Helix traveling wave tube simulation

Summary form only given. One of Litton´s goals is to collect a set of design codes that are sufficiently accurate to reduce the need for costly hardware iterations in the development of helix traveling wave tubes (TWTs). As a first step toward this goal, we are modeling existing TWTs to compare the various available design tools and to verify their accuracy. Where possible, multiple codes with similar capabilities are being examined and compared to measured data. HFSS and CTLSS are three-dimensional, finite-element based codes that solve for RF fields in general structures. They can each be used to calculate the phase velocity and interaction impedance of helix TWT circuits by modeling one turn of the helix and applying the appropriate symmetry conditions at the boundaries. Once the calculations and measurements of the cold circuit parameters are in agreement, the small signal characteristics of the TWT can be calculated. Stand alone small-signal codes have been developed in the past, however, with the advances in computing power, we have focussed on CHRISTINE1D, a one-dimensional, large-signal code developed by Science Applications, Inc. (SAIC) and the Naval Research Laboratory (NRL). This code typically calculates the output power for a single set of input parameters in a few seconds or less, making it an easy code to use for initial design optimization. After agreement is obtained for the small signal gain of the TWT, the large signal response can be investigated. Due to the growth of the beam size in the output circuit, good agreement between measurements and calculations is not expected, in general, with a one-dimensional code. Therefore, we have chosen to investigate CHRISTINE3D, currently a 2 1/2 dimensional, large-signal code developed by SAIC and NRL. We will present calculations and measured data comparing cold circuit parameters as well as small- and large-signal performance.