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A high-performance magnetic shield with large length-to-diameter ratio
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Image of FIG. 1.
FIG. 1.

Measured axial magnetic field along the vacuum tube axis. The dotted grey curve is the ambient magnetic field measured before the shields were installed. The dashed red curve is the field with the segmented shields. The solid black curve is the field with the welded shields.

Image of FIG. 2.
FIG. 2.

The magnetic shield. (a) When fully assembled, the shield is 342 in. (8.7 m) in length and consists of three layers. In this segmented version, each layer of the shield consists of 14 segments, plus bottom and top end caps. (b) Each segment has a row of PEM nuts welded to the bottom edge and a joining band welded to the top. The holes in this band allow the top of each segment to be fastened to the nuts at the bottom of the next. The three layers nest within each other. (c) The joining bands were removed and the segments were welded together and re-annealed. The three layers of the welded shield are pictured here in front of the annealing furnace (Solar Atmospheres).

Image of FIG. 3.
FIG. 3.

Measured magnetic field along the axis of a welded 14-segment, three-layer shield after degaussing for the (a) axial and (b) transverse magnetic fields. The inset of (a) shows the field improvement in uniformity after degaussing. In solid light grey is the measured field before degaussing. The solid red curve is the measured field after degaussing. The dashed black curve shows the simulated background expected from a perfectly uniform, finite length shield. In (b), the dashed black and solid red curves show the measured field components in the two transverse directions. The dashed black curve has been offset by 3.5 mG for clarity.

Image of FIG. 4.
FIG. 4.

Axial dynamic shielding ratio of the welded 14-segment, three-layer shield. In solid red is the dynamic shielding ratio inferred from measurement. In dashed black is a finite element model for a three-layer continuous shield with μ = 2.5 × 104 and radii, lengths, and thicknesses the same as the constructed shield. The dips in the dynamic shielding ratio correspond with welds between segments.

Image of FIG. 5.
FIG. 5.

Finite element simulation of an imperfect joint in a segmented shield with an applied field of B z, 0 = 500 mG. The axial magnetic field on-axis is shown in solid black. The peak occurs at the air gap (μ = 1, 0.010 in. in height) between the two segments. The magnetic fields for each segment alone are shown as dashed red and dotted grey curves. The peak is caused by the imperfect magnetic circuit between the segments allowing the field to escape the shield and into the shielded region.

Image of FIG. 6.
FIG. 6.

Finite element simulation of magnetic field lines in the presence of the inner layer of an 8.5 m shield (solid red) and a 1.2 m shield (black dashed). The former has the same length as the full, 14-segment shield, while the latter has the same length as a two-segment subset. The system is cylindrically symmetric about the left vertical axis. The triangle on the horizontal axis marks the radius of both shields (0.0927 m). Note that the longer shield strongly affects the ambient field at much further distances than the shorter shield.

Image of FIG. 7.
FIG. 7.

Measurement of the segmented shields compared to simulation. The dashed black curve is simulation; it has been offset by 150 mG for clarity. The solid red curve is a measurement of the initial segmented shield. The peaks in the simulated curve, which arise due to the modeled gaps between segments, align well with the measured peaks.

Image of FIG. 8.
FIG. 8.

Dependence of the axial shielding ratio S A on the wavelength of the field applied to an infinitely long shield. The open black circles show S A for a single shield with the same diameter and thickness as the inner layer of the constructed shield. The filled red circles show S A for the same system, but include two outer layers with the same diameters and thickness as the outer layers of the constructed shield. The solid curve is a fit to Eq. (1).

Image of FIG. 9.
FIG. 9.

Dependence of the axial shielding ratio S A on the wavelength of the field applied to a finite shield. The modeled shield has the same radius (3.68 in.) and length (334.7 in., or 8.5 m) as the inner layer of the constructed shield. The filled red (open black) circles describe the shield's response to an applied field with a node (antinode) centered on the shield. The dashed black curve represents the simulations for the infinitely long shield of the same radius (see Fig. 8). The system becomes nonlinear at wavelengths greater than the shield's length, and the method for calculating S A begins to fail.

Image of FIG. 10.
FIG. 10.

Residual axial magnetic field for multiple methods of joining the inner layers of a two-segment subset of the full shield. Labels (a)–(d) correspond to the subsections of Sec. V in the text. Arbitrary height and magnetic field offsets have been chosen for the best comparison of peak height. (a) Fasteners tightened with 30 in-lbs of torque. (b) A thermal interference fit. (c) Two long strips of Metglas wound helically around the center magnetometer support, topped by two layers of axial Metglas shielding. (d) The welded shield.

Image of FIG. 11.
FIG. 11.

Magnetic field on the axis of a two-segment, three-layer shield with all joints welded, before and after degaussing with 3.5 G applied axial field. The trace has been cropped to isolate the region near the joint. The dashed black curve is the field before degaussing. The solid red curve is the field after degaussing. The offset between the curves is a result of the degaussing procedure.

Image of FIG. 12.
FIG. 12.

Dynamic axial shielding ratio of two-segment, three-layer shields. The traces have been cropped to isolate the region near the joint. In solid red is the axial shielding ratio for three layers of welded shielding. In dashed black, the inner welded layer has been replaced by the inner layer joined by a thermal interference fit. Both measurements were taken at a high (3.5 G) applied field and after a degaussing sequence. While the field is measured to be flat for the welded shield, the dynamic shielding ratio is lower near the joint.


Generic image for table
Table I.

Theoretical shield properties of each layer of the constructed shield and all three combined. The transverse shielding ratio (S T ) and axial shielding ratio (S A ) are calculated from each layer's geometry using the formulas presented in the supplementary material.21 For each layer, we indicate its outer diameter (OD), length (Len.), length-to-diameter ratio (γ), and inverse demagnetizing factor (1/N). The material thickness is 0.050 in., and magnetic permeability is assumed to be 2.5 × 104.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A high-performance magnetic shield with large length-to-diameter ratio