Undergraduate Research at Jefferson Lab

Evaluation of a Tunnel Diode Oscillator for the Measurement of SRF Niobium Penetration Depth

Student: Celia Mercovich

School: Rochester Institute of Tech

Mentored By: Pashupati Dhakal

The superconducting radio frequency (SRF) cavity is a niobium accelerator component whose performance is dependent upon the fundamental length scale of the superconductor: the coherence length (ξ) and the London penetration depth (λ_L). Comparable to these values, the microwave penetration depth of the SRF cavity is approximately 50 nm below the material's surface. RF penetration depth in a superconductor can be measured from frequency changes in an inductor-capacitor (LC) tank circuit as a sample is inserted into the inductor coil. A tunnel diode in its state of negative differential resistance can be implemented in this circuit to provide the positive feedback which allows for stable oscillation. This project aimed to develop a tunnel diode oscillator (TDO) for the measurement of penetration depth in SRF niobium in the MHz frequency range. The characteristic current-voltage curve was first measured in several tunnel diodes, from which a voltage divider was calculated to place oscillation in the negative resistance region when connected with the LC circuit. The complete TDO was then built using a coil separated from the tank circuit by coaxial cable to allow for testing at low temperatures in a liquid helium Dewar. The frequency was varied with different values for inductance and capacitance before a stable signal was found and amplified to MHz, and then recorded with an oscilloscope and frequency counter. The niobium sample tested in the coil produced a measurable increase in frequency, and evaluation of its stability over time and temperature determined the TDO to be a viable method for estimating RF penetration depth. High frequency noise in the TDO signal initially prevented accuracy to kHz in room temperature measurements, however electrically shielding the circuit and filtering the signal reduced this by an order of magnitude. The transition from superconducting to normal conducting occurred at 9.05K with the niobium sample in a 20 turn coil, and 9.23K in a 10 turn coil, both comparable to the accepted value of 9.3K. From these critical temperatures, it was possible to calculate a rough temperature dependence of penetration depth using the Gorter-Casimir two-fluid model. Tunnel diode oscillators have been shown to be practical tools in the measurement of penetration depth; this device demonstrates one method for better characterizing SRF niobium. Future tests with the tunnel diode oscillator addressing penetration depth in applied DC field or more detailed measurements over varying temperature could ultimately aid in the design of more efficient niobium cavities.