A novel approach to direct measurement of the plasma potential

Abstract

A novel probe and approach to the direct measurements of the plasma potential in a strong magnetic field is suggested. The principle of this method is to reduce the electron saturation current to the same magnitude as that of the ion saturation current. In this case, the floating potential of the probe becomes identical to the plasma potential. This goal is attained by a shield, which screens off an adjustable part of the electron current from the probe collector due to the much smaller gyro-radius of the electrons. First systematic measurements have been performed in the CASTOR tokamak.PACS : 52.70.Ds

Ball‐Pen Probe Measurements in L‐Mode and H‐Mode on ASDEX Upgrade

“…We observe that the ratio R is close to one and independent of the pin retraction, which is an indication that the BPP measures the plasma potential. In general, the behaviour of the BPP in the SOL during an L-mode discharge and in the limiter shadow during an ELMy H-mode discharge in ASDEX Upgrade is the same as during Ohmic discharges in the CASTOR tokamak [2]. Although the ratio R is not perfectly equal to one in L-mode or during ELMs in H-mode, the BPP measures a potential close to the plasma potential, and the influence of the electron temperature is strongly suppressed, assuming the relation V BP P fl = Φ pl − ln(R) T e .…”

“…In the small tokamak CASTOR [5], ball-pen probes were successfully used to determine the plasma potential [2,3] at the edge plasma, with ion and electron temperature below 30 eV. The pin arrangement reduced the electron saturation current I − sat to the probe.…”

“…These topics are relevant for ITER, in particular for the life-time of first wall components. The ball-pen probe (BPP) [1][2][3][4] for direct measurements of the plasma potential Φ pl was developed in the Institute of Plasma Physics in Prague. Here we compare and combine BPP data with measurements of Langmuir probes located on the same probe head and with data from the Thomson scattering diagnostic in ASDEX Upgrade.…”

Novel Technique for Direct Measurement of the Plasma Diffusion Coefficient in Magnetized Plasma

“…However, the present situation is substantially different, due to the fact that the shaft is made of a dielectric material, and also to the fact that ions penetrate more easily inside the shaft, due to their larger Larmor radius [2][3][4]. As a consequence, one can expect that a positive charge is deposited on the inner wall of the shaft, and this reduces the ion parallel flux to very low levels.…”

Particle‐In‐Cell Simulations of the Ball‐Pen Probe

“…Several techniques have been developed, such as the triple probe [1], emissive probe [2] or the heavy ion beam probe [3], each coming with its limitations. The aim of the development of the Ball-pen probe (BPP) [4] [5] was to create a robust diagnostics capable of direct measurement of V pl in the SOL of fusion devices. The geometry of BPP reduces the electron current to the diagnostic collector.…”