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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
8 Ω<br />
LN2<br />
Liquid<br />
Nitrogen<br />
Dewar<br />
Nozzle<br />
Figure 4.32: A schematic summary <strong>of</strong> the system used to cool the nozzle to 150 K.<br />
A 50 W 8Ω resistor provides a variable heat source in a liquid nitrogen dewar, which<br />
controls the rate <strong>of</strong> the boil<strong>of</strong>f. The cold g<strong>as</strong>eous nitrogen is forced to flow through<br />
the cryostat <strong>of</strong> the nozzle, cooling it. Incre<strong>as</strong>ing the current in the resistor incre<strong>as</strong>es<br />
the boil <strong>of</strong>f rate, and cools the nozzle more.<br />
advantageous to cool the nozzle significantly below room temperature to reduce the<br />
initial velocity <strong>of</strong> the beam. With a lower temperature limit <strong>of</strong> 120K from the krypton<br />
carrier g<strong>as</strong>, and due to a desire to avoid excessive clustering and heating <strong>of</strong> the beam,<br />
the nozzle is run at a temperature <strong>of</strong> 150 K. To run the nozzle at this temperature, a<br />
g<strong>as</strong> line is run from a tank <strong>of</strong> liquid nitrogen to the pool type cryostat <strong>of</strong> the nozzle.<br />
This directs the cold boil <strong>of</strong>f g<strong>as</strong> from the liquid nitrogen dewar into the cryostat,<br />
cooling the nozzle. The flow rate <strong>of</strong> the boil <strong>of</strong>f is controlled by inserting a 50 W<br />
8 Ω resistor into the nitrogen dewar, <strong>with</strong> greater current leading to more power in<br />
the resistor and f<strong>as</strong>ter boil <strong>of</strong>f. Currents <strong>of</strong> up to 3 A are run through the resistor,<br />
which exceeds the power rating and is only fe<strong>as</strong>ible because the resistor is immersed<br />
in liquid nitrogen. A schematic illustration <strong>of</strong> this method <strong>of</strong> cooling the nozzle is<br />
shown in figure 4.32.<br />
The final change in the apparatus concerns the detection method. Since the<br />
oxygen is not met<strong>as</strong>table, it will not create a free electron when it hits the surface<br />
112
Change language