Ph.D. Thesis - Physics

After loading these settings, the homogeneity can normally be fine-tuned in a matter of
minutes.
A final aspect of the magnet is that the static field B0 does slowly drift in time due
to the tiny amount of power dissipation present in the superconducting coil. The order of
magnitude for this drift is about 1 Hz/hour. Although this is tiny in relation to ω0, this small
change can have a negative impact on precision techniques such as quantum operations. To
correct this problem, there is one further room-temperature coil that adjusts the static field.
The current through this coil is locked to the resonance frequency of the deuterium nuclei
in the solvent of the sample. Although this locking method is not the only way to approach
the problem, it is quite convenient, since the process runs in the background after some
initial setup.
3.4.3 Probe
The probe is a multi-purpose device that supplies the rf fields that manipulate the nuclei,
reads out the magnetization signal from the sample, and also regulates the temperature of
the sample using air at a specific temperature. The rf coils are about 1.5 cm in diameter and
address a region within the sample that is about 2 cm long, called the “active region.” The
coils are made of a low-resistivity metal (such as copper) and have from 1 to 3 windings.
The coils are part of a resonant circuit with a Q factor on the order of 100. A circuit
that contains two adjustable capacitors is used; the first varies the resonant frequency of
the circuit, enabling one to tune to a specific nuclear resonance, while the second permits
impedance matching. The transmission lines, which are coaxial cables, have an impedance
of 50 Ω; thus, the circuit is matched to that impedance to avoid reflection of power from
the sample, maximizing the effectiveness of the rf signals, and to permit maximum transfer
of the signal from the nuclei to the receiver, maximizing the signal-to-noise ratio.
The probe actually contains two separate pairs of rf coils; the first is for higher-frequency
nuclei such as 1 H and 19 F, both of which have resonances above 200 MHz. The second is for
lower-frequency nuclei such as 13 C, which has a frequency below 200 MHz. The reason for
this is that it is hard to design resonant circuits that have a high Q factor over a very wide
bandwidth. The high-band and low-band coils are mounted at right angles to minimize
cross-talk. We used a commercial HFX probe from Nalorac. The “H” and “F” refer to
the high-band coils, which are tuned for hydrogen or fluorine. The “X” means that the
low-band coils are tunable over a wide range. In our experiments, we tuned this coil for
13 C.
Finally, the probe is responsible for regulating the temperature of the sample. Thermo-
couples measure the temperature of the sample, and air at a specific temperature is flowed
through the probe in accordance with the temperature measurement. Thus the tempera-
ture is actively stabilized. The probe may also contain coils for generating magnetic field
gradients in space, but this does not play a role in our work here.
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