A large 'Active Magnetic Shield' for a high-precision experiment: nEDM collaboration

Abstract

We present a novel Active Magnetic Shield (AMS), designed and implemented for the n2EDM experiment at the Paul Scherrer Institute. The experiment will perform a high-sensitivity search for the electric dipole moment of the neutron. Magnetic-field stability and control is of key importance for n2EDM. A large, cubic, 5 m side length, magnetically shielded room (MSR) provides a passive, quasi-static shielding-factor of about ${10}^5$ for its inner sensitive volume. The AMS consists of a system of eight complex, feedback-controlled compensation coils constructed on an irregular grid spanned on a volume of less than 1000 m${}^3$ around the MSR. The AMS is designed to provide a stable and uniform magnetic-field environment around the MSR, while being reasonably compact. The system can compensate static and variable magnetic fields up to $\pm 50\mu \text{T}$ (homogeneous components) and $\pm 5\mu \text{T/m}$ (first-order gradients), suppressing them to a few $\mu \text{T}$ in the sub-Hertz frequency range. The presented design concept and implementation of the AMS fulfills the requirements of the n2EDM experiment and can be useful for other applications, where magnetically silent environments are important and spatial constraints inhibit simpler geometrical solutions.

Fig. 1

Frequency-dependent shielding-factors of the MSR. The light green region and the colored curves for each spatial direction were obtained using external reference excitation coils to produce $2\mathrm{\mu }\text{T}$ peak-to-peak sinusoidal fields at the central position of the MSR final location prior to the MSR assembly. The dark green region is the expected improved shielding-factor provided by the AMS system for large disturbances of order several $10\mathrm{\mu }\text{T}$ in the low-frequency range. Adapted from Ref. [22]

Fig. 2

Picture taken during a magnetic-field mapping in UCN area South at PSI [18]. The area was emptied before the n2EDM experiment was set up. Two people are moving the ‘mapping tower’ around. About half height of the tower in the displayed position marks the center of n2EDM. The UCN source is behind the concrete shielding to the left, and a superconducting magnet (blue) to fully polarize UCN directed to n2EDM, is visible on the platform. See main text for further details

Fig. 3

The volume of interest for the target fields of the n2EDM AMS system are given by the outside wall of the MSR (depicted in violet). An external magnetic field ${\mathit{B}}_{\mathbf{e}}$ is detected by magnetic-field sensors (green). As an example, the yellow coils could aim to compensate the external field. In practice, the AMS coils are more complicated due to spatial limitations and their close proximity to the MSR

Fig. 4

Illustrations of the method of simple coil design  [24]. Left: Initial definition of the grid (bold tiles) around points of interest (blue dots). Right: a set of simple loops (different colors) obtained in the course of simple coil design, representing the current paths needed to create the desired field (here: homogeneous in the indicated direction). Numbers and arrows indicate values of the currents and their directions in the corresponding loops. More details can be found in [17]

Fig. 5

Photo of the AMS prototype  [17, 18] mounted in the ETH laboratory. The smaller side of the frame, facing the window as seen on the photo, was kept open without windings – to allow easy access to the inside of the system

Fig. 6

Simulation of the y-component of the magnetic field produced by the y-coil of the AMS prototype in the x–y midplane. Shown are deviations of the magnetic field from the target value of $50\mathrm{\mu }\text{T}$. The grey contour depicts the volume of a removable cubic mu-metal shield, which was not considered in the simulation. Figure is adapted from [17]

Fig. 7

The map of the magnetic field produced by the first-gradient coil of the AMS prototype (G₁, as defined in Table 1), measured at the central $x-z$ plane at $y=115$ cm. Adapted from [17]

Fig. 8

Visualization of the feedback matrix $\mathbf{M}$ (see Eq. 1 and [17]). The rows designate the coils of the AMS prototype and the columns correspond to the absolute readings in x, y and z directions of the eight fluxgates (FG)

Fig. 9

Frequency-dependent shielding factor of the AMS prototype along its x direction measured with the perturbation from a square-shaped coil with 1 m side at a distance of about 3 m, producing external sinusoidal fields of different frequencies. Adapted from [18]. See text for details

Fig. 10

The AMS grid (in yellow) around the MSR. Part of the grid is not shown in the picture to allow the view onto the MSR. The colored lines represent as an example the main simple loops of the Y-coil

Fig. 11

The largest deviations (‘min’ for negative and ‘max’ for positive) from the target field at the MSR are plotted against the $\lambda$-parameter for the procedure of optimizing currents of the X-coil. More details in [18]. A $\pm 1\mathrm{\mu }\text{T}$ performance goal is indicated by the green box. The axis on the right gives the average currents in the edges of the grid and the orange curve shows its decrease with increasing $\lambda$

Fig. 12

Schematic top view of the AMS grid. A connected door is shown in red. The volume of the POI is indicated in blue around the MSR (green). The orange area depicts part of the AMS, which has only a minor impact on the field homogeneity around the MSR. Adapted from [18]

Fig. 13

Histogram of the residual fields around the MSR for different numbers of most important simple loops used in the 4th gradient coil of the system. Adapted from [18]

Fig. 14

Two-dimensional maps of the calculated magnetic-field residuals at z = 0, which is the vertical center of the MSR. The x and y coordinates are given in meters. The outline of the MSR is depicted as a gray square. The residuals are plotted for the full field in all coils including cross compensation, see text. Adapted from [18]

Fig. 15

Photograph of the AMS coil system constructed on the walls of the thermohouse around the MSR. On the right-hand side, in the middle tiles, and on the back-side to the left of the MSR, a few DIN rails are visible which are wired to connect circuits, as explained in the main text. Of course, the AMS extends fully around the MSR, also on the entire floor. The platform visible in front of the MSR is about 2.5 m above floor level

Fig. 16

Simplified scheme of the bipolar current source, developed to power AMS coil. Each current source consists of three channels (here: 15, 5 and 1 A), with their output currents proportionally to their individual control voltage

Fig. 17

Example of an AMS validation measurement: comparison between the measured (crosses) and the simulated (dotted line) magnetic field values for $B_x$, $B_y$, and $B_z$ components produced by the Y-coil. The measurements were taken along the gray line in the inset (y-direction) at $z=-1\text{m}$. Adapted from [18]

Fig. 18

The AMS suppression of magnetic fields from SULTAN: a, b magnetic fields measured by the feedback fluxgates outside the MSR during two different SULTAN ramps with the AMS system in static (a) and dynamic (b) modes; c, d the magnetic fields of the SULTAN magnet for the two ramps (dotted grey line, right scale) along with the corresponding magnetic field measured by an optically-pumped (QuSpin) magnetometer [31] inside the MSR (black line, left scale)