Optics Calculations on the Reference Orbit
In many applications, linear optics computations (tunes, beta-functions, etc.) need to be carried out not only on the design (central)
orbit but also on one or many excited orbits. This arises when the optics change with beam momentum or when the beam is put on an helical orbit to achieve
beam separation at parasitic crossings in a collider. To perform such computations, the program needs to be put in the so-called reference orbit mode
(see Control). In that case, for each requested calculation the program performs the following actions:
- Parses the main window and creates a structure describing the beam optics on the central orbit.
- Finds a reference orbit. This is done in exactly the same manner as when the Tools/Trajectory command in sinvoked.
In line mode the program uses the parameters given in TrajParamBlock as initial values.
In ring mode the program first closes the trajectory and then builds the beam positions at every element.
- From the known properties of elements and beam positions at each element, the program builds a new lattice referenced to this new orbit.
Multiipole elements are added to account for feeddown effects arising from off-center trajectories.
In reference orbit mode the View/Lattice command shows element parameters in the reference mode. In generatal, each element in the origial lattice
is represented by a set of elements elements.
- once the reference mode lattice is built, subsequent computations are performed exactly as they were for the design (central) orbit.
Consequently, all commands can be used in the reference orbit mode. The only exception is Tool|Trajectory command which yields ambiguous results
since the same sources of orbit excitation are used to compute orbit change at parsing and then for a real calculation.