twiss_output. For cases where all methods are valid, the results will differ by a factor of 2 from the results obtained with this command. Also, the present command has more general validity because it includes dipole curvature effects.
The quantities computed are , where and are integers and is or . , with or .
&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 lines_only = 0; 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
y0-- The initial x and y amplitudes to use for determining the small-amplitude tunes.
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.
lines_only-- If nonzero, then instead of a full set of
grid_sizeparticles, only two lines of particles with and/or are tracked. In this case, no terms are computed (except for or ). However, in addition to being faster, the results may be more reliable, e.g., may be more closely satisfied.
sparse_grid-- If nonzero, then instead of a full set of
grid_sizeparticles, 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
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.)
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.
scaling_iterations-- These control automatic scaling of the amplitudes. If elegant sees a tune shift larger than
scale_down_limitit will decrease
y0) by the factor
scale_down_factor. If elegant sees a tune shift smaller than
scale_up_limitit will increase
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
twiss_output. The user should experiment with this option to see if the results are reliable for a particular lattice.