Session chairperson and summary author: Carol Kory
The first session at IVEC 2002 included several interesting papers demonstrating the ever-growing power and accuracy of modern computational codes in the area of vacuum electronics. The session began with a keynote talk by C. L. Chang from SAIC in collaboration with CPI and NRL, which described a validation of the suite of TWT design codes developed under ONR-NRL using a 750 W, Ku-Band, communications TWT manufactured by CPI as a model. The suite of codes includes Michelle - a 3D gun and collector code; CTLSS - a 3D cold test code; CHRISTINE - 1D and 3D large signal codes; and mPPM/lesPPM – a magnet design code. Simulation results were presented, and the challenge of developing a proper interface between codes for accurate computations was discussed.
C. T. Chevalier from Analex Corp. in collaboration with NASA Glenn Research Center, described a comparison of simulated cold test characteristics using two computer codes from Computer Simulation Technology (CST), MAFIA and Microwave Studio (MWS). The dispersion, interaction impedance and attenuation were compared for three TWT slow-wave circuits: a coupled-cavity circuit, a novel finned-ladder circuit and a folded waveguide circuit. It was shown that MWS provided increased accuracy in dispersion compared to experiment over MAFIA, with an average decrease in computation time by a factor of four.
R. Harper from Triton Services, Electron Technology Division spoke of work being done with NRL on the optimization of a dynamic velocity taper for a wide-band TWT using CHRISTINE1D. The optimization tool uses a modified steepest descent method to search for increased efficiency as TWT parameters are varied. For this application, the helical pitch was varied using the optimizer, resulting in an increased predicted efficiency by a factor of 1.5 across a three-to-one bandwidth.
D. Smithe from Mission Research Corporation presented physics studies with Northop Grumman Electron Devices on hollow-beam helix TWTs using the 3D particle-in-cell (PIC) code, MAGIC3D. Traditional dispersion and small signal gain studies were discussed to provide initial benchmarking and confidence in the model.
To conclude the session, C. B. Wilsen (Northop Grumman Electron Devices) presented a paper in association with SAIC, University of Maryland and Mission Research Corporation on a simulation study of beam loading in a cavity using the 2D version of the PIC code, MAGIC. The concept of beam loading is important in high and medium power microwave sources where the intense electron beam alters resonant frequency, quality factor and gap transit time factor of cavity structures. Wilsen et al. observed that the beam loading is a function of perveance. Simulations were compared to two theoretical approaches.