Faculty Publications, Department of Physics and AstronomyCopyright (c) 2016 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsfacpub
Recent documents in Faculty Publications, Department of Physics and Astronomyen-usWed, 13 Jan 2016 01:37:34 PST3600Unphysical kinetic effects in particle-in-cell modeling of laser wakefield accelerators
http://digitalcommons.unl.edu/physicsfacpub/159
http://digitalcommons.unl.edu/physicsfacpub/159Mon, 11 Jan 2016 11:22:17 PST
Unphysical heating and macroparticle trapping that arise in the numerical modeling of laser wakefield accelerators using particle-in-cell codes are investigated. A dark current free laser wakefield accelerator stage, in which no trapping of background plasma electrons into the plasma wave should occur, and a highly nonlinear cavitated wake with self-trapping, are modeled. Numerical errors can lead to errors in the macroparticle orbits in both phase and momentum. These errors grow as a function of distance behind the drive laser and can be large enough to result in unphysical trapping in the plasma wake. The resulting numerical heating in intense short-pulse laser-plasma interactions grows much faster and to a higher level than the known numerical grid heating of an initially warm plasma in an undriven system. The amount of heating, at least in the region immediately behind the laser pulse, can, in general, be decreased by decreasing the grid size, increasing the number of particles per cell, or using smoother interpolation methods. The effect of numerical heating on macroparticle trapping is less severe in a highly nonlinear cavitated wake, since trapping occurs in the first plasma wave period immediately behind the laser pulse.
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Estelle Cormier-Michel et al.HfCo<sub>7</sub>-Based Rare-Earth-Free Permanent-Magnet Alloys
http://digitalcommons.unl.edu/physicsfacpub/158
http://digitalcommons.unl.edu/physicsfacpub/158Mon, 11 Jan 2016 11:18:42 PST
This study presents the structural and magnetic properties of melt-spun HfCo_{7}, HfCo_{7-x}Fe_{x }(0.25 < x < 1), and HfCo_{7}Si_{x} (0.2 < x < 1.2) alloys. Appreciable permanent-magnet properties with a magnetocrystalline anisotropy of about 9.6–16.5 Mergs/cm^{3}, a magnetic polarization J_{s} = 7.2 –10.6 kG, and coercivities H_{c} = 0.5–3.0 kOe were obtained by varying the composition of these alloys. Structural analysis reveals that the positions of x-ray diffraction peaks of HfCo_{7 }show good agreement with those corresponding to an orthorhombic structure having lattice parameters of about a = 4.719 A, b = 4.278 A, and c = 8.070 A. Based on these results, a model crystal structure for HfCo_{7} is developed and used to estimate the magnetic properties of HfCo_{7 }using density-functional calculations, which agree with the experimental results.
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B. Das et al.Quasi Monoenergetic and Tunable X-rays by Laser
Compton Scattering from Laser Wakefield e-beam
http://digitalcommons.unl.edu/physicsfacpub/157
http://digitalcommons.unl.edu/physicsfacpub/157Mon, 11 Jan 2016 11:07:22 PST
Quasi monoenergetic and tunable x-ray beams are reported by inverse-Compton scattering from laser wakefield accelerated electrons. The high peak brightness, ultrashort duration, and small size of the source make it uniquely suitable for many applications.
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Nathan D. Powers et al.Variational Formulation of Macroparticle Models
for Electromagnetic Plasma Simulations
http://digitalcommons.unl.edu/physicsfacpub/156
http://digitalcommons.unl.edu/physicsfacpub/156Mon, 11 Jan 2016 10:59:43 PST
A variational method is used to derive a selfconsistent macroparticle model for relativistic electromagnetic kinetic plasma simulations. Extending earlier work, discretization of the electromagnetic Low Lagrangian is performed via a reduction of the phase-space distribution function onto a collection of finite-sized macroparticles of arbitrary shape and discretization of field quantities onto a spatial grid. This approach may be used with lab frame coordinates or moving window coordinates; the latter can greatly improve computational efficiency for studying some types of laser-plasma interactions. The primary advantage of the variational approach is the preservation of Lagrangian symmetries, which in our case leads to energy conservation and thus avoids difficulties with grid heating. In addition, this approach decouples particle size from grid spacing and relaxes restrictions on particle shape, leading to low numerical noise. The variational approach also guarantees consistent approximations in the equations of motion and is amenable to higher order methods in both space and time. We restrict our attention to the 1.5-D case (one coordinate and two momenta). Simulations are performed with the new models and demonstrate energy conservation and low noise.
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Alexander B. Stamm et al.A Time-Implicit Algorithm for Solving the
Vlasov–Poisson Equation
http://digitalcommons.unl.edu/physicsfacpub/155
http://digitalcommons.unl.edu/physicsfacpub/155Mon, 11 Jan 2016 10:55:40 PSTB. A. Shadwick et al.Bayesian Spectrum Analysis for
Laser Vibrometry Processing
http://digitalcommons.unl.edu/physicsfacpub/154
http://digitalcommons.unl.edu/physicsfacpub/154Mon, 11 Jan 2016 10:53:24 PST
Laser vibration sensing provides a sensitive non-contact means of measuring vibrations of objects. These measurements are used in industrial quality control and wear monitoring as well as the analysis of the vibrational characteristics of objects. In laser vibrometry, the surface motion is monitored by heterodyne laser Doppler velocimetry, and the received heterodyne signal is sampled to produce a time-series which is processed to obtain a vibrational spectrum of the object under test. Laser vibrometry data has been processed with a traditional FM discriminator approach and by spectrogram and time-frequency distribution processing techniques. The latter techniques have demonstrated improved performance over the FM discriminator method, but do not take full advantage of the prior knowledge one has about the signal of interest. We consider here a statistical signal processing approach to laser vibrometry data. In this approach the quantities of interest are the frequencies of vibration, while the phase and quadrature amplitudes are considered nuisance parameters. Because of the optimal use of prior knowledge about the laser vibrometry signal, the frequencies can be determined with much greater precision and greater noise immunity than using Fourier- or time-frequency-based approaches. Furthermore, the statistical approach is known to have superior performance when the data extends over a small number of vibrational periods. We illustrate the method with data from a beroptic laser Doppler velocimeter. Our results show that while the choice of processing method for determining the instantaneous velocity is relatively unimportant, the Bayesian method exhibits superior performance in determining the vibrational frequency.
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Walter F. Buell et al.Higher-Order Explicit Methods for Laser-Plasma
Interactions
http://digitalcommons.unl.edu/physicsfacpub/153
http://digitalcommons.unl.edu/physicsfacpub/153Mon, 11 Jan 2016 10:46:04 PST
The evolution of a short, intense laser pulse propagating in an underdense plasma is of particular interest for laser-plasma accelerator physics and, in some circumstances, is well-modeled by the cold Maxwell-fluid equations. Solving this system using conventional second-order explicit methods in a three-dimensional simulation over experimentally-relevant configurations is prohibitively expensive. This motivated a search for more efficient numerical methods to solve the fluid equations. Explicit methods tend to suffer from stability constraints which couple the maximum allowable time step to the spatial grid size. If the dynamics of the system evolves on a time scale much larger than the constrained time step, an explicit method would require many more update cycles than is physically necessary. In these circumstances implicit methods, which tend to be unconditionally stable, may be attractive. But when physical situations (e.g., Raman processes) need to resolve the fast dynamics, implicit methods are unlikely to exhibit much improvement over explicit methods. Thus, we look for higher-order explicit methods in space that would allow coarser spatial grids and larger time steps. We restrict our discussion to the one-dimensional case and present a comprehensive survey of a wide range of numerical methods to solve the fluid equations, including methods of order two through six in space and two through eight in time. A systematic approach to determine the stability condition is presented using linear stability analysis of numerical dispersion relations. Three higher-order methods are implemented to show their behavior, in terms of numerical stability and energy conservation.
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J. Paxon Reyes et al.Higher-Order Explicit Numerical Methods for Laser-
Plasma Interactions
http://digitalcommons.unl.edu/physicsfacpub/152
http://digitalcommons.unl.edu/physicsfacpub/152Mon, 11 Jan 2016 10:44:05 PST
The evolution of a short, intense laser pulse propagating in an underdense plasma is of particular interest for laser-plasma accelerator physics. This case is well-modeled by the cold, Maxwell–fluid equations but, using conventional second-order explicit methods, a three-dimensional simulation for experimentally relevant configurations is prohibitively expensive. This motivated a search for numerical methods that might be used to solve the fluid equations more efficiently. Explicit methods tend to suffer from stability constraints which couple the maximum allowable time step to the spatial grid size. If the dynamics of the system evolves on a time scale much larger than the constrained time step, an explicit method may require many more update cycles than is physically necessary. In these circumstances implicit methods, which tend to be unconditionally stable, may be attractive. However, in many physical situations (e.g., Raman processes) it is necessary to fully-resolve the fast dynamics. In this case, implicit methods are unlikely to exhibit much improvement over explicit methods. Thus, we look for methods of higherorder in space that would allow the use of coarser spatial grids and thus larger time steps.
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J. Paxon Reyes et al.OPTICAL CONTROL OF ELECTRON TRAPPING
AND ACCELERATION IN PLASMA CHANNELS:
APPLICATION TO TUNABLE, PULSED SOURCES OF
MULTI-COLOR THOMSON GAMMA-RAYS
http://digitalcommons.unl.edu/physicsfacpub/151
http://digitalcommons.unl.edu/physicsfacpub/151Mon, 11 Jan 2016 10:41:01 PSTS. Y. Kalmykov et al.Accordion Effect in Plasma Channels:
Generation of Tunable Comb-Like Electron Beams
http://digitalcommons.unl.edu/physicsfacpub/150
http://digitalcommons.unl.edu/physicsfacpub/150Mon, 11 Jan 2016 10:37:53 PST
Propagating a short, relativistically intense laser pulse in a plasma channel makes it possible to generate comb-like electron beams for advanced radiation sources. The ponderomotive force of the leading edge of the pulse expels all electrons facing the pulse. The bare ions attract the ambient plasma electrons, forming a closed bubble of electron density confining the pulse tail. The cavity of electron density evolves slowly, in lock-step with the optical driver, and readily traps background electrons. The combination of a bubble (a self-consistently maintained, “soft” hollow channel) and a preformed channel forces transverse flapping of the laser pulse tail, causing oscillations in the bubble size. The resulting periodic injection produces a sequence of background-free, quasi-monoenergetic bunches of femtosecond duration. The number of these spectral components, their charge, energy, and energy separation is sensitive to the channel radius and pulse length. Accumulation of noise (continuously injected charge) can be prevented using a negatively chirped drive pulse with a bandwidth close to a one-half of the carrier wavelength. As a result of dispersion compensation, self-steepening of the pulse is reduced, and continuous injection almost completely suppressed. This level of control on a femtosecond time scale is hard to achieve with conventional accelerator techniques. These comb-like beams can drive high-brightness, tunable, multi-color -ray sources.
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Serge Y. Kalmykov et al.ALL-OPTICAL CONTROL OF ELECTRON
TRAPPING IN TAPERED PLASMAS AND
CHANNELS
http://digitalcommons.unl.edu/physicsfacpub/149
http://digitalcommons.unl.edu/physicsfacpub/149Mon, 11 Jan 2016 10:33:39 PST
The radiation pressure of a multi-terawatt, sub-100 fs laser pulse propagating in an under-dense plasma causes complete electron cavitation. The resulting electron density “bubble” guides the pulse over many Rayleigh lengths, leaving the background ions unperturbed while maintaining GV/cm-scale accelerating and focusing gradients. The shape of the bubble, and, hence, the wakefield potentials, evolve slowly, in lockstep with the optical driver. This dynamic structure readily traps background electrons. The electron injection process can thus be controlled by purely optical means.
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S. Y. Kalmykov et al.Magnetic and Structural Properties of Rapidly Quenched
Tetragonal Mn<sub>3-x</sub> Ga Nanostructures
http://digitalcommons.unl.edu/physicsfacpub/148
http://digitalcommons.unl.edu/physicsfacpub/148Mon, 11 Jan 2016 10:30:29 PST
Nanostructured Mn_{3-x} Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D0_{19} and cubic L2_{1} Heusler crystal structures based on the concentration of Mn in Mn_{3-x} Ga. Upon vacuum-annealing the samples at 450 C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D0_{22} structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn_{1.9} Ga and Mn_{2.1} Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn_{2.6} Ga and Mn_{3.0} Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn_{3-x} Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn_{2.6} Ga and Mn_{3.0} Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn_{3-x} Ga. The maximum magnetization of 57 emu/g (300 emu/cm^{3}) and the coercivity of 6.5 kOe were measured in the Mn_{1.9} Ga and Mn_{3.0} Ga ribbons, respectively.
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Y. Huh et al.LASER-WAKEFIELD ELECTRON ACCELERATOR
WITH INDEPENDENT BEAM-PARAMETER
CONTROL
http://digitalcommons.unl.edu/physicsfacpub/147
http://digitalcommons.unl.edu/physicsfacpub/147Mon, 11 Jan 2016 10:17:44 PST
It has been demonstrated that laser-wakefield acceleration (LWFA) can generate several electron beam parameters rivaling those generated by conventional RF-cavity accelerators. Tunable and monoenergetic electron beams have also been produced with LWFA. Currently, active research is being directed towards finding methods to generate e-beams that have independently controllable parameters, as is standard in the case of conventional accelerators. One promising approach is to separate and independently control the essential acceleration processes.
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G. Golovin et al.