Twiss Parameters

The example of Twiss parameters for an accelerator is a familiar one. Throughout these pages, it is assumed that two files, APS0.twi and APS.twi, exist containing the following data (a simplification of the Twiss output from the accelerator simulation code elegant):

• Parameters:
  • nux, nuy - Double-precision values of the x and y tunes.
  • alphac -- Double-precision values of the momentum compaction factor.
• Columns:
  • s - A double precision column of element positions. For simplicity, it is assumed to increase monotonically through the file.
  • ElementName - A string column of element names.
  • ElementType - A string column of element type identifiers.
  • betax, betay -- Double-precision columns of the beta functions for the horizontal and vertical planes, respectively.
  • psix, psiy -- Double-precision columns of the betatron phase advance.
  • etax, etay -- Double-precision columns of the dispersion functions.

To make it more interesting, APS0.twi is a single-page file containing the APS design lattice, while APS.twi is a multi-page file with each page corresponding to a different configuration.

In passing, it is appropriate to mention the style of the names used. It has been found helpful to use capitalization at word boundaries to make long names more readable. (In some cases, like betax, a certain case is used because it is significant.) When doing so will not create confusion, we also tend to capitalize the first letter of a name, which helps the name to stand out on the command line. Abiding by these conventions tends to result in readable names being created by Toolkit programs that have automatic name generation. Underscores in names are avoided because they increase the length of a name while adding less readability than capitalization.