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Manuscript Title: A numerical code for multiple water bag gravitational systems.
Authors: S. Cuperman, A. Harten
Program title: WATER BAG MODEL
Catalogue identifier: ACRU_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 8(1974)307
Programming language: Fortran.
Computer: CDC 6600.
Operating system: SCOPE 3.3.
RAM: 150K words
Word size: 60
Peripherals: magnetic tape.
Keywords: Astrophysics, Plasma physics, Water bag distributions, Nonhomogeneous Vlasov systems, One dimensional, Distribution function, Gravitational systems, Collisionless plasma.
Classification: 1.5, 19.3.

Nature of problem:
The non-linear stability analysis and evolution of non-homogeneous collisionless many-body systems are, as yet, unsolved problems. In fact even a rigerous linear analysis is missing. This is true for both self- gravitating systems of stars and plasmas (thermonuclear or astrophysical). The reason for this is that the mathematical methods available so far are notable to treat such complex problems as the collective interaction of the many bodies constituting the non- homogeneous systems mentioned above.

Solution method:
A convenient numerical integration method for studying the collective behaviour of a strongly non-homogeneous, finite size, non-maxwellian system is to follow the motion of the boundary curves defining the system in phase space. This method of integration enables us to investigate a system possibly consisting of a very large number of particles (enclosed by a boundary curve) without having to treat them explicitly. The method was first proposed by Roberts and Berk who also applied it to the study of the two-stream instability in one-dimensional plasma systems. This paper describes the computer code developed to investigate one-dimensional, collisionless systems of stars, consisting of regions of constant density matter (in phase space).

Restrictions:
The code may be used only for the investigation of one-dimensional (two- dimensional phase-space), collisionless systems of stars consisting of regions of constant density matter (in phase space).

Running time:
Execution times depend on the number of Eulerian strips used and on the number of mark points required to describe the system with the desired accuracy. If the number of Eulerian strips used is 215, the execution time per timestep for one mark point is about 1.5 X 10**-3 s.