Craig Bisset Wilsen

A simulation study of beam loading on a cavity

Beam loading exerts a strong influence on the operation of high-power and medium-power microwave sources. This paper reports a simulation study of beam loading on a cavity using the two-dimensional particle-in-cell code, MAGIC. We vary the beam voltage, the beam current, the degree of current modulation on the dc beam before the beam enters the cavity, and the degree of charge neutralization on the beam. We deduce the beam-loaded quality factor Q and the beam-loaded resonant frequency from a Lorentzian fit of the numerical data on the gap voltage response as a function of the driving frequency. The MAGIC simulations have revealed several unanticipated results. The beam loading is observed to be a function of perveance. Constant perveance beams, of varying voltage and current, exercise about the same degree of beam loading on the model klystron cavity (except, of course, for the cases with very small beam current). The inclusion of an ac component on the dc beam current has no effect on the degree of beam loading; neither does the neutralization of the electron beam. Many of these simulation results cannot be explained by existing theories that ignore ac space charge effects.

Ab initio investigation of the surface properties of dispenser B-type and scandate thermionic emission cathodes

Scandate cathodes (BaxScyOz on W) are important thermionic electron emission materials whose emission mechanism remains unclear. Ab initio modeling is used to investigate the surface properties of both scandate and traditional B-type (Ba–O on W) cathodes. We demonstrate that the Ba–O dipole surface structure believed to be present in active B-type cathodes is not thermodynamically stable, suggesting that a nonequilibrium steady state dominates the active cathode’s surface structure. We identify a stable, low work function BaxScyOz surface structure, which may be responsible for some scandate cathode properties and demonstrate that multicomponent surface coatings can lower cathode work functions.Scandate cathodes (BaxScyOz on W) are important thermionic electron emission materials whose emission mechanism remains unclear. Ab initio modeling is used to investigate the surface properties of both scandate and traditional B-type (Ba–O on W) cathodes. We demonstrate that the Ba–O dipole surface structure believed to be present in active B-type cathodes is not thermodynamically stable, suggesting that a nonequilibrium steady state dominates the active cathode’s surface structure. We identify a stable, low work function BaxScyOz surface structure, which may be responsible for some scandate cathode properties and demonstrate that multicomponent surface coatings can lower cathode work functions.

AC space charge effects on beam loading of a cavity

A calculation of the loading of a resonant cavity by an electron beam is presented. This calculation is the first to include all effects of ac space-charge. The fields in the cavity are determined in the presence of the beam, including the effects of higher order structure modes. The formulation also explicitly includes Ramos space-charge waves of all ranks. The result reduces to the conventional ballistic beam loading result for low beam current. Comparison of the complete theory developed here with the ballistic result demonstrates that the ballistic theory is sufficiently accurate, typically accounting for the quality factor to within 5%, and the resonant frequency to within 1%. In addition, the theory is shown to agree well with particle-in-cell simulations.

A note on current modulation from nonlinear electron orbits

Current modulation on an electron beam, including the effects of charge overtaking, harmonics, and intermodulation products, may be calculated exactly in a one-dimensional geometry once the electron orbit is given. This paper provides a simple proof together with a numerical example in a model that is commonly used in the klystron literature.

Input Circuit for a Wideband IOT

Emission gated microwave amplifiers, such as the inductive output tube (IOT), use density modulation to establish an AC current on the electron beam directly at the cathode surface. These amplifiers are compact and highly efficient, even in the linear regime. L-3 Electron Devices Division is developing a wide instantaneous bandwidth IOT (WBIOT) which will provide output power levels suitable for UHF radar applications with a 1 dB bandwidth in excess of ten percent. In this paper we describe the input circuit of the WBIOT.

High power X-band klystron

L-3 Communications Electron Devices Division (EDD) is developing a 9.3 GHz, 5 MW peak, 20 kW average power klystron suitable for use in linear accelerator and radar applications. Generating RF power of this magnitude within the confines of a relatively small sized X-band RF circuit is a significant challenge. A discussion of the design methodology and simulation tools used to accomplish this task will be presented, as will hot test data from the first prototype.

Development of a wideband inductive output tube

Inductive output tubes (IOTs) employ emission gating to modulate the electron beam directly at the cathode surface. The high efficiency of these compact devices has motivated their use in UHF television broadcast and particle accelerators. The instantaneous bandwidth of broadcast IOTs is two percent (6 MHz). Many radar and communications applications - in which the linearity and efficiency of the IOT are highly desirable - require a significantly larger bandwidth. L-3 Electron Devices Division (EDD) is therefore developing a wideband IOT (WBIOT) which will provide output power levels suitable for UHF radar with a 1 dB bandwidth greater than ten percent.

The wideband inductive output tube

L-3 Electron Devices Division (EDD) is developing a wideband IOT (WBIOT) with a 1 dB bandwidth greater than ten percent. High efficiency, good linearity, and compact size are provided by emission gated modulation of the electron beam directly at the cathode surface. Through use of a high impedance input circuit and an extended interaction output circuit, the instantaneous bandwidth of existing IOTs is increased by a factor of five, making the WBIOT suitable for use in a broad range of radar and communications applications.

IOT Development at Electron Devices

The inductive output tube, or IOT, has long been established as the amplifier of choice for UHF television broadcast. IOTs are also employed in an increasing number of accelerator applications. For example, L-3 has recently provided several IOTs, optimized for accelerator use, to the Thomas Jefferson National Accelerator Facility. Many of these accelerator projects have power and frequency requirements beyond the conventional IOT parameter regime. In particular, there have been a number of accelerator designs proposed utilizing the superconducting 1.3 GHz cavity developed at DESY

12.1: A high power wideband inductive output tube

L-3 Communications, Electron Devices Division (EDD) has developed an inductive output tube (IOT) with large instantaneous bandwidth for radar and communications applications. As in conventional IOTs, this wideband IOT (WBIOT) provides high efficiency, good linearity, and compact size through emission-gated modulation of the electron beam at the cathode surface. The extended interaction output circuit can be configured to meet required gain and bandwidth profiles.