13  Beam Command

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The BEAM command defines the particle species, reference energy or momentum, and global beam properties used by tracking and diagnostics.

label: BEAM, PARTICLE=name, MASS=real, CHARGE=real,
      ENERGY=real, PC=real, GAMMA=real, BCURRENT=real,
      NPART=real, BFREQ=real;
label: BEAM, PARTICLE=name, MASS=real, CHARGE=real,
      ENERGY=real, PC=real, GAMMA=real, BCURRENT=real,
      NPART=real, BFREQ=real, SOURCES=name;

The label is optional and defaults to UNNAMED_BEAM.

13.1 Particle Definition

PARTICLE selects one of the predefined species such as ELECTRON, POSITRON, MUON, PROTON, DEUTERON, HMINUS, H2P, ALPHA, CARBON, XENON, or URANIUM.

For custom species, the user can provide:

  • MASS in GeV
  • CHARGE in elementary-charge units

OPALX also defines PARTICLE=PHOTON. For photons, the beam is neutral and massless, and the beam command follows the specialized photon rules described below.

13.2 Beam Energy

There are three standard energy attributes, with the usual priority:

  • GAMMA
  • ENERGY
  • PC

For PARTICLE=PHOTON, only ENERGY is allowed. MASS, CHARGE, PC, and GAMMA are rejected by the beam validation path.

13.3 Other Attributes

The other important beam attributes are:

BFREQ
bunch frequency in MHz.
BCURRENT
bunch current in A.
NPART
number of macro particles used in the simulation.
NSLICE
number of slices per bunch.
MOMENTUMTOLERANCE
tolerance used when checking consistency between the distribution and the reference momentum.
SOURCES
name of the EMISSIONSOURCELIST that initializes the beam. This is required for non-photon beams in the current OPALX path.

Photon beams may be defined at the BEAM level, but the current TRACK implementation still rejects them.

13.3.1 CAIN Photon-Energy Convention

For PARTICLE=PHOTON, OPALX follows the CAIN convention and interprets ENERGY in eV.