Field Uniformity

The development of multiple-beam devices is required due to the increasing demand of compact, high-frequency, and high-power vacuum devices. A Ka-band multiple-beam extended-interaction oscillator which operates in TM01 mode with a large diameter (as the value is 14.6 mm which is larger than the operating wavelength of 8 mm) to obtain high output power has been put forward. In previous studies, the performance differences of single-beam extended-interaction oscillator with different electric field uniformity can be as high as 70%. Simulation results predicted the multiple-beam device has an average output power of 7.594 kW when a total beam of 3 A, 18 kV and the uniformity parameter is 0.064. Meanwhile, it predicted that the difference of output power of multiple-beam devices with different field uniformity (corresponding uniformity parameter is within 0.064~0.278) is within 2.53% when other operating conditions are unchanged. The results show that the multiple-beam device substantially decreases the influence of the field uniformity, which is an important factor for the performance in the single-beam device. A cold test experiment has been carried out based on perturbation theory to obtain the electric field distribution curves of this device, and this provides a method for studying multiple-beam devices.

Quasi-3-D Modeling of Higher Order Mode Instability in a Two-Gap Input Cavity of Broadband MBK

We present the results of our comprehensive study of the stability of a two-gap input cavity used in an experimental broadband multiple-beam klystron (MBK) with 18 beams and 7 cavities, which was designed for projected >500 kW maximum output power and ~13% bandwidth (so-called NRL MBK-3 design). The device was experimentally observed to be prone to self-excitation at all beam voltages below its designed operational value of ~42 kV. Postexperiment examination of the two-gap input cavity revealed traces of breakdown on its face. Detailed simulations performed using a 3-D computational electromagnetic code HFSS have shown presence of a few high-Q higher order modes (HOMs), which could explain self-excitation of the two-gap input cavity in a wide range of beam voltages. To explore HOM instability of the two-gap input cavity of the MBK in greater details, we applied the code TESLA-MB, which represents a parallel extension of the 2-D large-signal klystron code TESLA and which is suitable for an accurate modeling of multiple-beam devices with unique distributions of R/Qs over gaps in all its beam-tunnels.

Design of Doubly Convergent Multiple-Beam Electron Guns

In an effort to achieve higher power RF source performance, designers are utilizing distributed beam devices, such as sheet beams and multiple beams. A limitation is the amount of current that can be emitted by the cathode while still achieving long cathode lifetimes. The desire is to develop distributed beam devices that utilize fundamental mode cavities in the RF circuit. For multiple-beam devices, where the individual beams propagate at the same radius as the cathode, a limitation is reached, where the size of the cathode becomes limited by the space available. A solution is to place the cathodes at a larger radius and compress the beams toward the radius required for fundamental mode cavities. This paper describes the design of a multiple-beam gun where the ensemble of beams is compressed toward the device axis while still achieving parallel propagation through the RF circuit.

Wave coupling in sheet- and multiple-beam traveling-wave tubes

To increase the power level of the sources of coherent electromagnetic radiation at frequencies from 100 GHz up to the terahertz range it makes sense to develop devices with a spatially extended interaction space. Sheet-beam and multiple-beam devices belong to the category. In the present paper the small-signal theory of traveling-wave tubes with sheet-beam and multiple sheet-beam configurations is developed. It is shown that in such tubes the wave coupling on electron beams may occur even in small-signal regimes. The wave coupling and its role for amplification of forward and excitation of backward waves in such amplifiers is studied. Also the effect of transverse nonuniformity of the electromagnetic field on the device operation is analyzed and illustrated by several examples.

Experimental Study of Phase Pushing in a Fundamental-Mode Multiple-Beam Klystron

The authors present the results of experimental measurements of the radio frequency phase as a function of cathode voltage for an eight-beam four-cavity multiple-beam klystron (MBK) operated at a driven frequency of 3.25 GHz. The phase-pushing factor was measured in both the small- and large-signal regimes of amplifier operation and was found to be 0.0134deg/V and 0.0148deg/V, respectively. The experiment was also modeled with a simple analytic function and with telegraphist's equations solution for linear beam amplifiers, a multiple-beam 2.5-D nonlinear klystron code, with both methods yielding good agreement with the measured data. The low values for the phase-pushing factor are a benefit of the MBK's high-perveance operation that results in a shorter circuit length relative to single-beam devices of comparable power. These advantages contribute to the growing interest in the use of multiple-beam devices for high-power phase-sensitive applications such as advanced radar and high-data-rate digital communications

Three-dimensional simulation of laser-produced plasma for extreme ultraviolet lithography applications

Laser-produced plasma (LPP) from a tin target is being considered as the light source for the next generation of extreme ultraviolet (EUV) lithography. An integrated model was developed to simulate the plasma behavior and the EUV radiation output in LPP devices. The model includes plasma heat conduction and hydrodynamic processes in a two-temperature approximation, as well as detailed photon radiation transport using Monte Carlo methods. Multiple laser beams incident on a single target have been simulated in full three-dimensional geometry, using the total variation-diminishing scheme for the plasma hydrodynamics and an implicit scheme for heat conduction processes. Numerical simulations showed that EUV conversion efficiency increases for multiple-beam devices with specific optimum laser locations and direction compared to a single-beam device.