Langmuir Probe Array Research Articles

Turbulent filament properties in L and H-mode regime in the RFX-mod operating as a tokamak

The dynamics of turbulent filaments in L and ELM-free H-mode in the RFX-mod device operating as a tokamak is discussed in the present work. L-H transition and sustained H-mode have been achieved in the RFX-mod with the aid of an external electrode biasing (Spolaore et al 2017 Nucl. Fusion 57 116039). Through advanced statistical techniques, turbulent filaments are detected and tracked from the edge to the scrape-off layer (SOL) in a two-dimension floating potential map (measured with a 2D array of Langmuir probes) related with extreme events of a fixed ion saturation current signal in the SOL. While in L-mode filaments travel almost freely, their motion becomes restricted to the near SOL during the induced biasing H-mode. In this region, the background shear decorrelation time becomes shorter than the convective filament time, favouring its suppression. However, the experimental observation of a nearly ‘trapped’ monopole potential points out the possible role of the vortex selection mechanism. The sign and magnitude of the turbulent parallel vorticity are consistent with the E × B background vorticity in the SOL in this regime, while in L-mode, the inferred turbulent vorticity is always above the background one. The sparse filamentary transport in the far SOL during H-mode is further confirmed from the measurements of a poloidally symmetric array of electrostatic sensors on the wall. Lastly, the radial velocity of filaments in both regimes was discussed in the framework of analytical scalings. Filaments in L-mode exhibit a reasonable agreement with the sheath-limited regime in the cold ion approximation, with a non-negligible associated parallel current density measured with a set of 3-axial pick-up coils installed in the probe head. In H-mode, the collisionality increases near the separatrix while filaments seem to follow the inertial regime before become trapped.

Enhancements of residual Reynolds stresses by magnetic perturbations in the edge plasmas of the J-TEXT tokamak

Enhancements of residual Reynolds stresses and symmetry breaking with resonant magnetic perturbations (RMPs) are presented. These experiments were performed using a Langmuir probe array in the edge plasmas of the J-TEXT tokamak. This study aims to explain the mechanisms for generating residual stresses and intrinsic rotations. It is observed that with RMPs, peaked residual stresses, steeper pressure gradients and stronger shear layers all appear around the resonant surface. The toroidal rotation is accelerated in the direction of the plasma current and shows a correlation with turbulence and the measured residual stresses near the resonant surfaces of RMPs. Symmetry breaking induced by magnetic islands is proposed as a new mechanism, which qualitatively explains the enhanced residual stresses and the acceleration of the toroidal rotation. The changes to the residual stresses and symmetry breaking with RMPs near the last closed surface are also discussed.

Observation of inward transport flux accompanied with the long-lived mode during the L-H transition in the HL-2A tokamak

The inward transport flux accompanied by the long-lived mode (LLM) has been observed by the edge Langmuir probe array during the L-H transition in the HL-2A tokamak. The cross phase between the poloidal electric field fluctuation and the electron density fluctuation in the LLM frequency changed greatly during the L-H transition, which led to the radial transport flux in the LLM frequency to change its transport direction in a given radial region near the pedestal barrier. By using the long distance correlation method, the phase shift of poloidal electric field fluctuation proved to be the key reason, which led to the cross-phase term to change in the LLM frequency. Furthermore, the shear effect has a mode-selecting characteristic on the LLM and turbulence because the direction of turbulent transport still remains radially outward during the H-mode. In addition, when the inward conducted heat flux accompanied by LLM occurs, the poloidal electric field fluctuation and electron temperature fluctuation both have a positive gradient in the LLM frequency; this may indicate that the positive gradient of radial velocity and electron temperature in the LLM frequency play an important role during the entire transport direction change process.

Effect of resonant magnetic perturbation on boundary plasma turbulence and transport on J-TEXT tokamak

The effect of resonant magnetic perturbation (RMP) on boundary turbulence and transport in J-TEXT plasma is experimentally investigated. Edge plasma fluctuations in discharges with and without the (m/n = 3/1) RMP currents are diagnosed by using Langmuir probe arrays. It was found that fluctuations in the edge and scrape-off layer (SOL) regions decrease with the application of a 6 kA RMP. The broadband turbulence at the radial location of which has a characteristic frequency of 40–150 kHz was strongly suppressed when applying RMP, as was the radial turbulent particle flux and blob transport in the near-SOL region. These experimental findings make RMP a promising method of suppressing and controlling turbulence and particle transport in a plasma boundary.

Overview of recent TJ-II stellarator results

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components.As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

An overview of diagnostic upgrade and experimental progress in the KTX

The Keda Torus eXperiment (KTX) is still operated in the commissioning phase, and preparation for the operation capability of the KTX phase II upgrade is underway. The diagnostics in the KTX have been greatly developed: (1) the terahertz interferometer has been upgraded to seven chords for electron density profile inversion; (2) a Thomson scattering system with a 5 Joule laser has been installed and commissioning is in progress; (3) a 3D movable probe system has been developed for the electromagnetic turbulence measurement; (4) double-foil soft x-ray imaging diagnostics have been set up and a bench test has been completed; (5) an edge capacitive probe has been installed for the radial electrical field measurement; (6) a multi-channel spectrograph system has been built for detecting impurities of carbon and oxygen. In addition, the design of a new compact torus injection system has been completed for feeding and momentum driving. Pilot research, such as the 3D reversed field pinch physics and electromagnetic turbulence, etc, have been conducted in the discharge status of the KTX. The 3D spectra characters of electromagnetic turbulence are firstly measured using a classical two-point technique by Langmuir probe arrays set on the 3D movable probe system and edge magnetic sensors. The forward scattering is collected by the interferometer system, which shows the potential for turbulence research. The electromagnetic turbulence is tentatively investigated in the KTX. The formation of a quasi-single-helicity state in the KTX regime is also preliminarily explored in simulation.

Measurements of ion velocity and wave propagation in a hollow cathode plume

The mechanism responsible for the production of energetic ions in the plume of hollow cathodes for electric propulsion is still an open issue. These ions are of concern to cathode and thruster lifetime, particularly for cathodes operating at high (>20 A) discharge currents. Recent theoretical and experimental investigations suggest that there is a correlation between ion energy gain and ion acoustic turbulence. In this paper we present measurements of the evolution of the ion velocity distribution function in the near plume of a 100 A-class hollow cathode, operated in a regime in which the dominant mode is ion acoustic turbulence. Ion flow and thermal properties were related to measurements of the background plasma, fluctuation spectra, and dispersion relations obtained from an array of Langmuir probes. We found ions to flow outward from the cathode and accelerate downstream, to supersonic speeds, approximately aligned with the acoustic wave group velocity vectors. The directions of the ion flow and wave propagation were similar for a range of discharge currents and mass flow rates in the jet region of the plume. One operating condition showed a significant temperature increase, also in the direction of acoustic wave propagation, corresponding to the highest wave energy condition. These results are interpreted in the context of ion acoustic turbulence as a contributing mechanism for ion energy gain.

Plasma fluctuations in the scrape-off layer and at the divertor target in Alcator C-Mod and their relationship to divertor collisionality and density shoulder formation

Abstract Experiments indicate that the formation of density shoulders in the far scrape-off layer (SOL) is associated with an increase in cross-field transport and the propagation of coherent structures of high density known as blobs. It has been suggested that far SOL divertor collisionality (Λdiv) may be a controlling parameter in this physics, which could be challenging for heat flux mitigation schemes. At Alcator C-Mod, a new high spatial resolution divertor Langmuir probe array (rail probes) combined with a scanning Mirror Langmuir probe has been used to investigate this phenomenon in detail. Experiments were performed to mitigate divertor heat flux and thus increase Λdiv by two different methods and observe the divertor and scrape-off layer plasma response: (1) a 3-point density scan is found to increase the fluctuation intermittency at both the midplane and divertor, and produce a density shoulder at elevated Greenwald fraction; (2) a 3-point N2 seeding scan is found to increase the divertor strike point collisionality, by over two orders of magnitude yet no noticeable trend is observed in either the density profile or the fluctuation statistics. It is concluded that Λdiv near the strike point is not by itself a controlling parameter for SOL fluctuations and shoulder formation which is important for localized heat flux mitigation schemes targeting the strike point where heat flux is the greatest.

The source of and a simple way to avoid ground loops in biasing Langmuir probe array

Biasing multiple Langmuir probes arranged in an array through one voltage source presents a more compact, reliable and convenient way to bias than using an individual source for each probe. This approach however is the subject of ground loops and several methods have been discussed to avoid them. The high frequency isolation transformer is considered to be a best way to break ground loops since it does not require an additional power supplies and, in general, any other electric circuit. The presented results demonstrate a considerable suppression of the ground loop noise using this technique. The following limitations should be considered in this approach: only an AC voltage bias should be applied, the relatively high internal resistance of the transformer should be taken into account, the frequency of the AC bias signal should correspond to the transforner bandwidth.

Shadowing effects on multi-step Langmuir probe array on HL-2A tokamak

Multi-step Langmuir probe arrays have been designed and installed on the HL-2A tokamak [1]–[2] to study the turbulent transport in the edge plasma, especially for the measurement of poloidal momentum flux, Reynolds stress Rs. However, except the probe tips on the top step, all other tips on lower steps are shadowed by graphite skeleton. It is necessary to estimate the shadowing effects on equilibrium and fluctuation measurement. In this paper, comparison of shadowed tips to unshadowed ones is presented. The results show that shadowing can strongly reduce the ion and electron effective collection area. However, its effect is negligible for the turbulence intensity and coherence measurement, confirming that the multi-step LP array is proper for the turbulent transport measurement.

The flush-mounted rail Langmuir probe array designed for the Alcator C-Mod vertical target plate divertor

An array of flush-mounted and toroidally elongated Langmuir probes (henceforth called rail probes) have been specifically designed for the Alcator C-Mod's vertical target plate divertor and operated over multiple campaigns. The "flush" geometry enables the tungsten electrodes to survive high heat flux conditions in which traditional "proud" tungsten electrodes suffer damage from melting. The toroidally elongated rail-like geometry reduces the influence of sheath expansion, which is an important effect to consider in the design and interpretation of flush-mounted Langmuir probes. The new rail probes successfully operated during C-Mod's FY2015 and FY2016 experimental campaigns with no evidence of damage, despite being regularly subjected to heat flux densities parallel to the magnetic field exceeding ∼1 GW m-2 for short periods of time. A comparison between rail and proud probe data indicates that sheath expansion effects were successfully mitigated by the rail design, extending the use of these Langmuir probes to incident magnetic field line angles as low as 0.5°.

Observations of a field-aligned ion/ion-beam instability in a magnetized laboratory plasma

Collisionless coupling between super Alfvénic ions and an ambient plasma parallel to a background magnetic field is mediated by a set of electromagnetic ion/ion-beam instabilities including the resonant right hand instability (RHI). To study this coupling and its role in parallel shock formation, a new experimental configuration at the University of California, Los Angeles utilizes high-energy and high-repetition-rate lasers to create a super-Alfvénic field-aligned debris plasma within an ambient plasma in the Large Plasma Device. We used a time-resolved fluorescence monochromator and an array of Langmuir probes to characterize the laser plasma velocity distribution and density. The debris ions were observed to be sufficiently super-Alfvénic and dense to excite the RHI. Measurements with magnetic flux probes exhibited a right-hand circularly polarized frequency chirp consistent with the excitation of the RHI near the laser target. We compared measurements to 2D hybrid simulations of the experiment.

Experimental study of cross phase influence on Reynolds stress in the HL-2A tokamak

The cross phase between radial velocity perturbation and poloidal velocity perturbation plays an important role in the radial distribution of Reynolds stress. Recently, a novel theory of cross-phase dynamics predicts that, depending on the strength of the E × B shearing, the cross phase can stay in two different states: phase locked state(weak shear regime) and phase slipping state(strong shear regime). For the first time we divided Reynolds stress into three parts, turbulence fluctuation, cross phase and coherence between and , and studied their relative influences on Reynolds stress in different regimes. The profile of Reynolds stress and its three parts are measured separately by using multi-tipped Langmuir probe array. We observe that the three parts contribute to the radial distribution of Reynolds stress differently. In strong shear layer, the cross phase is randomly scattered across the layer—a signature of incoherent phase slips. Correspondingly, the radial distribution of the Reynolds stress is determined by the cross phase dynamics. In the weak shear region, the cross phase tends to stay in a coherent state (i.e. the phase locked state), where the turbulence fluctuation and coherence play a more important role. Besides, a direct relation between the coherence and the cross phase scattering is observed. Once the scattering of the cross phase increases, the coherence decreases.

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.

Generation of slowly rotating microwave plasma by amplitude-modulated resonant cavity

Slow rotation of microwave plasma at a rotational frequency of Ω/2π = 0.1–1000 Hz is realized to improve plasma uniformity by using a resonant cylindrical cavity and a solid-state microwave generator at a frequency of ω/2π = 2.4–2.5 GHz. The microwave at ω/2π is modulated in amplitude at Ω/2π and injected into the cavity from two orthogonal positions, exciting the TE111 mode. The cavity fields rotate either clockwise or anticlockwise at a frequency of Ω/2π when the phase differences, Δϕ at ω and ΔΦ at Ω, between the input microwaves are properly set as calculated by a theoretical analysis and finite-difference time-domain simulation. Rotating plasmas are experimentally measured in the microwave discharges of argon at 0.1–20 Torr. When the rotational frequency is low (Ω/2π < 30 Hz), a plasma rotation is visible in the optical emission image; the azimuthal rotation of a local ion density is also confirmed by a rotatable Langmuir probe array. Conversely, when Ω/2π > 1000 Hz, the electron density measurement by a curling probe reveals that the plasma rotation disappears in the downstream region. This observation is supported by a simplified analysis based on the diffusion equation, proving a characteristic distance of plasma rotation disappearance to be (Da: ambipolar diffusion coefficient).

Temporal-spatial structures of plasmas flows and turbulence around tearing mode islands in the edge tokamak plasmas

The temporal-spatial structures of plasma flows and turbulence around tearing mode islands are presented. The experiments were performed using Langmuir probe arrays in the edge plasmas of J-TEXT tokamak. The correlation analyses clearly show that the flows have similar structures of m/n = 3/1 as the magnetic island does (m and n are the poloidal and toroidal mode numbers, respectively). The sign of the potential fluctuations for the flows inverses and the powers significantly reduce at q = 3 surface. Approaching to the last closed flux surface for the magnetic islands, the radially elongated flow structure forms. The flows are concentrated near separatrix and show quadrupole structures. The turbulence is concentrated near X-point and partly trapped inside the magnetic islands.

Electron temperature and heat load measurements in the COMPASS divertor using the new system of probes

A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy H-mode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ = 7. The ELM energy density was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

Theoretical calculation of electron density and temperature in the edge of tokamak

In this work, we use a method based on the concept of particle confinement time [Formula: see text] uniqueness to calculate the electron density and temperature in ohmically heated, edge plasma of the Hefei tokamak-7. Here, with the help of the data taken from Johnson and Hinnov’s table, we have done an extensive work to find electron densities and temperatures that satisfy the [Formula: see text] uniqueness to evaluate the temporal evolution of electron density [Formula: see text] and temperature [Formula: see text]. The results are in good agreement as measured from the Langmuir probe array in previous works.

Mesoscale electric fluctuations interacting with zonal flows, magnetic fluctuations and turbulence

New mesoscale electric fluctuations (MSEFs) are identified in the edge plasmas of the HL-2A tokamak using multiple Langmuir probe arrays. The MSEFs, resulting from the synchronization and having components of dominant geodesic acoustic mode (GAM) and m/n = 6/2 potential fluctuations, are found at the same frequency as that of the magnetic fluctuations of m/n = 6/2 (m and n are poloidal and toroidal mode numbers, respectively). The temporal evolutions of the MSEFs and the magnetic fluctuations clearly show the frequency entrainment and the phase lock between the GAM and the m/n = 6/2 magnetic fluctuations. The results indicate that GAMs and magnetic fluctuations can transfer energy through nonlinear synchronization. The nonlinear coupling analyses show that the MSEFs couple to turbulence and low frequency zonal flows (LFZFs). This suggests that the MSEFs may contribute to the LFZF formation, reduction of turbulence level, and thus confinement regime transitions. The analysis of the envelope modulation demonstrates that both the MSEFs and the LFZFs modulate the turbulence while the MSEFs are modulated by the LFZFs.

Studies on radial and poloidal particle transport at the edge of SST-1 tokamak

The radial and poloidal particle fluxes occurring at the plasma edge are essential towards understanding the plasma confinement in the tokamak device. In tokamaks, the edge transport barriers play a critical role in the transitions from low confinement (L-mode) to high confinement (H-mode). Recently, the edge plasma profiles have been studied for steady-state superconducting tokamak-1 (SST-1) with the help of an array of Langmuir probes. The floating potential and ion saturation current fluctuations have been measured at different radial and poloidal distance at the edge of SST-1. Increases in magnetic fluctuations associated with enhanced magneto-hydrodynamic (MHD) activities have been found to increase the radial particle flux drastically. It indicates that the MHD activity leads to anomalous particle transport during the tokamak discharge at SST-1. It is found that the average poloidal velocity decreases from ∼4 km/s to ∼3.7 km/s whereas the average radial velocity decreases from ∼7.2 km/s to 4.7 km/s. Further, during MHD activity, it is found that as the magnetic island grows, the radial electric field changes from negative to positive. It is observed that the turbulent particle flux at SST-1 tokamak is predominantly positive and bursty. The local flux probability distribution function shows a clear non-Gaussian character, and it is skewed negatively.