7.23 error_element

• type: setup command.
• sequence: must follow run_control.
• function: assert a random error defintion for the accelerator.
• Command syntax, including use of equations and subcommands, is discussed in 7.2.

• name — The possibly wildcarded name of the elements for which errors are being specified.
• element_type — An optional, possibly wildcarded string giving the type of elements to which the errors should be applied. E.g., element_type=*MON* would match all beam position monitors. If this item is given, then name may be left blank.
• item — The parameter of the elements to which the error pertains.
• type — The type of random distribution to use. May be one of “uniform”, “gaussian”, “plus_or_minus”, or “sampled”. A “plus_or_minus” error is equal in magnitude to the amplitude given, with the sign randomly chosen. A “sampled” error is drawn from a set of user-supplied values, as described below.
• amplitude — The amplitude of the errors.
• cutoff — The cutoff for the gaussian random distribution in units of the amplitude. Ignored for other distribution types.
• bind, bind_number, bind_across_names — These parameters control “binding” of errors among elements, which means assigning the same error contribution to several elements. This occurs if bind is nonzero, which it is by default! If bind is negative, then the sign of the error will alternate between successive elements. bind_number can be used to limit the number of elements bound together. In particular, if bind_number is positive, then a positive value of bind indicates that bind_number successive elements having the same name will have the same error value. Finally, by default, elegant only binds the errors of objects having the same name, even if they are assigned errors by the same error_element command (i.e., through a wildcard name). If bind_across_names is nonzero, then binding is done even for elements with different names.
• post_correction — A flag indicating whether the errors should be added after orbit, tune, and chromaticity correction.
• fractional — A flag indicating whether the errors are fractional, in which case the amplitude refers to the amplitude of the fractional error.
• additive — A flag indicating that the errors should be added to the prior value of the parameter. If zero, then the errors replace the prior value of the parameter.
• allow_missing_elements — A flag indicating that execution may continue even if no matching elements are found.
• after — The name of an element. If given, the error is applied only to elements that follow the named element in the beamline.
• before — The name of an element. If given, the error is applied only to elements that precede the named element in the beamline.
• sample_file, sample_file_column, sample_mode— If the error type is “sampled”, then sample_file must contain the name of an SDDS containing the numerical column named by sample_file_column. The values in this column from the first page of the file are used for assigning error values. sample_mode may be one of “random”, “shuffle”, or “sequentual”, with the following meanings:
  • random — Values are drawn randomly from the list as needed, without regard to reuse of a given value.
  • shuffle — Values are drawn from the list in a random order until all are used, then a new random order is created. This ensures that all values are used with equal probability.
  • sequential — Values are used in the order given until all are used, repeatedly as needed starting from the beginning of the list.

  This feature can be used to assign errors based on a set of measured values or, using sddssampledist, an arbitrary external distribution.