Ph.D. Thesis - Physics

weeks. Low pressures depend on choosing materials that have a low vapor pressure at room
temperature.
Before assembly, each part within the chamber must be carefully cleaned. We use a four-
step process involving four solvents. In order, they are: detergent solution, distilled water,
acetone, and methanol. Each step involves sonicating the trap in the solvent for 30 min.,
typically with heat (≈ 80 ◦ C) applied. The process is designed to remove a large variety
of impurities on the surface; water and detergent for both polar and non-polar substances,
acetone for non-polar molecules that are not soluble in water, and finally methanol, which
removes the “residue” that is left over after an acetone clean. Although other vacuum
cleaning techniques exist, some involving three methanol steps, some involving only water
followed by acetone, we feel that it is better to be “safe than sorry” with the cleanliness of
materials put into a UHV chamber. A single dirty component can ruin good vacuum. All
components put into the vacuum chamber are subjected to this cleaning process, and so are
the tools used inside the chamber.
The pressure is monitored with a Bayard-Alpert ionization gauge, which works by ioniz-
ing gas particles in the vacuum chamber and measuring the current induced by them across
a pair of charged electrodes. This current is proportional to the density of gas particles in
the chamber. Our gauge can measure pressures as low as 10 −11 torr.
Rf resonator
The trap is driven with a helical resonator that is supplied with voltage from a broadband
rf amplifier (MiniCircuits TIA-1000-1R8). The helical resonator is a helically wound trans-
mission line that supports a quarter-wave resonance at a specific frequency that is governed
by the capacitance and inductance per unit length of the transmission line, as well as the
load impedance (that of the trap and feedthrough). A practical guide for the construction
of such resonators is given in Ref. [Fis76].
This paper permits calculation of the unloaded frequency of the resonator. However,
this frequency changes to a generally lower value when the trap is attached. The frequency
drop depends on the electrical characteristics of the trap and feedthrough and is difficult
to predict, but it is usually less than a factor of two. Since it is not necessary for the
resonance to be at one specific frequency, a small amount of trial and error enables us to
get a resonance in the right range. A typical Q factor for the finished resonator is 100, with
a voltage step-up between 20 and 40. The circuit was impedance matched by minimizing
the power reflected from the circuit along a 50 Ω coaxial cable. The voltage on the trap
was measured by securing a wire near the high-voltage end of the resonator and calibrating
it at low voltage by simultaneously measuring the actual voltage using a 100X scope probe.
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