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Keywords: tracking vacuum galactic
Forum: Fields: Magnetic and Otherwise
Re: Question Magnetic Field in Volume filled with 'Galactic' (Gumplinger Peter)
Date: 21 Jun, 2001
From: John Apostolakis <John Apostolakis>

We strongly recommend that for volumes of vacuum a user adds a user limit that includes a maximum step size. The step limit can be a distance of the order of the size of the geometrical setup (typically it can be the diameter of a detector), and overrides the astronomical distance in which the physics interactions can occur in vacuum.

For releases before June 2001, this user limit has been required for the correct functioning of the propagation of charged particles in a field. The next release, Geant4 3.2, will contain a protection for the problem encountered.

(As we try to maintain a particular accuracy (default is order of millimeters, the relative accuracy will be determined by the proposed step length, and for astronomical distances was causing numerical errors.)

A user limit will remain a recommendation, as it will aid performance in (at least) the near future. (The ideal value would be the largest possible step that a curved track could take in a volume, but a rough approximation will work almost as well.)

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