Summary form only given. The Cold Test and Large Signal Simulation (CTLSS) eigenmode solver, developed by SAIC, has been used to model the dispersion and impedance characteristics of a Northrop Grumman X-Band helix TWT. Helix geometry complicates the solution for solvers of this type due to the fact that the objects being modeled, circular backwall and helix as well as rectangular dielectric rods, do not easily lend themselves to accurate representation by any one coordinate system. This is further complicated by the fact that helix geometry is axially periodic and therefore solution times can become quite long since an entire pitch period must be modeled to allow for the correct periodicity of the helical system. Measured dispersion and impedance data from a Northrop Grumman X-Band TWT is being used to benchmark the results generated by CTLSS for helix geometry. A GUI is included with the CTLSS distribution which generates a CTLSS input file for any type of geometry desired. Several templates are available for common structures including helix TWTs, coupled-cavity TWTs, and klystron cavities. The GUI allows one to easily modify the size and shape of individual components as well as set or modify numerical parameters necessary for solution. A visualization toolkit is also provided with the CTLSS distribution which allows one to view the resulting CTLSS structure. This allows for verification of the physical layout of the structure being modeled. A separate option displays the computed eigenmode fields in 3D contour format. Comments on use of both the CTLSS GUI and visualization toolkit will be presented. CTLSS can be run on both UNIX and PC platforms. Both platforms have been used for this study and will be compared. Information on setup times will be included as well as solution times. Proper gridding is always important when setting up problems of this type. Many different approaches can be used to determine the optimum grid features to allow for the most accurate representation of the physical structures involved. A discussion of some of these approaches as well as the features of the helical system which most strongly affect the computed results will be given. Interaction impedance is an important input parameter for modeling of the beam/RF interaction of any helix TWT and determines the coupling of the individual electron macroparticles, rings, or discs to the RF wave traveling on the surrounding helix. Once the eigenmodes have been computed, this interaction impedance can be determined by integrating the axial electric field along the axis of the helical system. This quantity has been experimentally determined for the X-Band TWT using a perturbation method and an automated phase measurement system. The results of the comparison of measurement and CTLSS calculation will be presented.
