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The amplification of third sound mode in superfluid helium requires high quality factor. It can be achieved by improving the resonator. The clean resonator has been built for the first time. This resonator has no dirt bridging across the capacitor gap. Several improvement in the detection system has been complished. The new bias plate has been installed. It has more mechanical stability and better thermal conduction. One of the key features for this disertation is development the method to measure frequency and quality factor of non-oscillating Tunnel Diode Oscillator. Due to resonator improvement, the highest quality factor of the third sound mode in open disk design was obtained. However, it is still too low for good data. The third sound amplification experiment needs Q in the order of 105. The thickness oscillation amplitude of the experimental third sound and disk mechanical resonance signal are not consistent with the theoretical calculation.

Improvements in the Construction and Detection Electronics of a Third Sound Resonator Used for the Observation of Stimulated Condensation in Superfluid ⁴He