Magnetic Field Correction Research Articles

Measurement and correction of magnetic fields in pulsed slotted-tube microwigglers

Pulsed electromagnetic microwigglers are capable of creating stronger fields with shorter periods than conventional wigglers. Wiggler field parameters of 1 T and 5-mm period are achieved by passing ∼ 25 kA down a slotted 3 mm outer diameter copper tube. The taut-wire technique is used to measure the wiggler magnetic field in a small aperture (∼ 1.7 mm) with the required spatial (0.1 mm) and temporal (∼ 5 μs) resolution. The absence of midplane symmetry, in contrast to permanent magnet wigglers, causes unique problems in our microwigglers. We have measured strong dipole (bending) and quadrupole (defocusing) field errors with both dc and time-dependent parts. The effects on the magnetic field of current redistribution caused by heating in the copper tube has been measured. We have identified causes for the various field errors and their sensitivities to fabrication tolerances. Improvements we have made towards meeting the required tolerances and our field-correction technique will be discussed.

5237275 Magnetic field correction device

5237275 Magnetic field correction device

4983922 Magnetic shim for magnetic field correction

Improvement in signal-to-noise ratio (SNR) and/or penetration depth can be achieved in medical ultrasound by using long coded waveforms, in a similar manner as in radars or sonars. However, the time–bandwidth product (TB) improvement, and thereby SNR improvement is considerably lower in medical ultrasound, due to the lower available bandwidth. There is still space for about 20 dB improvement in the SNR, which will yield a penetration depth up to 20 cm at 5 MHz [M. O'Donnell, IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 39(3) (1992) 341]. The limited TB additionally yields unacceptably high range sidelobes. However, the frequency weighting from the ultrasonic transducer's bandwidth, although suboptimal, can be beneficial in sidelobe reduction. The purpose of this study is an experimental evaluation of the above considerations in a coded excitation ultrasound system. A coded excitation system based on a modified commercial scanner is presented. A predistorted FM signal is proposed in order to keep the resulting range sidelobes at acceptably low levels. The effect of the transducer is taken into account in the design of the compression filter. Intensity levels have been considered and simulations on the expected improvement in SNR are also presented. Images of a wire phantom and clinical images have been taken with the coded system. The images show a significant improvement in penetration depth and they preserve both axial resolution and contrast.

Physical Effects of U(1)-Induced Gauge Field and Their Non-Adiabatic Corrections for the Cases with Arbitrary Spin11Work supported in part by the National Natural Science Foundation of China.

By making use of the improved Born-Oppenheimer approximate method, we dealwith the separation of spin variable and space variable of neutral particle with arbitrary spin in inhomogeneous magnetic field and non-adiabatic corrections for the problems are thereby computed as perturbations. We also analysis the geometricalproperties of loop phase of the induced gauge potential and point out its observable effects relating to Bitter and Dubbers’ experiment. Finally, we discuss the transitions between the instantaneous spin states as non-;-adiabatic effects resulting from inhomogeneity of the magnetic field.

Magnetic field correction using magnetized shims

The field dependence of magnetization, which is a serious handicap for a straightforward application of magnetizable materials in the process of basic field correction, is discussed. The principles for compensation of this dependence by an appropriate choice of magnetic material, the operating point on the hysteresis curve and the shapes of shims are specified. A design that eliminates undesired harmonics through the use of proper azimuthal configuration and symmetries of shim positions, as well as the choice of block parameters when using several shim blocks simultaneously, is described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simulation of a magnetic field correction scheme for a Halbach-type undulator

An electron-beam trajectory in a Halbach-type undulator with simulated magnetization errors in its component magnet blocks has been calculated using a three-dimensional magnetic field calculation program. The errors taken into account were deviations of the magnetization strength from a nominal value and a deviation of the magnetization direction from a nominal axis. Both uniform and Gaussian distributions of the error magnitudes are assumed for the component magnet blocks of an ideal reference undulator. A simple method for rearranging magnet pairs is discovered. The field integrals are reduced by factors of 4–15 on the average with the method, and the local distortion of the trajectory is strongly corrected. It is found that the distortion of the trajectory in the corrected undulator is almost independent of the magnet gap.

Lf and (simultaneous) rf excitation of Cs atoms in an atomic beam

The experimental frequency standard CSX of the PTB has been used to perform experiments in which a Cs atomic beam is subject to the usual rf excitation coupling the hyperfine levels of the ground-state, and to a low frequency (lf) excitation which couples the Zeeman sublevels. As the lf excitation is done within the drift region between the two rf interaction zones of the Ramsey-type microwave cavity, this is equivalent to a double resonance experiment performed on the Cs atoms. Experimental and theoretical results concerning line shape and centre frequency of the rf transitions are given. The If resonance and its use to determine the magnetic field correction of the clock transition frequency in a Cs atomic clock is also discussed.

Magnetic field corrections to solar oscillation frequencies

The presence of a magnetic field both deep within the Sun and in its atmosphere raises the question of the field's influence on the p- and g-modes of oscillation and the implications for helioseismology. Observations1,2 of p-modes, in particular, have permitted a theoretical determination3 of the sound speed within the solar interior, thus providing a seismological probe of the Sun's depths. Magnetic fields within the Sun are likely to be too weak to significantly affect this determination of the sound speed. Nonetheless, magnetic fields may modify the oscillation frequencies in a distinctive fashion, thereby raising the possibility of placing limits on interior field strengths through frequency measurements4. Recently, Woodard and Noyes5 have reported a slight but systematic decrease in frequencies of low-degree p-modes from 1980 to 1984. Here we argue that the frequencies of both p- and g-modes are modified by a magnetic field. In particular, we attribute the decrease in p-mode frequencies to a magnetic field within the solar interior evolving over the solar cycle. Field strengths at the base of the convection zone of at least 5×105 G are required.

Magnetic Field And Field Gradient Corrections within a Nonconducting Sensor Enclosure in a Conducting Fluid---Part III: Current Exclusion Contribution for an Arbitrary Axisymmetric Enclosure

Neglecting the velocity field perturbation, we examine the current exclusion contribution to the magnetic distortion field /→H/' along the symmetry axis of a streamlined enclosure in the shape of a general body of revolution. In contrast to the case of a spherical enclosure where all orientations of the (unknown) unperturbed current density /→J/0 relative to the enclosure surface are equivalent, for the present geometry the relative orientation of /→J/0 must be specified. On the symmetry axis (z-axis) the component of the distortion field parallel to that axis vanishes independent of the orientation of /→J/0. In regard to field gradients, all diagonal elements of the gradient dyad similarly vanish. However, only the combination ∂H'x/∂y + ∂H'y/∂x vanishes independent of orientation. Here (x, y) denote any two orthogonal directions perpendicular to the symmetry axis.

The Effect of Finite Larmor Radius on Gravitational Instability of a Finitely Conducting Magnetised Hall Medium in Presence of Suspended Particles

AbstractThe problem of stability of a self‐gravitating, infinite homogeneous gas in the presence of suspended particles is investigated. The medium is assumed conducting and effect of external magnetic field, Hall current and finite Larmor radius corrections are also considered. The equations of the problem are linearized and from linearized equations a general dispersion relation for a dusty gas‐particle medium is obtained. The dispersion relation is reduced for two special cases of wave propagations: Parallel and perpendicular to the direction of uniform magnetic field. The effect of suspended particles on the medium is investigated in both the cases. It is found that in the presence of finite Larmor radius corrections and suspended particles the condition of instability is determined by Jeans's criterion for a self gravitating finitely conducting magnetised Hall medium.

The precision of electron cyclotron emission measurements from DITE Tokamak

Corrections for the systematic errors associated with the measurements of electron cyclotron emission from DITE Tokamak using a Michelson interferometer are considered under the following headings. (a) Finite optical depth an isotropic reflection model is developed leading to a line-integral expression for the emission intensity which depends on the optical depth and the wall reflection coefficient. (b) Finite density and refraction: the error due to the distortion of the instrument antenna pattern is calculated from results of ray-tracing computations. (c) Relativistic resonance layer width: an expression is derived for the corresponding spatial error for emission perpendicular to the magnetic field. (d) Magnetic field corrections: an expression for the spatial error due to the poloidal and diamagnetic field contributions is given and its effect on the observed Shafranov shift is investigated. (e) Frequency response: the general principles of quasi-optics are applied to the Michelson interferometer demonstrating that a small-aperture blackbody oven may be used to calibrate the system. (f) Spatial resolution of the viewing optics: the equivalence of lens optics and waveguide antenna is shown both theoretically and experimentally and expressions are given for the resolution perpendicular to the viewing axis. (g) Frequency resolution: it is shown that the resolution of a Michelson interferometer can be better than that suggested by application of the Rayleigh criterion. These corrections are applied to experimental emission spectra and the derived electron temperature profiles compared with laser scattering measurements in a discharge where the plasma equilibrium is changing. The Shafranov shift of the corresponding temperature surfaces is shown to be in good agreement with that expected for the magnetic flux surfaces. Using the wall reflection model, the electron density profile is derived from the emission profile of the third harmonic cyclotron frequency.

Magnetic Field And Field Gradient Corrections within a Nonconducting Sensor Enclosure in a Conducting Fluid---Part I: Potential Flow

Measurements of ocean-generated magnetic fields in submerged enclosures are affected by the nonconducting environment within the sensor housing. There are two physical sources for the effect: the exclusion of the electrical current from the interior of the enclosure and the perturbation of the hydrodynamic flow field produced by the enclosure. Of particular interest is the distortion of gradient-type measurements. Due to the current exclusion effect, any observed gradient dyad must be symmetric. This result is unavoidable. In order to analytically examine these effects in detail, we have considered the particular case of a spherical goemetry. Furthermore, in the present paper we neglect any vorticity associated with the perturbed velocity field (potential flow).

Influence of magnetic field on exciton luminescence

The magnetic field dependence of the exciton emission intensity I ex ( H) has been investigated in Ge crystals stressed along the direction near 〈100〉. In the low field limit the magnetic field correction has been evaluated to the wave functions of the ground and some excited states of an isotropic exciton. The calculated dependence I ex ( H) in the case of Ge is in a good agreement with the experimental one at H ⩽ 0.5 T.

The ISABELLE Accelerator Software, Control System, and Beam Diagnostic Philosophy

The ISABELLE Project combines two large proton accelerators with two storage rings in the same facility using superconducting magnet technology. This combination leads to severe constraints on beam loss in magnets and involves complex treatment of magnetic field imperfections and correction elements. The consequent demands placed upon beam diagnostics, accelerator model programs, and the computer oriented control system are discussed in terms of an illustrative operation scenario.

Quantum theory of static shielding II: Intermediate magnetic field strength

A theoretical analysis of magnetic field effects on the Friedel-Kohn “wiggle” contribution to shielding by a quantum plasma is carried out here for conditions of intermediate magnetic field strength ( E F > ω c > kT). In this case, magnetic field quantum corrections to the Friedel-Kohn “wiggl” may be expected to be of two types: (1) monotonic magnetic field corrections associated with an expansion in powers of ω c E F and (2) DHVA-type oscillatory magnetic field corrections associated with oscillatory terms involving the DHVA oscillation frequency E F ω c . The analysis of the monotonic magnetic field corrections is carried out with an expansion in powers of ω c E F , and we find that the actual expansion parameter involved is not ω c E F which is small; but rather it is [ ω c E F ) · p Fr] which may be either small or large since p F r is large. For distances from the shielded impurity such that [( ω c E F ) · p Fr] ⪡ 1 , the power expansion is valid and the zero-field Friedel-Kohn “wiggle” is the dominant high-wavenumber shielding phenomenon, with monotonic and anisotropic magnetic field corrections given by the power series. However, for distances from the impurity such that [( ω c E F ) · p Fr] ⪢ 1 the power expansion is invalid, and we estimate the monotonic magnetic field corrections from a nonperturbative integral representation. The resulting estimate obtained shows that when [( ω c E F ) · p Fr ⪢ 1 the Friedel-Kohn “wiggle” no longer retains its zero-field form, but instead it takes on the properties of its counter-part in the quantum strong field limit, which has a rather different character in that the long range of the “wiggle” is destroyed by a decaying exponential envelope factor exp(−2p F| r|) , while the “wiggle” is still discernable through an anisotropic oscillatory factor cos(2 p F r z ). This is not very surprising since the effective magnetic field “mixed” parameter [( ω c E F ) · p Fr] ⪢ 1 is large enough to be associated with the quantum strong field limit, in spite of the fact that ω c E F ⪡ 1 is small in the intermediate field case under consideration. The analysis of DHVA-type oscillatory magnetic field corrections is of special physical interest, since the occurrence of such terms in conjunction with the Friedel-Kohn “wiggle” would suggest the observation of two of the most interesting specifically quantum phenomena of the quantum plasma in a single experiment. However, our investigation of the only mechanisms which could give rise to such DHVA terms shows that the Friedel-Kohn “wiggle” is in fact devoid of DHVA oscillatory effects at large distances from the shielded impurity [ p F r ⪢ 1 and ( ω c E F )p Fr ⪢ 1 ]. This analysis should serve to provide some insight into questions arising in connection with the recent work of M.E. Rensink [5] and M. L. Glasser [6].

Test Ion Energy Loss in a Plasma with a Magnetic Field

The energy loss by a test ion in a plasma in the presence of a homogeneous magnetic field is considered, and numerical results for the magnetic field corrections to the rate of energy loss are presented. This is done for strong fields and for weak fields (i.e., for the electron cyclotron frequency greater than, and less than, the plasma frequency), and for all values of x, the ratio between test ion speed and plasma electron thermal speed; previous work is confined to x ≫ 1 .

Frequency Modulation Method for Performing Adiabatic Demagnetization in the Rotating Reference Frame

A new method for preforming adiabatic demagnetization in the rotating reference frame in NMR experiments is presented in this paper. In this new method the frequency of a crystal oscillator rather than the actual magnetic field is pulsed off resonance. Using simple circuits which are described in this paper, the effective field in the rotating frame can be pulsed by as much as 50 G, depending on the nucleus studied. This method completely avoids the problem of external magnetic field correction due to a pulsed field in the magnetic gap.

Multipurpose system for magnetic field correction in high-energy synchrotrons

Multipurpose system for magnetic field correction in high-energy synchrotrons

The rotation of Spain: Palaeomagnetic evidence from the Spanish meseta

The magnetic properties of 39 oriented samples of Triassic siltstones and of 43 oriented samples of Silurian andesites and basalts from the Spanish Meseta were investigated. After a.c. magnetic field demagnetization up to 3,000 Oe, peak value, and after correction for the dip of the geological strata the Triassic samples revealed a characteristic direction of magnetization with an average declination ( D) of 359.5° and an inclination ( I) of 23°. One group of samples of Silurian volcanic rocks from the province of Guadalajara had, after a.c. magnetic field demagnetization and correction for the dip, a characteristic direction of magnetization with an average declination of 159°C and an inclination of 18.5°; another group from the province of Ciudad Real had, after a.c. magnetic field demagnetization and after correction for the dip, a characteristic direction of magnetization with an average declination of 130.5° and an inclination of 22.5°. The palaeomagnetic pole position derived from the Triassic rocks is situated at 63°N 177.5°E, and the pole positions of the first and second group of Silurian samples are situated at 35.5°N 157°W and 21°N 132°W, respectively. The divergency between these results and the already known data for Triassic and Silurian from stable (extra-Alpine) Europe highly supports a counterclockwise rotation of Spain.

Cosmotron Pole Face Windings Generator System Permitting Long Beam Spill

A scheme is described for maintaining a constant current in the magnetic field correction windings of the Cosmotron, during the process of beam ejection and long spill-out. A solid-state switch is used to apply compensation for the vanishing induced voltage in these correction windings. It is shown how this method could be applied in the event beam ejection and long spill-out would be performed at two different energies.