The Description of the Optical System

The Optical System consists of a sequence of ionoptical elements, like drift length, sectorfields and quadrupols. Real sectors and quadrupols have fringing fields, which extend into the nearby drift lenghts and cause distortions of the particle trajectories. GICO has two ways to take these effects into account. The first way is to calculate the actual shape of the fringing field by the use of fringing field coefficients, and than to obtain the transfer matrix of the fringing field by the use of an integrator. While this way leads to a very good calculation of the fringing field effects, it also uses a large amount of CPU time. The second way is the use of fringing field integrals, which offer a fast way to calculate the transfer matrix. Note also that the fringing-field effects are only small corrections so that usually approximate values of the integrals are satisfactory and that reasonably good results can be obtained if a set of standard fringing-field integrals are used, determined from a realistic fringing-field distribution, i.e. a field distribution in which Maxwell's equations are valid everywhere. However, before the system is finally built, one should perform calculations with fringing-field distributions that are as accurate as possible, obtained from measurements or explicit calculations. This will then modify the results slightly to first- and second-order where these modifications usually can be counterbalanced by varying the geometry or some quadrupole and hexapole parameters slightly. At last the accuracy of the calculation can be assured by the use of the coefficients and the integrator

The command to switch between both modes of calculation is: 3 F(RINGING_FIELD) T(YPE) <type> <mode> ; [example: F   F   I ;] [F   T   S   E ;]

<type> can be S(OFT) or I(NTEGRAL), where S means the use of the integrator and I denotes the calculation with integrals. The <mode> can be set to E(NABLE) or D(ISABLE). If the mode is DISABLE, the fringing fields are not calculated at all, although they appear in the input file. There is one exeption: if the fringing fields are used to describe the influence of an oblique edge of a sector, their calculation cannot be disabeled. Note that a fringing field can only be defined between a sector or quadrupole or hexapole and a field-free region.