7.74 tune_shift_with_amplitude

• N.B.: this command is deprecated, because it is too difficult to tune it to get reliable answers. The use of driving term computation in twiss_output is recommended instead, even though it doesn’t include all possibly relevant effects. For tune-spread calculations, the tune_footprint command provides more versatility.
• type: setup command.
• function: prepare for computation of tune shifts with amplitude.
• sequence: must follow twiss_output.
• Command syntax, including use of equations and subcommands, is discussed in 7.2.
• methods:
  • : 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.
  • : tune shifts are computed using a concatenated multi-turn third-order matrix. This appears to be reliable for many cases we’ve tested.
  • : tune shifts can be computed quickly using Bengtsson’s formulae [29] by setting compute_driving_terms=1 in twiss_output. For cases where all methods are valid, the results will be larger by a factor of 2 than the results obtained with this command, since J_q = , where q is x or y. Note that the present command has more general validity because it includes dipole curvature effects.

  The quantities computed are ν_p, where n ≥ 0 and m ≥ 0 are integers and p is x or y. A_q = (1 + α_q)q²∕β_q, with q = x or q = y.

• 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.
• lines_only — If nonzero, then instead of a full set of grid_size² particles, only two lines of particles with x = 0 and/or y = 0 are tracked. In this case, no A_xⁱ *A_y^j terms are computed (except for i = 0 or j = 0). However, in addition to being faster, the results may be more reliable, e.g., ∂ν_x∕∂A_y = ∂ν_y∕∂A_x may be more closely satisfied.
• sparse_grid — Deprecated. If nonzero, then instead of a full set of grid_size² particles, a sparse grid of particles is tracked. Will save time at the expense of inaccurate higher-order terms. Not recommended.
• 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.