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At least 577 records · Page 32
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DOE OSTI2011

A Scalable Parallel Wideband MLFMA for Efficient Electromagnetic Simulations on Large Scale Clusters

The development of the multilevel fast multipole algorithm (MLFMA) and its multiscale variants have enabled the use of integral equation (IE) based solvers to compute scattering from complicated structures. Development of scalable parallel algorithms, to extend the reach of these solvers, has been a topic of intense research for about a decade. In this paper, we present a new algorithm for parallel implementation of IE solver that is augmented with a wideband MLFMA and scalable on large number of processors. The wideband MLFMA employed here, to handle multiscale problems, is a hybrid combination of the accelerated Cartesian expansion (ACE) and the classical MLFMA. The salient feature of the presented parallel algorithm is that it is implicitly load balanced and exhibits higher performance. This is achieved by developing a strategy to partition the MLFMA tree, and hence the associated computations, in a self-similar fashion among the parallel processors. As detailed in the paper, the algorithm employs both spatial and direction partitioning approaches in a flexible manner to ensure scalable performance. Plethora of results are presented here to exhibit the scalability of this algorithm on 512 and more processors.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2011

Construction and Operational Experience with a Superconducting Octupole Used to Trap Antihydrogen

A superconducting octupole magnet has seen extensive service as part of the ALPHA experiment at CERN. ALPHA has trapped antihydrogen, a crucial step towards performing precision measurements of anti-atoms. The octupole was made at the Direct Wind facility by the Superconducting Magnet Division at Brookhaven National Laboratory. The magnet was wound with a six-around-one NbTi cable about 1 mm in diameter. It is about 300 mm long, with a radius of 25 mm and a peak field at the conductor of 4.04 T. Specific features of the magnet, including a minimal amount of material in the coil and coil ends with low multipole content, were advantageous to its use in ALPHA. The magnet was operated for six months a year for five years. During this time it underwent about 900 thermal cycles (between 4K and 100K). A novel operational feature is that during the course of data-taking the magnet was repeatedly shut off from its 950 A operating current. The magnet quenches during the shutoff, with a decay constant of 9 ms. Over the course of the five years, the magnet was deliberately quenched many thousands of times. It still performs well.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2011

Multiple error data analysis for RHIC low-energy operations

In FY10, RHIC operated with gold beams at three different energies below its regular injection energy, namely at {radical}s = 5 GeV; 7.7 GeV; and 11.5 GeV: While beam lifetimes of about 10 minutes were achieved at 11.5 GeV and 7.7 GeV; leading to stable collider operation, beam lifetimes during tests at {radical}s = 5 GeV were only of the order of several seconds. Though space charge effects are a major concern in low energy operations due to the 1/{gamma}{sup 2} scaling of the space charge force, these can effectively be ruled out as the main cause of this abysmal performance because of the tiny bunch intensities of approximately 10{sub 8} Au/bunch, compared to 10{sup 9} Au/bunch at the two higher energies. During these 5 GeV tests, large betatron tune spreads were observed, but the extremely short beam lifetime prevented chromaticity measurements. Instead, chromaticities were scanned over a range of 40 units, but to no effect. It was therefore suspected at that time that the large tune spread was caused by higherorder multipole errors in the RHIC magnets.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

The effects of realistic pancake solenoids on particle transport

Solenoids are widely used to transport or focus particle beams. Usually, they are assumed as being ideal solenoids with a high axial-symmetry magnetic field. Using the Vector Field Opera program, we modeled asymmetrical solenoids with realistic geometry defects, caused by finite conductor and current jumpers. Their multipole magnetic components were analyzed with the Fourier fit method; we present some possible optimized methods for them. We also discuss the effects of 'realistic' solenoids on low energy particle transport. The finding in this paper may be applicable to some lower energy particle transport system design.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Parameter Selection and Longitudinal Phase Space Simulation for a Single Stage X-Band FEL Driver at 250 MeV

Hard x-ray Free electron lasers (FEL) are being built or proposed at many accelerator laboratories as it supports wide range of applications in many aspects. Most of the hard x-ray FEL design is similar with the SLAC Linac Coherent Light Source (LCLS), which features a two (or multiple) stage bunch compression. For the first stage of the bunch compression, usually the beam is accelerated in a lower-frequency RF section (such as S-band for LCLS), and then the longitudinal phase space is linearized by a higher-frequency RF section (harmonic RF, such as X-band for LCLS). In this paper, a compact hard x-ray FEL design is proposed, which is based on X-band RF acceleration and eliminating the need of a harmonic RF. The parameter selection and relation is discussed, and the longitudinal phase space simulation is presented. The FEL coherence condition of the electron beam in the undulators requires a large charge density, a small emittance and small energy spread. The RMS electron bunch length from the injector is in the ps scale, with a bunch charge in the range of hundreds pC to several nC, which means that the current is roughly 0.1 kA. According to the requirement from soft x-ray lasing and hard x-ray lasing, a peak current of 1 kA and 3 kA is needed respectively. Thus the bunch has to be compressed. Usually a two stage bunch compression or multipole stage bunch compression is adopted. The z-correlated energy chirp is normally established by letting the beam pass through a section of RF cavities, with a RF phase off crest. As stated above, S-band RF (3 GHz) acceleration could be applied in this section. Due to the nature of RF acceleration wave, the chirp on the bunch is not linear, but has the RF curvature on it. In order to linearize the energy chirp, a harmonic RF section with higher frequency is needed. For LCLS a short X-band RF section (12 GHz) is used which is a fourth order harmonic. The linearized bunch is then passing by a dispersive region, in which the particles with different energy have different path length. A four dipole chicane is the natural choice for the dispersive region. As the example illustrated in Figure 1, the head of the bunch has smaller energy, and gets a stronger bending kick from the dipole magnet, then has a longer path length in the dispersive region. Similarly, the tail of the bunch has larger energy and shorter path length in the dispersive region. At the exit of the dispersive region, the relative longitudinal position of the head and tail of the bunch both move to the center of the bunch, so the bunch length will be shorter.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Homogenous BSCCO-2212 Round Wires for Very High Field Magnets

The performance demands on modern particle accelerators generate a relentless push towards higher field magnets. In turn, advanced high field magnet development places increased demands on superconducting materials. Nb 3 Sn conductors have been used to achieve 16 T in a prototype dipole magnet and are thought to have the capability for ~18 T for accelerator magnets (primarily dipoles but also higher order multipole magnets).

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

A hybrid data acquisition system for magnetic measurements of accelerator magnets

A hybrid data acquisition system was developed for magnetic measurement of superconducting accelerator magnets at LBNL. It consists of a National Instruments dynamic signal acquisition (DSA) card and two Metrolab fast digital integrator (FDI) cards. The DSA card records the induced voltage signals from the rotating probe while the FDI cards records the flux increment integrated over a certain angular step. This allows the comparison of the measurements performed with two cards. In this note, the setup and test of the system is summarized. With a probe rotating at a speed of 0.5 Hz, the multipole coefficients of two magnets were measured with the hybrid system. The coefficients from the DSA and FDI cards agree with each other, indicating that the numerical integration of the raw voltage acquired by the DSA card is comparable to the performance of the FDI card in the current measurement setup.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

A Very Fast and Angular Momentum Conserving Tree Code

There are many methods used to compute the classical gravitational field in astrophysical simulation codes. With the exception of the typically impractical method of direct computation, none ensure conservation of angular momentum to machine precision. Under uniform time-stepping, the Cartesian fast multipole method of Dehnen (also known as the very fast tree code) conserves linear momentum to machine precision. We show that it is possible to modify this method in a way that conserves both angular and linear momenta.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES

Here we use the PFSS model and photospheric data from Wilcox Solar Observatory, SOHO /MDI, SDO/HMI, and SOLIS to compare the coronal field with heliospheric magnetic field measured at 1 au, compiled in the NASA/NSSDC OMNI 2 data set. We calculate their mutual polarity match and the power of the radial decay, p , of the radial field using different source surface distances and different number of harmonic multipoles. We find the average polarity match of 82% for the declining phase, 78%–79% for maxima, 76%–78% for the ascending phase, and 74%–76% for minima. On an average, the source surface of 3.25 R{sub S} gives the best polarity match. We also find strong evidence for solar cycle variation of the optimal source surface distance, with highest values (3.3 R{sub S}) during solar minima and lowest values (2.6 R{sub S}–2.7 R{sub S}) during the other three solar cycle phases. Raising the number of harmonic terms beyond 2 rarely improves the polarity match, showing that the structure of the HMF at 1 au is most of the time rather simple. All four data sets yield fairly similar polarity matches. Thus, polarity comparison is not affected by photospheric field scaling, unlike comparisons of the field intensity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Hydrodynamic Simulations of the Central Molecular Zone with a Realistic Galactic Potential

We present hydrodynamic simulations of gas clouds inflowing from the disk to a few hundred parsec region of the Milky Way. A gravitational potential is generated to include realistic Galactic structures by using thousands of multipole expansions (MEs) that describe 6.4 million stellar particles of a self-consistent Galaxy simulation. We find that a hybrid ME model, with two different basis sets and a thick-disk correction, accurately reproduces the overall structures of the Milky Way. Through non-axisymmetric Galactic structures of an elongated bar and spiral arms, gas clouds in the disk inflow to the nuclear region and form a central molecular zone-like nuclear ring. We find that the size of the nuclear ring evolves into ∼240 pc at T∼1500 Myr, regardless of the initial size. For most simulation runs, the rate of gas inflow to the nuclear region is equilibrated to ∼0.02 M{sub ⊙} yr{sup −1}. The nuclear ring is off-centered, relative to the Galactic center, by the lopsided central mass distribution of the Galaxy model, and thus an asymmetric mass distribution of the nuclear ring arises accordingly. The vertical asymmetry of the Galaxy model also causes the nuclear ring to be tilted along the Galactic plane. During the first ∼100 Myr, the vertical frequency of the gas motion is twice that of the orbital frequency, thus the projected nuclear ring shows a twisted, ∞-like shape.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Galaxy Count Correlation Function in Redshift Space Revisited

In the near future, cosmology will enter the wide and deep galaxy survey era, enabling high-precision studies of the large-scale structure of the universe in three dimensions. To test cosmological models and determine their parameters accurately, it is necessary to use data with exact theoretical expectations expressed in observational parameter space (angles and redshift). The data-driven, galaxy number count fluctuations on redshift shells can be used to build correlation functions ξ(θ,z{sub 1},z{sub 2}) on and between shells to probe the baryonic acoustic oscillations and distance-redshift distortions, as well as gravitational lensing and other relativistic effects. To obtain a numerical estimation of ξ(θ,z{sub 1},z{sub 2}) from a cosmological model, it is typical to use either a closed form derived from a tripolar spherical expansion or to compute the power spectrum C{sub ℓ}(z{sub 1},z{sub 2}) and perform a Legendre polynomial P{sub ℓ}(cos θ) expansion. Here, we present a new derivation of a ξ(θ,z{sub 1},z{sub 2}) closed form using the spherical harmonic expansion and proceeding to an infinite sum over multipoles thanks to an addition theorem. We demonstrate that this new expression is perfectly compatible with the existing closed forms but is simpler to establish and manipulate. We provide formulas for the leading density and redshift-space contributions, but also show how Doppler-like and lensing terms can be easily included in this formalism. We have implemented and made publicly available software for computing those correlations efficiently, without any Limber approximation, and validated this software with the CLASSgal code. It is available at https://gitlab.in2p3.fr/campagne/AngPow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Synchrotron charge density studies in materials chemistry: 16K X-ray charge density of new magnetic metalorganic framwork material, [Mn[subscript 2](C[subscript 8] H[subscript 4]O[subscript 4])[subscript 2](C[subscript 3]H[subscript 7]NO)[subscript 2]]

A new magnetic metal-organic framework material, [Mn{sub 2}(C{sub 8} H{sub 4}O{sub 4}){sub 2}(C{sub 3}H{sub 7}NO){sub 2}], has been synthesized. The structure consists of chains of carboxylate-bridged Mn atoms interconnected with acid linkers, giving much larger interchain than intrachain Mn...Mn distances. Magnetic susceptibility data fitted to a Curie-Weiss law give {Theta} = -5.7 K and a total magnetic moment of 5.96 {micro}{sub B}. The heat capacity provides no evidence of magnetic ordering down to 2 K. The X-ray charge density was determined from multipole modeling of 16 (1) K single-crystal synchrotron-radiation data. The structural surroundings of the two unique Mn centers are different, but orbital population analysis reveals close to single electron occupation in all 3d orbitals of both Mn sites, in agreement with the magnetic susceptibility measurements. Bader topological analysis shows the presence of direct chemical Mn...Mn interactions only in two out of three intrachain contacts, which suggests a 'broken' chain. The topological measures and approximate energy densities at the metal-ligand bond critical points ({rho}, {del}{sup 2}{rho}, G, V and H) indicate ionic interactions. Formal electron counting suggests mixed-valence Mn sites, but this hypothesis is not supported by the Bader atomic charges [q(Mn) = +2.035 and +2.031].

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

On the Importance of Symmetrizing RF Coupler Fields for Low Emittance Beams

The input power of accelerator structure is normally fed through a coupling slot(s) on the outer wall of the accelerator structure via magnetic coupling. While providing perfect matching, the coupling slots may produce non-axial-symmetric fields in the coupler cell that can induce emittance growth as the beam is accelerated in such a field. This effect is especially important for low emittance beams at low energies such as in the injector accelerators for light sources. In this paper, we present studies of multipole fields of different rf coupler designs and their effect on beam emittance for an X-band photocathode gun being jointly designed with LLNL, and X-band accelerator structures. We will present symmetrized rf coupler designs for these components to preserve the beam emittance.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Design and Field Measurements of Printed-Circuit Quadrupoles and Dipoles

Air-core printed-circuit (PC) quadrupoles and dipoles have been developed for the University of Maryland electron ring, currently under construction. The quadrupoles and dipoles are characterized by very small magnetic fields (about 15 G at the aperture edge) and small aspect ratios (length/diameter < 1). We review the theory behind the design of the PC lenses and bending elements, and present general expressions for estimating the values of integrated field and integrated field gradient as functions of design parameters. The new quadrupole magnet represents an improvement over an earlier version which was based on an empirical approach. Further, we summarize the results of multipole content of the magnet fields as measured with a rotating coil apparatus of special construction. The results are compared with calculations with an iron-free magnetics code and are related to different types of errors in the manufacture and assembly of the PC magnets.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Dynamic Aperture and Tolerances for PEP-X Ultimate Storage Ring Design

A lattice for the PEP-X ultimate storage ring light source, having 11 pm-rad natural emittance at a beam energy of 4.5 GeV at zero current, using 90 m of damping wiggler and fitting into the existing 2.2-km PEP-II tunnel, has been recently designed. Such a low emittance lattice requires very strong sextupoles for chromaticity correction, which in turn introduce strong non-linear field effects that limit the beam dynamic aperture. In order to maximize the dynamic aperture we choose the cell phases to cancel the third and fourth order geometric resonances in each 8-cell arc. Four families of chromatic sextupoles and six families of geometric (or harmonic) sextupoles are added to correct the chromatic and amplitude-dependent tunes. To find the best settings of the ten sextupole families, we use a Multi-Objective Genetic Optimizer employing elegant to optimize the beam lifetime and dynamic aperture simultaneously. Then we evaluate dynamic aperture reduction caused by magnetic field multipole errors, magnet fabrication errors and misalignments. A sufficient dynamic aperture is obtained for injection, as well as workable beam lifetime.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Status of the ATF2 Lattices

The current status for the ATF2 Nominal and Ultra-low {beta}* lattices are presented in this paper. New lattice designs have been obtained in order to minimise the impact of the last interpretation of multipole measurements that have been included into the model. However, the new ATF2 Ultra-low design is not able to recover the expected vertical beam size at the IP with the current magnet distribution. Therefore, different quadrupole sorting have been studied. A significant gain is evident for the ATF2 Ultra-low lattice when sorting the magnets according to the skew-sextupolar components. The ATF2 Nominal lattice is also expected to benefit from the new sorting. Tuning results of the new ATF2 Ultra-low lattice under realistic imperfections are also reported.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Progress on muon parametric-resonance ionization cooling channel development

Parametric-resonance Ionization Cooling (PIC) is intended as the final 6D cooling stage of a high-luminosity muon collider. To implement PIC, a continuous-field twin-helix magnetic channel was developed. A 6D cooling with stochastic effects off is demonstrated in a GEANT4/G4beamline model of a system where wedge-shaped Be absorbers are placed at the appropriate dispersion points in the twin-helix channel and are followed by short rf cavities. To proceed to cooling simulations with stochastics on, compensation of the beam aberrations from one absorber to another is required. Initial results on aberration compensation using a set of various-order continuous multipole fields are presented. As another avenue to mitigate the aberration effect, we optimize the cooling channel's period length. We observe a parasitic parametric resonance naturally occurring in the channel's horizontal plane due to the periodic beam energy modulation caused by the absorbers and rf. We discuss options for compensating this resonance and/or properly combining it with the induced half-integer parametric resonance needed for PIC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Dynamic Simulation with on the fly Doppler Broadening treatment in RMC

In conventional transient analysis, thermal hydraulics calculations are generally coupled to deterministic neutronics solvers. Due to the limited accuracy of deterministic methods and the rapid development of computer power, Monte Carlo neutronics calculations have received increasing attention nowadays. One of challenges for Monte Carlo methods coupled to thermal hydraulics calculations is the temperature dependent cross sections. Taking the detailed temperature distribution feedback into account, each nuclide may be in a large range of temperatures, pre-generated libraries at every temperature point are no longer applicable owing to unaffordable memory footprint. Different solutions have been proposed to deal with this problem. TMS, accounting for Doppler broadening effect by the explicit treatment of thermal motion, was used to model fast transients in Serpent 2. Stochastic mixing, using two sets of libraries for each nuclide and mixing them to get the correct temperature, was used in transient analysis by Sjenitzer and Hoogenboom. Besides, there are other on the fly Doppler broadening methods being developed, including series expansion of cross sections, multipole representation, different integration algorithms based on Doppler broadening equation. In this paper, on the fly Doppler broadening treatment based on integration algorithms was applied to transient calculation in Reactor Monte Carlo code RMC . In the following section, the integration algorithms implemented and dynamic simulation method in RMC is briefly introduced. Finally, a pure physical transient fuel assembly model was chosen for numerical validation. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗