Ball-pen Probe Research Articles

Retraction Note to: Design and Fabrication of Comparative Langmuir Ball-Pen Probe (LBP) for the Tokamak

Retraction Note to: Design and Fabrication of Comparative Langmuir Ball-Pen Probe (LBP) for the Tokamak

Spontaneous formation of a transport barrier in helium plasma in a tokamak with circular configuration

We report on the first experimental observation of a spontaneously formed transport barrier in the tokamak with a circular configuration in helium plasmas. There was no external polarization of the plasma by electric field or other technique to form the barrier as it is typically used in tokamaks with circular plasma. In general, the transport barriers play an important role in plasma confinement especially in tokamaks with divertor configuration. In our experiments, we clearly observe distinct characteristics of a transport barrier, including a steep gradient of the electron temperature and an enhanced radial electric field along with the change in the plasma potential, floating potential, and electron temperature fluctuation. The electron temperature and the plasma potential are obtained by a combination of the ball-pen and Langmuir probe measurements with high temporal resolution on a shot-to-shot basis. This first experimental observation of the spontaneously formed transport barrier might bring new possibilities to obtain a fusion-relevant study of the edge plasma parameters and transport in helium plasmas even on small tokamaks.

Statistical properties of ion and electron temperature fluctuations in the edge of the COMPASS tokamak

This paper presents measurements of the plasma turbulence with an emphasis on the ion temperature fluctuations in comparison to the electron temperature and the ion saturation current fluctuations in the edge of the COMPASS tokamak. The ion temperature values are obtained by using a fast swept ball-pen probe, with temporal resolution up to 10 µs, accompanied by independent 1 µs measurements of the electron temperature and ion saturation current. The experimental results obtained for a wide range of line-averaged densities show that statistical properties of the fluctuations are in good agreement with a form of beta or gamma distribution function. In addition, the dependence of the averaged ion and electron temperatures on the line-averaged density is studied in the near and main scrape off layer region.

High-heat flux ball-pen probe head in ASDEX-Upgrade.

A new high heat flux ball-pen probe head installed on the midplane manipulator is currently being used in ASDEX-Upgrade (AUG). The probe was designed to withstand high heat fluxes making possible the investigation of the plasma edge under harsh conditions, such as low power H-mode. Composed of seven pins (four Langmuir probes, mounted in two Mach probe pairs, and three ball-pen probes), the new probe head allows us to measure several plasma parameters simultaneously and with high temporal resolution. A novel method to correct the sheath potential dynamically accounting for the total secondary electron emission is introduced together with applications to obtain the electron temperature and plasma potential profiles. The total secondary electron emission yield is obtained from particle in cell simulations in AUG condition and probe realistic impact angle with respect to the magnetic field. Finally, the probe capability to investigate turbulence around the separatrix of AUG is discussed.

Real-time feedback system for divertor heat flux control at COMPASS tokamak

The reduction of the incident heat flux onto the divertor will be a necessity for the future thermonuclear reactors. Impurity seeding is recognized as an efficient way to achieve the partial detachment regime, which allows to dissipate a large fraction of power flux by radiation. This paper presents a heat flux real-time feedback system (RTFS) based on impurity seeding controlled by a combined ball-pen and Langmuir probe divertor array in the COMPASS tokamak. A number of features of the system have been studied, such as the type of impurity, seeding location, constants used in the real-time controller and the diagnostic selections. A detailed description of the designed RTFS and the results of the implementation are presented. The findings confirm the applicability of the RTFS for reduction and control of the divertor heat fluxes. Another important implication of this research is the ability of installing such systems in next-step devices.

Ion temperature measurements in the tokamak scrape-off layer with high temporal resolution

We present a uniquely fast (10 μs) ion temperature measurements in the tokamak edge plasma. Our approach is based on the sweeping of a ball-pen probe, where the ion temperature is obtained by fitting the electron branch of the corresponding I–V characteristic. We have performed measurements on the COMPASS tokamak during L-mode discharge. The temperature histograms reveal a non-Gaussian shape with a high-temperature tail peaking at low values. The fitted values of fast I–V measurements can be used to reconstruct (emulate) the slow swept I–V characteristic of a retarding field analyzer. The resulting ion temperature profile is nearly flat and provides a ratio of ion to electron temperature close to 1–2 in the vicinity of the last closed flux surface during L-mode discharges, as observed on other tokamaks.

Systematic errors in tokamak magnetic equilibrium reconstruction: a study of EFIT++ at tokamak COMPASS

Uncertainties and errors in magnetic equilibrium reconstructions are a wide-spread problem in interpreting experimental data measured in the tokamak edge. This study demonstrates errors in EFIT++ reconstructions performed on the COMPASS tokamak by comparing the outer midplane separatrix position to the Velocity Shear Layer (VSL) position. The VSL is detected as the plasma potential peak measured by a reciprocating ball-pen probe. A subsequent statistical analysis of nearly 400 discharges shows a strong systematic trend in the reconstructed separatrix position relative to the VSL, where the primary factors are plasma triangularity and the magnetic axis radial position. This dependency is significantly reduced after the measuring coils positions as recorded in EFIT input are optimised to provide a closer match between the “synthetic” coil signal calculated by the Biot-Savart law in a vacuum discharge and the actual coil signal. In conclusion, we suggest that applying this optimisation may lead to more accurate and reliable reconstructions of the COMPASS equilibrium, which would have a positive impact on the accuracy of measurement analysis performed in the edge plasma.

Ball pen probe in strongly magnetised RF plasmas

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 (Te). For all BPPs at higher field strengths (≥ 250 mT) the maximum collector potential underestimated the emissive probe by more than 2.7 Te (7 V). At these field strengths all BPPs agree with each other to within 1.5 Te (4.1 V). Possible reasons for these disagreements are discussed.

Limit cycle oscillations measurements with Langmuir and ball-pen probes on COMPASS

This contribution presents the experimentally observed edge plasma evolution during limit cycle oscillations (LCO) measured with a new Langmuir and ball-pen multi-pin probe head at the COMPASS tokamak. The observed LCO regime modulates the intensity of density fluctuations , radial electric field Er and intensity of emission with a frequency 3–5 kHz. The density fluctuations grow after Er decreases in strength which appears to be strongly correlated with the evolution of the pressure gradient . The magnetic signature of the LCO shows a left–right asymmetry with propagation from the low to high field side. High-frequency (above 100 kHz) precursor-like oscillations are observed as well.

Progress in diagnostics of the COMPASS tokamak

The COMPASS tokamak at IPP Prague is a small-size device with an ITER-relevant plasma geometry and operating in both the Ohmic as well as neutral beam assisted H-modes since 2012. A basic set of diagnostics installed at the beginning of the COMPASS operation has been gradually broadened in type of diagnostics, extended in number of detectors and collected channels and improved by an increased data acquisition speed. In recent years, a significant progress in diagnostic development has been motivated by the improved COMPASS plasma performance and broadening of its scientific programme (L-H transition and pedestal scaling studies, magnetic perturbations, runaway electron control and mitigation, plasma-surface interaction and corresponding heat fluxes, Alfvenic and edge localized mode observations, disruptions, etc.). In this contribution, we describe major upgrades of a broad spectrum of the COMPASS diagnostics and discuss their potential for physical studies. In particular, scrape-off layer plasma diagnostics will be represented by a new concept for microsecond electron temperature and heat flux measurements - we introduce a new set of divertor Langmuir and ball-pen probe arrays, newly constructed probe heads for reciprocating manipulators as well as several types of standalone probes. Among optical tools, an upgraded high-resolution edge Thomson scattering diagnostic for pedestal studies and a set of new visible light and infrared (plasma-surface interaction investigations) cameras will be described. Particle and beam diagnostics will be covered by a neutral particle analyzer, diagnostics on a lithium beam, Cherenkov detectors (for a direct detection of runaway electrons) and neutron detectors. We also present new modifications of the microwave reflectometer for fast edge density profile measurements.

Plasma potential and electron temperature evaluated by ball-pen and Langmuir probes in the COMPASS tokamak

The radial distributions of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak are measured during L-mode and H-mode regimes by using both Langmuir and ball-pen probes mounted on a horizontal reciprocating manipulator. The radial profile of the plasma potential derived previously from Langmuir probes data by using the first derivative probe technique is compared with data derived using ball-pen probes. A good agreement can be seen between the data acquired by the two techniques during the L-mode discharge and during the H-mode regime within the inter-ELM periods. In contrast with the first derivative probe technique, the ball-pen probe technique does not require a swept voltage and, therefore, the temporal resolution is only limited by the data acquisition system. In the electron temperature evaluation, in the far scrape-off layer and in the limiter shadow, where the electron energy distribution is Maxwellian, the results from both techniques match well. In the vicinity of the last closed flux surface, where the electron energy distribution function is bi-Maxwellian, the ball-pen probe technique results are in agreement with the high-temperature components of the electron distribution only. We also discuss the application of relatively large Langmuir probes placed in parallel and perpendicularly to the magnetic field lines to studying the main plasma parameters. The results obtained by the two types of the large probes agree well. They are compared with Thomson scattering data for electron temperatures and densities. The results for the electron densities are compared also with the results from ASTRA code calculation of the electron source due to the ionization of the neutrals by fast electrons and the origin of the bi-Maxwellian electron energy distribution function is briefly discussed.

Improved understanding of the ball-pen probe through particle-in-cell simulations

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 . 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.

Bunker probe: A plasma potential probe almost insensitive to its orientation with the magnetic field.

Due to their ability to suppress a large part of the electron current and thus measuring directly the plasma potential, ion sensitive probes have begun to be widely tested and used in fusion devices. For these probes to work, almost perfect alignment with the total magnetic field is necessary. This condition cannot always be fulfilled due to the curvature of magnetic fields, complex magnetic structure, or magnetic field reconnection. In this perspective, we have developed a plasma potential probe (named Bunker probe) based on the principle of the ion sensitive probe but almost insensitive to its orientation with the total magnetic field. Therefore it can be used to measure the plasma potential inside fusion devices, especially in regions with complex magnetic field topology. Experimental results are presented and compared with Ball-Pen probe measurements taken under identical conditions. We have observed that the floating potential of the Bunker probe is indeed little affected by its orientation with the magnetic field for angles ranging from 90° to 30°, in contrast to the Ball-Pen probe whose floating potential decreases towards that of a Langmuir probe if not properly aligned with the magnetic field.

Profile measurements of the electron temperature on the ASDEX Upgrade, COMPASS, and ISTTOK tokamak using Thomson scattering, triple, and ball-pen probes

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (ΦBPP) and the floating potential (Vfl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula Te = (ΦBPP - Vfl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.

Comparative measurements of plasma potential with ball-pen and Langmuir probe in low-temperature magnetized plasma

The ball-pen probe (BPP) is used for direct plasma potential measurements in magnetized plasma. The probe can adjust the ratio of the electron and ion saturation currents Isat−/Isat+ to be close to one and therefore its I-V characteristic becomes nearly symmetric. If this is achieved, the floating potential of the BPP is close to the plasma potential. Because of its rather simple construction, it offers an attractive probe for measurements in magnetized plasma. Comparative measurements of plasma potential by BPPs of different dimensions as well as one Langmuir probe (LP) in an argon discharge plasma of a cylindrical magnetron were performed at various experimental conditions. An additional comparison by an emissive probe was also performed. All these types of probes provide similar values of plasma potential in a wide range of plasma parameters. Our results for three different BPP dimensions indicate that the BPP can be operated in a cylindrical magnetron DC argon discharge if the value of the ratio of the magnetic field and neutral gas pressure, B/p, is greater than approximately 10 mT/Pa.

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe.

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. 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 ∼1 kV/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 E × B 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.

RETRACTED ARTICLE: Design and Fabrication of Comparative Langmuir Ball-Pen Probe (LBP) for the Tokamak

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.

Design and validation of the ball-pen probe for measurements in a low-temperature magnetized plasma

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 (λ(D), ρ(ce)) are used.

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

AbstractIn 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)

Interpretation of fast measurements of plasma potential, temperature and density in SOL of ASDEX Upgrade

This paper focuses on interpretation of fast (1 µs) and local (2–4 mm) measurements of plasma density, potential and electron temperature in the edge plasma of tokamak ASDEX Upgrade. Steady-state radial profiles demonstrate the credibility of the ball-pen probe. We demonstrate that floating potential fluctuations measured by a Langmuir probe are dominated by plasma electron temperature rather than potential. Spatial and temporal scales are found consistent with expectations based on interchange-driven turbulence. Conditionally averaged signals found for both potential and density are also consistent; however, those for temperature show an unexpected ∼4 mm wide decrease by 10% at the very centre of a blob. In the wall shadow, temperature measured by the swept Langmuir probe yields values ∼10 eV, whilst the ball-pen temperature gradient is more steep and credible, dropping down to ∼1 eV.