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DOE OSTI2016

Sci-Sat AM: Radiation Dosimetry and Practical Therapy Solutions - 12: Suitability of plan class specific reference fields for estimating dosimeter correction factors for small clinical CyberKnife fields

Purpose: The purpose of this work is to investigate the utility of plan class specific reference (PCSR) fields for predicting dosimeter response within isocentric and non-isocentric composite clinical fields using the smallest fields employed by the CyberKnife radiosurgery system. Methods: Monte Carlo dosimeter response correction factors (CFs) were calculated for a plastic scintillator and microchamber dosimeter in 21 clinical fields and 9 candidate plan-class PCSR fields which employ the 5, 7.5 and 10 mm diameter collimators. Measurements were performed in 5 PCSR fields to confirm the predicted relative response of detectors in the same field. Results: Ratios of corrected measured dose in the PCSR fields agree to within 1% of unity. Calculated CFs for isocentric fields agree within 1.5% of those for PCSR fields. Large and variable microchamber CFs are required for non-isocentric fields, with differences as high as 5% between different clinical fields in the same plan class and 4% within the same field depending on the point of measurement. Non-isocentric PCSR fields constructed to have relatively homogenous dose over a region larger than the detector have very different ion chamber CFs from clinical fields. The plastic scintillator detector has much more consistent response within each plan class but still require 3–4% corrections in some fields. Conclusions: While the PCSR field concept is useful for small isocentric fields, this approach may not be appropriate for non-isocentric clinical fields which exhibit large and variable ion chamber CFs which differ significantly from CFs for homogenous field PCSRs.

61 RADIATION PROTECTION AND DOSIMETRY↗

DOE OSTI2022

Wound Field and Hybrid Synchronous Machines for EV Traction with Brushless Capacitive Rotor Field Excitation (Final Report)

This project focused on the development of wound field synchronous machines (WFSMs) and hybrid excitation synchronous machines (HESMs) with brushless capacitive power transfer for the field excitation. The target application for the machines developed is the main traction motor in electric vehicles. The magnetization in these types of machines is provided by a field winding on the rotor which is excited with DC current. The magnetization level in the machine can be varied by changing the magnitude of the field current. The variable magnetization or field is one of the key features of WFSMs. WFSMs are commonly used as generators however they have several attractive features for automotive traction applications. 1) No use of permanent magnet: Rare earth permanent magnets are primarily mined and processed in China. They have been subject to large price and supply variations and their export may be restricted during times of geopolitical tension. 2) Easy field weakening: Wound field synchronous machines have complete control of their field excitation. With proper design, this type of machine can electromagnetically have an infinite constant power speed range. 3) High power factor: With proper choice of the field excitation, WFSMs may be operated with high or even unity power factor. This potentially allows for the inverter connected to the stator winding to be downsized. In comparison induction machines and interior permanent magnet synchronous machines must supply reactive power to the stator increasing the kVA rating and cost of the inverter. 4) Reduced iron losses at high speed: By reducing the field excitation, the iron loss in the stator can be reduced. This is in comparison to interior permanent magnet synchronous machines which must use stator current to buck or reduce the flux produced by the permanent magnets. Generally, WFSMs have their highest efficiency at high speed. 5) Torque output at high temperatures: The magnetization provided by the field winding only depends on the field current and not on the field winding temperature. This is in contrast to permanent magnet machines where the permanent magnet flux decreases as the magnet temperature increases. Historically a number of approaches have been developed to provide DC current to the rotating field winding in WFSM’s including brushes and slip rings, low frequency brushless exciters, and high frequency rotary transformers and rectifiers. In this project, a different approach was used: brushless capacitive power transfer. Brushless capacitive power transfer uses two sets of rotating capacitors or electrodes in which an AC electric field is established by a high frequency inverter. A displacement current can flow through the airgap in the rotating capacitors which is rectified on the rotor using a diode bridge. The potential advantage of capacitive power transfer is that there is no need for heavy iron to guide magnetic flux. The electric flux lines terminate on the charges on the rotary capacitor surfaces. This should also limit the electric field outside the rotary capacitor airgaps. The main challenge with capacitive power transfer is that the capacitance and surface area of the rotating capacitors is small. Because capacitive power transfer systems are essentially a dual of a magnetic system, an Ampere per Hertz relationship is characteristic versus a Volts per Hertz relationship. A very high frequency power inverter must be used to provide sufficient excitation to the field winding. The concept of using capacitive power transfer to excite a high performance WFSM was initially developed in a previous U.S.A. Dept. of Energy project, DE-EE0006829. This project focused on increasing the power density of the WFSMs and reducing the cost and manufacturing complexity of the capacitive power transfer system. This project has demonstrated that WFSMs with brushless capacitive field power transfer can provide a high-power density and low-cost automotive powertrain technology.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2016

Efficient magnetic fields for supporting toroidal plasmas

The magnetic field that supports tokamak and stellarator plasmas must be produced by coils well separated from the plasma. However, the larger the separation, the more difficult it is to produce a given magnetic field in the plasma region, so plasma configurations should be chosen that can be supported as efficiently as possible by distant coils. The efficiency of an externally generated magnetic field is a measure of the field's shaping component magnitude at the plasma compared to the magnitude near the coils; the efficiency of a plasma equilibrium can be measured using the efficiency of the required external shaping field. Counterintuitively, plasma shapes with low curvature and spectral width may have low efficiency, whereas plasma shapes with sharp edges may have high efficiency. Two precise measures of magnetic field efficiency, which correctly identify such differences in difficulty, will be examined. These measures, which can be expressed as matrices, relate the externally produced normal magnetic field on the plasma surface to the either the normal field or current on a distant control surface. A singular value decomposition (SVD) of either matrix yields an efficiency ordered basis for the magnetic field distributions. Calculations are carried out for both tokamak and stellarator cases. For axisymmetric surfaces with circular cross-section, the SVD is calculated analytically, and the range of poloidal and toroidal mode numbers that can be controlled to a given desired level is determined. If formulated properly, these efficiency measures are independent of the coordinates used to parameterize the surfaces.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

ELECTRON ACCELERATION AT A CORONAL SHOCK PROPAGATING THROUGH A LARGE-SCALE STREAMER-LIKE MAGNETIC FIELD

Using a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with a streamer-like magnetic field featuring a partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of the magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of the efficient electron acceleration region along the shock front during its propagation. We also find that, in general, the electron acceleration at the shock flank is not as efficient as that at the top of the closed field because a collapsing magnetic trap can be formed at the top. In addition, we find that the energy spectra of electrons are power-law-like, first hardening then softening with the spectral index varying in a range of −3 to −6. Physical interpretations of the results and implications for the study of solar radio bursts are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Small field in-air output factors: The role of miniphantom design and dosimeter type

Purpose: The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in-air, S{sub c} is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured S{sub c} values for small fields and to validate the measurements against Monte Carlo simulations. Methods: Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5–100 mm) and SRS cones (4–30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict S{sub c} as measured by the FOD. Results: For all detectors, S{sub c} agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%. Conclusions: Miniphantom design is the most important variable for reproducible and accurate measurements of the in-air output ratio, S{sub c}, in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, S{sub cp,} the selection of detector type for S{sub c} is not critical, provided the active dosimeter volume is small relative to the field size.

61 RADIATION PROTECTION AND DOSIMETRY↗

NASA NTRS1993

Effects of a parallel electric field and the geomagnetic field in the topside ionosphere on auroral and photoelectron energy distributions

The consequences of electric field acceleration and an inhomogeneous magnetic field on auroral electron energy distributions in the topside ionosphere are investigated. The one-dimensional, steady state electron transport equation includes elastic and inelastic collisions, an inhomogeneous magnetic field, and a field-aligned electric field. The case of a self-consistent polarization electric field is considered first. The self-consistent field is derived by solving the continuity equation for all ions of importance, including diffusion of O(+) and H(+), and the electron and ion energy equations to derive the electron and ion temperatures. The system of coupled electron transport, continuity, and energy equations is solved numerically. Recognizing observations of parallel electric fields of larger magnitude than the baseline case of the polarization field, the effect of two model fields on the electron distribution function is investigated. In one case the field is increased from the polarization field magnitude at 300 km to a maximum at the upper boundary of 800 km, and in another case a uniform field is added to the polarization field. Substantial perturbations of the low energy portion of the electron flux are produced: an upward directed electric field accelerates the downward directed flux of low-energy secondary electrons and decelerates the upward directed component. Above about 400 km the inhomogeneous magnetic field produces anisotropies in the angular distribution of the electron flux. The effects of the perturbed energy distributions on auroral spectral emission features are noted.

Min, Q.-L.↗

NASA NTRS1993

Effects of a Parallel Electric Field and the Geomagnetic Field in the Topside Ionosphere on Auroral and Photoelectron Energy Distributions

The consequences of electric field acceleration and an inhomogencous magnetic field on auroral electron energy distributions in the topside ionosphere are investigated. The one- dimensional, steady state electron transport equation includes elastic and inelastic collisions, an inhomogencous magnetic field, and a field-aligned electric field. The case of a self-consistent polarization electric field is considered first. The self-consistent field is derived by solving the continuity equation for all ions of importance, including diffusion of 0(+) and H(+), and the electron and ion energy equations to derive the electron and ion temperatures. The system of coupled electron transport, continuity, and energy equations is solved numerically. Recognizing observations of parallel electric fields of larger magnitude than the baseline case of the polarization field, the effect of two model fields on the electron distribution function in investigated. In one case the field is increased from the polarization field magnitude at 300 km to a maximum at the upper boundary of 800 km, and in another case a uniform field is added to the polarization field. Substantial perturbations of the low energy portion of the electron flux are produced: an upward directed electric field accelerates the downward directed flux of low-energy secondary electrons and decelerates the upward directed component. Above about 400 km the inhomogencous magnetic field produces anisotropies in the angular distribution of the electron flux. The effects of the perturbed energy distributions on auroral spectral emission features are noted.

Min, Q.-L.↗

DOE OSTI2015

TU-F-CAMPUS-T-04: Variations in Nominally Identical Small Fields From Photon Jaw Reproducibility and Associated Effects On Small Field Dosimetric Parameters

Purpose: To investigate uncertainties in small field output factors and detector specific correction factors from variations in field size for nominally identical fields using measurements and Monte Carlo simulations. Methods: Repeated measurements of small field output factors are made with the Exradin W1 (plastic scintillation detector) and the PTW microDiamond (synthetic diamond detector) in beams from the Elekta Precise linear accelerator. We investigate corrections for a 0.6x0.6 cm{sup 2} nominal field size shaped with secondary photon jaws at 100 cm source to surface distance (SSD). Measurements of small field profiles are made in a water phantom at 10 cm depth using both detectors and are subsequently used for accurate detector positioning. Supplementary Monte Carlo simulations with EGSnrc are used to calculate the absorbed dose to the detector and absorbed dose to water under the same conditions when varying field size. The jaws in the BEAMnrc model of the accelerator are varied by a reasonable amount to investigate the same situation without the influence of measurements uncertainties (such as detector positioning or variation in beam output). Results: For both detectors, small field output factor measurements differ by up to 11 % when repeated measurements are made in nominally identical 0.6x0.6 cm{sup 2} fields. Variations in the FWHM of measured profiles are consistent with field size variations reported by the accelerator. Monte Carlo simulations of the dose to detector vary by up to 16 % under worst case variations in field size. These variations are also present in calculations of absorbed dose to water. However, calculated detector specific correction factors are within 1 % when varying field size because of cancellation of effects. Conclusion: Clinical physicists should be aware of potentially significant uncertainties in measured output factors required for dosimetry of small fields due to field size variations for nominally identical fields.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2018

Modeling the adsorbate coverage distribution over a multi-faceted catalytic grain in the presence of an electric field: O/Fe from first principles

The impact of an external electric field on the concerted behavior of oxygen over a multi-faceted catalytic Fe grain is determined via the interpolation of ab initio models of oxygen adsorption on Fe(100), Fe(110), and Fe(111) in the presence of an external electric field. The application of both negative and positive electric fields weaken the adsorption strength for oxygen on all three surface facets, with Fe(110) experiencing the greatest effect. Kinetic models of a multi-faceted catalytic Fe grain show that the average oxygen coverage over the grain surface is reduced under the influence of both a negative and positive electric field, which are consistent with phase diagram results at comparable pressures. Furthermore, we show that there is a weak synergistic effect between a Pd promoter and a positive electric field on the oxygen adsorption energy, i.e. the Pd promoter and electric field combination weaken the oxygen adsorption energy to a greater degree than the simple addition of both components separately. Overall, the work shows that the application of an applied external electric field may be a useful tool in fine-tuning chemical properties of Fe-based catalysts in hydrodeoxygenation applications.

Bray, Jacob E.↗

DOE OSTI2014

Probing the plasma near high power wave launchers in fusion devices for static and dynamic electric fields (invited)

An exploratory study was carried out in the long-pulse tokamak Tore Supra, to determine if electric fields in the plasma around high-power, RF wave launchers could be measured with non-intrusive, passive, optical emission spectroscopy. The focus was in particular on the use of the external electric field Stark effect. The feasibility was found to be strongly dependent on the spatial extent of the electric fields and overlap between regions of strong (>∼1 kV/cm) electric fields and regions of plasma particle recycling and plasma-induced, spectral line emission. Most amenable to the measurement was the RF electric field in edge plasma, in front of a lower hybrid heating and current drive launcher. Electric field strengths and direction, derived from fitting the acquired spectra to a model including time-dependent Stark effect and the tokamak-range magnetic field Zeeman-effect, were found to be in good agreement with full-wave modeling of the observed launcher.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

Improvement of dose distribution in breast radiotherapy using a reversible transverse magnetic field Linac-MR unit

Purpose: To investigate the improvement in dose distribution in tangential breast radiotherapy using a reversible transverse magnetic field that maintains the same direction of Lorentz force between two fields. The investigation has a potential application in future Linac-MR units. Methods: Computed tomography images of four patients and magnetic fields of 0.25–1.5 Tesla (T) were used for Monte Carlo simulation. Two patients had intact breast while the other two had mastectomy. Simulations of planning and chest wall irradiation were similar to the actual clinical process. The direction of superior-inferior magnetic field for the medial treatment beam was reversed for the lateral beam. Results: For the ipsilateral lung and heart mean doses were reduced by a mean (range) of 45.8% (27.6%–58.6%) and 26.0% (20.2%–38.9%), respectively, depending on various treatment plan setups. The mean V{sub 20} for ipsilateral lung was reduced by 55.0% (43.6%–77.3%). In addition acceptable results were shown after simulation of 0.25 T magnetic field demonstrated in dose-volume reductions of the heart, ipsilateral lung, and noninvolved skin. Conclusions: Applying a reversible magnetic field during breast radiotherapy, not only reduces the dose to the lung and heart but also produces a sharp drop dose volume histogram for planning target volume, because of bending of the path of secondary charged particles toward the chest wall by the Lorentz force. The simulations have shown that use of the magnetic field at 1.5 T is not feasible for clinical applications due to the increase of ipsilateral chest wall skin dose in comparison to the conventional planning while 0.25 T is suitable for all patients due to dose reduction to the chest wall skin.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2014

Self-generated magnetic fields in direct-drive implosion experiments

Electric and self-generated magnetic fields in direct-drive implosion experiments on the OMEGA Laser Facility were investigated employing radiography with ∼10- to 60-MeV protons. The experiment used plastic-shell targets with imposed surface defects (glue spots, wires, and mount stalks), which enhance self-generated fields. The fields were measured during the 1-ns laser drive with an on-target intensity ∼10{sup 15} W/cm{sup 2}. Proton radiographs show multiple ring-like structures produced by electric fields ∼10{sup 7} V/cm and fine structures from surface defects, indicating self-generated fields up to ∼3 MG. These electric and magnetic fields show good agreement with two-dimensional magnetohydrodynamic simulations when the latter include the ∇T{sub e} × ∇n{sub e} source, Nernst convection, and anisotropic resistivity. The simulations predict that self-generated fields affect heat fluxes in the conduction zone and, through this, affect the growth of local perturbations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Rapid change of field line connectivity and reconnection in stochastic magnetic fields

Magnetic fields without a direction of continuous symmetry have the generic feature that neighboring field lines exponentiate away from each other and become stochastic, and hence the ideal constraint of preserving magnetic field line connectivity becomes exponentially sensitive to small deviations from ideal Ohm's law. The idea of breaking field line connectivity by stochasticity as a mechanism for fast reconnection is tested with numerical simulations based on reduced magnetohydrodynamics equations with a strong guide field line-tied to two perfectly conducting end plates. Starting from an ideally stable force-free equilibrium, the system is allowed to undergo resistive relaxation. Two distinct phases are found in the process of resistive relaxation. During the quasi-static phase, rapid change of field line connectivity and strong induced flow are found in regions of high field line exponentiation. However, although the field line connectivity of individual field lines can change rapidly, the overall pattern of field line mapping appears to deform gradually. From this perspective, field line exponentiation appears to cause enhanced diffusion rather than reconnection. In some cases, resistive quasi-static evolution can cause the ideally stable initial equilibrium to cross a stability threshold, leading to formation of intense current filaments and rapid change of field line mapping into a qualitatively different pattern. It is in this onset phase that the change of field line connectivity is more appropriately designated as magnetic reconnection. Our results show that rapid change of field line connectivity appears to be a necessary, but not a sufficient condition for fast reconnection.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2019

High Precision Magnetic Field Measurement for the Muon g-2 Experiment

The Muon g-2 experiment at Fermilab will measure the anomalous magnetic moment of the muon, aµ with a precision of 140 ppb, a four-fold improvement over the former BNL E821 result. If the current discrepancy of about 3.5σ between the Standard Model value for aµ and the former experiments was real, then the new experiment would verify the deviation with >7σ significance. The experiment uses muons with the magic momentum of 3.09 GeV/c stored in a very homogeneous magnetic field of a 45 m long storage ring. The determination of aµ requires the precise knowledge of both the anomalous muon spin precession, ωa, and the magnetic field, ωp, expressed in terms of the free-proton Larmor frequency. The field measurement uses the high precision tool of pulsed Nuclear Magnetic Resonance (pNMR) and requires cross-calibrating a set of probes: a very accurate NMR probe calibrates the 17 probes on the in-vacuum field mapping system (the so-called trolley). This trolley in turn is used to map the field over the full azimuth of the storage ring. The trolley measurement also provides the calibration of the 378 fixed probes around the ring that constantly monitor the field drift. This article presents the summary of the entire field measurement system needed to determine ωp. This work was supported in part by the US DOE, Fermilab and US DOE OHEP under contract No. KA2201020.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2015

First frontier field constraints on the cosmic star formation rate density AT z ~ 10—the impact of lensing shear on completeness of high-redshift galaxy samples

In this work, we search the complete Hubble Frontier Field data set of Abell 2744 and its parallel field for $z\sim 10$ sources to further refine the evolution of the cosmic star formation rate density (SFRD) between $z\sim 8$ and $z\sim 10$. We independently confirm two images of the recently discovered triply imaged $z\sim 9.8$ source by Zitrin et al. and set an upper limit for similar $z\sim 10$ galaxies with red colors of ${J}_{125}-{H}_{160}\gt 1.2$ in the parallel field of Abell 2744. We utilize extensive simulations to derive the effective selection volume of Lyman-break galaxies at $z\sim 10$, both in the lensed cluster field and in the adjacent parallel field. Particular care is taken to include position-dependent lensing shear to accurately account for the expected sizes and morphologies of highly magnified sources. We show that both source blending and shear reduce the completeness at a given observed magnitude in the cluster, particularly near the critical curves. These effects have a significant, but largely overlooked, impact on the detectability of high-redshift sources behind clusters, and substantially reduce the expected number of highly magnified sources. The detections and limits from both pointings result in an SFRD which is consistent within the uncertainties with previous estimates at $z\sim 10$ from blank fields. The combination of these new results with all other estimates is also consistent with a rapidly declining SFRD in the 170 Myr from $z\sim 8$ to $z\sim 10$ as predicted by cosmological simulations and dark-matter halo evolution in ΛCDM. Once biases introduced by magnification-dependent completeness are accounted for, the full six cluster and parallel Frontier Field program will be an extremely powerful new data set to probe the evolution of the galaxy population at $z\gt 8$ before the advent of the James Webb Space Telescope.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Influence of classical anisotropy fields on the properties of Heisenberg antiferromagnets within unified molecular field theory

Here, a comprehensive study of the influence of classical anisotropy fields on the magnetic properties of Heisenberg antiferromagnets within unified molecular field theory versus temperature T , magnetic field H , and anisotropy field parameter h A1 is presented for systems comprised of identical crystallographically-equivalent local moments. The anisotropy field for collinear z-axis antiferromagnetic (AFM) ordering is constructed so that it is aligned in the direction of each ordered and/or field-induced thermal-average moment with a magnitude proportional to the moment, whereas that for XY anisotropy is defined to be in the direction of the projection of the moment onto the xy plane, again with a magnitude proportional to the moment. Properties studied include the zero-field Néel temperature T N , ordered moment, heat capacity, and anisotropic magnetic susceptibility of the AFM phase versus T with moments aligned either along the z axis or in the xy plane. Also determined are the high-field magnetization perpendicular to the axis or plane of collinear or planar noncollinear AFM ordering, the high-field magnetization along the z axis of a collinear z -axis AFM, spin-flop (SF), and paramagnetic (PM) phases, and the free energies of these phases versus T , H , and h A1 . Phase diagrams at T =0 in the H z – h A1 plane and at T > 0 in the H z – T plane are constructed for spins S =1/2. For h A1 =0, the SF phase is stable at low field and the PM phase at high field with no AFM phase present. As h A1 increases, the phase diagram contains the AFM, SF, and PM phases. Further increases in h A1 lead to the disappearance of the SF phase and the appearance of a tricritical point on the AFM-PM transition curve. Furthermore, applications of the theory to extract h A1 from experimental low-field magnetic susceptibility data and high-field magnetization versus field isotherms for single crystals of AFMs are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Three-Dimensional Charge Density Wave Order in YBa2Cu3O6.67 at High Magnetic Fields

Charge density wave (CDW) correlations have been shown to universally exist in cuprate superconductors. However, their nature at high fields inferred from nuclear magnetic resonance is distinct from that measured by x-ray scattering at zero and low fields. Here we combine a pulsed magnet with an x-ray free electron laser to characterize the CDW in YBa2Cu3O6.67 via x-ray scattering in fields up to 28 Tesla. While the zero-field CDW order, which develops below T ~ 150 K, is essentially two-dimensional, at lower temperature and beyond 15 Tesla, another three-dimensionally ordered CDW emerges. The field-induced CDW onsets around the zero-field superconducting transition temperature, yet the incommensurate inplane ordering vector is field-independent. This implies that the two forms of CDW and hightemperature superconductivity are intimately linked.

Gerber, S.↗

NASA NTRS1998

Constructing the Coronal Magnetic Field: by Correlating Parameterized Magnetic Field Lines with Observed Coronal Plasma Structures

The reconstruction of the coronal magnetic field is carried out using a perturbation procedure. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal fluxtubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures (1) that the normal component of the photospheric field remains unchanged, (2) that the field is given in the entire corona, (3) that the field remains divergence free, and (4) that electrical currents are introduced into the field. It is demonstrated that a simple radial parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 Nov 26. At a coronal height of 30 km, the resulting magnetic field is a non-force free magnetic field with the maximum Lorentz force being on the order of 2.6 x 10(exp -9) dyn resulting from an electric current density of $0.13 mu A/ sq m. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field.

Gary, G. A.↗