The description of forward permanent magnet of year 2012 run in the DIRAC software

For DIRAC setup run of 2011 the additional retractable permanent magnet was used. The software part of it was done in [1]. As during the run the field of this magnet was significantly degradated due the radiation the another magnet([2]) was used for the run of 2012 . The last magnet induces the horizontal field in the gap between Be target and Pt foil. A magnet with bending power of 0.02 Tm is to shift the QY value only for the pairs produced in Be target leaving practically unchanged this value for pairs produced in Pt foil. This magnet was installed in the vacuum target station just after the target and before the Pt foil where the atomic pairs are broken. The distribution of this magnet field is used in GEANT-DIRAC and in ARIANE but in different forms. For GEANT-DIRAC the field should be present as three-dimensional polynomials(x,y,z) and the polynomials which used in ARIANE allow to calculate for a track its position(x and y) and angles($\theta$x and $\theta$y) in the target(or in Pt foil) on dependence of this track position and its momentum at far end of this magnet field.