Richard Kowalczyk

Nonlinear Thomson scattering: A tutorial

Recent advances in table-top, ultrahigh intensity lasers have led to significant renewed interest in the classic problem of Thomson scattering. An important current application of these scattering processes is the generation of ultrashort-pulse-duration x rays. In this tutorial, the classical theory of nonlinear Thomson scattering of an electron in an intense laser field is presented. It is found that the orbit, and therefore its nonlinear scattering spectra, depends on the amplitude and on the phase at which the electron sees the laser electric field. Novel, simple asymptotic expansions are obtained for the spectrum of radiation that is backscattered from a laser by a counter-propagating (or co-propagating) electron. The solutions are presented in such a way that they explicitly show—at least in the single particle regime—the relative merit of using an intense laser and of an energetic electron beam in x-ray production. The close analogy with free electron laser/synchrotron source is indicated.

Modeling and experimental studies of magnetron injection locking

A phase-locking model has been developed from circuit theory to qualitatively explain the various regimes observed in magnetron injection-locking experiments. The experiments utilize two continuous-wave oven magnetrons: one functions as an oscillator and the other as a driver. The model includes both magnetron-specific electronic conductance and frequency-pulling parameter. Both time and frequency domain solutions are developed from the model, allowing investigations into the growth and saturation as well as the frequency response of the output signal. This simplified model recovers qualitatively many of the phase-locking frequency characteristics observed in the experiments.

A 100 Watt W-Band MPM TWT

L-3 EDD has developed a W-Band TWT with 100 W RF power over 4 GHz of bandwidth around 94 GHz suitable for MPM integration. The TWT employs an aperture grid modulated electron gun with mod-anode current control, a serpentine waveguide interaction circuit, and a single-stage depressed collector. Two TWTs have been built and tested to over 100 W pulsed output power. The first unit has been operated at high duty, producing 65 W CW output power and 75 W average pulsed power with reduced beam current. The TWT is designed for ease of manufacture, and is suitable for MPM integration, relying on conduction cooling and capable of operation to an altitude of 50k feet.

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.

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.

Effects of a finite axial magnetic field on the beam loading of a cavity

The effect of a finite axial magnetic field on the beam loading of a cavity is evaluated. The calculation extends Branchs classic paper on ballistic bunching in that both the conductive and susceptive components of the beam-loaded admittance are computed, for general values of axial magnetic field. Also included is a comparison of the analytic formulation with a two-dimensional particle-in-cell simulation. This paper suggests that the finite axial magnetic field used in linear beam tubes (typically exceeding 1.5 /spl times/ the Brillouin field) would only modify the beam-loaded admittance by about 20%, from that computed under the assumption of an infinite axial magnetic field.

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.

A 100 Watt W-Band MPM

L-3 EDD has developed a pulsed mode WBand microwave power module (MPM) providing 100W RF power over 92 to 96 GHz. The MPM includes a serpentine waveguide PPM-focused TWT and a modular electronic power conditioner (EPC). A breadboard power supply was previously built and successfully tested with a W-Band TWT. The final power conditioner design is completing development for packaging with a TWT as an MPM.