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Review of Scientific Instruments

A modified Bitter-type electromagnet and control system for cold atom experiments

Tian Luan, Tianwei Zhou, Xuzong Chen, Zhaoyuan Ma

Published Feb 1, 2014• 5 citations
Publisher

Abstract

We present a modified Bitter-type electromagnet which features high magnetic field, fine electronic properties and efficient heat removal. The electromagnet is constructed from a stack of copper layers separated by mica layers that have the same shape. A distinctive design of cooling channels on the insulating layers and the parallel ducts between the layers ensures low resistance for cooling water to flow. A continuous current control system is also made to regulate the current through the electromagnet. In our experiment, versatile electromagnets are applied to generate magnetic field and gradient field. From our measurements, a peak magnetic field of 1000 G and a peak gradient field of 80 G/cm are generated in the center of the apparatuses which are 7 cm and 5 cm away from the edge of each electromagnet with a current of 230 A and 120 A, respectively. With the effective feedback design in the current control system and cooling water flow of 3.8 l/min, the stability of the current through the electromagnets can reach 10(-5).

DOI
10.1063/1.4864051

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We present the design, construction, and characterization of Bitter-type electromagnets which can generate high magnetic fields under continuous operation with efficient heat removal for cold atom experiments. The electromagnets are constructed from a stack of alternating layers consisting of copper arcs and insulating polyester spacers. Efficient cooling of the copper is achieved via parallel rectangular water cooling channels between copper layers with low resistance to flow; a high ratio of the water-cooled surface area to the volume of copper ensures a short length scale (~1 mm) to extract dissipated heat. High copper fraction per layer ensures high magnetic field generated per unit energy dissipated. The ensemble is highly scalable and compressed to create a watertight seal without epoxy. From our measurements, a peak field of 770 G is generated 14 mm away from a single electromagnet with a current of 400 A and a total power dissipation of 1.6 kW. With cooling water flowing at 3.8 l/min, the coil temperature only increases by 7 °C under continuous operation.
We describe the design, construction, and characterization of a Bitter-type electromagnet that produces a spatially-dependent magnetic field used for Zeeman slowing in cold-atom experiments. The coil consists of stacked copper arcs separated by PTFE spacers of varying thicknesses, generating a near-optimal field profile using a single power supply. With an electrical resistance of 26.5(3) mΩ and self-inductance of 19.1(1) μH, our design achieves a fast electrical switching time of τ ≈ 100 μs in a compact, 30-cm-long package. Water circulating helically through holes in the copper and channels in the spacers ensures efficient thermal management, limiting the temperature rise to ∼5° C over 36 s of continuous operation at 200 A.
We create a pair of symmetric Bitter-type electromagnet assemblies capable of producing multiple field configurations including uniform magnetic fields, spherical quadruple traps, or Ioffe–Pritchard magnetic bottles. Unlike other designs, our coil allows both radial and azimuthal cooling water flows by incorporating an innovative 3D-printed water distribution manifold. Combined with a double-coil geometry, such orthogonal flows permit stacking of non-concentric Bitter coils. We achieve a low thermal resistance of 4.2(1) °C kW−1 and high water flow rate of 10.0(3) l min−1 at a pressure of 190(10) kPa.
Many Zeeman slower magnets are geometrically captured by a vacuum chamber, preventing modification or repair without breaking the vacuum. We describe a Bitter-type electromagnet coil design that can be easily disassembled, reconfigured, repaired, and replaced without disturbing the vacuum system. This coil, designed to slow lithium atoms, produces a near-ideal field profile with a single DC power supply. With a resistance of 5.7(9) mΩ, an inductance of 8.8(2) μH, and switching times as low as ∼100μs, the coil compares favorably to other designs and can be disassembled, modified, and reassembled repeatedly without loss of performance. With forced-air cooling, the coil experiences moderate heating. This coil design offers greater flexibility than traditional electromagnet designs, and it can be retrofitted onto existing ultra-high vacuum chambers.
We study a novel millimetre-scale magnetic trap for ultracold atoms, in which the current carrying conductors can be situated outside the vacuum region, a few mm away from the atoms. This design generates a magnetic field gradient in excess of \SI{1000}{G/cm} at a distance of \SI{2}{mm} from the conductors. We perform electromagnetic and thermo-mechanical characterisation using Finite Element Methods (FEM). The predicted behaviour has been verified by electrical and thermal measurements on a prototype, but has not been implemented on an apparatus with cold atoms. Operating this trap at the highest gradient allows for rapid evaporative cooling comparable to that achieved by atom chips.