Date of Award

Spring 2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Hani Elsayed-Ali

Committee Member

Helmut Baumgart

Committee Member

Gon Namkoong

Committee Member

Alex Gurevich

Abstract

Niobium-coated copper (Nb/Cu) superconducting radiofrequency (SRF) cavities are a promising alternative to bulk niobium cavities for next-generation particle accelerators due to their reduced material cost and superior thermal conductivity of the copper substrate. However, cavity performance is fundamentally limited by the microstructural quality of sputtered Nb films and thermal transport across the Nb/Cu interface. Reduced thermal diffusivity in thin films and high interfacial thermal resistance contribute to localized heating and performance degradation under high RF fields. This dissertation addresses these challenges through a combined experimental and methodological investigation of thermal transport in Nb thin films, focusing on interface engineering using laser-generated multicharged niobium ions.

The dissertation begins with a comprehensive review of Nb/Cu SRF cavity technology, highlighting limitations of conventional sputtered Nb films and the potential of ion-assisted deposition and interface modification strategies. Key microstructural and thermal transport mechanisms governing bulk and thin-film Nb are reviewed to establish scientific context.

A TDTR system at Old Dominion University incorporates a closed-loop cryogenic station enabling measurements down to 10 K. A one-dimensional heat diffusion model extracts thermal diffusivity from TDTR data. Using TDTR, thermal diffusivity of sputtered Nb films (100–800 nm) on Cu substrates was studied. Diffusivity increases with film thickness due to grain coarsening, approaching bulk values above 400 nm. Low-temperature measurements (down to 40 K) shows increased thermal transport, consistent with reduced phonon scattering. Grain boundary scattering further limits thermal transport below 80 K.

A laser-driven multicharged niobium ion (MCI) source is developed. Using nanosecond laser ablation, distinct ion populations are generated and analyzed via time-of-flight and electrostatic diagnostics. A dedicated beamline delivers Nb ion pulses with charge states from Nb¹⁺ to Nb⁸⁺ for deposition and shallow implantation.

A hybrid deposition strategy is introduced, depositing an ultrathin Nb interlayer via multicharged ions onto Cu prior to DC magnetron sputtering. This interface treatment enhances interfacial bonding and reduces defect-mediated thermal resistance. TDTR measurements show approximately 24% improvement in thermal diffusivity in ion-seeded films versus sputtered films, with structural characterization indicating correlation between enhanced thermal transport and modified microstructure.

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ISBN

9798197810236

ORCID

0000-0003-1313-4617

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