Advances in Magnum-PSI probe diagnosis in support of plasma–surface interaction studies

Abstract

Magnum-PSI is a linear plasma generator equipped with a superconducting magnet, assuring fusion devices relevant conditions at plasma–surface interface. The plasma column was diagnosed using 64 probes embedded in the target. The cross-sectional distributions of plasma parameters (floating potential, ion saturation current and electron temperature) were measured for hydrogen and deuterium plasmas under various discharge conditions. The radial profile of the floating potential across the plasma column can be described by a reversed Mexican hat-like wavelet, having the most negative potential at the center of the plasma column. The negativity of the floating potential diminishes when the discharge current increases or the magnetic field decreases. The axial gradient of the floating potential is reduced by increasing the magnetic field. The ion saturation current is maximum at the center of the plasma column, increasing with both the discharge current and magnetic field. The ion flux to the target, estimated from Thomson scattering data, was confirmed by probe measurements. The electron temperature estimated from the ion branch of the probe current-voltage characteristic is few times larger than that obtained from Thomson scattering. By increasing the gas pressure in the target chamber, the time-dependent ion saturation current measured by probes changes from a constant average current (when the plasma column is attached to the target) to a fluctuating average current with scattered peaks (in a partially detached regime) which vanishes completely in the fully detached regime. With respect to hydrogen, the plasma column is wider in deuterium and is characterized by less negative floating potential distributions.

Plasma rotation and axial flow velocities in Magnum-PSI from cross-correlation measurements

“…Figure 8 shows that the azimuthal electric drift is dominant with respect to the diamagnetic drift, the latter being about an order of magnitude smaller. With the increase of the discharge current, the plasma rotation velocity decreases due to a lower radial electric field in the plasma column [29]. Apart from a slight change in the radial profile, the values of the plasma rotation velocity obtained from the MCC method do not change much with the magnetic field.…”

“…The fluctuations of the Magnum-PSI plasma parameters were measured with a 64-probe matrix (visible as insert in figure 1(b)) embedded in a tungsten plate, the latter being used as a floating target. The multi-probe system and the measuring circuits were thoroughly described in [27] and they were used to measure the plasma parameters in both divergent [27,28] and constant [29] magnetic field configuration. Because the probe signals will be analysed with respect to their space-time correlation, only spatial details of the multi-probe system and temporal characteristics of the data acquisition system are provided here.…”

“…Due to the proportional relationship between I is and plasma density, the analysis of I is fluctuations provides information about the plasma density behaviour. The electronic circuit and the method used to measure current signals were detailed in [27,29]. For signal analysis, a probe was set as reference and the cross-correlation function was calculated for all other probe signals with respect to the reference probe signal.…”

“…In order to facilitate the derivation of the plasma potential and ion pressure, fitted radial profiles were used for the floating potential, electron density and electron temperature. The experimental data for the floating potential can be seen in [29]. The plasma rotation velocity estimated from the cross-correlation analysis was averaged over the probes situated at the same radial position (±0.5 mm) with respect to the plasma centre.…”

“…For this purpose, plasma fluctuations have been registered with a multi-probe system that replaced the target of Magnum-PSI. The system was originally designed to measure 2D distributions of plasma parameters at the target [27][28][29], but the spatial arrangement of the probes combined with temporal measurements allowed to apply the maximum cross-correlation (MCC) method [30] for tracking the plasma flow. Plasma rotation velocity was obtained by applying the MCC method to the ion saturation current fluctuations measured with a normal target (the multi-probe system was normal to the plasma column).…”