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IVEC Summary, Session 12, Collectors 
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Session chairperson and summary author: David Whaley

Accurate modeling and measurement of collector electron dynamics are important for efficient device design. This session contained four papers focusing on modeling and experimental validation of collector design tools applied to TWT collectors, and electron backscatter models in both a high power gyro device and an X-ray tube. The first paper, presented by Dr. Thouvenin of Thales Electron Devices, described collector simulation using a proprietary three-dimensional time dependent computer code COLLECT3D. This code allows for electron bunching, true secondaries, backscattered primaries and rereflected primaries. The case of a truly three dimensional problem was shown which included a non-axisymmetric magnetic field to reduce backstreaming. The amazing result was given that run times of only 5 minutes are required even with problems allowing for multiple generations of secondary electrons.

The second paper on 3D collector design using MICHELLE, a newly-developed NRL/SAIC electron gun and collector design tool, was given by Dr. Zhai of Boeing Electron Dynamic Devices. This paper represents the first use of MICHELLE for practical TWT collector design. The design process was described including the CAD interface and 3D magnetic field import. Useful information regarding setup and run times was given showing that fully relevant 3D problems are tractable with desktop UNIX or PC platforms. Memory requirements of 500 MB and run times of 30 minutes were shown to be typical. Comparison of predicted current distributions to experimental data was also given showing good correlation.

Dr. Valfells of The Institute for Research in Electronics and Applied Physics of the University of Maryland presented the model for backscattered secondary electrons and its implementation in depressed collector design. The model uses a Monte Carlo and ray coalescing approach to accurately treat both emission coefficients and angular distribution of the emitted electrons while limiting the number of traced rays. This model was implemented for a 100 GHz 1.5 MW gyro device and includes calculation of collector heat dissipation profiles which are important in such high power devices.

The final paper of the session was presented by Dr. Salasoo of the GE Global Research Center. This paper described the modeling and measurement of backscattered electrons from the target of a 100 keV 0.250 A X-ray tube. A Monte Carlo methodology is used to model the backscatter process and predict backscatter distributions. A 2D segmented collector was built and tested and data was presented which shows good general correlation of the backscattered distribution. Fine scale correlation showed discrepancy between experiment and prediction and will be investigated further. After the initial target impact, as much as 40 percent of the original current can be backscattered and collected on surrounding interior surfaces of the tube. This predictive modeling is therefore important in target-collector design of X-ray tubes at these high beam powers.

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