Speaker
Description
Collective Thomson Scattering (CTS) diagnostics require high signal fidelity, temporal coverage, and calibration robustness, particularly looking ahead toward long-pulse and steady-state fusion devices. Until 2024, the CTS fast-digitization chain employed at ASDEX Upgrade (AUG) was limited by a duty cycle below 6% over 10 s, constraining both temporal continuity and the statistical quality of CTS measurements. This limitation, in preparation for future systems, motivated the development of a new FPGA-based digitizer, capable of continuous acquisition. This continuous ultrafast sampling, and the additional on-board processing power offered by the FPGA open possibilities for calibration, which have consequences for the ITER CTS.
Here we present the implementation and experimental assessment of the FPGA-based continuous ultrafast digitizer for CTS applications. The system provides a 100% duty cycle with a 5 GHz analog bandwidth and 8-bit vertical resolution. A second-generation (V2) system, currently under development, achieves 12-bit precision, addressing dynamic range limitations identified in the first prototype. A key feature of the platform is its on-board FPGA processing capability, which enables real-time FFT and offering the possibility of in situ data reduction through spectral averaging or decimation. While these capabilities are essential for managing data rates in continuous operation, they enable a much more flexible fast-digitizer calibration process.
The implications of continuous ultrafast sampling are discussed in the context of ITER CTS. The diagnostic system foresees eight CTS receivers, each covering over 10 GHz of bandwidth, combined with a dedicated alignment and calibration line, unique to this system. With this many individual digitizers, cross-calibration within each and between receivers is critical, while absolute calibration remains equally important for quantitative physics interpretation.
The talk will contrast various digitization methods, both historical and prospective, and the applicable calibration methods.
