8.3
Setup design
In section 8.2, we have shown that the measured diffusion coefficient, Dobs, is
angle dependent as long as the magnetic field lines lie within the scattering plane. In that case, by measuring under different angles one obtains different values of Dobs, each consisting of a different combination of Dz (parallel to
the magnetic field) and Dxy (perpendicular to the magnetic field) as is given
by equation 8.19. If the magnetic field would be applied perpendicular to the scattering plane one would only measure Dxy, independent of the scattering
angle. Thus, by having the magnetic field lines within the scattering plane, one can obtain more information, especially when measuring at multiple angles. We therefore have chosen to apply this geometry to our DLS insert.
8.3.1 The DLS insert
The design of the insert is shown in Figure 8.7. To fixate the optical alignment, we use a single mode fiber to couple in the laser light in the insert. This has the advantage that the optical alignment will be maintained, even when the insert is taken out of the magnet. The light is focused by a small lens into the center of a cubical cuvette. A polarizer is placed between the lens and the cuvette to ensure that the light has the correct polarization when entering the cuvette. Some rotation of polarization is to be expected in the cuvette at high magnetic field due to Faraday rotation. To minimize this effect, we made the cuvette as small as possible (5x5x5 mm3) and we placed analyzers before the detection fibers (a polarizer with the same orientation as the first polarizer) to only select the light with the correct polarization.
Since the space in the radial direction is very limited we have chosen to use small adjustable mirrors to reflect the scattered light along the axis of the magnet where we have much more working space. Two rotatable mirrors on movable rods are used to select a specific scattering angle. The scattering angle can be increased or decreased by shifting the rod up or down respectively. After fixating the rod at a certain position, the mirror can be rotated in order to reflect the scattered light towards the detection fiber, which brings it to a single photon detector that is connected to an autocorrelation system. For both channels, any scattering angle between 60◦ and 117◦ can be selected. Outside this range, the mirrors cannot couple the light in the detectors anymore.
Where possible, the components of the insert were made from non-ferro- magnetic materials to reduce interactions between the insert and the magnetic field. The four rods on which the discs are mounted were made of carbon fiber. All discs containing the optical components were made from aluminum as were
8 Towards dynamic light scattering in high magnetic fields
Figure 8.7: Drawings of the DLS insert. Left: the design of the entire
insert by Michel Peters. Right: schematic picture showing all components within the DLS insert. The positions of the individual discs containing the optical components can all be shifted separately in the vertical direction. This allows DLS to be measured at different positions in the magnet and the optical alignment to be performed for every component separately. The scattered light is directed to a single photon detector and is analyzed by an autocorrelator which are both placed at least 4 meters from the magnet.
the holder of the cuvette and the rods containing the mirrors. The connectors that are normally mounted at the end of an optical fiber are very magnetic and therefore we replaced them by home-made aluminum connectors. In the final
8.3 Setup design
Figure 8.8: Schematic picture showing the placement of the insert within the magnet. There is no direct contact between the magnet and the insert. Transfer of vibrations via the tripod has been reduced by passive damping via rubber rings and the addition of extra weight on the tripod.
design, only the two fiber lenses (made from stainless steel) were still slightly ferromagnetic. These two lenses were therefore clamped very tightly by two discs to make sure they could not shift, twist or turn in a magnetic field.
8.3.2 Placement in the magnet
The Bitter magnet has an inner diameter of 50 mm. Inside, a temperature tube is placed that will stabilize the temperature of the insert and the sample. This will reduce the working diameter to 40 mm. The Bitter magnet is cooled by 135 L/s of water which introduces significant vibrations in the magnet. To prevent the transfer of these vibrations from the magnet to our DLS insert, we have chosen to decouple the complete insert from the magnet. The insert was designed to have a diameter of 36 mm to ensure a 2 mm wide gap between the insert and the temperature tube. The upper part of the insert was fixed to the tripod which is not connected to the magnet but is standing on the floor. This is shown schematically in Figure 8.8. The insert is connected to the tripod via flexible rubber rings which reduces the transfer of vibrational noise. Heavy sandbags were placed on top of the tripod to damp any vibrations that were
8 Towards dynamic light scattering in high magnetic fields
transferred from the ground to the tripod. The bottom of the insert was fixed to the floor by a special holder mounted to a heavy block made of lead. In this way, there was no direct contact between the magnet and the insert which should minimize vibrational noise.