Density Ramp-up Research Articles

Pedestal dynamics and turbulence in H-mode density ramp-up experiment on EAST

Abstract Recent results of density ramp-up (<n e>/n GW = 0.5–0.75) experiment in type-I ELMy H-mode on experimental advanced superconducting tokamak are presented, with a focus on pedestal dynamics and turbulence behavior. With the density increase, a general trend is that the pedestal electron pressure (p e ped) decreases while the ELM frequency (f ELM) increases. Especially when <n e>/n GW arrives at ∼ 0.68, a sudden decrease of p e ped and an increase of f ELM are observed. A quasi-coherent mode (QCM) with a frequency of 200–300 kHz is observed in the pedestal region and appears in both density and magnetic fluctuations. The time evolution of p e ped during the inter-ELM phase is analyzed for different densities. The analysis suggests that the pedestal behavior for <n e>/n GW < 0.68 is mainly attributed to pedestal stability but not due to pedestal transport. It is shown that the resistivity effect on the peeling-ballooning (PB) mode (Zhang et al 2017 Phys. Plasmas 24 062108) could be used to explain the present result. But the sudden decrease of p e ped and increase of f ELM at <n e>/n GW > 0.68 cannot be attributed to the resistivity effect. It is found that for the plasma with <n e>/n GW > 0.68, the QCM disappears just before ELM and the density pedestal becomes steeper. This implies that the QCM can drive outward particle transport. The steeper density pedestal leads to a narrower pedestal width. A narrower pedestal width will lead to a lower pedestal pressure since PB mode limits the pressure gradient and could explain the observed sudden decrease of p e ped and increase of f ELM.

Experimental investigation of PDI bifurcation of lower hybrid waves during electron density ramp-up in EAST

The effect of parametric decay instability (PDI) on the current drive efficiency of 4.6 GHz lower hybrid (LH) waves in EAST is investigated experimentally, showing the PDI channel bifurcation of LH waves for the first time in EAST. First, experiments with three platforms of LH power were performed, achieving the LH power required for the PDI occurrence. Second, PDI bifurcation experiments were further carried out by ramping up the plasma electron density. The loop voltage increases with an increase in density, implying a decrease in the current driven by the LH wave. PDI bifurcation during electron density ramp-up was studied by analyzing the parallel refractive index ( n∥ ) and the frequency spectrum broadening, which is measured with a radio frequency magnetic probe array recently installed close to the LH antenna. It is observed for the first time that they both first increase with density, then there is not much variation and a clear sideband in the frequency spectrum is also observed when the density is up to 4×1019 m−3 , suggesting a change in the PDI channel. Calculation of the mode growth rate driven by PDI shows that when the edge electron density is up to 1.9×1018 m−3 , the growth rate of the ion cyclotron quasi-mode (ICQM) will exceed that of the ion sound quasi-mode (ISQM), quantitively explaining that with an increase in density, the PDI channel partly transits from the ISQM to the ICQM channel. Studies provide a possible way to reduce the power deposition in the edge region and improve drive capability by means of mitigating PDI behavior.

First results of high density H-mode operation in metal-wall EAST tokamak

In metal-wall EAST superconducting tokamak, H-mode operation with plasma density close to the Greenwald density limit nGW has been achieved with radio frequency and NBI heating for the first time. Both gas puffing from horizontal plane and HFS pellet fueling were used for density ramp-up during the experiment. The confinement of H-mode gradually deteriorates with plasma density increasing. And the H-L transition can be observed after heating power dropping or pellet injection. In the density range of (0.6–1)×nGW, the divertor detachment occurs and causes an obvious confinement degradation. The maximum accessible density ne, max in H-mode phase deviates from the Greenwald scaling. It has been observed that the fraction ne, max/nGW is almost independent of the total heating power, but a high heating power is helpful to extend the duration of high density H-mode. And ne,max/nGW has an increase relation with the safety factor q95 varied by changing plasma current. Besides, it is also found that the discharges with low plasma current have a higher ne, max/nGW than those with high plasma current. All these findings will provide a good reference to the high density plasma operation for future metal-wall fusion devices.

Prediction of multifaceted asymmetric radiation from the edge movement in density-limit disruptive plasmas on Experimental Advanced Superconducting Tokamak using random forest

Multifaceted asymmetric radiation from the edge (MARFE) movement which can cause density limit disruption is often encountered during high density operation on many tokamaks. Therefore, identifying and predicting MARFE movement is meaningful to mitigate or avoid density limit disruption for the steady-state high-density plasma operation. A machine learning method named random forest (RF) has been used to predict the MARFE movement based on the density ramp-up experiment in the 2022’s first campaign of Experimental Advanced Superconducting Tokamak (EAST). The RF model shows that besides Greenwald fraction which is the ratio of plasma density and Greenwald density limit, dβ p/dt, H 98 and dW mhd/dt are relatively important parameters for MARFE-movement prediction. Applying the RF model on test discharges, the test results show that the successful alarm rate for MARFE movement causing density limit disruption reaches ∼85% with a minimum alarm time of ∼40 ms and mean alarm time of ∼700 ms. At the same time, the false alarm rate for non-disruptive and non-density-limit disruptive discharges can be kept below 5%. These results provide a reference to the prediction of MARFE movement in high density plasmas, which can help the avoidance or mitigation of density limit disruption in future fusion reactors.

Characteristics of electron temperature profile stiffness in electron-heated plasmas on EAST

A very high core electron temperature (T e0 ∼ 10 keV) plasma has been established and stably sustained by applying both lower hybrid wave (LHW) and on-axis electron cyclotron resonance heating (ECRH) in the Experimental Advanced Superconducting Tokamak (EAST). In this work, power balance analysis shows that the increase of ECRH power can increase the normalized T e gradient significantly at the plasma core region (ρ < 0.6), but does not change the T e profile stiffness in the low-density L-mode plasmas. This has been considered to be due to a strong synergistic effect between ECRH and LHW. Furthermore, three distinguishable stages characterized by different T e profile stiffnesses can be identified from the density ramp-up in the electron-heated plasma on EAST. A stronger T e profile stiffness at ρ = 0.3 has been observed in the Stage-II, where the LHW power deposition gradually moves away from the plasma core region, following the electron density increases. Furthermore, the formation of an internal plasma density transport barrier inside ρ ∼ 0.6, accompanied by a sudden drop in core T e and a rise in both core plasma density and ion temperature, has been observed for the first time during the transition from the Stage-II to the Stage-III when the central line-averaged plasma density reaches a threshold of 2.2 × 1019 m−3. This finding strongly affects further development of high-performance gas-fueled electron-heated plasma scenarios in EAST and suggests an advanced operational regime with a wide internal plasma density transport barrier.

Modelling of tungsten impurity edge transport and screening for different divertor conditions in EAST

Tungsten (W) transport and screening in the edge plasma are investigated for EAST high dissipative divertor conditions. By combining the 1D impurity fluid model (1DImpFM) and the two-point model formatting (2PMF), W screening is proved to be enhanced for high upstream plasma density conditions, mainly because the impurity temperature gradient velocity decreases with the increase of the upstream plasma density. Based on dedicated EAST density ramp-up experiments, 2D simulations of W erosion and transport are carried out for different levels of dissipative divertor conditions by using the SOLPS-DIVIMP code package, and the modeling results are benchmarked with the 1DImpFM analytic model. The prompt-redeposition, the divertor screening, and the main SOL screening are quantitatively analyzed. For detached divertor conditions, the increase in the W ionization length reduces the prompt redeposition rate, but both the divertor screening and SOL screening are reinforced. The 1DImpFM can interpret well the W leakage in the near separatrix region; however, the 2D simulations suggest that the impurity pressure gradient force which is neglected by the 1DImpFM plays an important role, especially in the far-SOL region. With the divertor condition varied from the high-recycling regime to the deep detachment regime, the W source moves from the near strike point region to the far SOL, and thus makes the W transport in the far SOL more important. Therefore, the impurity pressure gradient force cannot be neglected for edge W transport analysis, especially for the detached divertor conditions.

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).

Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium (He) discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor. This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges. During density ramp-up, the particle flux shows a clear rollover, while the electron temperature around the outer strike point is decreasing simultaneously. The divertor detachment also exhibits a significant difference from that observed in comparable deuterium (D) discharges. The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power, and increases with the heating power. Moreover, detachment assisted with neon (Ne) seeding was also performed in L- and H-mode plasmas, pointing to the direction for reducing the density threshold of detachment in He operation. However, excessive Ne seeding causes confinement degradation during the divertor detachment phase. The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.

2D distributions of potential and density mean-values and oscillations in the ECRH and NBI plasmas at the TJ-II stellarator

2D plasma potential ϕ distribution was measured in the electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) plasmas of the TJ-II stellarator with the heavy ion beam probe for the whole radial range and wide area of the poloidal angle, and supported by Langmuir probe data at the edge. The whole operation domain for the on-axis ECRH was explored ( = 0.45–0.8 ×1019 m−3, P EC = 220–470 kW), in addition, NBI plasmas with = 0.9–1.3 × 1019 m−3 and P NBI = 510 kW were studied. In ECRH plasmas the density ramp-up is accompanied by the evolution of the potential from the bell-like to the Mexican hat profile, while the density profiles were flat or slightly hollow. The potential has the positive peak at the centre, and LFS-HFS (low field—high field sides) and up-down symmetry. Equipotential lines are consistent with vacuum magnetic flux surfaces. In the high-density NBI scenario, the ϕ profile was fully negative with a minimum up to −300 V at the centre, while at low-density ECRH plasma, ϕ has a maximum up to +0.9 kV at the centre. Fluctuations of potential and density are stronger in low-density scenarios and not poloidally symmetric. At the mid-radius (area of the maximum density), root mean square (RMS) of fluctuations were up to ϕ ∼ 15 V at LFS vs ∼20 V at HFS; RMS n e ∼ 2% at LFS vs ∼3% at HFS. In the NBI plasmas with the density rise, the asymmetry decreases and finally vanishing at = 1.2 × 1019 m−3. 2D distribution of the NBI-induced Alfvén eigenmodes (AEs) shows asymmetric ballooning structure: contrary to broadband turbulence, AE-associated potential perturbation dominates in the LFS with a factor up to 1.7 respect to the HFS. The electrostatic mode, excited in ECRH plasmas by suprathermal electrons also shows asymmetric structures: density perturbation dominates in the top-bottom direction compared to LFS-HFS direction.

Prediction of Radiative Collapse in Large Helical Device Using Feature Extraction by Exhaustive Search

A predictor model of radiative collapse of stellarator-heliotron plasmas has been developed by means of a machine learning technique and the feature of radiative collapse has been extracted with sparse modeling. The dataset used for training the model is constructed based on density ramp-up experiments in the Large Helical Device. As a result of feature extraction, the line averaged electron density, visible line emissions of CIV and OV, and the electron temperature at the edge have been selected as key parameters of radiative collapse. The likelihood of occurrence of radiative collapse has been quantified by using these parameters and this likelihood has been assessed in terms of predicting capability of the occurrence of radiative collapse. The collapse likelihood also implies the underlying physics of radiative collapse, therefore, the knowledge obtained by this data-driven study is expected to facilitate elucidation of the physics of the radiative collapse. In validation with discharges outside of the dataset, the predictor based on the likelihood has predicted over 85% of radiative collapse about 100 ms prior to this event on average while about 5% of stable discharges have been detected falsely as collapse discharges. The discharges in which the predictor made faults are discussed in order to investigate the cause of failure.

Optimization of the POlarimeter-INTerferometer diagnostics system on EAST

A double-pass, radial-view, 11-chord POlarimeter-INTerferometer (POINT) system has been routinely operated on the Experimental Advanced Superconducting Tokamak (EAST) to provide electron density and current density information for physics research. Stray light occurs due to spurious reflections from optical elements, including lenses, waveplates, windows, and even detectors, along the optical path and can seriously deteriorate the accuracy of the Faraday rotation measurements. As a result of stray light contamination, periodic oscillations can be observed in the Faraday rotation data during the density ramp-up phase. Aiming to realize real-time current profile feedback control using Faraday effect measurements, the contamination from stray light must be removed. Some optimization methods have been explored to decrease the error induced by stray light. The results show that the Faraday angle oscillation during the density ramp-up phase is reduced from 3̂–5̂ to 0.5̂–1̂ (double-pass). Measurement of the noncollinearity error is also performed for the POINT system on EAST. This optimization allows for more accurate current profile measurements for physics research.

Transient Gas Distribution in Porous Transport Layers of Polymer Electrolyte Membrane Electrolyzers

Here, we elucidated the dynamic gas transport behavior in the anode porous transport layer (PTL) of polymer electrolyte membrane (PEM) electrolyzers via in operando synchrotron X-ray imaging. The imaging results showed that the gas saturation in the PTL reached steady state more rapidly with a steep current density ramp-up and a shallow ramp-down (compared to a shallow ramp-up and a steep ramp-down, respectively). Additionally, the gas accumulation was characterized by a faster response time compared to that of the gas removal due to the relatively slow migration of residual gas from the catalyst layer-PTL interface to the flow channels. Furthermore, we investigated the impact of gas response behavior on gas saturation during intermittent electrolyzer operation. Intermittent operation led to an increase in residual gas accumulation, which negatively impacted the electrolyzer performance. Our results are key for informing design and operating strategies for mitigating such hysteresis effects and improving the performance of PEM electrolyzers when coupled with intermittent renewable energy sources.

Error correction associated with stray light for Faraday-effect polarimetry system on EAST

The polarimeter-interferometer system with 11 double-pass radial-view measurement chords has the ability to provide electron density and plasma current profiles, making it exceptionally useful in daily operation on the Experiment Advanced Superconducting Tokamak. However, due to limited optical access and intrinsic feedback, the stray lights arising from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lenses as well as the detectors) distort the Faraday rotation measurements. Furthermore, the feedback light from the retro-reflector which is used to realize the double-pass configuration makes it even worse. A data processing approach to decrease the stray light influence is reported in this paper. Based on the theoretical model developed, the Faraday rotation angle is extracted by subtracting the deviation term which can be calculated with a simplified model. With this approach, the Faraday rotation oscillation during density ramp-up can be reduced from 2°-5° to 0.5°-1.5°, which reduces the Faraday rotation measurement errors significantly.

Toroidal magnetic field ripple and ion orbit loss effects on edge toroidal rotation in J-TEXT Tokamak plasmas

The edge toroidal rotation on J-TEXT is in the counter-current direction and sensitive to electron density and safety factor. During a density ramp-up phase, the initial toroidal rotation in the edge region (e.g. r/a = 0.7–1), having one speed with little radial dependence, evolves to develop a radial profile when the edge density is sufficiently large. The effects of ripple-induced thermal loss on plasma rotation are predicted by neoclassical theory for those discharges, and comparisons with the measurements are performed. The predictions agree well with the measurements over a range of plasma conditions, confirming that using the ripple effect to interpret the phenomenon is appropriate in certain parameter regimes, while in others the neoclassical effects are not sufficient to predict the experimental observations.

Characterization of edge turbulence in different states of divertor detachment using reflectometry in the ASDEX Upgrade tokamak

Transport in the scrape-off layer (SOL) depends on the state of divertor detachment. L-mode discharges were analyzed where the state of divertor detachment is varied through a density ramp-up. By means of reflectometry measurements at the low (LFS) and the high field side (HFS), midplane density fluctuations are studied for the first time in ASDEX Upgrade simultaneously at both sides of the tokamak. Radial density fluctuation profiles () increase with radius in both the HFS and the LFS. It is found that in the SOL density fluctuations at the LFS have about a factor of two larger amplitude than at the HFS in agreement with ballooned transport. Density fluctuations at the LFS show a modest variation with increasing background density resulting mainly from a rise of low frequency components. Experimental results are in good agreement with an enhanced convection of filaments at the LFS at the beginning of outer divertor detachment leading to a flatter SOL density profile. In this phase of the discharge, density fluctuations measured at the HFS far-SOL display a strong increase, which may be associated with the presence of faster filaments originated at the LFS.

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas with linear density and temperature ramps

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas are investigated in the presence of various laser intensities and linear density and temperature ramps. Considering the ponderomotive force and using the momentum transfer and energy equations, the nonlinear electron density is derived. Taking into account the paraxial approximation and nonlinear electron density, a nonlinear differential equation, governing the focusing and defocusing of the laser beam, is obtained. Results show that in the absence of ramps the laser beam is focused between a minimum and a maximum value of laser intensity. For a certain value of laser intensity and initial electron density, the self-focusing process occurs in a temperature range which reaches its maximum at turning point temperature. However, the laser beam is converged in a narrow range for various amounts of initial electron density. It is indicated that the σ2 parameter and its sign can affect the self-focusing process for different values of laser intensity, initial temperature, and initial density. Finally, it is found that although the electron density ramp-down diverges the laser beam, electron density ramp-up improves the self-focusing process.

MHD limit cycles on FTU

The development of low-order tearing modes during density ramp-up in the high density regime on the Frascati Tokamak Upgrade is characterized by an initial ordinary stage, with a ‘one-to-one’ relation between mode amplitude and frequency, followed by the formation, on the amplitude/frequency plane, of ‘limit cycles’ with increasing area up to disruption for density limit if the density continues to grow. A critical mode amplitude for transition from smooth to cyclic behavior has been observed in experiments performed changing the line-averaged density, and the existence of such a threshold has been confirmed in experiments of real time control of tearing mode in the high density regime by means of electron cyclotron resonance heating.The amplitude and frequency modulations of the observed tearing mode (m and n are the poloidal and toroidal mode number, respectively) occur in few milliseconds, which is not in agreement with the diffusion resistive time of about two hundred milliseconds expected on the resonance from the non-linear theory. The origin of such modulations has been investigated, taking into account that in the high amplitude stages of the mode temporal evolution it is difficult to discriminate between non-linear effects and mode coupling mechanisms. Our analysis suggests that the formation of limit cycles could be due to a recursive island fragmentation, with a sort of self-healing phenomenon; in fact the island distortion increases before amplitude drops. Concerning the interaction with modes of different helicity, our experiments seem to indicate that the presence of the resonance in the plasma is necessary for the occurrence of deep and regular limit cycles for the 2/1 tearing mode.

Review of recent experiments on the T-10 tokamak with all metal wall

Transition to an all-metal wall was realized on T-10 by replacement of graphite limiters with tungsten ones. Light impurity levels remained high and W accumulation in the plasma core was revealed. A movable lithium (Li) limiter was added to investigate the possibility of the limitation of tungsten and light impurity levels in plasma. For the first time, tokamak results on tungsten protection with Li were obtained in OH and ECRH regimes. After lithization the tungsten density in the core dropped more than an order of magnitude, while W influx into plasma decreased 2–4 times. Drastic drops of light impurities in plasma were observed together with improvement of energy confinement time and density limit values. Nevertheless, Li levels in plasma remained low in both OH and ECRH regimes. Li density in the core as low as 0.5% of ne was obtained. Tungsten transport in T-10 plasma was investigated and results on prevention of W accumulation with central ECRH were obtained. Effects of plasma exposure on ITER-grade tungsten plates from limiters were studied. Investigations of density fluctuation with correlation reflectometry confirmed a decrease of fluctuation amplitude on high field side. Modeling showed that this effect can be, to a great extent, explained by the non-locality of reflectometry. Toroidal correlations at a distance of 2.5 m along field lines were studied. Three-wave interaction between geodesic acoustic modes and broad-band turbulence was found by analysis of heavy ion beam probe diagnostics data. The possibility of plasma current control and the prevention of non-thermal electron beams formation at density limit disruption by means of ECR heating and the controlled operation of OH power supply system has been demonstrated. The study of plasma density decay after gas puff switch off during density ramp-up phase in OH regimes and the effect of ‘density pump out’ during ECRH showed that both effects can be explained by the assumption regarding electron transport growth upon reaching marginal pressure profile. A new type of internal transport barrier in the narrow zone near q = 1 surface has been found.

Overview of the FTU results

Experiments on runaway electrons have been performed for the determination of the critical electric field for runaway generation. A large database of post-disruption runaway beams has been analyzed in order to identify linear dynamical models for new position and current runaway beam controllers, and experiments of electron cyclotron assisted plasma start-up have shown the presence of runaway electrons also below the expected electric field threshold, indicating that the radio-frequency power acts as seeding for fast electrons. A linear micro-stability analysis of neon-doped pulses has been carried out to investigate the mechanisms leading to the observed density peaking. A study of the ion drift effects on the MARFE instability has been performed and the peaking of density profile in the high density regime has been well reproduced using a thermo-diffusive pinch in the particle transport equation. The study of the density limit performed in the past has been extended towards lower values of toroidal magnetic field and plasma current. The analysis of the linear stability of the 2/1 tearing mode observed in high density plasmas has highlighted a destabilization with increasing peaking of the current profile during the density ramp-up, while the final phase of the mode temporal evolution is characterized by limit cycles on the amplitude/frequency plane. A liquid lithium limiter with thermal load capability up to 10 MW m−2 has been tested. The pulse duration has been extended up to 4.5 s and elongated configurations have been obtained for 3.5 s, with the X-point just outside the plasma chamber. A W/Fe sample has been exposed in the scrape-off layer in order to study the sputtering of Fe and the W enrichment of the surface layer. Dusts have been collected and analyzed, showing that the metallic population exhibits a high fraction of magnetic grains. A new diagnostic for in-flight runaway electron studies has allowed the image and the visible/infrared spectrum of the forward and backward synchrotron radiation to be provided simultaneously. A fast infrared camera for thermo-graphic analysis has provided the pattern of the toroidal limiter heating by disruption heat loads, and a triple-GEM detector has been tested for soft x-ray diagnostics. The collective Thomson scattering diagnostic has been upgraded and used for investigations on parametric decay instability excitation by electron cyclotron beams correlated with magnetic islands, and new capabilities of the Cherenkov probe have been explored in the presence of beta-induced Alfvén eigenmodes associated to high amplitude magnetic islands.

Measurement of the deuterium Balmer series line emission on EAST.

Volume recombination plays an important role towards plasma detachment for magnetically confined fusion devices. High quantum number states of the Balmer series of deuterium are used to study recombination. On EAST (Experimental Advanced Superconducting Tokamak), two visible spectroscopic measurements are applied for the upper/lower divertor with 13 channels, respectively. Both systems are coupled with Princeton Instruments ProEM EMCCD 1024B camera: one is equipped on an Acton SP2750 spectrometer, which has a high spectral resolution ∼0.0049 nm with 2400 gr/mm grating to measure the Dα(Hα) spectral line and with 1200 gr/mm grating to measure deuterium molecular Fulcher band emissions and another is equipped on IsoPlane SCT320 using 600 gr/mm to measure high-n Balmer series emission lines, allowing us to study volume recombination on EAST and to obtain the related line averaged plasma parameters (Te, ne) during EAST detached phases. This paper will present the details of the measurements and the characteristics of deuterium Balmer series line emissions during density ramp-up L-mode USN plasma on EAST.