Beam emission spectroscopy diagnostic design and capabilities for two-dimensional turbulence measurements on Wendelstein 7-X

A beam emission spectroscopy (BES) diagnostic is designed for studying two-dimensional
turbulent dynamics by measuring the Doppler-shifted Balmer-Alpha emission (𝑛 = 3 → 2) from neutral heating beams on Wendelstein 7-X (W7-X) stellarator. The BES viewing geometry has been determined in the conceptual design previously. However, the small Doppler shifts and small optical throughput compared to a typical BES diagnostic demand dedicated efforts on the optical assemblies and the detector module for the BES system. We present the detailed opto-mechanical design and specifications of BES, including a customized neutral beam viewing optical system, a semi-telecentric optical assembly, and a detector module for electronic amplification. The point spread function is calculated using the pyFIDASIM code with experimental parameters and W7-X magnetic configurations to estimate the BES spatial resolution and beam intensity. The as-manufactured interference filter is applied for the spectral isolated beam radiance calculation. Result shows that the BES system is capable of measuring the ion-scale turbulence for 𝑘⊥ 𝜌𝑖 ≤ 0.4 at 𝑟/𝑎 = 0.75 with reasonable spatial and wavenumber resolutions. ... mehrAn integrated detector module is fabricated where two 8 × 4 avalanche photodiode detectors (APD) are embedded into the custom-designed pre-amplifier
circuit to gain signals to the desired level. The detector noise measurement is performed and the signal-to-noise ratio (SNR) is evaluated. A detectable fluctuation level can be achieved as low as ˜𝑛𝑒/𝑛𝑒 ≈ 0.5% at frequency 𝑓 ≤ 400 kHz with a bandwidth of 1 MHz.