3.4.2 beam

The parameters related to the beam are set with the beam command

beam, <name>=value, ...;

There is a set of predefined distribution types that can be generated ¹. In this case one needs to specify the following parameters:

• particle - particle name, "e-","e+","gamma","proton", etc
• energy - particle energy
• distrType - type of distribution

and, in addition, other parameters that depend on the distribution type that has been chosen:

1. Global options:
   • X0 - Offset of distribution centre in x[m]
   • Y0 - Offset of distribution centre in y[m]
   • Z0 - Offset of distribution centre in z[m]
   • Xp0 - Angular offset from nominal axis in x-z plane
   • Yp0 - Angular offset from nominal z axis in y-z plane
   • Zp0 - Directional flag: Zp0 < 0 points the particle back up the beamline
   • T0 - Global time offset [s]
2. distrType=”reference”: a reference orbit particle, which has the offsets in the global options so
   • X0 - Offset of distribution centre in x[m]
   • Y0 - Offset of distribution centre in y[m]
   • Z0 - Offset of distribution centre in z[m]
   • Xp0 - Angular offset from nominal axis in x-z plane
   • Yp0 - Angular offset from nominal z axis in y-z plane
   • Zp0 - Directional flag: Zp0 < 0 points the particle back up the beamline
   • T0 - Global time offset [s]
3. distrType=”gauss”: a gaussian in x, x', y, y', energy and time, with given widths:
   • sigmaX - RMS of x distribution in [m]
   • sigmaXp - RMS of x' distribution in [rad]
   • sigmaY - RMS of y distribution in [m]
   • sigmaYp - RMS of y' distribution in [rad]
   • sigmaE - RMS of energy distribution divided by nominal beam kinetic energy
   • sigmaT - RMS of time distribution in [s]
4. distrType=”gausstwiss”: a gaussian bunch defined by twiss parameters 4, emittance, energy and time:
   • betx - \beta_x in [m]
   • bety - \beta_y in [m]
   • alfx - \alpha_x
   • alfy - \alpha_y
   • emitx - \epsilon_x in [m]
   • emity - \epsilon_y in [m]
   • sigmaE - RMS of energy distribution divided by nominal beam kinetic energy
   • sigmaT - RMS of time distribution in [s]
5. distrType=”gaussmatrix”: a gaussian bunch defined by N(N-1)/2 elements of sigma matrix, this overwrites sigmaX, sigmaXp, sigmaY, sigmaYp, sigmaE and sigmaT variables if they have been defined previously. It will also recalculate the Twiss parameters.
   • sigmaMN - \sigma_MN in [m] where M range between 1 and 6 and N ranges between M and 6
6. distrType=”eshell”: an infinitely thin elliptic shell (locus) in x,x' and y,y' with given semiaxes:
   • shellX
   • shellXp
   • shellY
   • shellYp
   • sigmaE
7. distrType=“ring”: in the x, y plane the particles are uniformly distributed in r and in \phi inside a ring with inner radius Rmin and outer radius Rmax. x', y' and time are exactly Xp0,Yp0 and T0 respectively for each generated particle. The kinetic energy distribution is a gaussian of width sigmaE centered about the nominal beam kinetic energy.
   • Rmin, Rmax - inner and outer radius in [m]
   • sigmaE - RMS energy spread [GeV]

Example:

     
     beam, particle="e+", energy=100*MeV, distrType="gauss", sigmaX=0.01,
                 sigmaXp=0.1, sigmaY=0.01, sigmaYp=0.1;
     
     

In alternative, one can pass to the simulation a file containing a list of particles to be generated. For more details see Bunch description formats.