Pedestal Temperature Research Articles

Effect of resistivity on the pedestal MHD stability in JET

The ELM triggering mechanism in tokamaks is not yet fully understood. For example, in the JET tokamak with ITER-like wall (commonly called JET-ILW), the ELMs are sometimes triggered before the ideal peeling-ballooning (PB) boundary is reached. This typically occurs for shots with high input power and high gas rate. The discrepancy between model and experiment has in previous works been clearly correlated with the relative shift between the electron temperature and density pedestals. The discrepancy has also been correlated with the resistivity in the middle-bottom of the pedestal. The present work shows that resistive MHD can have a significant impact on the PB stability of JET pedestals. The inclusion of resistivity removes the correlation between the discrepancy from the PB stability and the relative shift (the difference between the position of the electron temperature and density pedestals) and significantly improves the agreement between PB model and experimental results. The work also shows that the key parameter is the resistivity at the pedestal bottom, near the separatrix, while the resistivity near the middle/top of the pedestal has a negligible effect on the PB stability of JET plasmas.

Enabling adaptive pedestals in predictive transport simulations using neural networks

We present PEdestal Neural Network (PENN) as a machine learning model for tokamak pedestal predictions. Here, the model is trained using the EUROfusion JET pedestal database to predict the electron pedestal temperature and density from a set of global engineering and plasma parameters. Results show that PENN makes accurate predictions on the test set of the database, with R 2 = 0.93 for the temperature, and R 2 = 0.91 for the density. To demonstrate the applicability of the model, PENN is employed in the European transport simulator (ETS) to provide boundary conditions for the core of the plasma. In a case example in the ETS with varied neutral beam injection (NBI) power, results show that the model is consistent with previous studies regarding NBI power dependency on the pedestal. Additionally, we show how an uncertainty estimation method can be used to interpret the reliability of the predictions. Future work includes further analysis of how pedestal models, such as PENN, or other advanced deep learning models, can be more efficiently implemented in integrating modeling frameworks, and also how similar models may be generalized with respect to other tokamaks and future device scenarios.

Pedestal analysis of MAST ELMy regimes

Mega Ampere Spherical Tokamak (MAST) pedestal data has been analysed, where a pedestal database of 892 shots was obtained, using the upgraded MAST Thomson scattering (TS) diagnostic. Various edge localised mode (ELM) types are discussed, where characteristics and trends of MAST pedestals are shown. The data from the upgraded TS diagnostic confirms pedestal characteristics found in earlier analysis, using previous TS systems. Using the database, calculations of the bootstrap current are obtained using the different analytical formulae (Sauter and Redl), and comparisons are performed. The upgraded MAST TS system now spans the full plasma mid-plane, such that direct comparisons between inboard and outboard pedestals can be obtained, and asymmetries in the density pedestal width were found. To increase confidence in spherical tokamak pedestal predictions, the assumptions of Europed have been validated from the MAST pedestal data, and a value for the kinetic ballooning mode constraint () has been obtained. The first spherical tokamak pedestal predictions were performed in Europed and compared to experimental values. Using C = 0.145 the temperature pedestal height was predicted to within 10% of the experimental value. In addition type II ELMs on MAST are analysed, and stability analysis and parameter scans have been performed using ESSIVE. Similar magneto-hydrodynamic (MHD) stability properties are observed for type I and type II ELMs, originating from the mixed ELM regimes; it is therefore difficult to distinguish these ELMs using the ideal MHD codes.

Optimization of 3D controlled ELM-free state with recovered global confinement for KSTAR with n = 1 resonant magnetic field perturbation

Mitigation of deleterious heat flux from edge-localized modes (ELMs) on fusion reactors is often attempted with 3D perturbations of the confining magnetic fields. However, the established technique of resonant magnetic perturbations (RMPs) also degrades plasma performance, complicating implementation on future fusion reactors. In this paper, we introduce an adaptive real-time control scheme on the KSTAR tokamak as a viable approach to achieve an ELM-free state and simultaneously recover high-confinement (β N ∼ 1.91, β p ∼ 1.53, and H 98 ∼ 0.9), demonstrating successful handling of a volatile complex system through adaptive measures. We show that, by exploiting a salient hysteresis process to adaptively minimize the RMP strength, stable ELM suppression can be achieved while actively encouraging confinement recovery. This is made possible by a self-organized transport response in the plasma edge which reinforces the confinement improvement through a widening of the ion temperature pedestal and promotes control stability, in contrast to the deteriorating effect on performance observed in standard RMP experiments. These results establish the real-time approach as an up-and-coming solution toward an optimized ELM-free state, which is an important step for the operation of ITER and reactor-grade tokamak plasmas.

Ion thermal transport in the H-mode edge transport barrier on DIII-D

The power balance ion heat flux in the pedestal region on DIII-D increases and becomes increasingly anomalous (above conventional neoclassical) in experiments with higher temperature and lower density pedestals where the ion collisionality (νi*) is lowered toward values expected on ITER. Direct measurements of the main-ion temperature are shown to be essential on DIII-D when calculating the ion heat flux due to differences between the temperature of D+ and the more commonly measured C6+ impurity ions approaching the separatrix. Neoclassical transport calculations from NEO and non-linear gyrokinetic calculations using CGYRO are consistent with these observations and show that while neoclassical transport plays an important role, the turbulent ion heat flux due to ion scale electrostatic turbulence is significant and can contribute similar or larger ion heat fluxes at lower collisionality. Beam emission spectroscopy and Doppler backscattering measurements in the steep gradient region of the H-mode pedestal reveal increased broadband, long-wavelength ion scale fluctuations for the low νi* discharges at the radius where the non-linear CGYRO simulations were run. Taken together, increased fluctuations, power balance calculations, and gyrokinetic simulations show that the above neoclassical ion heat fluxes, including the increases at lower νi*, are likely due to weakly suppressed ion scale electrostatic turbulence. These new results are based on world first inferred ion and electron heat fluxes in the pedestal region of deuterium plasmas using direct measurements of the deuterium temperature for power balance across ion collisionalities covering an order of magnitude from high νi* values of 1.3 down to ITER relevant νi* ∼0.1.

Role of the separatrix density in the pedestal performance in deuterium low triangularity JET-ILW plasmas and comparison with JET-C

A reduction of the pedestal pressure with increasing separatrix density over pedestal density (n e sep/n e ped) has been observed in JET. The physics behind this correlation is investigated. The correlation is due to two distinct mechanisms. The increase of n e sep/n e ped till ≈0.4 shifts the pedestal pressure radially outwards, decreasing the peeling-balloning stability and reducing the pressure height. The effect of the position saturates above n e sep/n e ped ≈ 0.4. For higher values, the reduction of the pedestal pressure is ascribed to increased turbulent transport and, likely, to resistive MHD effects. The increase of n e sep/n e ped above ≈0.4 reduces ∇n e /n e, increasing η e and the pedestal turbulent transport. This reduces the pressure gradient and the pedestal temperature, producing an increase in the pedestal resistivity. The work suggests that the increase in resistivity might destabilize resistive balloning modes, further reducing the pedestal stability.

New understanding of inter-ELM pedestal turbulence, transport, and gradient behavior in the DIII-D tokamak

New observations of pedestal localized turbulence in the inter-ELM period of H-mode plasmas in DIII-D show that ion temperature gradient mode scale (ITG-scale) density fluctuation (ñ) increases immediately after each ELM crash and is quickly suppressed during the increase in local E × B shear. This excitation and subsequent suppression of ITG-scale ñ can explain the previously reported anomalous ion heat flux, Q i during the ELM (Viezzer et al 2017 Nucl. Fusion 57 022020). Shorter wavelength trapped electron mode scale (TEM-scale) ñ starts to increase at a critical pedestal temperature gradient (∇T e,ped ) and saturates as local E × B shear, ratio, and ∇T e,ped saturate. This TEM-scale ñ, which has the potential to cause electron (and also ion) heat transport, is consistent with driving an anomalous electron heat flux Q e, where Q e is estimated between ELMs using experimental profiles and power balance. Both ITG- and TEM-scale ñ amplitude variations with background T i/T e and ∇n e,ped are found to be consistent with theoretical predictions of these measured density fluctuations being ITG and TEM instabilities respectively. These new and unique observations on the nature of turbulence and their potential contributions to electron and ion heat fluxes at different ELM periods (i.e. collapse, recovery, and saturation phases of pedestal gradients) can significantly test and improve our pedestal predictive capabilities for ITER and other future fusion devices.

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

In a series of discharges on the DIII-D tokamak, fast vertical plasma jogs are used to induce current perturbations in the steep gradient region of the H-mode edge. These current perturbations directly influence the edge q profile, decoupling the resonant location and instability drive of pedestal-localized microtearing modes (MTMs). By exploiting this effect, we develop and apply a new experimental technique to track the dynamical frequency evolution of MTMs in the pedestal region, providing a compelling validation of the MTM model. The frequency of potential MTMs is calculated as the Doppler-shifted electron diamagnetic frequency at rational q = m/n surfaces, showing remarkable agreement with chirped frequency behavior of n = 3, 4 and 5 modes detected with fast magnetics. Data is collected throughout multiple ELM cycles in order to build robust statistics describing the time-dependent frequency evolution of MTMs, which can be explained by examining the recovery of pedestal gradients after an ELM event. MTMs have a dominant transport contribution in the electron thermal channel, so the presented results indicate that reduced models of pedestal transport must be electromagnetic in nature and constructed with accurate calculations of MTM stability; inclusion of this physics is essential for accurate predictions of the electron temperature pedestal profile. Supporting measurements of mode saturation, propagation direction and transport fingerprints are made to support the dynamic frequency determination, unambiguously and experimentally identifying MTMs in the pedestal region of DIII-D.

Self-consistent core-pedestal ITER scenario modeling

The purpose of these integrated ITER simulations is to identify dependencies that can impact the performance of ITER. The dependence of fusion power production, temperature and density pedestals, and core profiles on varying magnetic-q, edge density fueling strength, neoclassical transport, magnetic field strength, and alpha heating, are examined. It is noted that ion modes usually dominate the interior, while electron modes dominate the barrier. The electron mode without flow shear is substantially stronger. The q increase tends to cause more kinetic ballooning modes at the edge and the q decrease means that the peeling is getting worse, but the ballooning is getting better. The large edge q tends to provide hollow density while small edge q gives normal density profile. This is consistent with the observation that the density of particles is inversely proportional to the density of the plasma current. The rise in the source of the edge particle increases the density of the edge and the reaction rate close to the edge increases temperature fluxes, resulting in weak pedestal barriers. When neoclassical transport is turned off or the strength of B-field is increased, the pronounced edge barriers are identified. In addition, a distinct effect on the magnetohydrodynamic (MHD) modes is observed due to the increased B-field. The interesting aspect is that the barrier collapses about q = 2 as the stability of MHD has decreased to replace the barrier with a continuous slope but with roughly the same fusion power. This would actually be good for a reactor. The slope of the H-mode pedestal is found to be reduced due to the alpha heating. It is expected that the problem of edge-localized modes causing damage to the first wall will be eased. The effects of circular, elongated, and general geometry on plasma profiles are also compared and contrasted. The circular geometry has a hollow density profile and no edge or internal thermal transport barrier (ETB or ITB). However, both ETBs and ITBs can be found in the elongated geometry. In general geometry, there is a weaker ETB, comparable ITBs, but a higher edge particle transport barrier than in elongated geometry. Higher density near the edge, on the other hand, causes more wall erosion, so elongated geometry might be the best option.

Ballooning instability preventing the H-mode access in plasmas with negative triangularity shape on the DIII–D tokamak

Infinite toroidal mode number (n = ∞) ballooning mode analysis of negative triangularity discharges on DIII–D shows that the access to 2nd stability becomes strongly restricted when the top triangularity decreases even modestly from −0.18 to −0.36. This is observed in experiment to coincide with the suppression of the L–H-transition. Further theoretical analysis with ballooning mode limited pedestals shows that the threshold for opening the 2nd stability access rises from a pedestal temperature of 0.3–1 keV when the top triangularity is decreased from −0.18 to −0.36 indicating that to access the 2nd stability with the more negative triangular shape would require unrealistically high pedestal temperature. The pedestal predicted by the EPED code agrees with the experimental H-mode profile for the negative triangularity case but in contrast to positive triangular shapes the prediction shows no increase in pedestal height with increasing core pressure when triangularity is negative. This work provides a first model to predict when negative triangularity plasmas can be expected to access the H-mode.

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.

On the stability and stationarity of the Super H-mode combined with an ion transport barrier in the core

The Super H-mode, a regime with high pedestal pressure and stored energy, is explored on DIII-D and combined with an ion transport barrier in the plasma core to increase performance. A significant improvement of ion temperatures and confinement is facilitated by favorable conditions such as high rotational shear and high ion pedestal temperatures. As a result of a rise in density and simultaneous decrease in rotation, the ion transport barrier disappears during the discharge evolution, leading to a transition from a very high confinement state at early times, to a reduced but still high confinement phase. Additionally, in many discharges, a global magnetohydrodynamic (MHD) event consistent with the coupling of a destabilized internal mode to an edge localized mode causes a large energy loss and leads to a reorganization of the plasma into a lower temperature, higher density state. Depending on the magnitude of the global MHD event, the plasma edge collisionality can increase significantly and shift the operational boundary from the peeling to the ballooning side, which can be understood as a drop out of the Super H-mode channel into standard H-mode. Hence, in Super H-mode discharges with ion transport barriers, both the improved pedestal height and rotational shear contribute to the high stored energy. At very low levels of rotation, the confinement factor for SH modes is still expected to exceed standard H-mode by 20%–30%. With their overall stationarity and high-performance levels, Super H-mode discharges provide an attractive regime for ITER and may enable a more compact design of future fusion power plants.

Change in the pedestal stability between JET-C and JET-ILW low triangularity peeling-ballooning limited plasmas

This work discusses the possible mechanisms that have led to the degradation of the pedestal pressure height and pedestal stability of low triangularity peeling-ballooning (PB) limited pla-smas from JET with the carbon wall (JET-C) to JET with the ITER-like wall (JET-ILW). The work shows that the lower pedestal stability of JET-ILW is not directly caused by the presence of a metal wall, but rather due to the operational constraints that require operation with high gas fuelling. Recent results have suggested the possible role of the distance between the density and temperature pedestal positions (n e pos − T e pos, also called the ‘relative shift’) with increasing gas fuelling rate on the PB stability of JET-ILW baseline plasmas (Stefanikova et al 2018 Nucl. Fusion 58 056010). The work further extends the analysis of the role of the relative shift, showing that it plays an important role in the difference between the pedestal performance of JET-C and JET-ILW. Moreover, the work also shows that the pedestal density n e ped, Z eff, and pedestal pressure width w pe play an important role in this difference. The pedestal structure and stability have been studied both experimentally and by modelling. The modelling shows that the changes in n e pos − T e pos, n e ped, Z eff, and w pe are sufficient to explain the differences in the pedestal performance between JET-C and JET-ILW PB limited discharges. A hypothesis describing the possible mechanisms leading to the degradation of the pedestal pressure and stability from JET-C to JET-ILW in PB limited plasmas is put forward.

Accounting for Edge-Plasma Parameter Dynamics at JET Divertor Sputtering Estimation during Edge-Localized Modes

The sputtering of tungsten divertor tiles at the JET ITER-like wall (ILW) during edge-localized modes (ELMs) is one of the main sources of tungsten impurity in the core plasma. The analytical approach for interpretation of the Langmuir probe (LP) measurements of ion flux at the divertor tile is elaborated to model the ELM ion parallel transport and to estimate the flux of sputtered tungsten atoms from the divertor under inter-ELM and intra-ELM conditions. Accounting for the pedestal temperature and density decrease at the pedestal crash during ELM allows obtaining the profiles of the particle and heat fluxes at the divertor tile corresponding to the measurements at the physically correct magnetic connection length. Estimates of the tungsten sputtered flux under intra-ELM and inter-ELM conditions show that first type ELMs contribute significantly (~85%) to the overall tungsten sputtering, which is in a good agreement with the divertor optical emission spectroscopy of the 400.9 nm atomic neutral tungsten line in JET discharges.

Erosion and screening of tungsten during inter/intra-ELM periods in the JET-ILW divertor

Intra-ELM tungsten sources, which dominate the total W source, are quantified in the inner and outer divertor of JET-ILW. The amount of the sputtered W atoms for individual ELMs demonstrates a clear dependence on the ELM frequency. It decreases when the pedestal temperature is lower and, correspondingly, the ELM frequency is higher. Nevertheless, the entire gross erosion W source (the number of eroded W atoms per second due to ELMs) increases initially with ELM frequency and reaches its maximum at f ELM ≈ 50–55 Hz followed by its reduction in the high frequency range. The in/out asymmetry of the intra-ELM W sources during ELMs is a critical issue and is investigated in this contribution. At a lower ELM frequency of about 35 Hz the outer divertor gross W source is larger by a factor of 1.5. However the in/out asymmetry of the W erosion decreases strongly with ELM frequency demonstrating a nearly symmetric W source in both divertor legs at frequencies above 70 Hz. The screening of tungsten in the open magnetic configuration with strike points on tile 5 is more efficient by a factor of about 1.7 than in the corner configuration with the outer strike point at the pumping duct entrance.

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal magnetohydrodynamics equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.

Effects of triangularity on the fusion performance of CFETR

An important task of the China Fusion Engineering Test Reactor physics design is to develop operation scenarios with high fusion power (1 GW), high bootstrap current fraction for steady-state and a plasma edge compatible with heat and particle exhaust. To achieve these goals, triangularity (δ) effects on the fusion performance of two candidate scenarios, with or without reversed magnetic shear (RS), namely conventional H-mode and RS H-mode, are evaluated using core-edge coupled integrated modeling in this paper. For fixed pedestal density, it is shown that higher δ is favorable for higher fusion performance in the conventional H-mode scenario while the fusion performance decreases with increasing δ in the RS H-mode scenario. In conventional H-mode, the higher fusion performance at high δ mainly comes from a higher pedestal temperature as predicted by EPED in combination with stiff core kinetic profiles. In the RS H-mode scenario with a local reversed shear region, the profiles are non-stiff and a strong internal transport barrier (ITB) exists at low δ. This results in higher density and temperature inside the ITB for low δ, leading to higher fusion power. If the pedestal temperature is kept fixed, in both scenarios the significant increase in pedestal density, which extends into the core, dominates at high δ and leads to much higher fusion power. For conventional H-mode, destabilization from increasing δ is partially balanced by stabilization due to increasing ν*. Since the normalized heat sources are quite similar, it results in minimal changes in the temperature profiles except for the lowest density case. For RS H-mode, destabilization from increasing δ is approximately balanced by stabilization due to increasing ν* in foot region, but a strong temperature ITB is still evident for low δ. The ability to take advantage of the high pedestal density in conventional H-mode and reversed shear scenario depends on its compatibility with edge density requirements from efficient heat and particle exhaust. Transport analysis is presented to elucidate the roles of δ, collisionality and magnetic shear in altering the profiles and the ITB, which contribute to the different behavior in the two scenarios.

Power threshold and confinement of the I-mode in the EAST tokamak

Sixty-five discharges have been confirmed as I-mode discharges after searching the EAST database from 2016 to 2018, and some additional I-mode discharges were obtained in 2019. The I-mode regime features no edge localized modes (ELMs), high energy confinement, and a steep temperature pedestal, while particle confinement remains at L-mode levels. The I-mode regime has been obtained over a small parameter space (BT = 2.4 – 2.7 T, Ip = 0.4 – 0.6 MA, q95 = 4 – 6) using a configuration with drift away from the active X-point. Most of the discharges in EAST are dominated by lower hybrid wave (LHW) heating. In addition, some are heated with both LHW and neutral beam injection (NBI) heating, and a few discharges are heated by NBI and electron cyclotron resonance heating (ECRH). Turbulence suppression in density perturbation has been defined as signaling the onset of the L-I transition. The power threshold of the I-mode is slightly larger than that of the 2008 ITPA scaling law of H-mode. The minimum L-I power threshold varies only weakly with BT, and the power range for the I-mode increases with increasing BT. The I-mode is superior to the L-mode in terms of energy confinement but is still slightly inferior to the H-mode under similar conditions, although the energy confinement time decreases more slowly with increasing heating power than it does in the typical H-mode case. NBI has some benefit for achieving higher confinement. The I-mode is very sensitive to the density near the edge. Under a constant high heating power, a small enhancement in the density will terminate the I-mode, sometimes leading to the I-H transition.

Impurity transport and divertor retention in Ar and N seeded SOLPS 5.0 simulations for ASDEX Upgrade

Impurity seeding will be an important tool to reduce the peak power loads and temperatures at the divertor targets in future tokamak devices. To improve the physics understanding and provide predictive capabilities for the impact of impurities on the plasma, different impurity species have to be investigated and compared to each other. For this purpose SOLPS 5.0 simulations of argon (Ar) and nitrogen (N) seeded H-mode plasmas for ASDEX Upgrade have been performed. The (purely numerical) investigations extend previous studies dedicated to impurity transport and to the divertor impurity retention. An analysis of mixed Ar and N impurity seeding reveals that a trade-off between pedestal top temperature drop and fuel dilution can be achieved by an adjustment of the impurity mixture. Due to the impact of the impurities on the temperatures, the impurity seeding reduces the main ion ionization rates in the divertor regions, and therefore, the ion particle sources. Accordingly, this modification of the particle sources results in a main ion background flow inversion at higher seeding levels, which also strongly affects the impurity flow patterns. This mechanism explains a modification of the impurity density distribution at higher seeding levels, where Ar impurities are observed to be redistributed from the outer to the inner divertor. A less pronounced effect is observed for N, which can be explained by the radiation efficiency. The divertor impurity retention is determined by the relative positions of the ionization front of the neutral impurities and the impurity stagnation point. The impact of impurity seeding on the stagnation point position is studied in detail for the first time. Under the investigated conditions, decreasing the target temperature (i.e. increasing impurity seeding) always results in a reduction of the divertor impurity retention. This is a critical result making power exhaust even more challenging.

H-mode pedestal improvements with neon injection in DIII-D

Improvements in the H-mode pedestal pressure and global energy confinement were observed on DIII-D with both 3 and 6 MW of neutral beam (NBI) heating with neon injection. The pedestal pressure increased, primarily associated with a significant increase in the pedestal density with a relatively smaller decrease in the pedestal temperature. At the same neon injection rate, a rapid decrease in edge localized mode (ELM) frequency and ∼35% increase in plasma stored energy were observed in the 3 MW discharge, while in the 6 MW discharge, the ELM frequency was gradually decreased and the plasma stored energy gradually increased by up to 20%. The measured pedestal widths from both 3 and 6 MW discharges matched the EPED1.0 width scaling, , within 20% deviations before and after neon injection. The peeling-ballooning mode (PBM) stability analysis showed that after neon injection, the ballooning stability boundary was increased, while the peeling stability boundary only dropped once the neon level was relatively high. The ballooning stability boundary increase with neon injection is consistent with increase of the ion diamagnetic stabilization (i.e. including the contribution from carbon, neon as well as deuterium ions) which is used in the criterion to determine when the calculated linear growth rates of PBM are unstable. Consequently, the new ELITE computed PBM boundaries were more consistent with the operating points than when only deuterium was included. This in turn indicates that the impurity may help to improve pedestal pressure through affecting ion diamagnetic frequency in the pedestal region, which could positively lead to the ballooning stability boundary extension.