EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

Effect of the bv flag in MAD-X

When reversing the direction ("V") of a particle in a magnetic field ("B") while keeping its charge constant, the resulting force V * B changes sign. This is equivalent to flipping the field, but not the direction.

For practical reasons the properties of all elements of the LHC are defined in the MADX input as if they apply to a clockwise proton beam ("LHC beam 1"). This allows a single definition for elements traversed by both beams. Their effects on a beam with identical particle charge but running in the opposite direction ("LHC beam 2") must then be reversed inside the program.

In MADX this may be taken into account by setting the value of the BV attribute in the Beam commands. In the case of LHC beam 1 (clockwise) and beam 2 (counter-clockwise), treated in MADX both as clockwise proton beams, the Beam commands must look as follows:

   Beam, sequence=lhcb1, particle=proton, pc=450, bv=+1;
   Beam, sequence=lhcb2, particle=proton, pc=450, bv=-1;