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CRYOGENIC SOURCE
At the moment the bright beam machine is used to load the magneto-optical trap. The construction of an alternative MOT loading source will allow parallel experiments with the beam and the trap, and ultimately beam-trap scattering experiments.
The `low-tech' approach to this new source has been to adopt a liquid Helium cooled design. The LHe cooled source has a velocity distribution around 200m/s instead of the 1000m/s mean of the LN2 cooled source. Consequently the efforts required to slow and steer this beam within MOT trapping range are significantly less sophisticated than those required by the bright beam line.
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We have purchased continuous flow LHe cryostat to cool the source. Preliminary tests of the cryostat have indicated that we can dissipate almost 2W while maintaining a temperature of 4K at the cold finger. Given that the gas load contributes less than 0.1% of that, we anticipate that a low pressure source with ~1W discharge power will operate at the minimum possible temperature, and hence the lowest mean velocity. Since the flow regime, viscous or molecular, is indeterminate theoretically, we shall have to characterize the velocity distribution of the source operationally. |
As for the source itself we have followed a hollow cathode design based on our experience with designs for low pressure He discharge cells. A cavity is drilled in a solid block of copper such that an insulating hollow cylinder of boron nitride sits inside, maximising thermal contact with the copper block, which is in turn thermally connected to the cold finger. A carefully shaped hole in the nozzle plate acts as the anode. The He feed line, also made of copper, acts as the cathode point to commence the discharge. Another novel feature of the current design is to run the discharge to a grounded cathode. This simplifies construction and, we hope, will allow the source to run colder. |
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He* trajectory simulation
Blue: 90-110m/s. Green: 120-140m/s. Red: 160-180m/s.
Currently we imagine three beam optics between the LHe source and the trap: one lens between two collimating sections. The purpose of these is primarily to direct slow metastable atoms along a twisted path so that fast metastables and ground state atoms can be blocked and pumped away, thus increasing the effectiveness of the differential pumping stages required between the 10-5 torr source chamber to the 10-10 trap chamber.
With this new source operational we plan to conduct experiments which make specific use of the low velocities of atoms in the MOT. These include refined e- scattering cross-section measurements and ultracold atomic cooling techniques. For the latter we are pursuing sub-recoil momentum cooling of the atom cloud using velocity selective coherent population trapping (VSCPT). In the trap the atoms’ velocities, while typically well above the recoil momentum limit, are sufficiently close to it to allow significant interaction times with the VSCPT light fields. With these interaction times we should be able to produce slow moving (one recoil momentum) atomic clouds with narrow (sub-recoil) transverse momentum profiles, suitable for coupling into single atomic mode hollow optical fibers, loading silicon chip based waveguides, or acting as beamsplitters for a large area atomic interferometer
First time-of-flight (TOF)results from the new source, indicating an atom source temperature of 20 K. Shown on the left is the TOF spectrum, the peak at zero time indicating UV photons that are produced in the discharge that are also detected, and the peak between 500 and 1000 ms indicating the cold atom signal. On the right the TOF spectrum is translated into a velocity spectrum, showing the atomic velocity centered around 400 m/s. Note that this is the first attempt, and many improvements are currently being implemented.
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