Search

A study of ball pen probes (BPPs) in a rf strongly magnetised plasma is reported for the first time. These probes have been successfully used in fusion plasmas, with magnetic fields up to 2.5 T, to measure the plasma potential. In this paper experimental results of various ball pen designs (2 and 4 mm diameter with flat and conical collectors) are presented up to 0.5 T in a low pressure capacitively coupled rf plasma. A theory of the BPP is developed, showing that the increase of the collector potential and plateau region, with collector retraction, requires the electron current to decrease faster than the ion current. Experimentally, it is found that to develop effective electron screening the electron Larmor radius should be smaller than the tunnel internal diameter. Smaller tunnels improve screening due to the tunnel entrance wall sheaths. Inside the tunnel a plateau region forms at 81 mT reducing to a broad peak at higher field strengths. Ion shielding and surface losses (for small tunnel diameters) reduce the collector peak width and maximum potential with increasing magnetic field. Conical collectors were found to increase the length of the plateau region and broaden the peak. Particle in cell simulations were in good agreement with the experimental results. The electron shielding and plateau regions were reproduced but not the broad peak at higher field strengths. Good agreement between both 2 mm BPPs and an emissive probe was found only at 81 mT to within 3 V or 1.3 electron temperatures (T e). For all BPPs at higher field strengths (≥ 250 mT) the maximum collector potential underestimated the emissive probe by more than 2.7 T e (7 V). At these field strengths all BPPs agree with each other to within 1.5 T e (4.1 V). Possible reasons for these disagreements are discussed.

The first results of design and fabrication of comparative Langmuir Ball-Pen probe (LBP) for the IR-T1 tokamak were presented. Conventional Langmuir probe can’t measure the plasma potential precisely (due to temperature fluctuations). Therefore we used LBP for direct measurement of the plasma potential. First measurements show that the probe allows a modification of its collected electron and ion saturation currents. Regarding to the results, the ratio of R (up to down ion saturation currents) is close to one at a certain collector position (h = −0.7 mm). In this situation, the floating potential is very close to the plasma potential. Temporal and radial evolutions of plasma potential have been investigated and analyzed.

Ball-pen probes (BPP)have been deployed in the SOL of numerous tokamak experiments and low-temperature magnetised plasmas to make direct measurements of the plasma potential and electron temperature. Despite strong empirical evidence for the success of the BPP it lacks a theoretical underpinning of its collection mechanism. In this paper we investigate the capability of the probe to measure the plasma potential by means of particle-in-cell simulations. The BPP is found to float at a potential offset from the plasma potential by a factor TₑαBPP. By simulating BPPs and Langmuir probes, excellent agreement has been found between the measured electron temperature and the specified source temperature. The transport mechanism for both ions and electrons has been determined.E×B drifts are observed to drive electrons and ions down the tunnel. This mechanism is sensitive to the diameter of the probe

Experimental investigations of the plasma potential by means of a ball-pen probe and self-emitting Langmuir probe technique were performed on the tokamaks COMPASS and ASDEX Upgrade. A spontaneous transition between the cold and self-emitting Langmuir probe has been observed during the deep reciprocation in the vicinity or even inside the last closed flux surface (LCFS) of both tokamaks. These measurements of the plasma potential allow us to compare two independent probe techniques based on heating and non-heating processes. The observed different conditions of the spontaneous transition can be ascribed to different physical mechanisms. Nevertheless, the comparison of the power spectra of ball-pen probe and self-emitting Langmuir probe indicates very good agreement in all measurements and therefore these two probe techniques are interchangeable. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On ASDEX Upgrade first experiments were made using single probes with a voltage sweep frequency up to 100kHz. Possibilities and limitations using fast swept probes with a standard diagnostic and analysis tools are discussed. A good agreement between the data derived from fast swept single probe characteristics and floating as well as saturation current measurements was found. In a stationary (non ELMing) plasma the data of the fast swept probe are compared to standard slow swept probes (kHz range) showing an improvement of the measurement by faster sweeping. While ELM filaments already could be resolved the access of electron temperature fluctuations in small scale turbulence still has to be improved. Further comparisons are done in ELMy H-mode with combined ball-pen probe/floating potential measurements which can deliver electron temperatures with 25 μ s time resolution at reduced spatial resolution compared to pin probes. During ELMs the electron temperatures derived from the ball-pen probe and fast swept single probes agreed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present results of Particle-In-Cell simulations of a simplified model of the Ball-pen probe (BPP). The BPP is a probe designed for direct measurements of plasma potential in the scrape-off layer (SOL) of tokamaks. The probe is based on a standard Langmuir theory, which states that when the ratio of ion and electron saturation currents flowing to the probe collector approaches zero, the floating potential of the collector is close to the plasma potential. However, it is unclear whether the Langmuir theory is still valid in such a complex geometry, where the probe tunnel can work as a filter in velocity phase-space and as such violates the Maxwellian distributions of particle velocities. To verify the validity of the theory a simplified model of BPP has been studied using the SPICE2 code (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature and radial electric field on the Mega Amp Spherical Tokamak (MAST). The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile an estimate of the radial electric field is extracted which is shown to be of the order ~1kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from ExB motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

The ball-pen probe is a unique probe recently developed at the Institite of Plasma Physics in Prague. It has been designed for direct measurement of plasma potential at the CASTOR tokamak. Nowadays, it is used routinely at several tokamaks in Europe, and the first tests in low-temperature plasma have also already been performed. The aims of the thesis are primarily experimental. A ball-pen probe has been constructed from available materials, which is suitable for systematic measurements of radial profiles in the low-temperature plasma of a cylindrical magnetron. By means of comparison to other diagnostics, it was proved that ball- pen probe is able to directly measure plasma potential in a certain range of plasma parameters even though its current-voltage characteristic is not symmetric, which is in contradiction with the simplified theory for ball-pen probe. Powered by TCPDF (www.tcpdf.org)

Équipe 107 : Physique des plasmas chauds; International audience; Ball-pen probes have been used in fusion devices for direct measurements of the plasma potential. Their application in low-temperature magnetized plasma devices is still subject to studies. In this context, a ball-pen probe has been recently implemented on the linear plasma device Mirabelle. Produced by a thermionic discharge, the plasma is characterized by a low electron temperature and a low density. Plasma confinement is provided by an axial magnetic field that goes up to 100 mT. The principle of the ball-pen probe is to adjust the saturation current ratio to 1 by reducing the electron current contribution. In that case, the floating potential of the probe is close to the plasma potential. A thorough study of the ball-pen probe operation is performed for different designs of the probe over a large set of plasma conditions. Comparisons between ball-pen, Langmuir, and emissive probes are conducted in the same plasma conditions. The ball-pen probe is successfully measuring the plasma potential in these specific plasma conditions only if an adapted electronics and an adapted probe size to the plasma characteristic lengths (lambda(D), rho(ce)) are used.

The ball-pen probe represents a new unique probe recently developed at the Institute of Plasma Physics in Prague for direct measurement of plasma potential at the CASTOR tokamak. Presently is the probe utilized also on several other high-temperature devices in Europe. In recent years one has aimed to use ball- pen probe also in conditions for which it was not originally intended. The aim of this bachelor thesis is to experimentally demonstrate that the ball-pen probe is suitable for the direct measurement of the plasma potential in low-temperature weakly magnetized plasma generated by a DC cylindrical magnetron by perfor- ming comparative measurements using a Langmuir probe. 1

The ball-pen probe is a unique probe recently developed at the Institite of Plasma Physics in Prague. It has been designed for direct measurement of plasma potential at the CASTOR tokamak. It has also been succesfully tested on several other high-temperature plasma devices in Europe. The aims of the bachelor work are primarily experimental. A ball-pen probe has been constructed from available materials, which is suitable for measurement in the low-temperature plasma of a cylindrical magnetron. Although its parameters are much different from those in high-temperature plasma devices, the main principle of measurement with ball-pen probe has been proven to apply also in this brand-new conditions.

The ball-pen probe (BPP) is an innovative electric probe for direct measurements of the plasma potential. This probe was developed in IPP Prague and it is based on the Katsumata probe concept. Combined measurements of the plasma potential by a BPP and floating potential by a Langmuir probe provide also the value of the electron temperature. First test of the BPP on the RFX-mod reversed field pinch in Padova has been performed in November 2006. The BPP head, made of boron nitride, is equipped with four graphite collectors, which are positioned at four different radial positions h inside four shafts hollow into the probe head. The radial profile of the plasma potential and also the electron temperature were measured. Кулеподібний зонд – новий тип зонду для прямих вимірюаннь потенціалу плазми. Цей зонд розроблено у ІТП, в Празі, та засновано на концепції зонду Катсумати. Комбіновані вимірювання плазмового потенцілу кулеподібним зондом та плаваючого потенціалу Ленгмюрівським зондом дають також величину електронної температури. Перші випробування зонду в пінчі з оберненим полем на RFX-mod у Падові було проведено у листопаді 2006 р. Головка зонда, вироблена з нітриду бора, була оснащена чотирма графітовими колекторами, які були розташовані в чотирьох радіальних положеннях h усередині чотирьох порожнистих циліндричних отворів в голівці зонда. Виміряно радіальний профіль потенціалу плазми й електронну температуру. Шароподобный зонд – новый тип зонда для прямых измерений потенциала плазмы. Данный зонд разработан в ИФП, в Праге, и основан на концепции зонда Катсуматы. Комбинированные измерения плазменного потенциала шароподобным зондом и плавающего потенциала Ленгмюровским зондом дают также величину электронной температуры. Первые испытания зонда в пинче с обращенным полем на RFX-mod в Падове проведены в ноябре 2006 г. Головка зонда, сделанная из нитрида бора, оснащалась четырьмя графитовыми коллекторами, которые размещались в четырех различных радиальных положениях h внутри четырех пустотелых цилиндрических отверстий в головке зонда. Измерены радиальный профиль потенциала плазмы и электронная температура.