optimization_setup

• type: setup command.
• function: define overall optimization parameters and methods.

&optimization_setup
    STRING equation = NULL;
    STRING mode = "minimize";
    STRING method = "simplex";
    double tolerance = -0.01;
    double target = 0;
    long soft_failure = 1;
    long n_passes = 2;
    long n_evaluations = 500; 
    long n_restarts = 0;
    long matrix_order = 1;
    STRING log_file = NULL;
    long output_sparsing_factor = 0;
    long balance_terms = 0;
    double restart_worst_term_factor = 1;
    long restart_worst_terms = 1;
&end

• equation -- An rpn equation for the optimization function, expressed in terms of any parameters of any optimization variables and the ``final'' parameters of the beam (as recorded in the final output file available in the run_setup namelist). The optimization variables or covariables may appear in the equation in the form <element-name>.<parameter-name>, all in capital letters. Data from MARK elements with FITPOINT=1 may be used via symbols of the form <element-name>#<occurrence-number>.<parameter-name>, where <parameter-name> can be a Twiss parameter name (if the twiss command was given), a floor coordinate name (if the floor command was given, or a beam-size or centroid name. The parameter names are the same as those used in the corresponding output files. Beam sizes and centroids are from tracking the particle distribution.

  If the twiss command was given, one may also refer to statistics of Twiss parameters in the form <statistic>.<parameter-name>, where <statistic> is either min or max. One may also use the symbols nux, dnux/dp, (and corresponding symbols for y), alphac, and alphac2. These are the tune, chromaticity, and first- and second- order momentum compaction factors. The final values of the Twiss parameters are referred to simply as betax, etax, etc.

  If the twiss command was given and radiation integral computation was requested, one may use ex0 and Sdelta0 for the equilibrium emittance and momentum spread, plus J<plane> and tau<plane> for the damping partition and damping time, where <plane> is x, y, or delta.

  If the floor_coordinates command was given, one may use X, Z, and theta to refer to the final values of the floor coordinates.

  If the sasefel command was given, one may use variables of the form SASE.<property>, where <property> is one of gainLength, saturationLength, saturationPower, or lightWavelength.

  Finally, one may use any of the names from the ``final'' output file (see run_setup), e.g., Sx (x beamsize) or eny (y normalized emittance). These refer to tracked properties of the beam.

  The equation may be left blank, in which case the user must give one or more optimization_term commands. These use the same symbols, of course.

  There are several rpn functions that are useful in constructing a good optimization equation. These are ``soft-edge'' greater-than, less-than, and not-equal functions, which have the names segt, selt, and sene, respectively. The usage is as follows:

  • V1 V2 T segt. Returns a nonzero value if and only if value V2 is greater than V1. The returned value is $((V_1-V_2)/T)^2$. Typically used to constraint a quantity from above. E.g., to limit the maximum horizontal beta function to 10m with a tolerance of $T=0.1m$, one would use max.betax 10 .1 segt.
  • V1 V2 T selt. Returns a nonzero value if and only if value V2 is less than value V1. The returned value is $((V_1-V_2)/T)^2$.
  • V1 V2 T sene. Returns a nonzero value if and only if V1 and V2 differ by more than tol. If $V_1>V_2$, returns $((V_1-(V_2+T))/T)^2$. If $V_2>V_1$, returns $((V_2-(V_1+T))/T)^2$.
• mode -- May be either ``minimize'' or ``maximize''.

• method -- May be one of ``simplex'', ``grid'', ``powell'', ``randomwalk'', ``randomsample'', or ``sample''. Recommended methods are ``simplex'' and ``randomwalk''. The latter is very useful when the lattice is unstable or nearly so.

• tolerance -- The convergence criterion for the optimization, with a negative value indicating a fractional criterion.
• target -- The value which, if reached, results in immediate termination of the optimization, whether it has converged or not.
• soft_failure -- A flag indicating, if set, that failure of an optimization pass should not result in termination of the optimization.

• n_evaluations -- The number of allowed evaluations of the optimization function. If simplex optimization is used, this is the number of allowed evaluations per pass.

• n_passes -- The number of optimization passes made to achieve convergence (``simplex'' only). A pass ends (roughly) when the number of evaluations is completed or the function doesn't change within the tolerance. A new pass involves starting the optimization again using step sizes determined from the range of the simplex and the factor simplex_pass_factor.

• n_restarts -- The number of complete restarts of the optimization (simplex only). This is an additional loop around the n_passes loop. The difference is that a restart involves using the optimized result but the original step sizes. It is highly recommended that this feature be used if convergence problems are seen.

• restart_worst_term_factor, restart_worst_terms -- Often when there are convergence problems, it is because a few terms are causing difficulty. Convergence can often be obtained by increasing the weighting of these terms. If restart_worst_term_factor is positive, then elegant will multiply the weight of the restart_worst_terms largest terms by this factor at the beginning of a restart.

• matrix_order -- Specifies the highest order of matrix elements that should be available for fitting. Elements up to third order are available for the terminal point of the beamline, and up to secod order for interior fit points. Names for first-, second-, and third-order elements are of the form Rij, Tijk, and Uijkl.
• log_file -- A file to which progress reports will be written as optimization proceeds. For SDDS data, use the final output file from the run_setup namelist.
• output_sparsing_factor -- If set to a value larger than 0, results in sparsing of output to the ``final'' file (see run_setup). This can make a significant difference in the optimization speed.