Interpretation of flush-mounted probe current-voltage characteristics using four-parametric fits

Abstract

The evaluation of probe measurements in the swept regime can be performed via afit to the ion branch of the current-voltage (I-V ) characteristic, providing the most relevant plasma parameters such as the ion saturation current, the electron temperature and the floating potential. In this paper, we present a parametric study based upon the existing flush-mounted probe setup briefly operated at the COMPASS tokamak, aiming to compare relevant I-V formulas widely used for such analysis. Selected four-parametric fit descriptions were applied to synthetic data obtained by simulation of the probe in aparticle-in-cell model, SPICE, both in simplified 2D and more complex 3D geometries. Plasma parameters recovered by the fit were compared to the input from the model and the precision of this recovery was mapped to a wide range of parameters, especially with respect to the ratio of the probe size d pin to the Larmor radius r L . Results show that while the electron temperature and the floating potential can be obtained quite precisely regardless of the method, the ion saturation current can be identified incorrectly, mainly due to the fact that the sheath expansion description by optical approximation is not sufficient for almost grazing field line angles of incidence, especially when d pin ∼r L . We show, however, that operation of flush-mounted probes can be fruitful especially in high-field devices. Additionally, we have explored the sensitivity on the available bias potential range, showing that for proper analysis of I-V characteristic using four-parametric fit, the lower bound of the fit range should be at least k T e 4 B e below the floating potential. If this condition cannot be satisfied, threeparametric fit in the range close to the floating potential can still produce results with acceptable precision.

Embedded-probes diagnostics for the COMPASS upgrade tokamak: a concept

“…It can be compared with the experimental coefficient used in the fit, although more precise 3D simulations could be necessary. Previous comparison between 2D and 3D simulations indicated, that while the effect of real geometry on electron temperature and floating potential remains the same, the real value of 𝑅, as well as the 𝐼 sat are slightly different [5]. For angles of incidence smaller than 3 • , the absolute value of 𝑅 se increases threefold.…”

“…In view of predicting the work of the flush probes with realistic dimensions, the SPICE2 code [5,6] was used to calculate I-V characteristics. The simulations were performed in 2D3V, so that they represent the first approximation of the I-Vs expected.…”

“…Then, in section 3, calculations, using the PFCflux code [4], of the heat fluxes and the incident angle of the field lines at the different limiters are presented. In section 4, model I-V characteristics for flush-mounted probes are calculated using the SPICE2 code [5,6], and their dependence on the incident angle is studied. The last section presents a discussion on the capabilities and limitations of the electronics used for measuring with the probes envisaged to be used.…”

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

“…However, these models were based on a simplified treatment of processes occurring in the Debye sheath and the magnetic pre-sheath, leading to uncertainties in their predictions. In practice, the options for validation of measured quantities are either the comparison (or even calibration) against other diagnostics or dedicated 3D3V modelling using the particle-in-cell (PIC) method [7]. In this work we extend the modelling study on flush mounted probes [7] to the case of protruding probes which are employed in the divertor of the COMPASS tokamak.…”

“…In practice, the options for validation of measured quantities are either the comparison (or even calibration) against other diagnostics or dedicated 3D3V modelling using the particle-in-cell (PIC) method [7]. In this work we extend the modelling study on flush mounted probes [7] to the case of protruding probes which are employed in the divertor of the COMPASS tokamak. The divertor of the COMPASS tokamak [8] has been equipped with a combined probe array, which comprises 55 probe triplets, each triplet consisting of two LPs and one ballpen probe [9].…”