Retarding field analyzer for the wendelstein 7-X boundary plasma

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.