Elsevier

Journal of Nuclear Materials

Volumes 390–391, 15 June 2009, Pages 1114-1117
Journal of Nuclear Materials

Direct measurements of the plasma potential in ELMy H-mode plasma with ball-pen probes on ASDEX Upgrade tokamak

https://doi.org/10.1016/j.jnucmat.2009.01.286Get rights and content

Abstract

Experimental investigations of the plasma potential and electric field were performed for ELMy H-mode plasmas in the vicinity of the limiter shadow of ASDEX Upgrade. A fast reciprocating probe with a probe head containing four ball-pen probes (BPPs) [J. Adamek et al., Czech. J. Phys. 54 (2004) C95 – C99.] was used on the midplane manipulator. Different gradients of the plasma potential were observed during ELMs and in between them. The temporal resolution of the direct plasma potential measurements with BPP was determined by using power-spectra analysis.

Introduction

The investigation of the electric field and its fluctuations remains one of the most important tasks in the context of anomalous edge plasma and momentum transport. A precise knowledge of the plasma potential and its fluctuations in the edge plasma region is vital for a better understanding of transport phenomena and is thus also relevant for ITER, in particular for the life time of first wall components. Moreover, direct plasma potential measurements are necessary for understanding SOL physics and the experimental verification of different codes for modelling SOL [1], [2].

The ball-pen probe (BPP) [3], [4], [5], [6] for direct measurements of the plasma potential was recently developed in the Institute of Plasma Physics AS CR in Prague. Here we report on BPP measurements of the plasma potential in ELMy H-modes of ASDEX Upgrade. Radial profiles of the plasma potential are obtained for the time during an ELM event and in the quiet phase in between ELMs. Furthermore, frequency spectra of potential fluctuations are analyzed.

Section snippets

Principle of the ball-pen probe

The principle of the BPP (see Fig. 1) is to make use of the large difference between the gyro-radii of electrons and ions in magnetized plasma, which is based on the concept of the Katsumata probe [7]. The conically shaped collector of a BPP is retracted by a certain depth (h) inside a screening channel of ceramic (usually boron nitride – BN). Thereby a certain part of the electron current density is shielded from the collector, whereas the ions, due to their much larger gyro-radii are less

Probe construction and experimental set-up

The fast reciprocating probe shaft on the ASDEX Upgrade midplane manipulator was used to insert a probe head containing four BPPs with different retraction depth (h) of its collectors (h = −0.5 mm (BPP0), −1 mm (BPP1), −2 mm (BPP2) and −3 mm (BPP3)) and two Langmuir probes (LP1 and LP2) as shown in Fig. 2. The BPPs are made of stainless steel collectors with diameters of 4 mm, which are fixed inside the ceramic shielding tubes with inner diameters of 6 mm. The shielding tubes are made of one piece of

Experimental results

The temporal evolutions of the plasma potentials measured by the BPP0 (magenta line), BPP2 (green line) and BPP3 (black line) during stroke 2 are plotted in Fig. 3. The temporal resolution of plotted potentials is given by the sampling frequency f = 0.5 MHz of the data acquisition system. The exposure time of the probe head to the plasma outside the limiter shadow lasted from 3.11 < t < 3.13 s. The BPP1 with h = −1 mm (red line) started to work only at t = 3.2 s (the end of stroke 2) due to a short circuit

Conclusions

Ball-pen probes were used for measurements of the plasma potential during ELMy H-mode shots on ASDEX Upgrade. The probe head was exposed to the edge plasma in the vicinity of the limiter. It was found that all ball-pen probes directly measure the radial profile of the plasma potential independently of the position of their collectors (h = −0.5, −2, −3 mm), which is in good agreement with measurements on the CASTOR tokamak [1], [2]. The electric field close to the limiter in between ELMs is

Acknowledgements

This work was carried out within the EURATOM Associations IPP.CR, IPP and ÖAW and was supported by projects GA AV B100430601 of the Grant Agency of AS CR, and by the Austrian Science Fund under Grant No. L302-N02.

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