Small Edge Localized Modes Research Articles

Investigation of pedestal parameters and divertor heat fluxes in small ELM regimes in DIII-D

Abstract Divertor heat flux and its correlation with pedestal parameters within various small edge localized mode (ELM) regimes, including high beta poloidal, type-II and ELMs with negative triangularity H-modes were investigated in DIII-D. The parallel energy fluences of type-II and high beta poloidal small ELM regimes fall below the linear scaling with pedestal electron pressure for type-I ELMs put forward in Eich et al 2017 (Nucl. Mater. Energy 12 84–90). The negative triangularity of H-mode ELMs follow the Eich scaling for type-I ELMs. The parallel heat flux and total heat loads to the divertor were determined using high-time resolution infrared thermography, while pedestal parameters were obtained through self-consistent kinetic equilibrium reconstructions. Linear regressions for the type-II and high beta poloidal regimes demonstrate that an equivalent 7.5 MA small ELM scenario in ITER would fall below the ~5 MJ m − 2 leading edge melting limit for tungsten (Gunn et al 2017 Nucl. Fusion 57 046025). Utilizing fast thermography, the scrape-off layer power fall-off length for both inter-ELM and intra-ELM was determined and compared to the Eich scaling with poloidal magnetic field in Eich et al (ASDEX Upgrade Team and JET EFDA Contributors 2013 Nucl. Fusion 53 093031). Except for the high beta poloidal scenario, all the small ELM regimes during both inter- and intra-ELM periods had power fall-off lengths ( λ q ) larger then would be expected from the B pol , MP − 1 scaling associated with type-I ELMs, signifying their potential in managing heat loads and offering a solution for core–edge integration.

Dependence of ELM instability on separatrix density in EAST long-pulse H-mode plasmas

Abstract The transition from small edge-localized modes (ELMs) to large ELMs has been repetitively observed in minute-scale long-pulse high-confinement mode (H-mode) discharges in 2017 EAST campaign. The appearance of large ELMs is found to be strongly correlated with the decrease in separatrix density due to the gradual decrease in fuel recycling during the long-pulse H-mode operations (LPHOs). By numerical scan of separatrix density with a fixed temperature profile, it has been found that the dependence of ELM instability on separatrix density is related to the competition between the ion diamagnetic stabilizing effect and destabilizing effect of pressure gradient and current density in the pedestal region, shedding light on a comprehensive understanding of different roles of separatrix density in ELM instability observed in EAST experiments. With a high separatrix density, the ideal ballooning mode could be destabilized near the separatrix, which is thought to help achieve small ELMs in EAST LPHOs. In 2021 EAST campaign, the experiment of large ELMs control has been performed through actively changing fuel recycling by moving strike point location (SPL) on the lower tungsten divertor target plate. It has been demonstrated that the mitigation of large ELMs is strongly correlated with the significant increase in separatrix density, which is thought to be attributed to a higher ionization source in the scrape-off layer (SOL) region by SOLPS-ITER simulation. The high ionization source in the SOL region is believed to provide a strong fueling effect near the separatrix and thus raise the local density, which is considered as an important reason for triggering ballooning instabilities near the separatrix and achieving small ELMs.

A long-pulse small edge-localized-mode high-confinement plasma with detachment feedback control by floating potential in an experimental advanced superconducting tokamak in a metal wall environment

Abstract One of the key challenges facing the magnetic fusion research is to demonstrate the compatibility between high confinement and radiative divertor in long-pulse discharges with a metal wall environment. A small edge localized mode (ELM) high confinement plasma is obtained in the upgraded lower divertor of Experimental Advanced Superconducting Tokamak (EAST) with an energy confinement factor H98~1.1 and a Greenwald density fraction fGW ~ 0.65 maintained for 26 s, and periodical detachment is achieved through active control of neon impurity seeding in this long-pulse discharge. For the divertor region, partial detachment is achieved periodically on the outer divertor target plates with the plasma temperature near the outer strike point decreased to below 5 eV and peak surface temperature on the outer divertor target plates maintained below 350C. The peak heat flux of the lower outer divertor decreases significantly and its profile along the target becomes very flat in the detached state. Two low-frequency (<10 kHz) fluctuations that are related to rippling mode caused by a resistive instability appear in the detached state. For the pedestal region, the electron pressure profile is flatter and ELM amplitude is smaller in the detached state than that in the attached state. Edge coherent mode (ECM) appears in the attached state and disappears in the detached state. To achieve this experimentally, a new impurity seeding feedback control scheme is applied, where the floating potential measured by divertor Langmuir probes is used as the feedback sensor, which is more reliable in the long-pulse discharges with high heat fluxes, thus more suitable for application in future devices. This work provides a new approach for the actively controlled radiative divertor as a solution to the divertor heat loads of the future fusion reactors.

Nonlinear simulations of the peeling-ballooning instability of super H-modes in the HL-3 tokamak

As the newly built tokamak in China, HL-3 will explore high-performance operation scenarios, such as super H-mode. The energy confinement and core parameters in the super H-mode can be much larger than that in the normal H-mode. Based on the pedestal simulation code EPED, the operation space of the super H-mode is obtained in HL-3. Magnetic shear decreases with increasing triangularity; consequently, a super H-mode can be achieved. The threshold of triangularity for accessing a super H-mode in HL-3 is around 0.4. By using BOUT++, a nonlinear simulation study of the pedestal instabilities in the super H-mode equilibrium is executed for the first time. As expected, the low n peeling mode, which can cause much of the energy loss (17%) from the pedestal region, is dominant in the super H-mode. Such a large collapse in the pedestal region would lead to a transition from super H-mode to H-mode. It is crucial to expand the parameter space of the super H-mode or mitigate the edge-localized mode (ELM) size for sustaining the super H-mode operations. The E × B velocity shear is found to play an important role in controlling the ELMs in HL-3. The small E × B velocity shear leads to a large growth rate but results in a small ELM size around the peeling boundary. The ELM size is closely related to both the growth rate of peeling-ballooning mode and the duration time of the linear phase. In contrast, a large E × B velocity shear can stabilize the instabilities near the ballooning boundary. Next, the parameter space of the super H-mode can be enlarged.

Exploring the physics of a high-performance H-mode scenario with small ELMs at low collisionality in JET with Be/W wall

A new H-mode regime at low density and low edge safety factor (q 95 = 3.2, with Ip = 3 MA) that combines high energy confinement, stationary conditions for density and radiation and small Edge Localized Modes (ELMs) have been found in JET with Be/W wall. Such a regime is achieved by operating without external gas puffing, leading to a decrease in the edge density and a substantial increase in rotation and ion temperature in both the pedestal and the core region. Transport modelling shows a reduction of the turbulence, which starts in the pedestal region and extends into the plasma core, and outward impurity convection, consistent with the improved energy confinement and the lack of W accumulation observed in those conditions. In addition, large type I ELMs, typically found in gas-fuelled plasmas, are replaced by smaller and more frequent ELMs, whose appearance is correlated with a substantial reduction of the pedestal density and its gradient. Pedestals in this operating regime are stable to peeling–ballooning modes, consistent with the lack of large ELMs. This is in contrast to results in unfuelled JET-C plasmas that typically operated at higher pedestal densities and developed low frequency, large type I ELMs, thus pointing to the low density as one of the critical parameters for accessing this small ELMs regime in JET. This small ELMs regime exhibits the same low pedestal collisionality ( νe,ped∗∼0.1 ) expected in ITER and operates at low q 95, thus making it different from other small ELMs regimes that are typically obtained at higher q 95 and higher pedestal collisionality. These features make this newly developed H-mode regime in JET with Be/W wall a valuable tool for exploring the underlying transport, the different mechanisms of turbulence stabilization, as well as the physics associated with the appearance of small ELMs in high-temperature plasmas at ITER relevant pedestal collisionality.

Facilitated core-edge integration through divertor nitrogen seeding in the HL-2A tokamak

A divertor detachment with sustainable high-performance plasmas has been achieved through divertor nitrogen seeding in the HL-2A tokamak. The closed divertor structure facilitates achieving high divertor neutral pressure due to its efficient particle containment effect as demonstrated by both experimental and SOLPS simulation results, which aids in achieving the divertor detachment. The radiative divertor is conducive to high-frequency small edge localized modes (ELMs) operation, characterized by a reduced pedestal ion temperature gradient ∇Ti and the enhanced electron density gradient ∇ne . The presence of minority nitrogen at the edge plasma, which results from the divertor nitrogen seeding, contributes to the increase of ion temperature Ti in the confinement region by suppressing the turbulence through the impurity dilution effect and E×B shear. The increased ion temperature Ti and electron density ne compensate the energy loss resulting from the increased edge radiation during detachment, which contributes to the confinement sustainment combined with reduced pedestal energy loss caused by small ELMs. This work advances our understanding of the fundamental physics governing closed divertor detachment with sustainable high-performance plasmas through divertor nitrogen seeding.

Prediction of pellet mass thresholds for ELM triggering in low-collisionality, ITER-like discharges

In ITER, pellets are calculated to require more than 8 times the mass than currently planned to reliably trigger edge-localized modes (ELMs). Unmitigated heat flux impulses from ELMs are intolerable in ITER at full power and current. Therefore, ITER operation relies on multiple approaches to control ELM heat fluxes. One method is pellet ELM pacing to instigate small rapid ELMs with low heat flux. Predicting the performance of pellet pacing is critical for ITER, which is expected to operate in a regime with a low-collisionality, peeling-limited pedestal. However, to trigger ELMs the local pressure increase in the expanding pellet cloud pushes the equilibrium over the ballooning stability limit. In this work, linear and nonlinear M3D-C1 simulations are used to predict pellet mass thresholds in DIII-D discharges and ITER scenarios with peeling-limited pedestals. It is found that the distance of the equilibrium’s operational point from the ballooning branch of the pedestal stability boundary strongly changes thresholds. Linear M3D-C1 simulations find a strong dependence of the pellet mass threshold on the poloidal injection location for ITER’s 15 MA, Q = 10 scenario. The required pellet mass at the planned injection locations is 8 to 17 times larger than currently considered. However, such linear simulations do not include pellet ablation physics or time evolution of density and temperature. A new scheme of 2D nonlinear simulations, coupled with linear stability analysis at various steps throughout the nonlinear time evolution, was developed to include such physics and improve on the linear results. These new nonlinear-to-linear simulations confirm previous findings. This result suggests that pellet ELM triggering in ITER could require pellets much larger than those currently planned, which makes ELM-pacing operationally challenging. On the other hand, fueling pellets injected from the high-field side will likely not unintentionally trigger ELMs in an otherwise ELM-stable plasma.

Effect of hyper-resistivity on ballooning modes with resonant magnetic perturbations

The impact of hyper-resistivity on non-ideal ballooning modes (BMs) is studied in the presence of resonant magnetic perturbation (RMP) through considering the hyper-resistivity, resistivity and diamagnetic effect in the BM model with an equilibrium distorted by RMP, which is stable for ideal BMs. Similar to the resistivity, the hyper-resistivity is also destabilizing for the BMs, but RMPs make the mode spectrum of the BMs destabilized by the hyper-resistivity move towards the low toroidal mode number side on the flux surface with a safety factor slightly larger than the RMP resonance safety factor, where the growth rates of the BMs destabilized by the resistivity decrease due to RMP. When both the hyper-resistivity and the resistivity are considered, there is a sort of competitive relationship between them in determining the properties of BMs. If either of the hyper-resistivity term and the resistivity term is much larger than the other one, the instability of BMs is mainly determined by the larger one, and the effect of the smaller one is masked. The destabilizing mechanisms of the hyper-resistivity and the resistivity on BMs are similar, namely, the diffusion and dissipation of current and magnetic field weaken the stabilizing effect of magnetic field line bending. The research results may be important for understanding the enhancement of plasma transport and the mechanism of small edge localized mode (ELM) during ELM control with RMP.

Turbulence spreading effects on the ELM size and SOL width

BOUT++ turbulence simulations were performed to investigate the impact of turbulence spreading on the edge localized mode (ELM) size and divertor heat flux width $({\lambda _q})$ broadening in small ELM regimes. This study is motivated by EAST experiments. BOUT++ linear simulations of a pedestal radial electric field (Er) scan show that the dominant toroidal number mode (n) shifts from high-n to low-n, with a narrow mode spectrum, and the maximum linear growth rate increases as the pedestal Er well deepens. The nonlinear simulations show that as the net E × B pedestal flow increases, the pressure fluctuation level and its inward penetration beyond the top of the pedestal both increase. This leads to a transition from small ELMs to large ELMs. Both inward and outward turbulence spreading are sensitive to the scrape-off-layer (SOL) plasma profiles. The inward turbulence spreading increases for the steep SOL profiles, leading to increasing pedestal energy loss in the small ELM regime. The SOL width $({\lambda _q})$ is significantly broadened progressing from the ELM-free to small ELM regime, due to the onset of strong radial turbulent transport. The extent of the SOL width $({\lambda _q})$ broadening depends strongly on outward turbulence spreading. The fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the separatrix can be enhanced by increasing either pedestal Er flow shear or local SOL pressure gradient. The ${\lambda _q}$ is broadened as the fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the last close flux surface (LCFS) increases. Local SOL E × B flow shear will restrain outward turbulence spreading and the associated heat flux width broadening. Operating in H-mode with small ELMs has the potential to solve two critical problems: reducing the ELM size and broadening the SOL width.

Numerical simulation of ultrashort-pulse reflectometry (USPR) on EAST

The microwave reflectometer is a popular non-intrusive plasma density diagnostic instrument on tokamaks that provides centimeter and millisecond level resolution. The ultrashort-pulse reflectometer (USPR) achieves plasma density measurement by emitting a chirped wave containing a broadband signal and measuring the time of flight from different frequency components. A USPR system is currently being built on EAST (Experimental Advanced Superconducting Tokamak) to meet the needs of diagnostic of the pedestal density evolution, such as high-frequency small edge-localized modes. In order to predict the density reconstruction of the EAST USPR system, this work presents a numerical simulation study of the beam propagation of the chirped wave of extraordinary waves (X-mode) in the plasma based on Python. The electron density profile has been successfully reconstructed by the reflection signal interpretation. The small gap between the right-hand cut-off layer and the electron cyclotron resonance layer, due to the low plasma density on the plasma edge, causes unexpected leakage from the transmitting microwave beam to the pedestal and the core region. This kind of ‘tunneling’ effect will cause the reflected signal to have energy loss in the low-frequency band. The study also discusses the influence of the poloidal magnetic field on the reflected signal. The spatial variation of the poloidal magnetic field will lead to the conversion between extraordinary (X) waves and ordinary (O) waves, which leads to energy loss in the reflected signals. The simulation results show that the ‘tunneling’ effect and the O-X mode conversion effect have little effect on the EAST USPR system. Therefore, the currently designed transmit power meets the working requirements.

Study on ELM burn-through induced by small ELMs with divertor detachment in EAST

In the H-mode radiative divertor experiments, the transient heat flux induced by edge localized modes (ELMs) could cause the detached divertor plasma to re-attach in a few milliseconds, which is so-called ELM burn-through. The periodic hot plasma burning through the neutral front could bring significant damages to the divertor target and thus has attracted increasing attention. A systematic study on the burn-through of small ELMs during radiative divertor experiments has been conducted in the EAST tokamak. Firstly, the affected area of small ELMs on the divertor target has been evaluated, and the results show that the radial extension of small ELMs is restricted to a range of less than the particle flux fall-off width (λjs) along the divertor target. Secondly, it has been found that the occurrence of small ELM burn-through is highly correlated with the degree of divertor detachment and ELM frequency. When the detached area is larger than λjs, the small ELMs in the main plasma could be mitigated or even suppressed, avoiding the burn-through of small ELMs. When the divertor detachment only occurs near the strike point while the ELM frequency is high enough (fELM ≳ 800 Hz), there is also no burn-through of small ELMs. When the degree of divertor detachment is low and ELM frequency is not high simultaneously, the ELM burn-through induced by small ELMs could be observed and is correlated with the competition between ionization and recombination process of the divertor plasma. Further analysis reveals that the burn-through is triggered by the accumulated particles and energy of multiple small ELMs, not by that of a single small ELM under this condition. Finally, statistical analysis indicates that the small ELM burn-through occurs when the detached area is limited to a region of less than 0.4λjs and simultaneously ELM frequency is smaller than 800 Hz.

Investigation of the compatibility of pellet fueling with ELM-free H-mode plasmas in EAST tokamak

Experiments on pellet fueling have been carried out in edge localized mode (ELM)-free high-confinement mode (H-mode) plasmas with q 95 ∼ 6 in the EAST tokamak. Cryogenic deuterium pellets were injected into the ELM-free plasmas at a frequency of 10 or 5 Hz from ∼45 cm above the mid-plane on the low-field side. It is found that the ELM-free H-mode plasmas are still sustained even if both the edge and core plasma are impacted by the pellet injections (PIs). Several small ELMs would appear and the edge coherent mode accompanying the ELM-free phase fades or even disappears just after the pellet events, but the plasma would rapidly recover to the ELM-free state. Although the ELMing phase is very short, it may be an issue that still needs to be resolved in the future. Meanwhile, the 3/2 tearing mode often appearing in ELM-free discharges would be stabilized by the PIs, and the high-Z impurity concentration would be reduced during the PIs. All these results will be meaningful to International Thermonuclear Experimental Reactor and future fusion reactors.

Effect of pedestal density on the formation of small edge localized modes

Recent experiments have demonstrated that a high separatrix density and a large ratio of separatrix density to pedestal top density are two crucial conditions for achieving high confinement operation with small edge localized modes (ELMs). In order to identify the underlying physics of this phenomenon, a series of equilibria with different separatrix and pedestal top densities are constructed, and their peeling–ballooning (P–B) instabilities are analyzed through simulation. It is found that there is a threshold value of pedestal top density which comes from competition between ion inertia and diamagnetic effect, and ELM energy loss can be minimized at the threshold value for a fixed separatrix density. When the pedestal top density is smaller than the threshold value, the ion inertial effect induced by the density profile has a significant influence on the growth of ELMs, resulting in an increased linear growth rate and more ELM energy loss by trigging low-n modes (n being the toroidal mode number) in the nonlinear phase. When the pedestal top density is larger than the threshold value, the diamagnetic effect is the main factor determining the mode spectrum, which moves to the high-n region with a larger growth rate and the nonlinear ELM energy loss increases. However, for a fixed pedestal top density, a larger separatrix density leads to a wider mode spectrum with a smaller growth rate; thus ELM energy loss is reduced. The results of this research provide a new mechanism, namely that the P–B mode is possibly transferred to a resistive ballooning mode, to interpret the experimental findings during high pedestal density operation.

Quasicontinuous Exhaust Scenario for a Fusion Reactor: The Renaissance of Small Edge Localized Modes.

Tokamak operational regimes with small edge localized modes (ELMs) could be a solution to the problem of large transient heat loads in fusion reactors. A ballooning mode near the last closed flux surface governed by the pressure gradient and the magnetic shear there has been proposed for small ELMs. In this Letter, we experimentally investigate several stabilizing effects near the last closed flux surface and present linear ideal simulations that indeed develop ballooninglike fluctuations there and connect them with nonlinear resistive simulations. The dimensionless parameters of the small ELM regime in the region of interest are very similar to those in a reactor, making this regime the ideal exhaust scenario for a future device.

Quantifying heat and particle flux to primary and secondary divertors for various types of edge-localized-modes

Between 15% and 30% of edge-localized-mode (ELM) heat flux can be deposited to regions outside the main divertor including the far scrape-off-layer (SOL), private flux region (PFR), and secondary divertor inner target. Analysis shown here demonstrates that type-I and small ELM plasmas are transported to the PFR and to the secondary inner divertor, which is magnetically isolated from the outer divertor, leading to well-defined heat flux profiles and with peak values that can surpass those at the secondary outer target. Such features are consistently observed for pedestal collisionalities from νe* ∼ 1.5 to νe* ∼ 3.9. Heat flux profiles of the examined ELM types feature rippled structures due to bursts in the outer far-SOL region but not on the secondary inner target, causing long decay lengths in the time-averaged ELM profiles. The contribution of each ELM type to the total time-averaged power deposited to the secondary divertor has been evaluated, showing that grassy ELMs contribute ∼8%, small ELMs ∼67%, and type-I ELMs ∼85%. These findings imply that small ELMs may yet pose a concern for future machines if some regions of the main wall are not designed to withstand significant heat and particle fluxes. Due to the low intra-ELM heat flux contribution, however, the grassy ELM regime is an attractive option for an ELMing scenario in future machines.

The simulation of ELM suppression by ion cyclotron resonance heating in EAST using BOUT++

Edge localized modes (ELMs) are completely suppressed by ion cyclotron resonant heating (ICRH) during H-mode discharge, which is first observed in the Experimental Advanced Superconducting Tokamak (EAST). The electromagnetic turbulence model in the BOUT++ framework is applied to the simulations, with the aim of obtaining the key physics mechanisms of ELM suppression by ICRH. The simulations, for the EAST experiment with no ELMs after ICRH, reveal that the radio frequency (RF) sheath plays a key role in the suppression of ELMs. The simulation results show that: (a) using the flow-balanced electric field, the ELM sizes are about 3.4% before ICRH, and 2.1% after ICRH. Although the smaller ELM is obtained with ICRH, it still belongs to relatively large ELMs; (b) instead of the flow-balance condition, the radial electric field E r is calculated using the RF sheath model, and the ELM size is about 0.36% with ICRH, much closer to the small/no ELM regime. After considering the effect of ICRH, the larger E × B shear rate in the pedestal and scrape-off layer induced by the RF sheath reduces the linear growth rate, and enhances the nonlinear mode coupling. Furthermore, we find that there is an effective range of the sheath potential, in which the ELM can be well suppressed, but beyond this range the ELM may be only mitigated rather than suppressed. This can reasonably explain why the probability of ELM suppression by ICRH appearing in the experiments is low.

New H-mode regimes with small ELMs and high thermal confinement in the Joint European Torus

New H-mode regimes with high confinement, low core impurity accumulation, and small edge-localized mode perturbations have been obtained in magnetically confined plasmas at the Joint European Torus tokamak. Such regimes are achieved by means of optimized particle fueling conditions at high input power, current, and magnetic field, which lead to a self-organized state with a strong increase in rotation and ion temperature and a decrease in the edge density. An interplay between core and edge plasma regions leads to reduced turbulence levels and outward impurity convection. These results pave the way to an attractive alternative to the standard plasmas considered for fusion energy generation in a tokamak with a metallic wall environment such as the ones expected in ITER.

High resolution density pedestal measurements during edge localized modes by short-pulse reflectometry in the TCV tokamak

This publication presents high spatio-temporal resolution (mm µs−1) density profile measurements of the pedestal top during type I, III, and small edge localized mode (ELM) H-mode plasmas in the tokamak à configuration variable (TCV). These measurements were performed using a novel short-pulse reflectometer. Average inter-ELM density profiles are obtained via conditional averaging using the D trace as ELM indicator. Changes to the pedestal density profile gradients prior to type-III ELMs reveal unique pedestal dynamics leading to the ELM crash which can provide important experimental data for validation of non-linear MHD ELM simulations. The small-ELM scenario is found to feature a ∼25–35 kHz quasi-coherent density fluctuation near the separatrix 0.993–1.05 not observed during a similar type-I ELM discharge. This oscillation is also found in low-field-side magnetic pick-up probes displaying a ballooning character and n = +1 toroidal mode number. This oscillation could help explain the markedly different pedestal dynamics observed in the small-ELM regime.

Type-I ELM mitigation by continuous lithium granule gravitational injection into the upper tungsten divertor in EAST

Large edge-localized modes (ELMs) were mitigated by gravitational injection of lithium granules into the upper X-point region of the experimental advanced superconducting tokamak (EAST) device with tungsten plasma-facing components. The maximum ELM size was reduced by ∼70% in high β N H-mode plasmas. Large ELM stabilization was sustained for up to about 40 energy confinement times, with constant core radiated power and no evidence of high-Z or low-Z impurity accumulation. The lithium granules injection reduced the edge plasma pedestal density and temperature and their gradients, due to increased edge radiation and reduced recycling from the plasma-facing components. Ideal stability calculations using the ELITE code indicate that the stabilization of large ELMs correlates with improved stability of intermediate-n peeling-ballooning modes, due to reduced edge current resulting from the profile changes. The pedestal pressure reduction was partially offset by a core density increase, which resulted in a modest ∼7% drop in core stored energy and normalized energy confinement time. We surmise that the remnant small ELMs are triggered by the penetration of multiple Li granules just past the separatrix, similar to small ELMs triggered by deuterium pellet Futatani et al (2014 Nucl. Fusion 54 073008). This study extends previous ELM elimination with Li powder injection Maingi et al (2018 Nucl. Fusion 58 024003) in EAST because (1) use of small, dust-like powder and the related potential health hazards were eliminated, and (2) use of macroscopic granules should be more applicable to future devices, due to deeper penetration than dust particles, e.g. inside the separatrix with velocities ∼10 m s−1 in EAST.

Observation of fully detached divertor integrated with improved core confinement for tokamak fusion plasmas

Integration of divertor detachment with a high-performance (βN ∼ 3, βp > 2, H98 ∼ 1.5) core plasma has been demonstrated in DIII-D high-βp (poloidal beta) plasmas associated with a sustained core internal transport barrier (ITB) and an H-mode edge transport barrier (ETB). Such good core-edge integration has been achieved for both neon and nitrogen seeding, for both favorable and unfavorable B-field directions, independently from the impurity puffing locations, though these variations play important roles on divertor characteristics. Compared to the standard H-mode plasmas, the high-βp plasma exhibits a much wider window of detachment compatible with high confinement core. Fully detached divertor plasmas with low plasma temperature (Te < 5 eV), low particle flux, and low heat flux across the entire divertor target plate were obtained by using nitrogen seeding. This detached high-βp plasma is compatible with a newly developed detachment control system which can help optimize the nitrogen gas flow rate. Several features, i.e., the high edge safety factor in the high-βp scenario, impurity injection, closed divertor and reduced heating power requirement due to the high confinement, facilitate the achievement of full divertor detachment at lower density. Instead of degrading global performance, the divertor detachment facilitates the access to an even stronger ITB at large radius with a relatively weak ETB through self-organized synergy between ITB and ETB, leading to sustained high confinement. The strengthening of the large-radius ITB compensates for the ETB degradation associated with divertor detachment. In addition, a weak ETB naturally has smaller edge localized modes (ELMs). In particular, with neon injection, a long-period no-ELM H-mode phase has been achieved simultaneously with high-performance core and partially detached divertor plasmas. These results demonstrate the possibility of integrating excellent core plasma performance with an effective divertor solution, an essential step toward steady-state operation of reactor-grade plasmas.