Buffer-Gas Cooling of Atomic Chromium
5.3 Cryogenic setup
− mS 2kBT ·
d t
2
dt ,
where d is the distance from the chromium chip to the beam. The absorption coefficientα should then be proportional topS(t), so that the time-dependence of the transmission through the chromium plume can qualitatively be obtained from the Lambert-Beer law (4.14). With d = 5 cm in the current setup, an accordingly fitted curve yields a temperature of 976 K for the data plotted in Figure 5.3.
5.3 Cryogenic setup
Figure 5.4 gives a schematic overview of the complete setup intended for the pro-duction, buffer-gas loading, magnetic trapping and in situ detection of atomic
74 BUFFER-GAS COOLING OF ATOMIC CHROMIUM
ablation pulse
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
85 90 95 100
Transmission[%]
Time [ms]
Figure 5.3: Typical room-temperature ablation signal. As explained in the text, it is fitted to a one-dimensional Maxwell velocity distribution. The ablation pulse is seen by the detector even though it is protected with a green filter.
chromium in this thesis. Its major components have already been introduced in Chapter 3.
The copper cell is centered inside the magnet bore with a special stainless-steel centering ring countersunk into the magnet flange of the IVC. Three dental floss2 strands stretch from it to the cell’s copper rod to lock its position with respect to the magnet and prevent fatal thermal contact to 4.2 K parts. The copper rod’s length was chosen such that the cell is also centered vertically.
This design allows a relatively simple installation of the cell assembly in the cryostat without significantly compromising thermal isolation.
5.3.1 Buffer-gas injection
The helium buffer-gas is injected into the cell via a 1/8” thin-walled stainless steel capillary, that is thermally anchored at 1 K, 4.2 K, and inside the tubing leading to room-temperature through the cryostats upper LHe dewar. It can in fact be seen in Figure 3.2 next to the dilution unit. Intermediate circular windings provide elasticity as well as the required thermal isolation between the cell and the 1 K level. A removable connection to the cell is realized with Swagelok VCR fittings and corresponding stainless silver plated gaskets, which have found to be extremely reliable at cryogenic temperatures.
The amount of buffer-gas admitted to the cell has been controlled by measur-ing the pressure inside the capillary with gauge situated at room-temperature.
While the cell is constantly cooled with the dilution unit, small quantities of helium are repeatedly added until the displayed pressure does not change any-more, indicating that it has reached the equilibrium vapor pressure.
2Dental floss has excellent material properties at cryogenic temperatures and is ideally suited to make strong, thermally isolating connections.
CRYOGENIC SETUP 75
¸/2 ¸/4 50mm
25.4 mm
35 mm 35 mm
100 mm 1000 mm
PBS
FND-100 FND-100
FND-100
Mixing chamber
Cell window Silver mirror
4K window
77K window
300K window above table
below table
Green filter Green filter
Greenfilter
circular polarization linear polarization
alternatively
Transmission signal / 400 mm
100 mm
AOM
1st order
from 425nm source
532nmpulsedYAG
Spectra-Physics INDI-30-10
Centering ring He buffer-gas
Figure 5.4: Overview of the complete chromium buffer-gas loading setup.
76 BUFFER-GAS COOLING OF ATOMIC CHROMIUM 5.3.2 Optics
For sample production, the output of the ablation laser is guided below the optical table with two mirrors (not shown in Figure 5.4), where optics is fixed head-over on two breadboards. A 1000 mm lens then focuses the collimated beam onto one of two available chromium chips inside the experimental cell.
The remaining space on the cell top is occupied by solid lumps of dysprosium and holmium for future trapping experiments.
The detection beam from the 425 nm laser source is first focused through a generic acousto-optic modulator (AOM). The AOM allows to quickly switch the subsequently used 1st order deflection on and off, both manually as well as triggered. A two-mirror elevator then transfers the blue light to below the optical table. Two alternative configurations can be chosen here, depending on whether linearly or circularly polarized light is used.
High quality linear polarization is obtained with a polarizing beam-splitter (PBS), which in combination with a half-wave plate also allows an adjustment of the power level. A telescope then expands the beam to a diameter of about 8.5 mm, before it is send into the cryostat via a 2” semipermeable mirror. The latter was favored over a standard beamsplitter due to the large separation of its two wedged surfaces, which helps to avoid any etalon effects. After the detection beam has propagated once through the chromium sample, it is retro-reflected from a silver mirror mounted to the top of the experimental cell, and passes the sample again. It finally reaches a photodiode behind the 2” mirror. An iris and a 532 nm filter (T <1 %) protect it from unwanted scattered light.
For circularly polarized light, an additional quarter-wave plate is introduced behind the PBS. It is then possible to much more sensitively detect the retro-reflected light at the corresponding output of the PBS, as shown in Figure 5.4.
To compensate for power fluctuations in the detection beam, it is constantly monitored by another photodiode picking up the transmitted fraction of the light incident on the 2” mirror. Together with the raw transmission signal from one of the other photodiodes, its output is processed in a self-made analog divider for online normalization. It is built around an AD538 real-time compu-tational unit together with a preceding offset compensation, helping to further improve the final transmission signal quality.
5.4 Results
Initial experiments with the fully equipped dilution refrigerator have allowed to test the interplay of all technical components and explore first domains of the vast buffer-gas loading parameter space. Various factors affecting the cell temperature have been studied, and buffer-gas cooling of ablated chromium in
4He has been observed.
RESULTS 77