| Advanced
Computing for 21st Century Accelerator Science and Technology
This project is working on simulating particle accelerators,
some of the largest and most complex scientific instruments.
A new generation of accelerator simulation codes will promote
more efficient use of existing accelerators and will strongly
impact the design, technology, and cost of future accelerators.
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| Figure
17 Simulation showing halo particles
being tracked backward in an accelerator to their starting
points. Such simulations and associated visualizations
provide insight into the halo formation mechanism in high
intensity beams. (Click on image to view animation.) |
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Macroparticle simulation plays an important role in modern
accelerator design and operation. Most linear rf accelerators
have been designed based on macroparticle simulations using
longitudinal position as the independent variable. The choice
of an independent variable affects the accuracy of the simulation
with regard to the calculation of the space-charge forces
and hence the accuracy of the particle trajectories. This
leads to changes in the prediction of the evolution of the
beam distribution, both in the core and in the halo (Figure
17).
Qiang et al. have done a systematic comparison between using
longitudinal position as the independent variable and using
time as the independent variable in macroparticle simulations.
They found that, for an rms-matched beam, the maximum relative
moment difference for second and fourth moments and for beam
maximum amplitudes between these two types of simulations
is 0.25% in a 10 m reference transport system with physical
parameters similar to the Spallation Neutron Source linac
design. The maximum z-to-t transform error
in the space-charge force calculation of the position dependent
simulation is about 0.1% in such a system. This might cause
a several percent error in a complete simulation of a linac
with a length of hundreds of meters. The error may be several
times larger in simulations of mismatched beams. New algorithms,
such as a particle advance method based on the use of multiple
reference particles, are being studied in order to improve
the accuracy of position-dependent simulations of high-intensity
beams.
INVESTIGATORS
K. Ko, Stanford Linear Accelerator Center; R. D. Ryne and
E. G. Ng, Lawrence Berkeley National Laboratory; A. Dragt,
University of Maryland; G. H. Golub, Stanford University;
K.-L. Ma, University of California, Davis; W. Mori, University
of California, Los Angeles.
PUBLICATION
J. Qiang, R. D. Ryne, and R. W. Garnett, “Systematic
comparison of position and time dependent macroparticle simulations
in beam dynamics studies,” Phys. Rev. ST Accel. Beams
5, 064201 (2002).
URL
http://scidac.nersc.gov/accelerator/ |
