Buffer-Gas Loading and Magnetic Trapping
3.3 Experimental cell
The experimental cell is the container for the helium buffer-gas and its walls represent the thermal reservoir determining the buffer-gas temperature. As has already been indicated, it should possess a number of crucial properties to make it suitable for buffer-gas loading and magnetic trapping.
Most importantly, it should be nonmagnetic and leave as much trap depth available on the inside as possible, while at the same time not jeopardizing the thermal isolation from the magnet. Furthermore, induced temperature changes from an incorporated resistive heater should quickly and uniformly spread over the complete cell, which requires a high thermal conductivity. The thermal contact to the cryostat cold plate on the other hand has to be chosen with care to avoid excessive heating of the mixing chamber at higher cell temperatures and still allow efficient re-cooling.
When no magnetic field ramps are necessary in the experiment, the best choice of cell material certainly is oxygen-free high conductivity (OFHC) copper with its superior thermal properties (see Table 3.3). Since the use of copper allows to fall back on mature cryogenic techniques, it was chosen for the cell built for the initial experiments towards buffer-gas loading of atomic chromium reported in Chapter 5. Its simple layout is illustrated in Figure 3.12.
Optical bottom access for ablation and detection is provided with a 2” in-dium sealed window. A silver mirror mounted to the cell top retro-reflects the detection beam. Resistive heating of the cell body is provided by a twisted pair Manganin wire (R= 284.5 Ω) wrapped around its circumference and fixed with thermally conducting Delta Bond 152 glue. An embedded 1 kΩ ROX sensor
Thermal Conductivity Specific Heat
[mW/cm·K] [mJ/mol·K]
OFHC Copper 500 0.1
Sapphire 0.1 10−5
4He II 100 0.6
3He 0.05 2500
Table 3.3:Thermal properties of selected materials at 200 mK. The numbers represent typical values and strongly depend on the purity of the substance.
52 EXPERIMENTAL REALIZATION
buffer-gas capillary
thermal connection to mixing chamber
solid chromium
silver mirror
indium sealed window
70 mm monolithic
copper cell body
Figure 3.12:OFHC copper cell as used for the chromium experiments.
reads the cell temperature. Thermal contact to the mixing chamber is estab-lished with a copper rod extending directly from the cell top. The cell tem-perature is usually set by running the dilution unit at full cooling power and accordingly adjusting the heating current. Like this, optimal cooling rates are achieved when the heater is switched off.
The use of a conducting cell material is prohibited when the experiment requires varying magnetic fields, as in evaporative cooling, for example. They would induce eddy currents causing an intolerable resistive heating of the cell.
Although the implementation of RF evaporation would in principle avoid these issues, it is rather impractical in the context of buffer-gas loading, since the strong magnetic fields near the trap edge lead to large level splittings with unusually high resonance frequencies.
In view of future applications, an alternative experimental cell has therefore been constructed in this thesis from insulating G-10 material (Figure 3.13).
To compensate its low thermal conductivity, the cell makes use of the ideally infinite thermal conductivity of superfluid4He II, which is filled into a double wall from a gold plated and indium sealed reservoir attached to the mixing chamber. Here, sinter provides a large surface area improving thermal contact, which otherwise would be inhibited by a large Kapitza boundary resistance.
EXPERIMENTAL CELL 53
electrical feedthrough LHe link buffer-gas capillary
solid precursor for ablation silver mirror
1.5 mm thick double wall
50 mm FS window
165mm
bottom view heat exchanger interior complete cell assembly
Figure 3.13:Metal-free G10 experimental cell.
The use of metal at this position is in general uncritical, since the magnet’s quadrupole field drops quite rapidly in the far field. As can for example be verified by numerically evaluating equations (2.12), it vanishes with 1/zAH4 on the symmetry axis and equally with 1/ρ4 in the symmetry plane.
A temperature sensor and a resistive heater are incorporated into the cell jacket, both realized with 1 kΩ ROX chip resistors (LakeShore RX-102A-BR) directly immersed into liquid helium. Six Vespel needle standoffs keep the outer cell wall centered in the magnet core. They extend from G-10 holders glued to the top and bottom of the cell. Thermal contact to the 4.2 K bath is thus minimized.
All cell components are connected with either Stycast 1266 or Stycast 2850 FT epoxy. Special care has to be taken here to avoid any leaks, that can in general not be fixed with additional glue once the first epoxy has dried. Liquid helium sometimes propagates along the layers making up the G-10, which should
54 EXPERIMENTAL REALIZATION thus be positioned such that it cannot find access to these. The cell shown in Figure 3.13 has passed helium leak testing both at room-temperature and at 4.2 K and like this has demonstrated its usefulness for future buffer-gas loading experiments involving evaporative cooling.
To avoid the leak issues encountered with G-10 material, it has also been considered to machine a cell from monolithic sapphire. Such a design should also be favorable when more sophisticated detection methods are needed. High quality mirrors and windows on a sapphire substrate could be directly bonded to the cell body and would not suffer from differential contraction upon cooldown.
In fact, monolithic sapphire resonators have already been used at 4.2 K [87].
Due to the high cost of sapphire and the relatively low thermal conductivity, these possibilities have so far not been pursued further, though.