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The paper describes an efficient algorithm to integrate the equations of a fast moving charge cloud of small size in a large electron gun. Particle in cell computations of a realistic electron gun is challenging due to the large discrepancy between the size of the cavity and the size of the cloud. A fine grid must be used to resolve the small volume of the charge, with a grid spacing in the order of 0.1 mm. The cavity has extensions of about 100 mm. Therefore one has to deal with about 1000 million gridcells. Such a large grid is handled best with parallel systems. Each node of the parallel system computes the electromagnetic field in its subvolume. As the extension of the charge keeps being small during the flight, at each timestep the charged particles will be located in only a few subvolumes of the nodes of the parallel system. This would lead to a strong load imbalance, if the particle related computations for each particle would be performed by the node where the particle is in. GdfidL instead spreads the data of all particle over all processors, which perform the particle related computations, and send back the results to the processors where the particles are in.
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