Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
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operated. More detail on the magnetic and rotational states <strong>of</strong> molecular oxygen can<br />
be found in [86].<br />
Since high-field-seeking states are incompatible <strong>with</strong> the coilgun in its current<br />
method <strong>of</strong> operation, the avoided crossing sets a limit on the maximal value <strong>of</strong> the<br />
magnetic fields that can be used to slow oxygen. The me<strong>as</strong>ured peak value <strong>of</strong> the<br />
magnetic fields created on the axis <strong>of</strong> the coilgun solenoids is 5.2T, and finite element<br />
calculations indicate that the maximal magnetic field adjacent to the coil windings<br />
is just below 6 T. This maximal field which an atom can encounter in the coilgun is<br />
well below the limit <strong>of</strong> 8 T. Thus, the avoided crossing is not expected to play a large<br />
role in the operation <strong>of</strong> the coilgun. In order to achieve the highest slowing efficiency,<br />
coil timing is tuned for molecules in the ground rotational J =2,MJ = 2 sub-level.<br />
4.6.2 Oxygen Beam Creation and Detection<br />
Since the ground state <strong>of</strong> oxygen is magnetic, a discharge is not required<br />
to excite the molecule to a met<strong>as</strong>table state to enable its slowing by the coilgun.<br />
However, because the boiling point <strong>of</strong> oxygen is ≈ 90K,thenozzlecannotbecooled<br />
to 77 K by liquid nitrogen. Additionally, the m<strong>as</strong>s <strong>of</strong> molecular oxygen is greater<br />
than that <strong>of</strong> met<strong>as</strong>table neon, and so oxygen h<strong>as</strong> a greater kinetic energy at the same<br />
velocity. The magnetic moment <strong>of</strong> oxygen is less than that <strong>of</strong> met<strong>as</strong>table neon <strong>as</strong><br />
well, which means that less energy will be removed per coil. These two issues make it<br />
highly advantageous to start <strong>with</strong> <strong>as</strong> slow an initial beam <strong>as</strong> possible. For this re<strong>as</strong>on,<br />
rather than continue to use neon <strong>as</strong> a carrier g<strong>as</strong>, krypton is used instead <strong>as</strong> its higher<br />
m<strong>as</strong>s leads to a slower initial beam. Oxygen is mixed <strong>with</strong> krypton at a 15:75 ratio,<br />
and the valve is run <strong>with</strong> a backing pressure <strong>of</strong> 34 psi.<br />
Oxygen is a liquid at 77 K (krypton is too, its boiling point is ≈ 120 K),<br />
so cooling the nozzle <strong>with</strong> liquid nitrogen is not a possibility. However, it is still<br />
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