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arXiv:1901.08791 · SciPost Physics

Compact bulk-machined electromagnets for quantum gas experiments

Kévin Roux, Barbara Cilenti, Victor Helson, Hideki Konishi, Jean-Philippe Brantut

Published Apr 24, 2019• 7 citations
arXivHTMLPDF

Abstract

We present an electromagnet combining a large number of windings in a constrained volume with efficient cooling. It is based on bulk copper where a small pitch spiral is cut out and impregnated with epoxy, forming an ensemble which is then machined at will to maximize the use of the available volume. Water cooling is achieved in parallel by direct contact between coolant and the copper windings. A pair of such coils produces magnetic fields suitable for exploiting the broad Feshbach resonance of ^6 6 Li at 832.2\, 832.2 G. It offers a compact and cost-effective alternative solution for quantum gas experiments.

DOI
10.21468/scipostphys.6.4.048

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We present an electromagnet combining a large number of windings in a constrained volume with efficient cooling. It is based on bulk copper where a small pitch spiral is cut out and impregnated with epoxy, forming an ensemble which is then machined at will to maximize the use of the available volume. Water cooling is achieved in parallel by direct contact between coolant and the copper windings. A pair of such coils produces magnetic fields suitable for exploiting the broad Feshbach resonance of ^6 6 Li at 832.2, 832.2 G. It offers a compact and cost-effective alternative solution for quantum gas experiments.
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Cold atom experiments commonly use broad magnetic Feshbach resonances to manipulate the interaction between atoms. In order to induce quantum dynamics by a change in the interaction strength, rapid (∼μs) magnetic field changes over several tens of Gauss are required. Here, we present a compact design of a coil and its control circuit for a change in the magnetic field up to 36 G in 3 µs. The setup comprises two concentric solenoids with minimal space requirements, which can be readily added to existing apparatuses. This design makes the observation of non-equilibrium physics with broad Feshbach resonances accessible.
We describe an experimental setup and the cooling procedure for producing 39K^{39}\mathrm{K}39K Bose-Einstein condensates of over $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms. Condensation is achieved via a combination of sympathetic cooling with 87Rb^{87}\mathrm{Rb}87Rb in a quadrupole-Ioffe-configuration (QUIC) magnetic trap and direct evaporation in a large-volume crossed optical dipole trap, where we exploit the broad Feshbach resonance at 402 G to tune the 39K^{39}\mathrm{K}39K interactions from weak and attractive to strong and repulsive. In the same apparatus we create quasipure 87Rb^{87}\mathrm{Rb}87Rb condensates of over $8\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms.
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.
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).