Both mean and oscillatory components of radial electric field Er play important roles in the formation of the turbulent and transport processes in toroidal devices, tokamaks and stellarators. Previous experiments [1, 2] and gyrokinetic modeling [3] have shown that the electrostatic potential φ (and Er) may vary along the flux surfaces that may affect the transport. In addition to the...
As magnetic fusion devices operate at higher power, it is increasingly important to achieve an improved theoretical understanding of edge plasma turbulence and transport in order to control core energy confinement and to maintain the necessary limit on plasma heat flux to surrounding material components. Making use of advanced numerical methods from the computational fluid community and...
The transport of energetic particles induced by the toroidal Alfvén eigenmode (TAE) excited by multiple super-Alfvénic particles was for the first time in stellarator/helical devices studied using comprehensive neutron and energetic particle diagnostics in Large Helical Device (LHD). It was clearly shown that both super-Alfvénic hydrogen and deuterium beam ions are transported to the outer...
Edge Localized Modes (ELMs) comprising of repetitive plasma eruptions from the edge of a tokamak plasma are a very common feature of high confinement mode (H-mode) operation in advanced tokamak devices. Each ELM outburst is associated with an expulsion of a large amount of energy and particles in a very short time that can potentially cause serious damage to plasma facing components and...
Wendelstein 7-X (W7-X) is an optimized stellarator of the HELIAS type. ASDEX Upgrade is a medium sized tokamak. In electron cyclotron heated (ECRH) hydrogen plasmas the central ion temperature is clamped at Ti,0 ~ 1.5 keV in both W7-X (Figure-1) and AUG. These findings are found to be virtually independent of heating power and electron density, and for W7-X, independent of magnetic...
Due to the limited power exhaust capability of the divertor, a future DEMO reactor needs a high core/pedestal radiation level, controlled and tailored by an appropriate seed impurity mix. The core/pedestal seeding has to be integrated with substantial divertor radiative cooling and a no/very small ELM plasma regime. Required boundary conditions of the seeding scenario are sufficient energy...
In this paper we will present nonlinear full-$f$ electromagnetic gyrokinetic simulations of turbulence in the pedestal and scrape-off layer (SOL) region of a tokamak. The algorithms in the Gkeyll code solve the electromagnetic gyrokinetic equations using a continuum high-order discontinuous Galerkin scheme. The equations are written in a sympletic form in which the particle parallel momentum...
The capability to suppress edge localized modes (ELMs) is crucial for the success of ITER because the transient heat loads on the divertor due to ELMs would reduce the lifetime of plasma facing components to unacceptable levels. ELMs can be suppressed with the application of resonant magnetic perturbations (RMPs). But a side effect of RMPs is enhanced particle flux, or density pump-out, that...
E-mail: ysna@snu.ac.kr
Edge Localized Modes (ELM) are rapid MHD events occurring at the edge region of tokamak plasmas, which can result in damages on the divertor plates. Therefore, to fully suppress ELM via resonant magnetic perturbation (RMP) [1-4] is of great help to reach and sustain high-performance H-mode plasmas. It was found that certain conditions must met for the RMP-driven ELM...
A fusion reactor based on a stellarator design has the advantage of an easier access to long pulse scenarios. In fact, one of the main goals of Wendelstein 7-X (W7-X), the largest advanced stellarator in the world, is to demonstrate the steady-state capabilities of the stellarator line. From 2022 onward, all plasma-facing components will be water-cooled, which should enable pulse duration of...
Simultaneous high ion temperature ($T_i$) and high electron temperature ($T_e$) regime has been significantly extended due to the optimized heating scenario in the LHD deuterium (D) campaign. Such high-temperature plasmas were realized by the simultaneous formation of an electron ITB (e-ITB) and an ion ITB (i-ITB) by the combination of high power NBI and ECRH. In the high $T_i$ operation the...
Sudden increase of the energy confinement time by 38% is observed during divertor detachment operation with RMP application in deuterium plasmas in LHD. Edge transport barrier (ETB) is formed during the RMP induced H-mode phase at the inner separatrix of magnetic island created by the RMP, leading to steepening of pressure profile while the detachment is maintained. Such behavior has been...
EAST is typically operated on the radio-frequency (RF) waves heating scenarios, which is also highly related to ITER and CFETR. For the recent experiments of EAST H-mode operations, RF waves are found to be efficient to mitigate and suppress the edge instabilities, such as ELMs [1,2]. From the simulation point of view, the RF heating and driving effects can be studied from 2 aspects: directly...
Highlight of this work: This work predicts the optimal coil phasing, semi-empirical threshold coil current and ‘favorable’ $q_{95}$ window for ELM mitigation for HL-2M 1MA discharge scenario. It is found that pressure gradient may play an important role on determining the peeling-tearing displacement near X-point, due to the curvature effect (GGJ effect) of equilibrium magnetic field....
Transition between isotope-mixing and non-mixing states in hydrogen-deuterium mixture plasmas is observed for the first time in the world in the isotope (hydrogen and deuterium) mixture plasma in Large Helical Device. In the non-mixing state, the isotope density ratio profile is un-uniform when the beam fueling isotope species differs from the recycling isotope species and the profile varies...
Visible magnetic-dipole (M1) lines can serve as a novel diagnostic mean of tungsten in ITER and future DEMO reactors. Here we report that a local tungsten density in core plasmas of the Large Helical Device (LHD) is successfully assessed with the measurement of visible M1 lines emitted by W$^{26+}$ and W$^{27+}$. By such a measurement, (i) the radial profile of total tungsten density in the...
The question of Power EXhaust (PEX) in future fusion reactors is an open issue. Current technology can exhaust only ∼ 10 MW/m2, but reaching such low power deposition density would be impossible if only the ∼ 1 m2 surface of the divertor targets is available to accommodate it. Suitable strategies are currently under investigation and, in the near future, the Divertor Tokamak Test (DTT)...
The feature of radiation collapse has been extracted from high-density NBI heated experiments in LHD using sparse modeling technique. Accessible density limited by radiation collapse has been evaluated using extracted features. The extracted feature is described by the combination of plasma parameters such as line averaged electron density $\bar{n}_\mathrm{e}$, impurity line emission intensity...
Wendelstein 7-X (W7-X) is an advanced stellarator [1] designed to explore the reactor viability of optimized stellarators. The device started operation in December 2015 with a simple uncooled limiter for an integral commissioning phase of about three months. In 2016/17 an uncooled divertor, the so-called test-divertor-unit (TDU), was installed which has the same geometry as the water-cooled...
Injection of sub-millimeter-sized powder grains of boron composites is performed in the Large Helical Device (LHD) showing wall conditioning effects in real time during long plasma discharges. This alternative boronization technique has been recently employed in both tokamaks (1-4) and stellarators (5), showing beneficial effects on plasma performance both through the modification of the edge...
Understanding the scrape-off layer (SOL) physics in current fusion devices is essential to solve the exhaust problem for future reactors. The SOL configuration determines the intersections between the magnetic field lines and the device targets, opening up parallel transport channels for plasma heat and particles, which represent the most direct way for the exhaust to reach the divertor...
In the Large Helical Device (LHD), deuterium (D) plasma experiment has been conducted since 2017. This is the first opportunity in stellarator / heliotron devices to reveal the transport of tritium (T) generated by DD fusion reactions. Dedicated analyses of the remaining T in the LHD vacuum vessel have been conducted, and results of the analyses suggest that T remains densely in divertor tiles...
Recent improvements of the EMC3-EIRENE code have allowed to assess for the first time the
detached divertor scenario foreseen for ITER during ELM suppression by resonant magnetic
perturbation (RMP) fields. This is a major breakthrough because ELM suppression is required
for ITER to maintain the integrity of the plasma wall interface. The ITER divertor has been
designed based on extensive...
Energy confinement and energetic-particle (EP) driven MHD modes have been experimentally studied from the viewpoint of magnetic configuration effect in the Heliotron J helical device. Experiments scanning the bumpiness component in magnetic Fourier spectra show that (i) maximal kinetic energy in electrons is achieved when the medium bumpiness is chosen where the neoclassical (NC) diffusion is...
This paper first studies the coupling effects between the impurities and the background plasma under BOUT++ framework by extending the six-field two-fluid MHD model [1]. The extended model is applied to study the effect of pedestal Li impurity on ELMs during real-time Li powder injection. The extended BOUT++ MHD model is first validated by the good agreement between analytical results and...
Tungsten (W)-coated divertor tiles were installed in the wide area of the closed divertor of one inner-toroidal section out of the ten toroidal sections in the Large Helical Device (LHD). The visible and extreme ultraviolet (EUV) spectroscopic measurements show the different dependence of emission intensity of neutral W atoms and highly ionized W ions (W$^{24+}$~W$^{29+}$) on the line-averaged...
In addition to the existing hybrid electromagnetic (EM) scheme that uses gyrokinetic ions and fluid electrons (Ref. 1), two new EM schemes, which use kinetic electrons, have been implemented in the whole-volume particle-in-cell (PIC) code XGC (Ref. 2). In particular, these new EM schemes in XGC will allow higher-fidelity global simulation of long-wavelength modes, which are important for...
Edge localised mode (ELM) instabilities are a concern for future tokamak operations due to the predicted high heat loads and excessive erosion to the first wall/divertor [1]. A solution to address this focuses on divertor design; the ‘Super-X’ magnetic configuration is designed to alleviate high heat loads and will be tested on MAST-U [2]. Plasma detachment has been predicted in the MAST-U...
Scaling laws for the energy confinement time using engineering parameters as input are of great importance for fusion research. While they provide only limited insight in the actual physics determining the energy confinement, they are necessary tools for system studies and control schemes. It has become clear for both tokamaks and stellarators, however, that the scaling behavior is...
![Divertor density evolution from Stark broadening analysis of a horizontal line of sight at the target plate shows indications of high recycling ($n_{e,tar} \lg n_{e,up}$). The red curves are modeling predictions for W7-X and W7-AS adapted from [11].][f1]
New experimental evidence based on spectroscopy, see Fig. 1, and other edge diagnostics indicate the existence of a high-recycling...
The Wendelstein 7-X (W7-X) experiment commissioned the first of two neutral beam boxes [1] in the previous divertor campaign, providing 3.6 MW of heating power, achieving of densities above $2\times10^{20}$ $m^{-3}$, and providing the first initial assessment of fast ion confinement in the device. Demonstration of the confinement of fast ions is key to forwarding the stellarator concept as a...
Flux driven pedestal formation in tokamaks:
Turbulence simulations validated against the isotope effect
C. Bourdelle1, G. De Dominici1, G. Fuhr2, P. Beyer2, L. Chôné3, F. Cianfrani2, G. L. Falchetto1, X. Garbet1, Y. Sarazin1
1 CEA, IRFM, F-13108 St-Paul-Lez-Durance, France
2 CNRS, Aix-Marseille Univ., PIIM UMR7345, Marseille, France
3 Department of Applied Physics, Aalto University,...
W7-X is the first large fusion device where stationary reactor relevant densities have been achieved with electron heating only as it will be also the case for alpha particle heated fusion reactors. Employing the multi-pass ECRH scenario in the second harmonic O-mode (O2-ECRH), stationary (12 s) densities of up to 1.5 1020 m-3 had been achieved with hydrogen gas fueling. This scenario also...
To improve the performance of ECRH, a real-time control system for the deposition location of ECRH was newly developed to compensate the effect of refraction in standard oblique injection of ECRH on LHD. By the control absorption power higher than that without control was maintained under line-averaged electron density $n_{\rm e,avg}\simeq 3\times 10^{19}$ m$^{-3}$. In addition to that, a...
In non axis-symmetric, magnetic confinement fusion devices like the optimized stellarator Wendelstein 7-W (W7-X), recent theoretically predicted aspects on impurity transport, as the existence of a mixed collisionality regime $^1$ or the build up of a radial electric field $E_r$ $^2$, have been addressed in several initial experimental studies. Based on measurements of impurity transport times...
This work reports on advances in the validation of QH-mode modeling and the integration of momentum transport effects from ion-orbit loss (IOL), neutral interaction and neoclassical poloidal ion-flow damping with the NIMROD code [Ref 1]. Present understanding of steady-state tokamak edge-plasma solutions without edge-localized modes (ELMs) is interpretive. While MHD modeling is successful in...
Isotope effect on energy confinement time and thermal transport has been investigated for L-mode plasmas in LHD. Detailed and comprehensive assessment in stellarator-heliotron plasmas has progressed since the last IAEA-FEC [1]. Plasmas of hydrogen(H), deuterium(D) and their mixture including helium (He) have exhibited no significant dependence on the isotope mass $M$ in thermal energy...
The influence of plasma scenarios on the amplitude and radial structure of Zonal Flows has been investigated in the plasma edge of the TJ-II stellarator. The main results reported in this paper include: i) first experimental observation of the influence of the ion mass in the radial width of Long Range Correlations (LRC) as proxy of zonal flows; ii) the investigation of the influence of...
Strength of the internal transport barrier (ITB) is quantified in deuterium (D) and hydrogen (H) plasmas using a unique criterion based on the transport nonlinearity on the temperature. Two distinct isotope effects are found: (i) Stronger ITBs in D plasmas and (ii) edge confinement degradation accompanied by the ITB formation emerging in H plasmas. Principal component analysis reveals the...
Long-pulse high performance plasma operation of future fusion power plants requires a solution for tolerable plasma exhaust, including steady-state and transient heat- and particle-fluxes on plasma-facing components. Recently, applications of three-dimensional (3D) magnetic topologies for controlling the edge plasma transport, stability, and plasma-wall interactions (PWIs) have attracted much...
Different characteristics of turbulence driven transport were found in LHD and W7X from identical experiments of ECRH plasma. The kinetic pressure was higher in W7X in the central region ($\rho$<~0.4) indicating that anomalous transport is lower in W7X in this region. On the other hand, the kinetic pressure is higher in LHD in outer region ($\rho$>~0.4) indicating that anomalous transport is...
Stellarator of Costa Rica 1 (SCR-1) is the first (and so far, the only) stellarator device in Latin American region and has been operational since June 29, 2016. SCR-1 has a microwave heating system composed of two magnetrons of 2 kW and 3 kW and each generates microwaves at 2.45 GHz [1] with a mean magnetic field = 41.99 mT. This work presents the latest MHD calculations and simulations...
The divertor peak heat load during an ELM is one of the most critical problems of future fusion reactors, the heat flux width of plasma boundary is about 1-2mm, and the heat flux is about 100MW. Under the standard magnetic configuration of diverter, the deposition width of the heat flux on the target plate expands about 10 times, and the heat flux intensity is about 100MW/m$^2$. When ELM is...
Over the past few years, the need to develop reliable external actuators to control Alfvén Eigenmodes (AEs) in burning plasmas has triggered a substantial amount of experimental activity in several tokamaks and stellarators [1]. This contribution summarizes the experiments related to the AEs control topic that have been carried out in the TJ-II stellarator, paying special attention to the...
Plasmas flows have been extensively studied in magnetic confinement fusion and found to be critical to edge transport barriers and global MHD stability. In non-symmetric systems, the flow-defining radial electric field and net parallel rotation are expected to be set by neoclassical mechanisms and are related to the radial particle transport and parallel bootstrap current. In this respect, the...
The non-linear MHD simulations are performed to model the pellet triggered ELMs in JT-60SA. The target plasma equilibrium is reproduced from the calculation of ELMy H-mode by CRONOS simulation.
Energetic particles, including energetic ions and electrons produced by the fusion reaction and auxiliary heating, can excite various Alfv\'en instabilities in magnetic confinement fusion plasma such as ITER, which may induce significant transport of energetic particles and degrade the overall plasma confinement. Increased energetic particle transport enhanced by Alfv\'en eigenmodes has been...
A first attempt to observe a nuclear-elastic-scattering (NES) effect caused by energetic protons on deuteron slowing-down behavior was conducted on the Large Helical Device (LHD). We confirmed experimentally that the energetic ion slowing-down behavior in a fusion plasma is influenced by the NES. A neutron decay time of DD neutrons after turning off the deuterium beam (that was injected into a...
MAST Upgrade is scheduled to start operating in October 2020 followed by the start of the first experimental campaign in December. Experiments will emphasise addressing key physics issues for ITER and the design of future fusion reactors [b] by leveraging the unique capabilities of this new device. The main aims of the early experimental campaigns will be to study the benefits of alternative...
Core fuelling is a critical issue on the pathway to the development of steady-state operational scenarios in magnetically confined plasma devices. Although gas puffing has been the established tool for creating and maintaining plasmas in small and medium sized tokamaks and stellarators, its location at the plasma periphery portends that it will become inefficient for larger reactor-type...
The stellarator W7-X has conducted, so far, experiments under both limiter and divertor conditions$^{1-2}$. While in the former case, smooth flux surfaces at the plasma edge, being free of low-order resonances, are cut by five local limiters, in the latter case, a lower-order island chain is intersected with ten sophisticated divertor units. Here, an interesting question emerges as how and to...
Recently analysis of ion-neutral momentum exchange explained electric field formations not only for the tokamak boundary but also for the arc discharge and earth ionosphere [1]. The radial electric field (Er) formation is induced by the ion gyro-center shift of ion-neutral charge exchange reactions in which only perpendicular momentum transfer is counted. In parallel momentum exchange between...
Wendelstein 7-X (W7-X), the currently largest operating stellarator in the world, finished successfully its first operational phase in divertor configuration using the so-called test divertor unit (TDU) [Klinger]. Plasma-facing components (PFCs) made of fine grain graphite, designed to sustain without active cooling power loads of $10 MWm^{-2}$, were used to exhaust the plasma particle and...
Studies of the effects of hydrogen (H) isotopes are an important issue on achievement of high
performance in the next Deuteriun–Tritium (DT) and Tritium-Tritium (TT) high beta
operations at JET with ITER-like Wall. Experiments with mixed H-D plasmas have been
recently performed at JET to understand the dependence of the confinement on these isotopes
by varying the gas, beam [1] and pellet...
A notable improvement in the plasma performance has been observed after the injection of a series of frozen hydrogen pellets into ECH heated plasmas in the stellarator W7-X [1]. In these experiments, the pellet series rise the plasma density considerably and the central fuelling results in a peaked density profile. At the end of the pellet series, the ion and electron temperatures rise and...
It is likely that fusion reactors will benefit from, indeed perhaps require, ‘advanced’ divertor geometries to improve the power handling of the divertor and the control of detachment. One candidate is the Super-X divertor, in which the target major radius, $R_t$, is increased from the conventional value, which is similar to the X-point value $R_x$, to larger major radius where the total...
Effects of an external RMP (Resonant Magnetic Perturbation) on the slowing-down duration time ($\Delta t_{\rm slowing}$) of locked-mode-like instabilities with and without the island structure are investigated in LHD. For both instabilities, the $\Delta t_{\rm slowing}$ decreases with the increase of the external RMP and the RMP dependence is consistent with the braking force model due to the...
The plasma in the superconducting optimized stellarator Wendelstein 7-X [1] is mainly heated by an electron cyclotron resonance heating (ECRH) system, which allows up to 7.5 MW of injected power. The heating system can also be used to drive net toroidal current in the plasma (electron cyclotron current drive, ECCD) [2]. Toroidal current is not necessary for plasma confinement in stellarators,...
Divertor detachment was successfully sustained using higher-Z (krypton, Kr) and lower-Z (neon, Ne) superimposed seeding. Plasma radiation could be enhanced at the upstream region in the edge plasma with suppression of impurity accumulation toward the core plasma. Here, the pre-seeded Kr emission was drastically enhanced after the subsequent Ne seeding. Formation of negative radial electric...
Synthetic observations of modelled scrape-off layer (SOL) turbulence were compared to experimental beam emission spectroscopy (BES) measurements taken on ASDEX-Upgrade and EAST tokamaks and feasibility of similar studies are discussed on JET.
The behavior and dynamics of the plasma in the SOL contribute to the performance of a plasma discharge and mostly determines the plasma loads on the...
Pre-ionization method using non-resonant 2.45 GHz microwave heating ($P_\mathrm{2.45GHz}$ < 20 kW) enables us to achieve NBI plasma start-up in helical systems for the first time even in low NB power ($P_\mathrm{NB}$) and without resonance heating conditions. Three physical phenomena are essential to achieving the start-up: (1) high-energy electrons are formed by the non-resonant heating,...
The suppression of the energetic particle (EP) driven mode is essential for the realization of the fusion reactor. Modification of the local plasma parameters, based on the stability analysis using FAR3D, successfully suppresses the EP driven interchange mode observed in the Large Helical Device (LHD). The achieved temperature is thereby improved significantly. Control of the EP driven...
Both the exploration of previously unattained plasma conditions in stellarators and high-reliability requirements (for safe, long-pulse operation at higher plasma performance) pose new challenges for plasma control in Wendelstein 7-X (W7-X). With upgrades being installed at present, the development of predictive tools for safe long-pulse operation of the optimized stellarator W7-X becomes more...
Comprehensive toroidal modeling using 3D MHD codes MARS-F and MARS-Q offers useful physics insights and quantitative understanding into ITER ELM control by RMP. Type-I edge localized modes (ELMs) release large bursts of heat and particle flux to the plasma facing components in tokamaks, potentially causing significant material erosion in future devices such as ITER$^1$. ELM mitigation or...
Turbulence properties against the variation of isotope ratio and zonal flow activity are elucidated in Heliotron J. The turbulence amplitudes for density and potential fluctuations reduce as the hydrogen/deuterium(H/D) gas ratio is varied from H to D dominant plasmas and zonal flow activity is enhanced. Two-point correlation analysis reveals that the correlation of the fluctuations decreases...
The perpendicular transport in the Scrape-Off Layer (SOL) of toroidal fusion devices is widely assumed to be dominated by turbulent transport. In tokamaks, a particularly important role in this regard is played by blob-filaments exhibiting coherent radial propagation. These filaments have been shown to significantly contribute to the perpendicular SOL transport and impact fundamental SOL...
The pedestal plays an important role in determining the confinement in tokamak H-mode plasmas. However, the steep pressure gradients in this transport barrier also lead to edge localized modes (ELMs) [1]. There is good understanding of the pedestal in type I ELM regimes [2], however, type I ELMs are known to damage plasma facing components and so future large tokamaks must operate with small...
- INTRODUCTION
The super-X divertor (SXD) configuration features larger plasma wetted area than conventional SN divertor, due to large $R_{target}$. The wetted area is further increased with the longer connection length, which allows more time for cross-field transports. Other main features of SXD include baffling and large outer divertor volume. This means potentially higher maximum...
