bunched_beam

• type: setup command.
• function: set up for tracking of particle coordinates with various distributions.

&bunched_beam
    STRING bunch = NULL;
    long n_particles_per_bunch = 1;
    double time_start = 0;
    STRING matched_to_cell = NULL;
    double emit_x  = 0;
    double emit_nx  = 0;
    double beta_x  = 1.0;
    double alpha_x = 0.0;
    double eta_x   = 0.0;
    double etap_x  = 0.0;
    double emit_y  = 0;
    double emit_ny  = 0;
    double beta_y  = 1.0;
    double alpha_y = 0.0;
    double eta_y   = 0.0;
    double etap_y  = 0.0;
    long use_twiss_command_values = 0;
    double Po = 0.0;
    double sigma_dp = 0.0;
    double sigma_s = 0.0;
    double dp_s_coupling = 0;
    double emit_z = 0;
    double beta_z = 0;
    double alpha_z = 0;
    long one_random_bunch = 1;
    long symmetrize = 0;
    long halton_sequence[3] = {0, 0, 0};
    long halton_radix[6] = {0, 0, 0, 0, 0, 0};
    long randomize_order[3] = {0, 0, 0};
    long limit_invariants = 0;
    long limit_in_4d = 0;
    long enforce_rms_values[3] = {0, 0, 0};
    double distribution_cutoff[3] = {2, 2, 2};
    STRING distribution_type[3] = {"gaussian","gaussian","gaussian"};
    double centroid[6] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0};
    long save_initial_coordinates = 1;
&end

• bunch -- The (incomplete) name of an SDDS file to which the phase-space coordinates of the bunches are to be written. Recommended value: ``%s.bun''.
• n_particles_per_bunch -- Number of particles in each bunch.
• time_start -- The central value of the time coordinate for the bunch.
• matched_to_cell -- The name of a beamline from which the Twiss parameters of the bunch are to be computed.
• emit_X -- RMS emittance for the X plane.
• emit_nX -- RMS normalized emittance for the X plane. Ignored if emit_X is nonzero.
• beta_X, alpha_X, eta_X, etap_X -- Twiss parameters for the X plane.
• use_twiss_command_values -- If nonzero, then the values for , , , and are taken from the twiss command. It is an error if no twiss command has been given.
• Po -- Central momentum of the bunch.
• sigma_dp, sigma_s -- Fractional momentum spread, , and bunch length. Note that sigma_s is actually the length in , so that for the length of the bunch in time will be greater than one might expect.
• dp_s_coupling -- Specifies the coupling between s and , defined as .
• emit_z, beta_z, alpha_z -- Provide another way to specify the longitudinal phase space, either separately from or in combination with sigma_dp, sigma_s, and dp_s_coupling.

  Basically, which values elegant uses depends what you set to nonzero values. If you set emit_z, then sigma_dp, sigma_s, and dp_s_coupling are ignored. If you don't set emit_z, then elegant uses sigma_dp and sigma_s; it additionally uses alpha_z if it is nonzero, otherwise it uses dp_s_coupling. For reference, the relationship between them is . Note that to impart a chirp that results in compression for (e.g., a normal four-dipole chicane), you must have or .

• one_random_bunch -- If non-zero, then only one random particle distribution is generated. Otherwise, a new distribution will be generated for every simulation step.
• enforce_rms_values[3] -- Flags, one for each plane, indicating whether to force the distribution to have the specified RMS properties.
• distribution_cutoff[3] -- Distribution cutoff parameters for each plane.
• distribution_type[3] -- Distribution type for each plane. May be ``gaussian'', ``hard-edge'', ``uniform-ellipse'', ``shell'', or ``dynamic-aperture''.
• limit_invariants -- If non-zero, the distribution cutoffs are applied to the invariants, rather than to the coordinates.
• limit_in_4d -- If non-zero, then the transverse distribution is taken to be a 4-d gaussian or uniform distribution. One of these must be chosen using the distribution_type control. It must be the same for x and y.
• symmetrize -- If non-zero, the distribution is symmetric under changes of sign in the coordinates. Automatically results in a zero centroid for all coordinates.
• halton\_sequence[3] and halton\_radix[6] -- This provides a ``quiet-start'' feature by choosing Halton sequences in place of random number generation. There are three new variables that control this feature. halton\_sequence is an array of three flags that permit turning on Halton sequence generation for the horizontal, vertical, or longitudinal planes. For example, halton\_sequence[0] = 3*1 will turn on Halton sequences for all three planes, while halton\_sequence[2] = 1, will turn it on for the longitudinal plane only.

  halton\_radix is an array of six integers that permit giving the radix for each sequence (i.e., x, x', y, y', t, p). Each radix must be a prime number. You should never use the same prime for two sequences (with one exception, see below). If these are left at zero, then elegant chooses values that eliminate phase-space banding to some extent.

• randomize\_order[3] -- Allows randomizing the order of assigned coordinates for the pairs (x, x'), (y, y'), and (t,p). 0 means no randomization; 1 means randomize (x, x', y, y', t, p) values independently, which destroys any x-x', y-y', and t-p correlations; 2 means randomize (x, x'), (y, y'), and (t, p) in pair-wise fashion, which retains correlations between x-x' etc but destroys those between x-y, x-p, etc. This is used with Halton sequences to remove banding.

• centroid[6] -- Centroid offsets for each of the six coordinates.
• save_initial_coordinates -- A flag that, if set, results in saving initial coordinates of tracked particles in memory. This is the default behavior. If unset, the initial coordinates are not saved, but are regenerated each time they are needed. This is more memory efficient and is useful for tracking very large numbers of particles.