Longitudinal Field Gradient Research Articles

Modelling PM dipoles with longitudinal field gradient for SILA facility

Computational models are described which were built for parametric simulations of permanent magnet (PM) dipoles with a longitudinal field gradient. The dipoles will be used as bending magnets in the synchrotron facility SILA, a new project of the National Research Center “Kurchatov Institute”. The dipoles employ a Sm-Co permanent magnet material and pure iron. The need for high magnetic performance and anticipated interference between neighbouring components is a challenge for design and construction of the magnet system. For this reason the magnet designs must rely on precision 3D simulations. The dipoles are described in detail with parameterized models. Parametric simulations are to be applied at all stages of the project till the commissioning to provide realistic prediction of the magnet behaviour.

Spectroscopy of nanoantenna-covered Cu2O : Towards enhancing quadrupole transitions in Rydberg excitons

Plasmonic antennas channel incoming light into a confined near field that comes along with strong field gradients at the edges. Transverse field gradients are capable of driving quadrupole transitions in a more effective way than the weak longitudinal field gradient in dipolar light. Cuprous oxide Rydberg excitons with principal quantum number $n$ between 5 and 15 are several tens to a few hundreds of nanometers in diameter and can have different orbital angular momentum quantum numbers. Usually, only $P$- and $F$-excitons are electric dipole allowed, while $S$- and $D$-excitons remain dark. Here, we use 30-nm-wide and 60- to 110-nm-long plasmonic aluminum nanoantennas deposited on the cuprous oxide crystal surface to create transverse field gradients in similar spatial dimensions as the size of the Rydberg excitons. This way, light field and matter wave function overlap spatially and quadrupole transitions of $S$excitons are enhanced. The relative enhancement of the $S$-exciton oscillator strength with respect to the $P$-exciton oscillator accounts for a factor of up to 1.4.

Towards magnetic-field-modulated radiotherapy (MagMRT) with an MR-LINAC—a Monte Carlo study

Objective. The feasibility of magnetic-field-modulated radiotherapy (MagMRT) with an MR-LINAC was investigated by studying the effects of dose enhancement and reduction using a transverse magnetic field with a longitudinal gradient applied along a photon radiation beam. Approach. Geant4 simulation toolkit was used to perform Monte Carlo simulations on a water phantom with the energy spectrum of a 7 MV flattening-filter-free photon beam from an Elekta Unity system as the source of radiation. Linear magnetic field gradients with magnitudes ranged from 1 to 6 T cm−1 and spatial extents of 1–3 cm were used to study the dependence of dose modulation on these two parameters. The effects of radiation field size and the ability of dose modulation through optimizing the waveform of magnetic field variation were also explored. Main results. Our results show that dose enhancement and reduction can be achieved by applying a transverse magnetic field with a longitudinal field gradient along a photon beam. The steeper the gradient, the more prominent is the effect. A dose enhancement of 33% and a dose reduction of 22% are found for a magnetic gradient of 6 T cm−1 and −6 T cm−1 respectively. The spatial extent of the dose modulation effect which is greater than 3% is found to be around 1–2 cm. Both the dose enhancement and reduction effects are independent of the radiation field sizes, but they exhibit different behaviors with the spatial extents of the gradient. Multiple locations of dose enhancement and reduction can be produced by modulating the waveform of the magnetic field variation along the radiation beam, demonstrating a vast degree of freedom in the modulation aspect of MagMRT. Significance. MagMRT is a conceptually feasible and promising new radiotherapy modulation technique along the direction of the radiation beam.

On the use of twisted photons for spectroscopy of impurity centers in crystals

Absorption of twisted photons for quadrupole $^{4}f\text{\ensuremath{-}}^{4}f$ transitions in $\mathrm{R}{\mathrm{e}}^{3+}$ impurity ions in micro- and macrocrystals is considered. Two types of twisted photon beams---the Laguerre-Gaussian and the Bessel-Gaussian beams---are considered and compared. Twisted photons have a nonzero transverse field gradient, which leads to selection rules for quadrupole transitions other than those in the case of the usual quadrupole transitions caused by a longitudinal field gradient. This allows one to observe new spectral lines which are absent in the usual absorption spectra. For microcrystals placed at the beam axis of twisted photons the otherwise dominating forced dipole $^{4}f\text{\ensuremath{-}}^{4}f$ transitions are absent, which simplifies the observation of new spectral lines.

Precise measurement of magnetic field gradients from free spin precession signals of 3He and 129Xe magnetometers

We report on precise measurements of magnetic field gradients extracted from transverse relaxation rates of precessing spin samples. The experimental approach is based on the free precession of gaseous, nuclear spin polarized $^3$He and $^{129}$Xe atoms in a spherical cell inside a magnetic guiding field of about 400 nT using LT$_C$ SQUIDs as low-noise magnetic flux detectors. The transverse relaxation rates of both spin species are simultaneously monitored as magnetic field gradients are varied. For transverse relaxation times reaching 100 h, the residual longitudinal field gradient across the spin sample could be deduced to be$|\vec{\nabla}B_z|=(5.6 \pm 0.4)$ pT/cm. The method takes advantage of the high signal-to-noise ratio with which the decaying spin precession signal can be monitored that finally leads to the exceptional accuracy to determine magnetic field gradients at the sub pT/cm scale.

The Project of a Magnetic System for Compact x-ray Light Source

The low-emittance lattice cell concept with usage of superconducting bending magnets with longitudinal field gradient and anti-bends is applied to develop a compact storage ring for x-ray light source of high brilliance. Its synchrotron radiation generation potential is compared with existing compact light source.

CORONAL INFLOWS DURING THE INTERVAL 1996-2014

We extend our previous counts of coronal inflows from the 5 yr interval 1996-2001 to the 18 yr interval 1996-2014. By comparing stackplots of these counts with similar stackplots of the source-surface magnetic field and its longitudinal gradient, we find that the inflows occur in long-lived streams with counting rates in excess of 18 inflows per day at sector boundaries where the gradient exceeds 0.22 G rad{sup –1}. These streams are responsible for the high (86%) correlation between the inflow rate and the longitudinal field gradient. The overall inflow rate was several times larger in sunspot cycle 23 than it has been so far in cycle 24, reflecting the relatively weak source-surface fields during this cycle. By comparison, in cycles 21-22, the source-surface field and its gradient had bursts of great strength, as if large numbers of inflows occurred during those cycles. We find no obvious relation between inflows and coronal mass ejections (CMEs) on timescales of days to weeks, regardless of the speeds of the CMEs, and only a 60% correlation on timescales of months, provided the CMEs are fast (V > 600 km s{sup –1}). We conclude that most of the flux carried out by CMEs is returnedmore » to the Sun via field line reconnection well below the 2.0 R {sub ☉} inner limit of the LASCO field of view, and that the remainder accumulates in the outer corona for an eventual return at sector boundaries.« less

Temporal and spatial expansion of a multi-dimensional model for electron acceleration in the bubble regime

AbstractAn extended analytical model for particle dynamics in fields of a highly-nonlinear plasma wakefield (the bubble or blow out regime) is derived. A recently proposed piecewise model is generalized to include a time dependent bubble radius and full field solution in the acceleration direction. Incorporation of the cavity dynamics in the model is required to simulate the particle trapping properly. On the other hand, it is shown that the previously reported piecewise model does not reproduce the formation of a mono energetic peak in the particle spectrum. The mono energetic electron beams are recovered only when the full longitudinal field gradient is included in the model.

Longitudinal wake field for an electron beam accelerated through an ultrahigh field gradient

Electron accelerators with higher and higher longitudinal field gradients are desirable, as they allow for the production of high-energy beams by means of compact and cheap setups. Laser-plasma acceleration technique appears to constitute the more promising breakthrough in this direction, delivering unprecedent field gradients up to TV/m. In this article we give a quantitative description of the impact of longitudinal wake fields on the electron beam. Our paper is based on the solution of Maxwell's equations for the longitudinal field. Our conclusions are valid when the acceleration distance is much smaller than the overtaking length, that is the length that electrons travel as a light signal from the tail of the bunch overtakes the head of the bunch. This condition is well verified for laser-plasma devices. We calculate a closed expression for the impedance and the wake function that may be evaluated numerically. It is shown that the rate of energy loss in the bunch due to radiative interaction is equal to the energy emitted through coherent radiation in the far zone. Furthermore, an expression is found for the asymptotic limit of a large distance of the electron beam from the accelerator compared with the overtaking length. Such expression allows us to calculate analytical solutions for a Gaussian transverse and longitudinal bunch shape. We describe an application of our analytical asymptote by studying the feasibility of a table-top free-electron laser based on laser-plasma driver. Numerical estimations presented in this paper indicate that the effects of the time-dependent energy change induced by the longitudinal wake pose a serious threat to the operation of this device.

Brine diffusion in first-year sea ice measured by Earth's field PGSE-NMR

Pulsed gradient spin echo (PGSE) measurements on first-year Antarctic sea ice have been performed using a portable Nuclear Magnetic Resonance probe that utilises the terrestrial magnetic field. This probe is a new device that incorporates independent transverse and longitudinal field gradient coils. A depth profile of the self-diffusion coefficient of liquid in brine inclusions has been measured through the 2002 season sea ice sheet in McMurdo Sound, near Cape Evans, Ross Island. The data is comprised of both horizontal and vertical brine diffusivity that have been measured in situ and with reduced invasive impact on the ice sheet, which constitutes a significant refinement of the Earth's field NMR technique. The observation that the depth dependence of the diffusion coefficient matched the expected magnitude with the temperature gradient through the ice sheet both confirmed the validity of our experiment and suggested that the sheet is best modelled as a matrix consisting of liquid inclusions in which the water molecules move in a Brownian manner.

Time evolution and morphological characteristics of white light flare on 18 January 1989

Some properties of the white light flare (WLF) that occurred on 18 January 1989 are presented. The flare consisted of 7 patched-shape kernels, with sizes (1.5 similar to 4) 10(17) cm(2). The lifetimes of the kernels were from several minutes to more than 40 minutes. The initial bright points of the WLF kernels originated between the umbra and penumbra in penumbra fibrils, between penumbra fibrils and the photosphere, or on the photosphere and light bridge. These kernels appeared 2 to 3 minutes after H alpha flares, and reached their maxima respectively before the first maximum and 1 to 2 minutes before the second maximum of the H alpha flare, and disappeared slowly before the disappearance of the H alpha flare. The direction and size of material motion in the kernels are different. The precur sors of the WLF are similar to those of ordinary flares. The magnetic pattern of the photosphere in the active region is in delta- structure, and the kernels lie at or near the changing magnetic neutral lines. The small longitudinal field gradient may be constrained by the small transverse field on the photosphere. The flare exhibited both temporal and spatial properties in the center reversal and line-width of H alpha profiles, and has characteristics of both class I and II. Thus it may be a mixed-class white light flare.

Photospheric and Chromospheric Activity Associated with 3B Flare of February 27, 1992

We have analyzed the Hα filtergrams and vector magnetograms of the active region NOAA 7070, in which a 3B/X3.3 flare occurred on February 27, 1992. The average area per sunspot of this active region was in declining phase at the time of the flare. The vector magnetograms indicate that the magnetic field was non-potential at the flaring site. Besides non-potentiality, the longitudinal field gradient was found to be the highest at the region showing initial Hα brightening. Further, in Hα filtergrams no appreciable change in the morphology of the filament tracing the magnetic neutral line was noticed in the post-flare stage. Also, the photospheric vector magnetograms show considerable shear in post-flare magnetic field of the active region. In this paper we present the observations and discuss the possible mechanism responsible for the 3B/X3.3 flare.

Characterization of Longitudinal Field Gradients from Electrical Stimulation in the Normal and Injured Rodent Spinal Cord

The purpose of this experiment was to characterize the longitudinal field gradients from exogenously applied electrical stimulation in the normal and injured rodent spinal cord. In addition, we compared the field gradients arising from stimulation with two different types of stimulating electrodes. Twenty normal rats underwent the surgical implantation of either extradural disc (n = 10) or cuff (n = 10) electrodes in the lower cervical and upper thoracic spinal cord. Sine waves of 1.5 to 50 microA and 0.5 to 50 Hz were used for stimulation. Field gradients were measured differentially from two extracellular glass microelectrodes, positioned stereotactically in the spinal cord at different locations between the stimulating electrodes. The effect of acute spinal cord injury on local field strength was studied in five animals from each group. The field gradients from stimulation with disc electrodes were greatest in close proximity to the discs and decreased markedly toward the point equidistant between the electrodes. In contrast, the gradients produced by cuff electrodes were much more evenly distributed along the spinal cord, increasing only slightly in proximity to the electrodes. These fields were also more evenly distributed throughout the spinal cord in cross-section but were generally weaker than those arising from disc electrodes. Acute spinal cord injury significantly increased the field gradients arising from both disc and cuff electrodes. However, the observed gradients remained substantially lower than those reported to enhance neurite growth in vitro. We conclude that the position and design of stimulating electrodes has a profound effect on longitudinal field gradients within the mammalian spinal cord, as does the presence of an acute spinal cord injury.

Characterization of Longitudinal Field Gradients from Electrical Stimulation in the Normal and Injured Rodent Spinal Cord

THE PURPOSE OF this experiment was to characterize the longitudinal field gradients from exogenously applied electrical stimulation in the normal and injured rodent spinal cord. In addition, we compared the field gradients arising from stimulation with two different types of stimulating electrodes. Twenty normal rats underwent the surgical implantation of either extradural disc (n = 10) or cuff (n = 10) electrodes in the lower cervical and upper thoracic spinal cord. Sine waves of 1.5 to 50 μA and 0.5 to 50 Hz were used for stimulation. Field gradients were measured differentially from two extracellular glass microelectrodes, positioned stereotactically in the spinal cord at different locations between the stimulating electrodes. The effect of acute spinal cord injury on local field strength was studied in five animals from each group. The field gradients from stimulation with disc electrodes were greatest in close proximity to the discs and decreased markedly toward the point equidistant between the electrodes. In contrast, the gradients produced by cuff electrodes were much more evenly distributed along the spinal cord, increasing only slightly in proximity to the electrodes. These fields were also more evenly distributed throughout the spinal cord in cross-section but were generally weaker than those arising from disc electrodes. Acute spinal cord injury significantly increased the field gradients arising from both disc and cuff electrodes. However, the observed gradients remained substantially lower than those reported to enhance neurite growth in vitro. We conclude that the position and design of stimulating electrodes has a profound effect on longitudinal field gradients within the mammalian spinal cord, as does the presence of an acute spinal cord injury.

Characterization of Longitudinal Field Gradients from Electrical Stimulation in the Normal and Injured Rodent Spinal Cord

Characterization of Longitudinal Field Gradients from Electrical Stimulation in the Normal and Injured Rodent Spinal Cord