tune_shift_with_amplitude

• type: setup command.
• function: prepare for computation of tune shifts with amplitude.
• note: must be given prior to the twiss_output command.
• method: tune shifts with amplitude are computed via tracking a series of particles at different amplitudes or by a matrix method. NAFF is used to determine the tunes from the tracking data. It is the user's responsbility to optimize the parameters to ensure that results are reasonable. Using tracking to determine tune shifts is more accurate than analytical methods as it includes multi-turn effects that are important in some rings (e.g., the APS).

&tune_shift_with_amplitude
    long turns = 2048;
    double x0 = 1e-6;
    double y0 = 1e-6;
    double x1 = 3e-4;
    double y1 = 3e-4;
    long grid_size = 6;
    long sparse_grid = 0;
    long spread_only = 0;
    double nux_roi_width = 0.02;
    double nuy_roi_width = 0.02;
    double scale_down_factor = 2;
    double scale_up_factor = 1.05;
    double scale_down_limit = 0.01;
    double scale_up_limit = 1e-4;
    long scaling_iterations = 10;
    long use_concatenation = 0;
    long verbose = 0;
    long order = 2;
    STRING tune_output = NULL;
&end

• turns -- The number of turns to track. If zero, then the concatenated matrix is used instead of tracking, and all other parameters of this command are irrelevant. The matrix method doesn't work well with all lattices. The order of the concatenated matrix is given by the concat_order control in twiss_output.
• x0, y0 -- The initial x and y amplitudes to use for determining the small-amplitude tunes.
• x1, y1 -- The initial x and y amplitudes to user for determining the tune shifts. These values should be small enough to ensure linearity in the tune shift.
• grid_size -- Size of the grid of points in x and y.
• sparse_grid -- If nonzero, then instead of a full set of grid_size$^2$ particles, a sparse grid of particles is tracked. Will save time at some expense in accuracy.
• spread_only -- Compute the tune spread only and don't bother with the tune shift coefficients. These tune spreads can be optimized and appear in the twiss output file under the names nuxTswaLower, nuxTswaUpper, and similarly for the y plane. This is the recommended way to reduce tune shift with amplitude, as the tune spread is more reliable than the coefficients of the expansion. (Particles that get lost are automatically ignored in both types of computations.)
• nux_roi_width, nuy_roi_width -- Widths of the region of interest for x and y tunes. As the grid is filled in, elegant finds the tune for each tracked particle on the grid. Successive tune values are looked for in the region of the given width around the previous tune value. This prevents jumping from the main tune peak to another peak, which can happen when the tune spectrum has many lines.
• scale_down_factor, scale_up_factor, scale_down_limit, scale_up_limit, scaling_iterations -- These control automatic scaling of the amplitudes. If elegant sees a tune shift larger than scale_down_limit it will decrease x0 (or y0) by the factor scale_down_factor. If elegant sees a tune shift smaller than scale_up_limit it will increase x0 (or y0) by the factor scale_up_factor. Suggestion: if you find yourself playing with these values and the initial amplitudes in order to get reliable TSWA coefficients, try just using the tune spread.
• verbose -- If nonzero, information about the progress of the algorithm is printed to the screen.
• use_concatenation -- If nonzero, then tracks with the concatenated matrix instead of element-by-element. The order of the concatenated matrix is given by the concat_order control in twiss_output. The user should experiment with this option to see if the results are reliable for a particular lattice.