Langmuir Probe Array Research Articles

Reduction of turbulence by enhanced low-frequency zonal flow-like structures in HL-2A edge plasmas

Abstract A low-frequency zonal flow-like (LFZF-like) structure peaking at f ≈ 2.0 kHz has been observed in HL-2A ohmically heated deuterium plasmas using a combined Langmuir probe array. This time-varying potential structure, which has axisymmetric characteristics (n = 0) and a finite radial correlation length (less than 1 cm), was identified to be generated by the three-wave interaction in small-scale turbulence. The results illustrate that the amplitude of the LFZF-like structure dramatically increases with the influence of impurity ions, which is mainly due to the increased strength in the nonlinear energy transfer by the turbulence vortex symmetry-breaking process. Consequently, the enhanced LFZF-like structure has the ability to stabilize the local turbulence via the shearing decorrelation mechanism as demonstrated in this experiment. The observed results given here reveal the essential role played by the LFZF-like structure in the reduction of turbulence levels, which could advance our understanding of the multi-scale physics governing turbulence and the resulting transport in magnetically confined plasmas.

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density < ne > ∼0.7–1 × 1018 m−3, about 20%–40% of the current Ibias driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current ISOL . The fact ISOL is noticeably lower than Ibias indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E ×B drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.

Turbulence spreading and its effects on the edge flows and turbulence during sawtooth cycles in the J-TEXT tokamak plasmas

Turbulence spreading and its effects on the edge flows and turbulence during sawtooth cycles in the J-TEXT tokamak plasmas are presented. These experiments were performed by Langmuir probe array and electron cyclotron emission. This study aims to understand the rapid transport phenomenon and the role of turbulence in driving flows. Beyond the mixing radius, the turbulence pulse moves faster than the sawtooth heat pulses. The results confirm the existence of turbulence spreading during sawtooth cycles. As the turbulence and heat pulses propagate to the edge, the edge turbulence, radial electric fields, pressures, and geodesic acoustic modes are all enhanced. Hysteresis relationships between the intensities of the turbulence and the radial electric fields are observed. The radial electric field lags behind the turbulence, and its intensity increases/decreases almost linearly with the increase/decrease in the turbulence intensity. The observation suggests that the edge flows are driven dominantly by turbulence during sawtooth cycles. The weakening/enhancement of the edge flows accompanies the increase/decrease in the ion collision rates during sawtooth cycles.

Effect of the E × B drift on the redistribution of the divertor particle flux in the HL-2A ECRH plasmas

Double-peaked distribution (DPD) of particle flux has only been observed on the outer divertor target in electron cyclotron resonance heating deuterium plasmas with B×∇B toward the X-point in the HL-2A tokamak using high spatiotemporal Langmuir probe arrays. The experimental results demonstrate that the formation of the DPD is mainly due to the enhanced poloidal E×B drift flow stimulated in the divertor region, which is dependent on the plasma density, heating power, and divertor structure. The experimental results are qualitatively consistent with the SOLPS simulation. The experiment also shows that the formation of the DPD might be related to the enhanced cross field transport in the far scrape-off layer. This experimental findings presented here reveal the crucial role played by the synergistic effect of poloidal E × B drift flow and the closed divertor structure in the redistribution of the particle flux, which provides a potential way for the control of high heat flux in future fusion devices.

Effects of negative and positive bias voltages on turbulence characteristics in J-TEXT tokamak

The effects of positive and negative bias voltages on the turbulence characteristics in the edge plasmas of J-TEXT are studied using Langmuir probe arrays. The experimental results illustrate that the turbulent particle transport fluxes are suppressed in both positive and negative bias cases, while the suppression mechanisms are quite different. For positive bias (+300 V), long-range correlation (LRC) structures with frequencies of about 10–30 kHz are generated, which are beneficial for turbulent transport suppression. In the −300 V bias case, no LRCs appear, while a strong mean flow shear exists which can reduce the amplitudes of potential and density fluctuations and thus decrease the turbulent transport.

Langmuir Probes as a Tool to Investigate Plasma Uniformity in a Large Negative Ion Source

The Source for the Production of Ions of Deuterium Extracted from an RF plasma (SPIDER) experiment is the full-size test-bed of the negative ion sources of ITER neutral beams. It aims to extract and accelerate more than 350 A/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> of negative hydrogen ions from 1280 apertures, distributed over a surface of approximately 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> . To assess plasma uniformity, SPIDER is equipped with bi-dimensional arrays of Langmuir probes, covering the plasma grid (PG) and the bias plate (BP) plasma-facing surfaces. These sets of diagnostics are routinely used during the ion source operation and allow to investigate the plasma properties in the so-called source expansion and extraction region. The dependencies of plasma parameters on source operation parameters such as RF power, pressure, magnetic filter field, PG, and BP biases are reported and used to evaluate the uniformity of the plasma, showing the presence of plasma drifts due to the magnetic field. Finally, the correlation between plasma and beam uniformity is assessed.

Effects of sawtooth heat pulses on edge flows and turbulence in a tokamak plasma

Enhancements of edge zonal flows, radial electric fields, and turbulence are observed in electron cyclotron resonance heating-heated plasmas (Zhao et al 2013 Nucl. Fusion 53 083011). In this paper, the effects of sawtooth heat pulses on flows and turbulence are presented. These experiments are performed using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak. The edge zonal flows, radial electric fields, and turbulence are all enhanced by sawteeth. Propagation of the zonal flow and turbulence intensities is also observed. The delay time of the maximal intensity of the electric fields, zonal flows, and turbulence with respect to the sawtooth crashes is estimated as ∼1 ms and comparable to that of the sawtooth-triggered intermediate phases. Not only the zonal flows but also the radial electric fields lag behind the turbulence. Furthermore, the intensities of both the zonal flows and electric fields nearly linearly increase/decrease with the increase/decrease of the turbulence intensity. A double-source predator–prey model analysis suggests that a relatively strong turbulence source may contribute to the dominant zonal flow formation during sawtooth cycles.

Isotope effects on turbulence and zonal flows in HL-2A edge plasmas

The effect of isotope mass on the interaction between turbulence and geodesic acoustic mode (GAM) zonal flows has been investigated in HL-2A ohmically heated deuterium (D) and hydrogen (H) plasmas using a double-step Langmuir probe array. The experimental results indicate that the level of GAM zonal flows and the turbulence eddy size together with the eddy tilting angle are all increased in the edge region in D plasmas compared to those in H plasmas under similar discharge parameters involving plasma current, magnetic field and line-averaged density. Evidence shows that in D plasmas, the nonlinear energy transfer is the main cause of the stronger excitation of GAM zonal flows, which extract more energy from ambient turbulence and, consequently, lead to lower turbulent transport and better confinement in D plasmas. The experimental findings may contribute to the understanding of the isotopic physics and associated turbulent transport in tokamak plasmas.

Experimental investigation of scrape-off layer blob high density transition in L-mode plasmas on EAST

Lithium Beam Emission Spectroscopy systems in the outer midplane and divertor Langmuir probe arrays embedded in the divertor target plates, are utilized to investigate the scrape-off layer (SOL) blob transition and its relation with divertor detachment on EAST. The blob transition in the near SOL is observed during the density ramp-up phase. When the plasma density, normalized to the Greenwald density limit, exceeds a threshold of f GW ∼ 0.5, the blob size and lifetime increases by 2 – 3 times, while the blob detection rate decreases by about 2 times. In addition, a weak density shoulder is observed in the near SOL region at the same density threshold. Further analysis indicates that the divertor detachment is highly correlated with the blob transition, and the density threshold of blob transition is consistent with that of the access to the outer divertor detachment. The potential physical mechanisms are discussed. These results could be useful for the understanding of plasma-wall interaction issues in future devices that will operate under a detached divertor and high density conditions (over the blob transition threshold).

Effect of impurity ions on the coupling between zonal flow and local turbulent transport in HL-2A plasmas

The effect of impurity ions on the coupling between geodesic acoustic mode (GAM) zonal flow and local turbulent transport has been studied using a Langmuir probe array in HL-2A ohmically heated deuterium plasmas. The experimental results illustrate that both the frequency and amplitude level of the GAM zonal flow significantly reduce with increase in the carbon ion concentration, which is qualitatively consistent with theoretical predictions (Xie et al 2018 Plasma Phys. Control. Fusion 60 025015). Meanwhile less energy is transferred from turbulence to GAM zonal flow due to the reduction in tilting and stretching of the turbulent vortex. Consequently, the impurity ions enhance the turbulence and turbulent transport owing to the reduced GAM zonal flow, as demonstrated by experiment. The experimental results presented here therefore reveal the dual roles played by impurity ions in the dynamics of GAM zonal flow, which could contribute to the understanding of inherent mechanisms governing turbulent transport in the presence of impurity ions.

Development and application of limiter Langmuir probe array in EAST

A newly developed limiter Langmuir probe (LLP) array has been applied to the scrape-off layer (SOL) plasma measurement in EAST tokamak. The LLP system consists of two poloidal arrays located at opposite sides of the low field side guard limiter, and each poloidal array has 26 Langmuir probe channels with a spatial resolution of 35 mm in the vertical direction. The probe tip is polished to match the local graphite tile plane and avoid the leading edge and the corresponding high heat load. The probe can be assembled easily and has an excellent electric insulating property. Langmuir probes have three operation modes, i.e., floating potential mode, ion saturation current mode, and swept Langmuir probe mode. The LLP array could be operated in different modes according to the experimental purpose. The LLP array has been successfully applied to the EAST experiment in 2021, and the three-dimensional plasma structure and the plasma parameters (electron density and temperature) in the SOL have been measured.

Two-dimensional deflection of a plasma plume exhausted from a magnetically steered radiofrequency plasma thruster

Two-dimensional steering of a magnetic nozzle in a radio frequency plasma thruster is performed by mounting four small additional solenoids in addition to a main solenoid providing the axisymmetric magnetic nozzle. The cross-sectional profile of the ion saturation current is measured by a Langmuir probe array aligned along the radial axis, which is rotated to perform the spatial scan in the cross section of the plume. The measurements show that the horizontal, vertical, and diagonal deflections of the plume can be controlled by the electric currents supplied to the four additional solenoids. Since the deflections of the magnetic nozzle and the plume can provide the thrust vector control as demonstrated in previous studies, the present result leads a two-dimensional thrust vector control in the magnetic nozzle radio frequency plasma thruster.

Validation of edge turbulence codes in a magnetic X-point scenario in TORPEX

Transport processes around the magnetic X-point of tokamaks, such as turbulence and mean-field drifts, are scarcely understood. The assessment of the capability of turbulence codes to quantitatively reproduce these dynamics has been hampered by limitations in computational power and available experimental data. In this paper, we present a rigorous validation of full-scale simulations of a newly developed X-point scenario in the basic toroidal plasma device TORPEX, performed with the four state-of-the-art codes FELTOR, GBS, GRILLIX, and STORM. High-resolution Langmuir probe array measurements of various time-averaged and fluctuating quantities and across the entire cross section of TORPEX show that this X-point scenario features the key ingredients of X-point dynamics, such as small-scale fluctuations and background drifts. The codes are able to qualitatively reproduce some characteristics of the time-averaged fields, such as the ion saturation current profiles at mid-height, the plasma up-down asymmetry, and the blob trajectories. A quantitative agreement is found for the background E × B velocity pattern, while the fluctuation levels are generally underestimated typically by factors of 2 or more, and thus, background fluxes are found to dominate over turbulent ones in simulations. The sensitivity of the simulation results on the plasma collisionality and on the position of the sources is tested in GBS, showing a mild effect on the overall quantitative agreement with the experiment. Overall, this validation reveals the challenges to reproduce the plasma dynamics near an X-point and provides a clear path to a quantitative and computationally relatively inexpensive assessment of future developments in turbulence codes.

Dynamic interaction between fluctuations with different origins in a linear magnetized plasma

This paper presents the experimental identification of the dynamic interaction between three fluctuations excited by different thermodynamical forces in a linear plasma. The observed fluctuations are characterized as an axial symmetric mode, a drift wave, and an azimuthally symmetric potential fluctuation from the spatiotemporal structures measured with Langmuir probe arrays. The intermittent burst of the axial symmetric mode is observed by the instantaneous wave number spectrum. The energy transfer analysis revealed that the axially symmetric mode gains energy from the other mode during the burst while the drift wave loses energy. The intermittent burst synchronizes with the azimuthal symmetric potential fluctuation.

Toroidal component of velocity for geodesic acoustic modes in the edge plasmas of the J-TEXT tokamak

The toroidal component of the velocity for geodesic acoustic modes (GAMs) is first demonstrated. Multiple Langmuir probe arrays set up near the top tokamak of the J-TEXT were utilized for this study. A significant peak at the GAM frequency is observed in Mach number fluctuations. The toroidal velocity for the GAMs is estimated as ∼10–100 m s−1 and increases with the poloidal velocity. The ratio of toroidal component to the poloidal one of the velocity is mainly located in the interval between 0.3 and 1.0. With higher safety factors q, the ratio almost does not change with decreasing the safety factor, whereas it goes up sharply at low q. The coherencies between poloidal electric fields and Mach number fluctuations in turbulence frequency bands are also evaluated, and are higher than those between radial electric fields and Mach number fluctuations.

Development of a Langmuir probe array for radial potential profile measurement in the collisional merging formation FRC.

The radial electric field in a field-reversed configuration (FRC) plasma plays an important role in the global stability and confinement properties. Herein, we developed a new Langmuir probe array named "Skewered probe" employed in measuring the radial potential profile in the collisional merging formation of an FRC in the FAT-CM (FRC Amplification via Translation - Collisional Merging) device. Because an FRC has a strong toroidal flow, the skewered probe consists of alternately skewered ring electrodes and ceramic beads on a thin stainless-steel tube to neutralize the effect of plasma flow. The developed array has nine electrodes, one every 2cm from r = 9-25cm, and it measures the FRC boundary in the case when the radius of the excluded flux ranges from 10 to 20cm. The skewered probe also has one additional electrode that measures the potential near the chamber wall as a reference for the other electrodes. The radial potential profile of the FRC formed by the collisional merging method in the FAT-CM device was measured using the probe, and the results showed that the region of negative potential gradually changed to a positive potential after merging the FRCs. It was also shown that a strong outward electric field is formed near the separatrix at n = 2 rotational instability.

Langmuir probe array for the small angle slot divertor in DIII-D.

The DIII-D small angle slot (SAS) divertor is designed for divertor physics studies with enhanced neutral confinement and special target geometries in a closed divertor. The closed nature of the SAS makes optical diagnostic measurements difficult, so a specially designed, multipurpose array of Langmuir probes has been implemented to study the plasma conditions in and around the slot. The probes are spaced to provide at least 2mm resolution (shorter than the energy decay length) of the near scrape-off layer when mapped to the outer mid-plane. Due to space limitations at the bottom of the slot, a novel spring-loaded probe and tile design was developed to clamp several short rooftop probe tips and insulators to the cooled baseplate. Initial probe measurements revealed tile to tile edge shadowing, especially where magnetic field line surface angles were less than 1°. Additionally, it was found, using three Langmuir probes (at 90°, 180°, and 270°), that the strike point variation of ±5mm radially around the torus was not well aligned with the circular slot geometry [Watkins et al., Nucl. Mater. Energy 18, 46 (2019)]. These issues were resolved by (1) designing tiles with all probes mounted near the tile center instead of near the edges and (2) aligning these new custom tiles to the measured strike point toroidal surface with a very accurate laser scanning alignment tool. Post-alignment Langmuir probe measurements and plasma behavior demonstrated close agreement at two separate toroidal locations that were 45° apart.

A fast-reciprocating probe array for two-dimensional measurements in the divertor region of the Tokamak à configuration variable.

A detailed description of the construction and commissioning of the fast-moving reciprocating divertor Langmuir probe array on the Tokamak à Configuration Variable (TCV) is presented. A horizontal array of 24 Langmuir probes (12 Mach probes) combined with a fast vertical movement provides unprecedented two-dimensional measurements of the plasma properties across the entire divertor outer leg volume and up to the X-point. The L-shape probe arm has to be very compact to minimize plasma perturbations and, at the same time, has to house 24 coaxial transmission lines, withstand time-averaged heat fluxes of up to ≈30MW/m2 and accelerations of up to 8g, and be resilient to violent plasma disruptions. For the fast vertical motion of the probe arm, extending up to 38cm into the TCV vacuum vessel, an assembly with a precise and powerful linear electric motor (up to 4000 N of force) was mounted in a 4m high structure within the limited space below the TCV. The diagnostic has already performed measurements in ≈200 plasma discharges and has been hit by ≈20 disruptions without suffering any critical damage. The measurements are qualitatively and quantitatively consistent with other reference diagnostics, such as upstream electron Thomson scattering and downstream wall-embedded Langmuir probes, and are used extensively on the TCV for experimental divertor studies and for comparisons with edge transport and turbulence code results.

Diagnosis of Post-Arc Residual Plasma in Vacuum Breaker

The residual plasma in the gap between the anode and cathode has an important influence on the recovery characteristics of the medium after the arc current zero in the vacuum arc extinguishing chamber. A diagnostic system for post-arc residual plasma is proposed and designed to diagnose the density of residual plasma during sheath expansion. The experimental platform is built on the basis of the synthetic circuit and detachable vacuum chamber. The diagnostic system uses Langmuir probe array to diagnose the post-arc plasma density change in vacuum interrupter, which reflects the dynamic process of sheath expansion in the gap. The effect of different current sizes and contact spacing on residual plasma is discussed in this paper. The plasma densities diagnosed by different position probes are used to analyze the post-arc characteristics of vacuum circuit breakers from a micro-level. The accuracy and reliability of the diagnostic system are verified by comparing with the previous studies, which provides some reference value for the future research on the post-arc micro-characteristics of vacuum breaker.

Temporal Profiling of Electron Temperatures Using the Hα–Hβ Line Emission and Triple Langmuir Probe Array in the Pre-Ionization Discharge of the MT-I Spherical Tokamak

Photodiodes masked with narrow-band filters have been used to obtain the temporal profiles of Hα and Hβ line emissions in the pre-ionization phase of hydrogen discharge in the MT-I Spherical Tokamak. The line ratio method relating the emission intensities of the above mentioned lines, having different excitation thresholds, provides the temporal profile of electron temperature. A triple Langmuir probe array having three individual sets of triple probes, arranged linearly, has also been used to measure the temporal profile of electron temperature at three different radial positions simultaneously. Additionally, a spectrometer HR4000+ is used to get the line-integrated emission intensity of H-Balmer lines. The objective of this experiment is to demonstrate the successful development of the optical and electric probe as a diagnostic tool for tokamak discharge.