Frequency control in the process of a multicell superconducting cavity production

Abstract

Modifications in the geometry of a superconducting RF cavity due to various processing procedures are presented in a convenient matrix formulation. Specifically, the effect of chemical etching, cooling down, and preloading are characterized, while the corresponding frequency shifts are calculated with a reliable software. This matrix method was used in the fabrication of the first cornell energy recovery linac (ERL) 7-cell cavity. Cavity fabrication can be broken down into three main stages: deep-drawing cups, welding the cups in pairs to obtain "dumbbells" and end groups, and, finally, welding the obtained components into a completed cavity. Frequency measurements and precise machining were implemented after the second stage. A custom RF fixture and data acquisition system were designed and validated for this purpose. The system comprised of a mechanical press with RF contacts, a network analyzer, a load cell and custom LABVIEW and MATLAB scripts. To extract the individual frequencies of the cups from these measurements, the established algorithm of calculations was analysed and corrected. Corrections for the ambient environment were also incorporated into the measurement protocol. Using the procedure presented, the frequency deviation of the completed 1.3 GHz 7-cell cavity was 360 kHz, corresponding to an average error about 75 μm in length for every cell.

Use of quartz tuning forks for sensitive, in-situ measurements of helium properties during SRF cavity tests

“…Designing and building cavities consist of several stages and involve much effort. The cavities are made from optimized elliptic cells of niobium [1] that are welded together by electron beam to form multi-cell cavities. The cavity is then processed through three stage of tumbling, followed by buffer chemical polishing, electro-polishing and high pressure water rinses [2].…”

Design and construction of the main linac module for the superconducting energy recovery linac project at Cornell

“…The first 1.3 GHz, 7-cell main-linac cavity was fabricated based on a design that maximized the beam-break up current. After stamping half-cells and welding them into dumbbells, their resonant frequencies were measured and trimmed accordingly [11] before welded together to form an entire cavity. The cavity received a bulk bufferchemical polish (BCP) of 150 ȝm, was outgassed at 650 C for 12 hours, and was tuned to 1297.425 MHz, and then received a final 10 ȝm BCP and two eight-hour high pressure rinses.…”