100 research outputs found
Perturbation Analysis of Orthogonal Matching Pursuit
Orthogonal Matching Pursuit (OMP) is a canonical greedy pursuit algorithm for
sparse approximation. Previous studies of OMP have mainly considered the exact
recovery of a sparse signal through and , where is a matrix with more columns than rows. In this paper,
based on Restricted Isometry Property (RIP), the performance of OMP is analyzed
under general perturbations, which means both and are
perturbed. Though exact recovery of an almost sparse signal is no
longer feasible, the main contribution reveals that the exact recovery of the
locations of largest magnitude entries of can be guaranteed under
reasonable conditions. The error between and solution of OMP is also
estimated. It is also demonstrated that the sufficient condition is rather
tight by constructing an example. When is strong-decaying, it is proved
that the sufficient conditions can be relaxed, and the locations can even be
recovered in the order of the entries' magnitude.Comment: 29 page
Statistical Analysis of Crossed Undulator for Polarization Control in a SASE FEL
There is a growing interest in producing intense, coherent x-ray radiation
with an adjustable and arbitrary polarization state. In this paper, we study
the crossed undulator scheme (K.-J. Kim, Nucl. Instrum. Methods A {\bf 445},
329 (2000)) for rapid polarization control in a self-amplified spontaneous
emission (SASE) free electron laser (FEL). Because a SASE source is a
temporally chaotic light, we perform a statistical analysis on the state of
polarization using FEL theory and simulations. We show that by adding a small
phase shifter and a short (about 1.3 times the FEL power gain length),
rotated planar undulator after the main SASE planar undulator, one
can obtain circularly polarized light -- with over 80% polarization -- near the
FEL saturation.Comment: 14 pages, 4 figure
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SASE FEL Polarization Control Using Crossed Undulator
There is a growing interest in producing intense, coherent x-ray radiation with an adjustable and arbitrary polarization state. In this paper, we study the crossed undulator scheme for rapid polarization control in a self-amplified spontaneous emission (SASE) free electron laser (FEL). Because a SASE source is a temporally chaotic light, we perform a statistical analysis on the state of polarization using FEL theory and simulations. We show that by adding a small phase shifter and a short (about 1.3 times the FEL power gain length), 90{sup o} rotated planar undulator after the main SASE planar undulator, one can obtain circularly polarized light--with over 80% polarization--near the FEL saturation
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Analysis of Slice Transverse Emittance Evolution in a Photocathode RF Gun
This report is an analysis of slice transverse emittance evolution in a photocathode RF gu
Nonlinearly Shaped Pulses in Photoinjectors and Free-Electron Lasers
Photoinjectors and Free Electron Lasers (FEL) are amongst the most advanced
systems in accelerator physics and have consistently pushed the boundaries of
emittance and x-ray peak power. In this paper, laser shaping at the cathode is
proposed to further lower the emittance and reduce electron beam tails, which
would result in brighter x-ray production. Using dispersion controlled
nonlinear shaping (DCNS), laser pulses and beam dynamics were simulated in
LCLS-II. The photoinjector emittance was optimized and the resulting e-beam
profiles were then simulated and optimized in the linac. Finally, the expected
FEL performance is estimated and compared to the current technology: Gaussian
laser pulses on the cathode. The e-beams produced by DCNS pulses show a
potential for 35% increase in x-ray power per pulse during SASE when compared
to the standard Gaussian laser pulses
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