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Description
In this paper we present the results of European Spallation Source (ESS) Superconducting Linac (SCL) cavities measurements which allowed us to determine crucial parameters for future SCL operation and Low-Level Radio Frequency (LLRF) systems tuning. The campaign of measurements was performed during the SCL commissioning for the second beam on dump campaign and prior to beam on target. All the measurements taken involved the usage of the cavity fast tuning system – the piezo tuner. Additionally, measurements of mechanical modes were done for all cavity types, which allowed to determine their longitudinal natural frequencies and prepare accordingly for optimal piezo tuners usage in the presence of the Lorentz Force Detuning (LFD). To confirm the frequency range of possible LFD, the LFD coefficient has been determined, and to ensure the piezo tuners are able to compensate for it, their characterisation has been carried out. SCL cavities passband modes were measured, in order to fine tune notch filter frequency position for all cavity types (expected pi-mode neighbour position shows, as expected, a natural spread due to small variation in the cavity geometries). Piezo capacitance as function of temperature was measured, which can be used as a fault tuning tool in case of piezo aging effects, causing unexpected cavity detuning at high gradient. The described tools and measurement outcome are presented with their future implementation in the ESS Linac environment.
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