9 Accelerator and Element Description

As mentioned in the introduction, elegant uses a variant of the MAD input format for describing accelerators. With some exceptions, the accelerator description for one program can be read by the other with no modification. Among the differences:

• elegant does not support the use of MAD-style equations to compute the value of a quantity. The link_elements namelist command can be used for this purpose, and is actually more flexible than the method used by MAD. Also, rpn-style equations may be given in double-quotes; these are evaluated once only when the lattice is parsed.
• elegant does not support substitution of parameters in beamline definitions.
• elegant contains many elements that MAD does not have, such as kick elements, wake fields, and numerically integrated elements.
• The length of an input line is not limited to 80 characters in elegant, as it is in MAD. However, for compatibility, any lattice created by elegant will conform to this limit.
• The maximum length of the name of an element or beamline is 100 characters.

elegant’s lattice parser translates all input into upper case, except where the input is protected by double quotes. However, various commands (such as vary_element or link_elements) that accept element names as input do not perform any translation. Hence, when referring to element names in commands, the user must enter the names in upper case unless they are protected by double quotes in the lattice file.

When the lattice file is very complex, it is sometimes convenient to separate it into several files. These can then be imported into a main lattice file using the #INCLUDE directive, as in

The rules for naming elements and beamlines are as follows:

• The name should start with an alphabetic character (i.e., a-z A-Z).
• The name may contain any of the following characters in addition to alphabetic characters and numbers: ~ @ $ % ^ & - _ + = { } [ ] \ | / ? < > . : |\verb
• The name should not contain any of the following: # * ! ‘‘ ’ ‘
• The name should not contain spaces, tabs, or non-printing characters.

If using unusual characters in a name, it is a good idea to enclose the name in double quotes. This is required if : is in the name.

elegant’s print_dictionary command allows the user to obtain a list of names and short descriptions of all accelerator elements recognized by the program, along with the names, units, types, and default values of all parameters of each element. The present output of this command is listed in the next section. The reader is referred to the MAD manual[2] for details on sign conventions for angles, focusing strength, and so forth.

Comments may be embedded in the lattice file by starting a line with an exclamation point (“!”). Rpn expressions may be embedded separately from an element definition by starting a line with a percent sign (“%”). For example

Note that to use an RPN expression the value of a parameter, one must enclose the expression in double quotes.