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

Abstract

This manuscript presents a new method of interpreting the ion temperature (Ti) 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 Ti probability density function (PDF), which we use as the Ti representation. The boundary conditions that we place on the I–V characteristics when calculating the Ti values impact the resulting Ti PDF. If the boundaries are insensitive to the plasma fluctuations, then the most probable Ti value of the PDF (20 eV–25 eV) is similar to the Ti 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 Ti 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 Ti measurements.

Ion temperature measurements in the tokamak scrape-off layer with high temporal resolution

“…This is to be expected since the fitting procedure applied on the tail of the I -V characteristic is dominated by blob temperatures, as pointed out in [26]. However, the T i histograms may differ if the RFA (-like) I -V characteristics are fitted with different current cut-off or voltage cut-off, as noted in [8]. We see in figure 7 that the ratio T i /T e is larger in the main-SOL than in the vicinity of the LCFS.…”

“…Conversely, ion temperature measurements are typically limited to a few kHz using e.g. a retarding field analyzer (RFA) [6][7][8][9]. Applying a fastswept voltage to this diagnostic is challenging because the measured current-on the level of tens of microamps-is very small compared to the parasitic current of the coaxial cable.…”

“…∼30 mA of parasitic current over a 5 m coaxial cable. However, recent measurements on ASDEX Upgrade using RFA with a higher sweeping frequency of 10 kHz [8] show that only fast sweeping should be used in intermittent scrape-off layer (SOL) plasmas for an accurate reconstruction of the ion temperature histograms.…”

Statistical properties of ion and electron temperature fluctuations in the edge of the COMPASS tokamak

“…In fusion devices, the radial profiles of T i are typically measured using RFAs [25,26] with a temporal resolution in the order of 1 ms, too slow to study T i fluctuations. In the main SOL [27], the resulting RFA T i values are significantly higher than the T e ones, observation also supported by simulations of blobs including ion dynamics [6].…”

“…Fast T i measurements gives therefore similar results than the ones obtained by actual RFA measurements. The interpretation of the signification of RFA data, as mentioned in [25], can be illustrated by comparing the RFA-like T i to its mean values (orange squares in figure 1) calculated on the same datasets and enabled thanks to fast probe measurements. It is clear that both values are quite different, due to the fact that the RFA diagnostic measures only on the 'hot tail' of the ion temperature distribution (high temperature), while the average value is a mixture of all available temperatures.…”