Ion Saturation Current Density Research Articles

Impact of Bt direction on the detachment threshold during H-mode discharge on EAST with SOLPS-ITER modeling

Abstract Upper single-null H-mode detachment experiments on EAST were carried out with an ITER-like tungsten divertor, and it was found that the detachment threshold of the upper outer divertor was significantly lower under an unfavorable toroidal magnetic field (B t) than a favorable B t. In this work, SOLPS-ITER simulations are carried out to study the effects of B t direction on the detachment threshold to explain the experimental observations. The simulation results are compared with the experiments, showing reasonable agreement with the diagnostics, and the simulation setups and parameters are thus determined. Based on the simulation data, two-point model formatting is applied to the flux tube where detachment occurs. A quantitative comparison highlights the critical roles of the upstream parallel heat flux and the downstream momentum and energy loss factors on the ion saturation current density at the target ( j sat,t). Further analysis of the simulation results indicates that due to the influence of E×B drift on the convective heat flux, combined with the presence of heat transport associated with parallel current, the upstream heat flux is reduced under unfavorable B t, while in the downstream region the divergence of the E×B drift flux acts as a particle source under unfavorable B t and as a sink under favorable B t, leading to increased momentum and energy losses under unfavorable B t. These factors collectively cause a lower detachment threshold under unfavorable B t.

Characterization of the TJ-II stellarator plasma by means of reciprocating Langmuir probes

Experiments were conducted in deuterium plasma in the TJ-II stellarator by means of swept Langmuir probes mounted on reciprocating probes manipulators. The results were processed using the four-parameter fit, as well as the triple-probe and the first-derivative probe techniques. The parameters determined were the floating potential, the ion saturation current density, the electron temperature and density, and the plasma potential. The results were obtained for two plasma heating techniques – electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) heating. In the case of ECRH, employing the first-derivative probe technique resulted in finding that the electron-energy distribution function (EEDF) was not Maxwellian, but rather a bi-Maxwellian one with thermal (14-25 eV) and cold (4-5 eV) electrons. In comparison, during NBI heating we found a Maxwellian EEDF with the electron temperature being around 5 eV and slightly increasing in the confined plasma, but always remaining below 15 eV. We present a detailed analysis and discussion of the data for the plasma parameters as acquired by different techniques of using the reciprocating probe manipulator.

Development of charge-discharge circuitry based on supercapacitor and its application to limiter probe diagnostics in EAST

The EAST limiter probe is installed on the front surface of guard limiter, which consists of two columns and can operate in floating potential mode, ion saturation current mode, and swept single-probe mode simultaneously. When Langmuir probe operates in the ion saturation current mode, it requires a stable biasing voltage. To meet this requirement, a large capacitor is used to provide a biasing voltage for the probe. Comparing with the 9 V dry batteries that are commonly used in magnetic confinement fusion devices, employing a large capacitor offers advantages such as flexible voltage adjustment, easy maintenance, and environmental friendliness. Therefore, we have designed and tested a complete set of supercapacitor charge-discharge control circuitry. In this work, a control software is developed for the supercapacitor charge-discharge control circuitry based on the Python language to enable the remote and automatic controlling of the circuitry operation. As demonstrated in experiments, the capacitor charge-discharge control circuitry can supply stable biasing voltage output for the probe under long-pulse discharge, and it is workable in complex electromagnetic environment of magnetic confinement fusion device. By implementing the supercapacitor charge-discharge control circuitry in EAST limiter probe diagnostics, the three-dimensional distributions of plasma parameters are measured, such as ion saturation current, floating potential, electron temperature, and plasma density. In a lower hybrid wave (LHW) heating experiment, the 2.45 GHz LHW is found to generate larger electron density than the 4.6 GHz LHW, and the largest electron density appears when both the 2.45 GHz and 4.6 GHz LHWs are turned on simultaneously. These experimental results confirm that supercapacitor charge-discharge control circuitry can be operated reliably and stably.

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.

Argon–seeded detachment during ELM control by RMPs in KSTAR

In this study, we demonstrate argon-seeded discharges that exhibited a detached divertor during the full suppression and mitigation of edge-localized modes (ELMs) by an International Thermonuclear Experimental Reactor-like, three-row resonant magnetic perturbation (RMP) configuration in KSTAR. During the ELM suppression phase, the peak heat flux on the divertor target was successfully reduced from 1.6 MW m−2–0.5 MW m−2 via argon seeding. Further, the ion saturation current densities corresponding to the particle fluxes on both targets were reduced by more than 50%. During the RMP grassy-ELM regime, a further reduction to 0.1 MW m−2 in the divertor heat load was successfully achieved. A highly localized radiation zone near the X-point was also observed during divertor detachment. The calculated degree of detachment based on the two-point model increased to levels of approximately 3 and 2.3 for the outer target and inner target cases, respectively. These results provide valuable information regarding the effect of mid-Z impurities on RMP-detachment-compatible discharges.

Effect of repetitive irradiation on ion saturation current of laser ablation plasma using 1 kHz repetitive laser

We demonstrated 100 shots of burst laser irradiation with an Al target using a 1 kHz repetitive pulse laser. By using the laser, we generated ablation plasmas with a repetition rate of 1 kHz. The time-evolution of ion saturation current density in the generated ablation plasma was measured using the orifice probe. The result shows that the waveforms on 100 spikes were observed. The ion saturation current density decreased rapidly from the first to the third shot and reached a minimum value at the fourth shot. After the fourth shot, it reached 1 μA/mm2 at the 53rd shot and settled at 0.5 μA/mm2. We extracted the time-evolution of ion saturation current densities of the first, 10th, 50th and 100th shots. The drift velocities were estimated by Time of Flight from the time-evolution of current. The result shows that the drift velocity of the ablation plasma increases with the increase in the number of shots.

Measurements of electron energy distribution on electron cyclotron resonance plasma operated with hydrogen gas

This article deals with the production of electron cyclotron resonance hydrogen plasma in a closed gas chamber called Menja device. For this purpose, 2.45 GHz frequency and various power i.e. 0 - 500 W are employed. The flow of hydrogen gas is controlled manually with the range 1.5 – 10 sccm maintaining pressure range 10-5 – 10-4 mbar. The Langmuir probe technique is used to drag the electrons and ions by biasing the probe positively and negatively to study the key parameters such as ion saturation current, floating potential, plasma potential, electron temperature and plasma density. Besides these, to obtain information on electron energies and their interaction with plasma, the electron energy distribution function (EEDF) has been studied with the help of 2nd derivative of current obtained from the differentiator. For this purpose, an analogue differentiation circuit was built and tested.

Characteristics of a radio frequency magnetized double-ring-shaped hollow cathode plasma source with permanent magnets for high-density hydrogen plasma generation

A rf magnetized double ring-shaped hollow cathode plasma with permanent magnets has been developed for high-density hydrogen plasma generation at various hydrogen gas pressures, pH2. The discharge breakdown voltage with magnet decreases with increasing pH2 and is lower than that without magnet at 1 ≤ pH2 ≤ 30 Pa and is otherwise higher than that. Spatial distributions of the ion saturation current density have also been investigated at various hydrogen gas pressures of pH2 = 5, 10, 20, 30 and 40 Pa. The high-density double peaks of the ion density due to the double ring-shaped hollow trenches are observed near the hollow electrode at pH2 ≥ 10 Pa. The laterally uniform high-density hydrogen plasma with a density of 1011cm−3 is attained at the distance z = 30 mm from the electrode under low gas pressure of 5 Pa and 10 Pa at rf power of 60 W.

Simulations of COMPASS vertical displacement events with a self-consistent model for halo currents including neutrals and sheath boundary conditions

The understanding of the halo current properties during disruptions is key to design and operate large scale tokamaks in view of the large thermal and electromagnetic loads that they entail. For the first time, we present a fully self-consistent model for halo current simulations including neutral particles and sheath boundary conditions. The model is used to simulate vertical displacement events (VDEs) occurring in the COMPASS tokamak. Recent COMPASS experiments have shown that the parallel halo current density at the plasma-wall interface is limited by the ion saturation current during VDE-induced disruptions. We show that usual magneto-hydrodynamic boundary conditions can lead to the violation of this physical limit and we implement this current density limitation through a boundary condition for the electrostatic potential. Sheath boundary conditions for the density, the heat flux, the parallel velocity and a realistic parameter choice (e.g. Spitzer’s resistivity and Spitzer–Härm parallel thermal conductivity) extend present VDE simulations beyond the state of the art. Experimental measurements of the current density, temperature and heat flux profiles at the COMPASS divertor are compared with the results obtained from axisymmetric simulations. Since the ion saturation current density () is shown to be essential to determine the halo current profile, parametric scans are performed to study its dependence on different quantities such as the plasma resistivity and the particle and heat diffusion coefficients. In this respect, the plasma resistivity in the halo region broadens significantly the profile, increasing the halo width at a similar total halo current.

Experimental observation of heat flux mitigation during divertor detachment in the DIII-D small angle slot divertor

Abstract Employing an array of surface eroding thermocouples (SETCs), the heat flux measured at the outer strike point of the DIII-D Small Angle Slot (SAS) divertor was found to be reduced by ~50% when the plasma reached the detachment regime compared to the attached state. Reduction in heat flux of similar magnitude has been found in both toroidal field directions (obtained by ramping up the plasma density) but with different characteristics: With the ion B × ∇B drift direction toward the SAS divertor, the heat flux measured by the SETCs first increased as plasma density increased, which may due to reduction in energy losses in the upstream and hence increase in the power flow into the divertor, and then began to roll over concurrent with a reduction in ion saturation current density on nearby Langmuir probes. In contrast, with the ion B × ∇B drift directed away from the SAS, the heat flux began at a higher value but decreases monotonically with increasing plasma density. This leads to lower heat flux in this BT direction at up to 20% lower line-averaged density. These significant differences in the onset of detachment in opposing BT directions are believed to be largely determined by the impacts of E × B drifts. Because ITER’s divertor must be operated with some degree of plasma detachment to radiate most of the power arriving in the scrape-off-layer, it is crucial to further understand the impact of magnetic drifts on plasma detachment behavior in a tight divertor geometry.

Artificial neural network based predictive negative hydrogen ion helicon plasma source for fusion grade large sized ion source

Ion source operation and control is a challenging task, and the manual search of the optimized input parameter by the operator becomes a time consuming and cumbersome process. HELicon Experiment for Negative Ion Source (HELEN-I) with single driver developed at the Institute for Plasma Research (IPR) for the production negative hydrogen ion involves a multitude of interacting systems (such as Radio Frequency (RF) power, gas feed pressure, magnetic fields). This paper presents an Artificial Intelligence (AI) driven multiple-input and multiple-output model for HELEN-I to get optimized values of ion saturation current (mA) and high plasma density of order 1018/m3. The developed Artificial Neural Network (ANN) based model predicts the outputs of the HELEN-I and is further utilizes a heuristic global optimization Particle Swarm Optimization (PSO) algorithm to select optimal source parameters to predict the desired outputs without conducting real-time experiments. The experimental studies further validate the results obtained by the proposed approach.

Effect of the gas puff location on the divertor plasma properties in COMPASS tokamak

Langmuir probes are used to study the plasma parameters in the divertor during deuterium gas puff injection on the high- (HFS) or low-field sides (LFS). The probe data were processed to evaluate the plasma potential and the electron temperatures and densities. A difference was found in the plasma parameters depending on the gas puff location.In the case of a gas puff on the LFS, the plasma parameters changed vastly, mainly in the inner divertor – the plasma potential, the ion saturation-current density and the electron temperature dropped. After the gas puff, the electron temperature changed from 10-15 eV down to within the 5-9 eV range. As a result, the parallel heat-flux density decreased. At the same time, in the outer divertor the plasma parameters remained the same. We thus concluded that using a gas puff on the LFS will facilitate reaching a detachment regime by increasing the density of puffed neutrals.When the deuterium gas puff was on the HFS, the plasma parameters in the divertor region remained almost the same before and during the puff. The electron temperature decreased with just few eV as a result of the increased amount of gas in the vacuum chamber.

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.

Generation of Supersonic Plasma Flow by an Ion Extraction Electrode

A concept of ion extraction has been adopted to generate and analyze supersonic plasma flow (M∞ > 1) in steady state condition. A capacitively coupled plasma (CCP) with weakly magnetic intensity (B = 100 G) was used as a plasma source along the axial direction and a cylindrical electrode was installed to generate negative potential within plasma by depleting ions. Characteristics of ion velocity distribution have been analyzed by equation of energy conservation from depletion ratio of ions within an ion extraction electrode. The normalized velocity (M∞ = vi/Cs) of flowing plasmas was deduced by using a Mach probe (MP) of parallel type using an exponential formula of the ratio of upstream and downstream ion saturation current densities. The first result on the measurement of ion velocity distribution with supersonic plasma flow (M∞ =~ 1.1–1.2) is obtained.

Discharge runaway in high power impulse magnetron sputtering of carbon: the effect of gas pressure, composition and target peak voltage

Pressure and target voltage driven discharge runaway from low to high discharge current density regimes in high power impulse magnetron sputtering of carbon is investigated. The main purpose is to provide a meaningful insight of the discharge dynamics, with the ultimate goal to establish a correlation between discharge properties and process parameters to control the film growth. This is achieved by examining a wide range of pressures (2–20 mTorr) and target voltages (700–850 V) and measuring ion saturation current density at the substrate position. We show that the minimum plasma impedance is an important parameter identifying the discharge transition as well as establishing a stable operating condition. Using the formalism of generalized recycling model, we introduce a new parameter, ‘recycling ratio’, to quantify the process gas recycling for specific process conditions. The model takes into account the ion flux to the target, the amount of gas available, and the amount of gas required for sustaining the discharge. We show that this parameter describes the relation between the gas recycling and the discharge current density. As a test case, we discuss the pressure and voltage driven transitions by changing the gas composition when adding Ne into the discharge. We propose that standard Ar HiPIMS discharges operated with significant gas recycling do not require Ne to increase the carbon ionization.

Simulations of particle and heat fluxes in an ELMy H-mode discharge on EAST using BOUT++ code

In order to study the distribution and evolution of the transient particle and heat fluxes during edge-localized mode (ELM) bursts on the Experimental Advanced Superconducting Tokamak (EAST), the BOUT++ six-field two-fluid model is used to simulate the pedestal collapse. The profiles from the EAST H-mode discharge #56129 are used as the initial conditions. Linear analysis shows that the resistive ballooning mode and drift-Alfven wave are two dominant instabilities for the equilibrium, and play important roles in driving ELMs. The evolution of the density profile and the growing process of the heat flux at divertor targets during the burst of ELMs are reproduced. The time evolution of the poloidal structures of T e is well simulated, and the dominant mode in each stage of the ELM crash process is found. The studies show that during the nonlinear phase, the dominant mode is 5, and it changes to 0 when the nonlinear phase goes to saturation after the ELM crash. The time evolution of the radial electron heat flux, ion heat flux, and particle density flux at the outer midplane (OMP) are obtained, and the corresponding transport coefficients D r, χ ir, and χ er reach maximum around 0.3 ∼ 0.5 m2 s−1 at ΨN = 0.9. The heat fluxes at outer target plates are several times larger than that at inner target plates, which is consistent with the experimental observations. The simulated profiles of ion saturation current density (j s) at the lower outboard (LO) divertor target are compared to those of experiments by Langmuir probes. The profiles near the strike point are similar, and the peak values of j s from simulation are very close to the measurements.

Three-dimensional simulations of plasma turbulence in the RFX-mod scrape-off layer and comparison with experimental measurements

The tokamak scrape-off layer (SOL) plasma dynamics is investigated in a circular limiter configuration with a low edge safety factor. Focusing on the experimental parameters of two ohmic tokamak inner-wall limited plasma discharges in RFX-mod [Sonato et al., Fusion Eng. Des. 74, 97 (2005)], nonlinear SOL plasma simulations are performed with the GBS code [Ricci et al., Plasma Phys. Controlled Fusion 54, 124047 (2012)]. The numerical results are compared with the experimental measurements, assessing the reliability of the GBS model in describing the RFX-mod SOL plasma dynamics. It is found that the simulations are able to quantitatively reproduce the RFX-mod experimental measurements of the electron plasma density, electron temperature, and ion saturation current density (jsat) equilibrium profiles. Moreover, there are indications that the turbulent transport is driven by the same instability in the simulations and in the experiment, with coherent structures having similar statistical properties. On the other hand, it is found that the simulation results are not able to correctly reproduce the floating potential equilibrium profile and the jsat fluctuation level. It is likely that these discrepancies are, at least in part, related to simulating only the tokamak SOL region, without including the plasma dynamics inside the last close flux surface, and to the limits of applicability of the drift approximation. The turbulence drive is then identified from the nonlinear simulations and with the linear theory. It results that the inertial drift wave is the instability driving most of the turbulent transport in the considered discharges.

Effect of the resonant magnetic perturbation on the plasma parameters in COMPASS tokamak’s divertor region

The resonant magnetic perturbation (RMP) has proven to be a useful way to suppress edge-localized modes that under certain conditions can damage the device by the large power fluxes carried from the bulk plasma to the wall. The effect of RMP on the L-mode plasma parameters in the divertor region of the COMPASS tokamak was studied using the array of 39 Langmuir probes embedded into the divertor target. The current-voltage (IV) probe characteristics were processed by the first-derivative probe technique to obtain the plasma potential and the electron energy distribution function (EEDF) which was approximated by a bi-Maxwellian EEDF with a low-energy (4-6 eV) fraction and a high-energy (11-35 eV) one, the both factions having similar electron density. Clear splitting was observed during the RMP pulse in the low-field-side scrape-off-layer profiles of the floating potential Ufl and the ion saturation current density Jsat; these two quantities were obtained both by direct continuous measurement and by evaluation of the IV characteristics of probes with swept bias. The negative peaks of Ufl induced by RMP spatially overlaps with the local minima of Jsat (and ne) rather than with its local maxima which is partly caused by the spatial variation of the plasma potential and partly by the changed shape of the EEDF. The effective temperature of the whole EEDF is not correlated with the negative peaks of Ufl, and the profile of the parallel power flux density shows secondary maxima due to RMP which mimic those of Jsat.

Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas.

A Child-Langmuir law-based method for accounting for Debye sheath expansion while fitting the current-voltage I-V characteristic of proud Langmuir probes (electrodes that extend into the volume of the plasma) is described. For Langmuir probes of a typical size used in tokamak plasmas, these new estimates of electron temperature and ion saturation current density values decreased by up to 60% compared to methods that did not account for sheath expansion. Changes to the collection area are modeled using the Child-Langmuir law and effective expansion perimeter lp, and the model is thus referred to as the "perimeter sheath expansion method." lp is determined solely from electrode geometry, so the method may be employed without prior measurement of the magnitude of the sheath expansion effects for a given Langmuir probe and can be used for electrodes of different geometries. This method correctly predicts the non-saturating ΔI/ΔV slope for cold, low-density plasmas where sheath-expansion effects are strong, as well as for hot plasmas where ΔI/ΔV ∼ 0, though it is shown that the sheath can still significantly affect the collection area in these hot conditions. The perimeter sheath expansion method has several advantages compared to methods where the non-saturating current is fitted: (1) It is more resilient to scatter in the I-V characteristics observed in turbulent plasmas. (2) It is able to separate the contributions to the ΔI/ΔV slope from sheath expansion to that of the high energy electron tail in high Te conditions. (3) It calculates the change in the collection area due to the Debye sheath for conditions where ΔI/ΔV ∼ 0 and for V = Vf.

Ion current density distribution in a pulsed non-self-sustained glow discharge with a large hollow cathode

The paper reports on azimuthal and radial distributions of the ion saturation current density measured with a single cylindrical and a plane probe in the plasma of a low-pressure (≈1 Pa) pulsed non-self-sustained glow discharge with a large (0.21 m3) hollow cathode at discharge currents higher than 100 A. It is shown that increasing the discharge operating voltage and decreasing the operating pressure in the range 0.4–1 Pa improves the uniformity of the ion current density distribution.