³He compact polarizing facility
Contrary to the currently used polarizing facility for centralized hyperpolarized gas production a compact and mobile facility can be used to polarize 3He next to a user’s application. The main advantages of such an “on-site” polarizer are independence from gas deliveries from the central production facility and better polarization degrees for experiments as the shipping time is saved, in which polarization decays due to relaxation losses. The new compact polarizing facility follows the same design principles and aims at the same performance data as its predecessor for centralized polarizing. However, a decisive reduction in size compared to the currently used facility is achieved by embedding the polarizing assembly into a solenoid with a homogeneous field all over its volume.
Such a homogeneous field is realized by enclosing the solenoid into a magnetic shield composed of a soft magnetic material with a high magnetic permeability . It consists of a cylinder with two endplates, in which the solenoid is fitted in (Fig.1). One endplate is designed as a door in order to be able to remove polarized gas from the apparatus and the other endplate has a whole for supply lines and the support structure for the polarizing assembly in it. These endplates act as mirrors for the magnetic flux, which creates a homogeneous magnetic field inside the complete volume of the solenoid as the solenoid in between two mirrors appears as an infinitely long solenoid. Because the whole in one of the endplates disturbs the homogeneity of the field a chimney with a correction coil is attached to the endplate.
Fig.1: Solenoid enclosed in soft magnetic material
The optical pumping requires a circularly polarized laser beam at a wavelength of λ=1083 nm. Unlike the current polarizing facility, which uses two fiber lasers, the compact apparatus will be equipped with only one pump laser. The gas is optically pumped in six 1.25 m long glass tubes with a total volume of about 19 L (Fig.2).
Fig.2: Schematic drawing of the optical assembly
The beam from a 15W fiber laser is magnified with a telescope to a diameter of about 35 mm and then guided through the six glass tubes. Polarizing beam splitters with attached λ/4-plates are put ahead of each glass cell to restore a perfectly circular polarization of the incoming beam and to deflect the returning outgoing beam towards the next cell. After each cell a dichroic mirror reflects the 1083 nm pumping-light back into the cell in order to maximize absorption in the gas. It transmits the 668 nm fluorescence light from the gas discharge whose circular polarization can be measured by a monitor behind the mirror. This polarization is directly correlated to that of the helium.
Fig.3: Current view of the compact polarizing facility
1. S.Hiebel et al., Journal of Magnetic Resonance 204 (2010) 37
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