Deployment and future prospects of high performance diagnostics featuring serial I/O (SIO) data acquisition (DAQ) at ASDEX Upgrade

Ion temperature measurement techniques using fast sweeping retarding field analyzer (RFA) in strongly intermittent ASDEX Upgrade tokamak plasmas

This manuscript presents a new method of interpreting the ion temperature (T_i) measurement with a retarding field analyzer (RFA) that accounts for the intermittent/turbulent nature of the scrape off layer (SOL) plasmas in tokamaks. Fast measurements and statistical methods are desirable for an adequate description of random fluctuations caused by such intermittent events as edge localized modes (ELMs) and blobs. We use a RFA that can sweep its current-voltage (I-V) characteristics with up to 10 kHz. The RFA uses an electronics compensation stage to subtract the capacitive pickup due to the finite connecting cable capacitance, which greatly improves the signal-to-noise ratio. In the 10 kHz case, a single I-V characteristic is obtained in time, which is an order of magnitude faster than the ELM cycle. The fast sweeping frequency allows us to reconstruct the T_i probability density function (PDF), which we use as the T_i representation. The boundary conditions that we place on the I-V characteristics when calculating the T_i values impact the resulting T_i PDF. If the boundaries are insensitive to the plasma fluctuations, then the most probable T_i value of the PDF (20 eV-25 eV) is similar to the T_i value obtained via the classical conditional averaging method (20 eV-27 eV). However, if the boundary conditions follow the fluctuations, then the PDF-based method gives a substantially higher most probable T_i value (35 eV-60 eV). Overall, we show that a fast sweeping RFA diagnostic should be used in intermittent SOL plasmas to reconstruct the PDF for accurate T_i measurements.

Helium line ratio spectroscopy for high spatiotemporal resolution plasma edge profile measurements at ASDEX Upgrade (invited)

The thermal helium beam edge diagnostic has recently been upgraded at the ASDEX Upgrade (AUG) tokamak experiment. Line ratio spectroscopy on neutral helium is a valuable tool for simultaneous determination of the electron temperature and density of plasmas. The diagnostic now offers a temporal resolution of 900 kHz with a spatial resolution of up to 3 mm at 32 lines of sight (LOS) simultaneously. The LOS covers a radial region of 8.5 cm, starting at the limiter radius and reaching into the confined region beyond the separatrix. Two components are of particular importance for the aforementioned hardware improvements. The first is the optical head, which collects the light from the experiment. Equipped with an innovative clamping system for optical fiber ends, an arbitrary distribution pattern of LOS can be achieved to gain radial and poloidal profiles. The second major development is a new polychromator system that measures the intensity of the 587 nm, 667 nm, 706 nm, and 728 nm helium lines simultaneously for 32 channels with filter-photomultiplier tube arrays. Thus, the thermal helium beam diagnostic supplements the AUG edge diagnostics, offering fast and spatially highly resolved electron temperature and density profile measurements that cover the plasma edge and scrape-off layer region. Plasma fluctuations, edge localized modes, filaments, and other turbulent structures are resolved, allowing analysis of their frequency and localization or their propagation velocity.