The session opened with a keynote paper presented by Dr. Baruch Levush from the Naval Research Laboratory, Washington DC. Dr. Levush reviewed the results obtained using codes which had been developed under a program supported by the Office for Naval Research. He showed examples of comparisons between measured and simulated results for cold and hot tests and for automatic optimization of tube parameters. The agreement between the computed and measured results was generally very good. Dr. Levush went on to point out the uncertainties in the data input to these codes and to show that small changes to the input data can have big effects on the results. For example in a helix TWT: a 10% error in the relative permittivity of the support rods produces a 1.5 dB change in the gain, manufacturing tolerances of 3% lead to 0.5 dB change in gain, and uncertainty about the profile of the attenuator can also lead to uncertainties in the gain of several dB. His conclusion was that reliable computer simulation requires accurate data and focussed validation of the codes being used.

The four other papers delivered in this session reported advances in some of the codes whose development is being sponsored by the Office for Naval Research. Dr. Henry Freund described GATOR, a non-linear, time domain, analysis of coupled-cavity TWTs, which is able to model the initial transient signals in a tube. He showed excellent agreement with measured results for the dependence of output power on input power and also the gain ripples resulting from changes in the termination matches caused by the electron beam. It was expected that the model would be useful for studying tube stability and operation within the cold stop-band of the slow-wave structure. Dr David Chernin described the use of the CHRISTINE code to investigate regenerative, backward wave and band edge instabilities in helix TWTs. New developments in space-charge modelling in CHRISTINE were described by Dr Simon Cooke. Most current models of TWTs assume that the space-charge fields are bounded by a conducting tube at the radius of the helix. The errors in the space-charge forces arising from this assumption have been examined and an improved model was described. Finally Dr. Alexander Vlasov showed how the code MAGY, which had originally been developed to model gyro-devices, was being adapted to model other types of tubes with cylindrical symmetry. Promising results were presented for a Ka-band klystron.