Flush-mounted Langmuir Probes Research Articles

Refining light impurity content estimates at the lower divertor based on experimental data in WEST

WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten (W). One of the aims of WEST is to study plasma operation with W PFCs in preparation for long-pulse operation on W divertor devices like ITER (Bucalossi et al., 2014) [1]. For long-pulse operation, the W impurity content and transport to the core plasma are critical concerns that require further measurement and interpretation in order to improve plasma performance and PFC durability. In low confinement mode (L-mode) operation, the sputtering of tungsten is not expected to be driven by the plasma itself (D), but by the main light impurities (C, B, O, N) observed at trace levels in WEST. Identifying the main W sources and understanding W transport in the SOL are key metrics for plasma performance, which are simulated with Scrape-Off Layer (SOL) impurity transport codes (SOLEDGE-ERO, OEDGE-DIVIMP). Previous studies have used O as proxy for light impurity with a global content varying from 3 to 5% in SOL transport simulations (Genova et al., 2021; Rooij et al., 2020; Klepper et al., 2022) [2–4]. A power scan was performed in a standard WEST magnetic equilibrium with LH injected power up to 5 MW. Density and temperature were measured with flush-mounted Langmuir probes (Dejarnac et al., 2021) [5]. The impurity fluxes are estimated by applying the number of ionizations per photons (S/XB) coefficients calculated with the collisional-radiative model “ColRadPy” (Johnson et al., 2019) [6] to the spectral radiance measured with the WEST visible spectroscopy system (Meyer et al., 2016) [7]. This work details how the low-Z impurity content (C, O) near the divertor target was refined using the visible spectroscopy for several impurity charge states, coupled with ColRadPy to infer the content of all impurity charge states at the divertor. First results shows that the main light impurity is C and not O as originally assumed. This inferred impurity content is then used to evaluate the W sputtering at the divertor, considering the sheath at the divertor. A quantitative comparison is performed with the W flux at the divertor measured with visible spectroscopy that shows a qualitative agreement, but not fully consistent at the strike point position. This, in turn, suggests that the impurity content is not complete with O and C only.

Divertor power loads and scrape off layer width in the large aspect ratio full tungsten tokamak WEST

WEST is a full W tokamak with an extensive set of diagnostics for heat load measurements especially in the lower divertor. It is composed by infrared thermography, thermal measurement with thermocouples and fibre Bragg grating embedded few mm below the surface and flush mounted Langmuir probes. A large database including different magnetic equilibrium and input power is investigated to compare the heat load pattern (location, amplitude of the peak and heat flux decay length) on the inner and outer strike point regions: from the first ohmic diverted plasma (obtained during the second experimental campaign C2 in 2018) up to the high power (8 MW total injected) and high energy (up to 90 MJ injected energy in lower single null configuration) experiments performed in the last experimental campaign (C4 in 2019). Concerning the peak location, a good agreement (<1 cm) is obtained between thermal inversions and flush-mounted LP measurements. The peak heat flux from the whole set of diagnostics is in good agreement and mainly in the ±20% range, while the heat flux decay length reported on the target shows significant discrepancy between diagnostics and location in the machine (±40% range). Despite such discrepancy, heat flux decay length at target is found to scale mainly with the magnetic flux expansion through the variation of the X-point height, as expected. The improved plasma performances achieved during C4 enabled to reach significant heat load in the divertor, up to 6 MW m−2 with 4 MW of additional heating power showing the capability to reach the ITER relevant heat load (10 MW m−2 steady state) with about 7 MW of additional power in L-mode discharge. The heat load distribution is clearly asymmetric with a 3/4 and 1/4 distribution on the outer and inner strike point region respectively for the parallel heat flux.

Angular dependence measurements of Magnum-PSI plasmas using MAST-U angled-tip Langmuir probes

Abstract Measurements made using flush-mounted Langmuir probes in tokamaks are difficult to interpret when operating at grazing angles of magnetic field incidence due to the effects of sheath expansion on the probe collection area. The Super-X divertor on the upgraded Mega Amp Spherical Tokamak (MAST-U) can have very shallow angles of magnetic field incidence to plasma facing components (1–10 ° ), making the use of conventional flush-mounted probes problematic. A novel probe tip geometry, based on the angled-tip design used successfully on JET and DIII-D, has therefore been used in MAST-U to mitigate sheath expansion effects by increasing the projected probe extent. To verify whether the new design of probe tip allows temperature ( T e ) and density ( n e ) measurements to be performed accurately at low angles of incidence, a 4-probe array based on this design was used on Magnum-PSI. Parameter scans were made on a range of hydrogen plasmas in conditions comparable to those expected in MAST-U. Plasma parameters were extracted from the measured IV characteristics - using a novel fitting approach which minimised a goodness-of-fit parameter ( T e δ T e ) - and compared to Magnum-PSI Thomson scattering measurements. The measured plasma parameters show that the MAST-U angled-tip design seems to successfully mitigate the effects of sheath expansion at low angles of magnetic field incidence. The standard MAST-U Langmuir probe also shows an apparent upper operational limit of θ = 8 ° . This covers the vast majority of expected plasma configurations in MAST-U when running both conventional and Super-X configurations.

Flush-mounted Langmuir probes in the WEST tokamak divertor

The design of the Langmuir probes for the lower divertor of the WEST tokamak is presented, in which uncooled, tungsten-coated graphite targets were installed for the first phase of operation in anticipation of full-tungsten, actively cooled, ITER-like divertor components. Due to the long plasma discharges and high heat loads, the probe tips are flush with the divertor tiles in which they are embedded, as it is foreseen and indeed required in ITER. Tantalum metal was chosen for the probe tips because of its high melting point, and ductility at room temperature which allows simple crimping to electrically connect them to cables, as well as being compatible with the full metal environment of WEST. High heat flux tests of Ta Langmuir probe prototypes confirmed the robustness of the design for energy flux densities up to 8 MW/m2 for 5.5 s, and at lower 3.5 MW/m² energy flux density for 40 s long discharges. The probes were operational from the first discharge in WEST and the good alignment of all probes with the surrounding surface was confirmed by a good match of each adjacent probe's data when the strike points were swept across the target.

Three-dimensional plasma edge turbulence simulations of the Mega Ampere Spherical Tokamak and comparison with experimental measurements

The STORM module of BOUT++ (Easy et al 2014 Phys. Plasmas 21 122515) is generalized to simulate plasma turbulence at the periphery of tokamak devices in diverted configurations and it is used to carry out three-dimensional nonlinear flux-driven simulations in double null configuration with realistic experimental parameters of an L-mode plasma discharge in the Mega Ampere Spherical Tokamak. The reliability of STORM in modeling the scrape-off layer (SOL) plasma dynamics is assessed by comparing the numerical results with experimental measurements from a reciprocating Gundestrup probe and from flush-mounted Langmuir probes. This is the first time that a thorough comparison between experimental measurements and three-dimensional simulations in double null configuration is attempted. It is found that the simulations correctly capture most of the statistical properties of plasma turbulence at the outer mid-plane, whereas ion saturation current and floating potential time-averaged profiles at the outer mid-plane are steeper in the simulations than in the experiment. In particular, it is found that the ion saturation current and floating potential probability distribution functions, as well as the power spectra and several statistical properties of intermittent events in the tokamak SOL, such as the shape, duration and separation of burst events are correctly described by the STORM model. Good qualitative agreement is also obtained for the time-averaged ion saturation current density profiles at the divertor plates. On the other hand, the ion saturation current decay length is approximately 4 times smaller in the numerical results than in the experimental measurements. Additionally, the level of the fluctuations is smaller in the simulations than in the experiment. Finally, possible areas of improvement for the STORM model are identified.

Upgrade Design of Lower Divertor Langmuir Probe Diagnostic System in the EAST Tokamak

A flush-mounted Langmuir probe system has been built on the lower graphitic divertor targets in the experimental advanced superconducting tokamak (EAST) in 2016, which is revised from the previous divertor Langmuir probes, aimed to reduce the erosion from high-energy particle and strong heat flux on the probe surfaces exposed in the plasma, and explore new structure applications. The theories of single and triple flush-mounted probes relevant to their dimensions and the plasma parameters in the EAST divertor region are reviewed. During the 2016 EAST campaign, the flush-mounted probe system measured the plasma parameters by using single and triple probes, respectively, to obtain plasma parameters. The results were compared with the previous measurements by the domed probe in the similar condition. The results showed a good agreement between measurements using different probe geometries and techniques. This demonstrated the reliability of the flush-mounted probe system as a diagnostic tool in the EAST divertor. Optimization of these Langmuir probe system for the next generation of EAST lower divertor, which will be upgraded to a full tungsten divertor with active water cooling, is also discussed.

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°.

Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

In inboard-limited plasmas, the scrape-off layer (SOL) shows two regions: the near SOL, extending a few mm from the last closed flux surface (LCFS), characterized by a steep gradient of the parallel heat flux radial profile, and a far SOL, typically some cm wide, with flatter heat flux profiles. The physics of the near SOL is investigated in TCV with two series of experiments featuring deuterium and helium plasmas, in which the plasma current, density and elongation have been varied. The parallel heat flux profiles are measured on the limiter by means of infrared thermography. For the first time, the near SOL is reported to disappear for low plasma current or at high density, for values of the SOL collisionality corresponding to a conduction-limited regime. The power in the near SOL is shown to decrease with the normalized Spitzer resistivity ν as . The floating potential profiles, measured at the limiter using flush-mounted Langmuir probes (LP), show the presence of non-ambipolar currents, and their relation to the presence of a velocity shear layer is discussed. The shearing rate is shown to strictly correlate with the power in the near SOL , consistently with a recent theoretical model. Measurements of the near SOL on the Low Field Side (LFS) are performed using a reciprocating Langmuir probe (RP). The near SOL is reported to vanish simultaneously at the LFS and at the limiter. The near and far SOL widths are compared with the predictions from existing theoretical models, to which empirical corrections with resistivity and elongation are proposed.

Resilient Batch-Fabricated Planar Arrays of Miniaturized Langmuir Probes for Real-Time Measurement of Plasma Potential Fluctuations in the HF to Microwave Frequency Range

We report the design, fabrication, and characterization of miniaturized, flush-mounted Langmuir probe arrays for RF diagnosis of plasmas in the HF to microwave range of frequencies. We developed probes of radii ≥125 μm by electroless nickel metallization of ultrasonically drilled through-substrate vias. Planar arrays with as many as 25 probes spaced 1.6 mm apart (39 probes/cm2) in Pyrex, silicon carbide, and alumina substrates were produced. The sensor system was built to have a frequency response between 2 MHz and 3 GHz, and a probe impedance greater than or within close range of the plasma sheath impedance for plasma densities ≥1016 m -3 . We characterized a self-biasing nickel probe part of a 2×2 array with alumina substrate using a high-density magnetized helicon plasma source; we found that the measurement of the plasma potential from the MEMS probe compares well with independent measurements using a hot emissive probe and an ion sensitive probe. The sensor technology can be used to monitor plasma-based manufacturing systems, plasma-based energy generation systems, and as on-board plasma diagnostics in spacecraft including nanosatellites.

Divertor power and particle fluxes between and during type-I ELMs in the ASDEX Upgrade

Particle, electric charge and power fluxes for type-I ELMy H-modes are measured in the divertor of the ASDEX Upgrade tokamak by triple Langmuir probes, shunts, infrared (IR) thermography and spectroscopy. The discharges are in the medium to high density range, resulting in predominantly convective edge localized modes (ELMs) with moderate fractional stored energy losses of 2% or below. Time resolved data over ELM cycles are obtained by coherent averaging of typically one hundred similar ELMs, spatial profiles from the flush-mounted Langmuir probes are obtained by strike point sweeps. The application of simple physics models is used to compare different diagnostics and to make consistency checks, e.g. the standard sheath model applied to the Langmuir probes yields power fluxes which are compared with the thermographic measurements. In between ELMs, Langmuir probe and thermography power loads appear consistent in the outer divertor, taking into account additional load due to radiation and charge exchange neutrals measured by thermography. The inner divertor is completely detached and no significant power flow by charged particles is measured. During ELMs, quite similar power flux profiles are found in the outer divertor by thermography and probes, albeit larger uncertainties in Langmuir probe evaluation during ELMs have to be taken into account. In the inner divertor, ELM power fluxes from thermography are a factor 10 larger than those derived from probes using the standard sheath model. This deviation is too large to be caused by deficiencies of probe analysis. The total ELM energy deposition from IR is about a factor 2 higher in the inner divertor compared with the outer divertor. Spectroscopic measurements suggest a quite moderate contribution of radiation to the target power load. Shunt measurements reveal a significant positive charge flow into the inner target during ELMs. The net number of elementary charges correlates well with the total core particle loss obtained from highly resolved density profiles. As a consequence, the discrepancy between probe and IR measurements is attributed to the ion power channel via a high mean impact energy of the ions at the inner target. The dominant contributing mechanism is proposed to be the directed loss of ions from the pedestal region into the inner divertor.

Two-dimensional particle simulation of the current flow to a flush-mounted Langmuir probe in a strong oblique magnetic field

The current flow to flat Langmuir probes mounted flush into divertor target plates or limiter in a tokamak is studied with particle-in-cell simulations. The simulation model is two-dimensional in real space and three-dimensional in velocity space resolving the gyration of electrons and ions. The following two effects are studied: (i) The lack of saturation of the ion current at grazing incidence of the magnetic field. Simulations are performed with realistic values of the dimensionless parameters (ratios between probe size, gyroradius and Debye length) and the grazing angle of the magnetic field. (ii) The influence of the self-consistent E×B drift. A contribution to the ion current nonsaturation (at negative bias) is found as well as a reduction of the electron current (at positive bias).

Linearized magnetohydrodynamic theory of Langmuir probes with resistivity, friction, and polarization

Standard Langmuir probe analysis interprets the entire voltage drop to occur in the sheath and neglects any voltage drop due to bulk resistivity. In a magnetized plasma this implies a strong mechanism to damp the E×B drift around the probe’s flux tube. Application of linearized magnetohydrodynamics to the bulk plasma allows the consideration of a variety of geometries and damping mechanisms on an equal footing and the treatment of plasma turbulence without the inappropriate recourse to anomalous transport coefficients. An analytic expression for the apparent temperature determined by long flush mounted Langmuir probes can be found if the boundary conditions are also linearized. The friction on neutrals is always stronger than viscosity and is able to account for all the I–V characteristics measured in the ASDEX Upgrade [M. Kaufmann et al., Plasma Physics and Controlled Fusion 35, 205 (1993)] divertor. At all but the lowest densities, the polarization term associated with E×B drifts is stronger still.

An experimental study of ion currents to a flush mounted Langmuir probe in an ECR discharge

Calculations based on a recently derived theory for the non-saturating ion current of a flush mounted Langmuir probe in an oblique magnetic field are compared with the measured ionic branches of probe characteristics obtained in an electron cyclotron resonance (ECR) discharge. It turns out that the theoretical formula reproduces the shape of the measured curves well and also yields ionic densities in the outer region of the core plasma consistent with values obtained by other means in the same discharge under similar conditions.

The Ion Current of a Flush Mounted Langmuir Probe in an Oblique Magnetic Field

For a small Debye length and a small ion gyro radius an analytical formula for the ion currrent of a flush mounted Langmuir probe is presented. The formula is compared with other approaches.

Flush mounted Langmuir probes in an oblique magnetic field

Due to the high heat flux to the target plates of present day divertor tokamaks, the use of thermally robust flush mounted Langmuir probes becomes more and more important. The surface normal of the probe defines a direction in space which is generally neither parallel nor perpendicular to the magnetic field, and the angle between these two vectors plays an important role for the physics in front of the probe. An analytic description of the sheath physics in front of a target plate is presented, and a model for the analysis of the I–V characteristics of such probes is derived therefrom. The model includes, on the one hand, considerably more physics than previous descriptions, and is, on the other hand, much simpler and more practical than numerical solutions and simulations. Subsequently the application of this model to triple probes is discussed. It will become evident that flush mounted probes can be used as reliably as domed probes to determine the plasma parameters in front of the target plates.

Study of gross and net erosion in the ASDEX upgrade divertor

Erosion of carbon and tungsten in the divertor of ASDEX Upgrade was investigated by surface analysis of target tile probes. The probes were exposed in the divertor using a manipulator system. Erosion of carbon was measured by graphite probes covered with a 150 nm layer of 13C isotope to distinguish from the intrinsic carbon content. Tungsten erosion was measured by similar probes with 1–20 nm tungsten markers evaporated on the surface. From the measured erosion, sputtering yields were determined using plasma parameters at the target plates obtained from flush mounted Langmuir probes.

Flush mounted probes in ASDEX Upgrade - Can they be operated as triple probes?

Due to the high heat flux in the divertor region of present day tokamaks, the use of thermally robust flush mounted Langmuir probes becomes more and more important. We will present a model for the analysis of the I-V-characteristics yielded by such probes, discuss some of their special features and their suitability as triple probes. It will become evident that flush mounted probes can be used as well as domed probes to determine the plasma parameters in front of the target plates.

Flush-mounted probes in the divertor plates of Tokamak de Varennes

Arrays of 12 flush-mounted Langmuir probes have been installed in both the upper and lower outboard divertor plates of TdeV (Tokamak de Varennes). Measurements of electron temperature and density are obtained using a novel method of analysis which assumes that charge collection is dominated by the finite size of the electrostatic sheath. The technique is validated by comparing measurements from a cylindrical and a flush probe which are operated simultaneously in the same plasma.