Peaked Density Profiles Research Articles

Preliminary compact torus injection experiments on the Keda Torus eXperiment reversed field pinch

A new compact torus injector (KTX-CTI) has been built for injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP). The aim is to study the fundamental physics governing the compact torus (CT) central fueling processes. In experiments conducted under the sole influence of a 0.1 T toroidal magnetic field, the injected CT successfully penetrated the entire toroidal magnetic field, reaching the inner wall of the KTX vacuum vessel. Upon reaching the inner wall, the CT diffused both radially outward and toroidally within the vessel at a discernible diffusion speed. Moreover, the inherent helicity within the CT induced a modest KTX plasma current of 200 A, consistent with predictions based on helicity conservation. CT injection demonstrated the capability to initiate KTX discharges at low loop voltages, suggesting its potential as a pre-ionization and current startup technique. During RFP discharges featuring CT injection, the central plasma density was found to exceed the Greenwald density limit, with more peaked density profiles, indicating the predominant confinement of CT plasma within the core region of the KTX bulk plasma.

Inward particle transport driven by biased endplate in a cylindrical magnetized plasma

The inward particle transport is associated with the formation of peaked density profiles, which contributes to improve the fusion rate and the realization of steady-state discharge. The active control of inward particle transport is considered as one of the most critical issues of magnetic confinement fusion. Recently, it is realized preliminarily by adding a biased endplate in the Peking University Plasma Test (PPT) device. The results reveal that the inward particle flux increases with the bias voltage of the endplate. It is also found that the profile of radial electric field ( ) shear is flattened by the increased bias voltage. Radial velocity fluctuations affect the inward particle more than density fluctuations, and the frequency of the dominant mode driving inward particle flux increases with the biased voltage applied to the endplate. The experimental results in the PPT device provide a method to actively control the inward particle flux using a biased endplate and enrich the understanding of the relationship between shear and turbulence transport.

Progress in the development of the ITER baseline scenario in TCV

Under the auspices of EUROfusion, the ITER baseline (IBL) scenario has been jointly investigated on AUG and TCV in the past years and this paper reports on the developments on TCV. Three ITER shapes, namely the JET, AUG and ITER IBL have been reproduced in TCV, illustrating that the higher the triangularity the larger the ELM perturbation and the more difficult it is to reach stationary states with q 95< 3.6. It is found that the performance of TCV IBL is mainly limited by (neoclassical) tearing modes, in particular 2/1 modes which are triggered after a large ELM. It is demonstrated that the shorter the ELM period the larger β N at the NTM onset. We show that these modes can be avoided with central X3 EC heating at relatively high q 95 and moderate β N . However, the lack of significant ECH at the high central densities obtained in TCV IBL scenario limits the duration of low q 95 cases to about four confinement times. During this time, density usually keeps peaking until (neoclassical) tearing modes are triggered. Nevertheless, the TCV IBL database covers the ITER target values ( H98y2∼ 1, βN∼ 1.8 at q95∼ 3) and a slightly better confinement than requested for ITER is reported. Integrated modelling results show that ITG modes are the dominant instabilities, and show that, in TCV, fuelling also plays a role to sustain peaked density profiles. The role of profiles, sawteeth and ELMs regarding MHD stability are also discussed.

Self-consistent investigation of density fueling needs on ITER and CFETR utilizing the new Pellet Ablation Module

Self-consistent modeling using the stability, transport, equilibrium, and pedestal (STEP) workflow in the OMFIT integrated modeling framework (predicting pedestal with EPED, core profiles with TGYRO, current profile with ONETWO, and EFIT for equilibrium) suggests ITER and future devices such as China Fusion Engineering Test Reactor (CFETR) Zhuang et al (2019 Nucl. Fusion 59 112010) will benefit from high-density operation (Greenwald limit fraction 0.7−1.3). Regimes with an operational density near the Greenwald limit will likely need peaked density profiles so that the pedestal density remains below the Greenwald limit. Peaked density profiles can be achieved with the help of pellet injection. A flexible Pellet Ablation Module (PAM), which predicts the density source based on a comprehensive analytical pellet ablation model, has been developed for predicting pellet fueling for transport studies, and has been incorporated into the STEP workflow for predictive modeling. This workflow is applied to DIII-D and finds good agreement with experiments. On ITER the effect of pellet fueling is examined in an advanced inductive scenario, where a fusion gain of up to Q = 9 is predicted with strong central pellet fueling. On CFETR, with a mid-radius density source, an average of electrons s−1 are required to achieve the density and temperature profiles necessary for the 1000 MW advanced scenario with a tritium burn-up fraction of .

Evidence for AGN-regulated Cooling in Clusters at z ∼ 1.4: A Multiwavelength View of SPT-CL J0607-4448

We present a multiwavelength analysis of the galaxy cluster SPT-CL J0607-4448 (SPT0607), which is one of the most distant clusters discovered by the South Pole Telescope at z = 1.4010 ± 0.0028. The high-redshift cluster shows clear signs of being relaxed with well-regulated feedback from the active galactic nucleus (AGN) in the brightest cluster galaxy (BCG). Using Chandra X-ray data, we construct thermodynamic profiles and determine the properties of the intracluster medium. The cool-core nature of the cluster is supported by a centrally peaked density profile and low central entropy ( keV cm2), which we estimate assuming an isothermal temperature profile due to the limited spectral information given the distance to the cluster. Using the density profile and gas cooling time inferred from the X-ray data, we find a mass-cooling rate yr−1. From optical spectroscopy and photometry around the [O ii] emission line, we estimate that the BCG star formation rate is yr−1, roughly two orders of magnitude lower than the predicted mass-cooling rate. In addition, using ATCA radio data at 2.1 GHz, we measure a radio jet power erg s−1, which is consistent with the X-ray cooling luminosity ( erg s−1 within r cool = 43 kpc). These findings suggest that SPT0607 is a relaxed, cool-core cluster with AGN-regulated cooling at an epoch shortly after cluster formation, implying that the balance between cooling and feedback can be reached quickly. We discuss the implications for these findings on the evolution of AGN feedback in galaxy clusters.

Progress on physics understanding of improved confinement with fishbone instability at low q 95 < 3.5 operation regime in EAST

Improved confinement at the low q 95 < 3.5 operation regime with fishbone instability compared to sawtooth oscillation has been observed and investigated on the EAST under the dominant electron heating condition with a tungsten divertor. The formation of an internal transport barrier in the ion thermal channel strongly correlates to the excitation of the fishbone, accompanied by reduced particle outward transport in the core region identified by a central peaked density profile. Current density distribution is found to change from a monotonic shape with q 0 < 1 during sawtooth oscillation to a central flat structure, magnetic shear s ∼ 0 at ρ < 0.4, with the fishbone instability at where the higher off-axis bootstrap current fraction might play a critical role. Linear gyrokinetic simulation by NLT code was carried out to investigate the turbulent transport characteristics, which is qualitatively in good agreement with experimental measurements from CO2 laser collective scattering diagnostics. The electron-scale trapped electron mode that dominates the turbulent transport during sawtooth is found to be stabilized with the fishbone at identical heating power and plasma configurations.

Role of E × B drift in double-peak density distribution for the new lower tungsten divertor with unfavorable B t on EAST

Doubly peaked density distribution is expected not only to affect the plasma-wetted area at divertor plates, but also to correlate with the upstream density profile and hence characteristics of magnetohydrodynamic activities in tokamak plasmas (Wang et al 2020 Phys. Rev. Lett. 124 195002). Clarifying its origin is important to understand the compatibility between power/particle exhausts in divertor and high-performance core plasmas required by present-day and future tokamak devices. In this paper, we analyze the double-peak density profile appearing in the modeling during the physics design phase of the new lower tungsten divertor for EAST by using a comprehensive 2D SOLPS-ITER code package, including full drifts and currents, with a concentration on an unfavorable magnetic field (ion B × ∇B drift is directed away from the primary X-point). The results indicate that E × B drift induced by the plasma potential gradient near the target, which is closely related to the divertor state, plays essential roles in the formation of a double-peak profile at the target: (1) large enough radial E p × B drift produces a broadened high-density region; (2) strong poloidal E r × B drift drives a significant particle sink and creates a valley on the high-density profile. Thus, the simulation results can explain why this kind of doubly peaked density profile is usually observed at the high-recycling divertor regime. In addition, features of the double-peak ion saturation current distribution measured in preliminary experiments testing the new lower tungsten divertor are qualitatively consistent with the simulations.

Inward diffusion driven by low frequency fluctuations in self-organizing magnetospheric plasma

The new findings for dynamic process of inward diffusion in the magnetospheric plasma are reported on the Ring Trap 1 (RT-1) experiment: (i) the evolution of local density profile in the self-organized process has been analyzed by the newly developed tomographic reconstruction applying a deep learning method; (ii) the impact of neutral-gas injection excites low-frequency fluctuations, which continues until the peaked density profile recovers. The fluctuations have magnetic components (suggesting the high-beta effect) which have two different frequencies and propagation directions. The phase velocities are of the order of magnetization drifts, and both the velocities and the intensities increase in proportion to the electron density. The self-regulating mechanism of density profile works most apparently in the naturally made confinement system, magnetosphere, which teaches the basic physics of long-lived structures underlying every stationary confinement scheme.

Tungsten impurity effects on the coupling of TEM and ITG mode in reversed magnetic shear tokamak plasmas

The trapped electron mode (TEM) and ion temperature gradient (ITG) mode may couple into one hybrid mode or coexist simultaneously, depending on the regime of tokamak plasma parameters. A gyrokinetic integral eigenvalue equation is applied to investigate the coupling of TEM and ITG mode in the presence of tungsten (W) impurity in tokamak geometry of reversed magnetic shear (RMS). Systematic analysis of W impurity effects on the mode characteristics and induced quasi-linear particle flux has been performed in a wide regime of plasma parameters such as density gradient , charge concentration , and charge number of impurity ions, and poloidal wave number spectrum. It is found that both RMS and the impurity ions with inwardly peaked density profile and high charge concentration have strongly stabilizing effects on the modes. Furthermore, the increasing tends to decouple the hybrid mode into coexisting modes or dominating TEM/ITG mode in different parameter regions. The significance of the results for the particle transport of impurity and main ions as well as electrons in advanced tokamaks with W components is discussed.

Overview of density pedestal structure: role of fueling versus transport

How much of the density pedestal structure is determined by fueling versus transport is still an open question. In most current tokamaks, the scrape-off layer is not opaque enough to screen the neutrals and limit their penetration past the separatrix. As such, recycled neutrals and additional gas puffing can directly affect the pedestal structure through ionization. However, in ITER and other future burning plasma magnetic confinement devices, the ionization source will be pushed further out into the scrape-off layer. This poses the question: how much of the density pedestal structure is governed by this edge ionization source versus plasma transport effects? Theoretically, several turbulent modes have been identified which could provide a ‘pinch-like’ up-gradient transport mechanism. Up-gradient transport is necessary in a source-free region to obtain a peaked density profile, which is often observed in the core of tokamaks. In an opaque scrape-off layer, without the existence of such a pinch, the density pedestal structure would eventually disappear. Not only would this limit our ability to fuel a fusion reactor, it would also affect the pedestal stability. In this paper we will give an overview of the state-of-the-art on what sets the density pedestal structure based on experimental observations as well as theoretical models. We will complement these studies with new results from Alcator C-Mod and DIII-D where the opaqueness is increased to reach values similar to those in ITER.

First Evidence of Local E×B Drift in the Divertor Influencing the Structure and Stability of Confined Plasma near the Edge of Fusion Devices.

The structure of the edge plasma in a magnetic confinement system has a strong impact on the overall plasma performance. We uncover for the first time a magnetic-field-direction dependent density shelf, i.e., local flattening of the density radial profile near the magnetic separatrix, in high confinement plasmas with low edge collisionality in the DIII-D tokamak. The density shelf is correlated with a doubly peaked density profile near the divertor target plate, which tends to occur for operation with the ion B×∇B drift direction away from the X-point, as currently employed for DIII-D advanced tokamak scenarios. This double-peaked divertor plasma profile is connected via the E×B drifts, arising from a strong radial electric field induced by the radial electron temperature gradient near the divertor target. The drifts lead to the reversal of the poloidal flow above the divertor target, resulting in the formation of the density shelf. The edge density shelf can be further enhanced at higher heating power, preventing large, periodic bursts of the plasma, i.e., edge-localized modes, in the edge region, consistent with ideal magnetohydrodynamics calculations.

High-performance plasmas after pellet injections in Wendelstein 7-X

A significant improvement of plasma parameters in the optimized stellarator W7-X is found after injections of frozen hydrogen pellets. The ion temperature in the post-pellet phase exceeds 3 keV with 5 MW of electron heating and the global energy confinement time surpasses the empirical ISS04-scaling. The plasma parameters realized in such experiments are significantly above those in comparable gas-fuelled discharges. In this paper, we present details of these pellet experiments and discuss the main plasma properties during the enhanced confinement phases. Local power balance is applied to show that the heat transport in post-pellet phases is close to the neoclassical level for the ion channel and is about a factor of two above that level for the combined losses. In comparable gas-fuelled discharges, the heat transport is by about ten times larger than the neoclassical level, and thus is largely anomalous. It is further observed that the improvement in the transport is related to the peaked density profiles that lead to a stabilization of the ion-scale turbulence.

Steady state densities in a plasma confined by a dipole magnet: Diffusion induced transport explored through direct measurements and modeling

Steady state densities in a plasma confined by a permanent dipole magnet are determined through detailed experiments and modeling. Two diffusion models are developed, and the resulting equations are solved numerically to yield the radial and angular plasma density profiles, which are compared with those obtained experimentally. The models consider the fluid and continuity equations along with Fick’s law and take into account the experimentally determined electron temperature (Te) and the static dipole magnetic fields (B) in space, as a common input. In model I, the equation of motion for both charges (ions and electrons) is used to self-consistently generate the ambipolar electric field (E), while model II considers the electron equation of motion and takes into account the experimentally determined plasma potential (Vs) as another input, whose gradient provides the ambipolar electric field. Results indicate that the plasma density peaks around r ∼ (2–12) cm depending on the polar angle and the discharge pressure and decreases for large r, while its angular variation shows a maximum in the equatorial plane (θ = 90°) and decreases toward the polar regions. Te and Vs are higher in the polar cusp regions and decrease toward the equatorial plane, with the profiles becoming more spherically symmetric away from the magnet. The numerically obtained density profiles from the models agree well with those obtained experimentally. The phenomenon of inward diffusion resulting in peaked density profiles as reported by earlier authors is found to be a natural outcome of the solution of the diffusion equation.

Short wavelength ion temperature gradient mode in tokamak plasmas with hollow density profiles

By the gyrokinetic integral eigenmode equation, we numerically investigate the short-wavelength ion-temperature-gradient (SWITG) mode in tokamak plasmas with hollow density profiles. This paper finds that the critical ion temperature gradient (ITG) R/LTiC exists for the hollow density profile. Over such a gradient, the SWITG mode is unstable. The R/LTiC for a hollow density profile is slightly lower than that for a peaked density profile, though far away from the threshold, the mode in the latter case is more unstable. Furthermore, the scaling of the ITG threshold increasing with ion-to-electron temperature ratio Ti/Te has been calculated, suggesting that the SWITG mode is more difficult to excite in hot ion plasmas as compared with that in hot electron plasmas. For the slightly hollow density profiles, it is shown that in the case of flat (peaked) electron temperature profiles, the SWITG modes are destabilized (stabilized) by the trapped electrons. For the steep hollow density profiles, however, the trapped electrons reduce the growth rate substantially, and thus, the SWITG stability threshold is raised significantly. In addition, the dependence of the threshold on local plasma parameters is analyzed in detail. It is found that the SWITG modes become more unstable for the high safety factor, small Larmor radius, weak magnetic shear, and high electron-to-ion temperature ratio.

Density peaking in JET—determined by fuelling or transport?

Core density profile peaking and electron particle transport have been extensively studied by performing several dimensionless collisionality (υ*) scans with other matched dimensionless profiles in various plasma operation scenarios on the Joint European Torus (JET). This is the first time when electron particle transport coefficients in the H-mode have been measured on JET with high resolution diagnostics, and therefore we are in a position to distinguish between the neutral beam injection (NBI) source and inward electron particle pinch in contributing to core density peaking. The NBI particle source is found to contribute typically 50%–60% to the electron density peaking in JET H-mode plasmas where Te/Ti ~ 1 or smaller and at υ* = 0.1–0.5 (averaged between r/a = 0.3–0.8), and being independent of υ* within that range. In these H-mode plasmas, the electron particle transport coefficients, De and ve, are small, thus giving rise to the large influence of NBI fueling with respect to transport effect on peaking. In L-mode plasma conditions, the role of the NBI source is small, typically 10%–20%, and the electron particle transport coefficients are large. These dimensionless υ* scans give the best possible data for model validation. TGLF simulations are in good agreement with the experimental results with respect to the role of NBI particle source versus inward pinch in affecting density peaking, both for the H-mode and L-mode υ* scans. It predicts, similarly to experimental results, that typically about half of the peaking originates from the NBI fuelling in the H-mode and 10%–20% in the L-mode. GENE simulation results also support the key role of NBI fuelling in causing a peaked density profile in JET H-mode plasma (Te/Ti ~ 1 and υ* = 0.1–0.5) and, in fact, give an even higher weight on NBI fuelling than that experimentally observed or predicted by TGLF. For the non-fuelled H-mode plasma at higher Te/Ti = 1.5 and lower βN and υ*, both TGLF and GENE predict peaked density profiles, therefore agreeing well with experimental steady-state density peaking. Overall, the various modelling results give a fairly good confidence in using TGLF and GENE in predicting density peaking in quite a wide range of plasma conditions in JET.

Influence of the safety factor profile on the particle and heat transport in the Globus-M spherical tokamak

The advanced tokamak scenario is a promising operation scenario for ITER and fusion neutron sources. In this scenario the minimum value of the safety factor in the center of the plasma exceeds unity. In the compact spherical tokamak Globus-M, the formation of such conditions is possible with neutral beam injection at the current ramp-up phase. Due to the slower diffusion of current inside the plasma, a zone is formed with reduced heat and particle transport across the magnetic field, which affects the temperature and density profiles of the plasma. This leads to the peaked density profile formation and improvement of the energy confinement time. To achieve a high fraction of the bootstrap current, it is necessary to increase the plasma pressure. At the same time, the maximum allowable pressure is limited to the normalized beta limit.

Impurity effects on ion temperature gradient driven multiple modes in transport barriers

The impurity effects on ion temperature gradient (ITG) driven instability in transport barriers (TBs) are numerically investigated with the gyrokinetic integral eigenmode equations in tokamak plasmas. In particular, the effects of temperature and density gradients of the main ions ( and ) are analyzed independently to understand the physical mechanisms better, instead of keeping their ratio as carried out in previous works, when the parameters of impurity ions vary. It is found that the effect of impurity ions with outwardly peaked density profiles on ITG modes depends on the competition between the destabilizing effect of the impurity density gradient and the stabilizing effect induced by the dilution of main ions from impurity ions when is fixed, which is in significant contrast with the results for a fixed . The destabilizing effects include enhancement of ITG modes and coupling to the impurity mode (IM) in weak ITGs (big ) and strong impurity density gradient regimes. In addition, the stability boundaries for ITG modes, including high-order modes, are discussed in detail, and compared with previous works (Fröjdh et al 1992 Nucl. Fusion 32 419). Furthermore, the impurity ions with either inwardly or slightly outwardly peaked density profiles have weaker and stronger stabilizing effects on small and big poloidal wave vector modes, respectively. However, the impurity ions with steeper outwardly peaked density profiles have stronger stabilizing effects on big modes. Moreover, the inwardly peaked impurity ion density profiles are beneficial for main ion confinement and impurity decumulation, due to the main (impurity) ions flowing inwardly (outwardly). Finally, analyses of eigenmode structure and the quasi-linear particle flux are performed in detail. The results show that impurity ions have non-negligible effects, especially on higher-order ITG modes.

Dynamic evolution of microturbulence with an improved confinement mode in the ramp-down phase of plasma current on EAST

Micro-instabilities are often invoked to account for the anomalous transport of energy and particles in fusion plasmas, whose origin and contributions to anomalous transport remain uncertain. The dynamic features of the microturbulence in the plasma core and the gradient region are investigated simultaneously by a poloidal CO2 laser coherent scattering diagnostic system on the experimental advanced superconducting tokamak (EAST). In 2018, an improved confinement mode characterized by a peaked density profile is observed in the current ramp-down phase with no auxiliary heating, which has rarely been studied. The experimental result suggests that the gradual shut off of the LHW heating might trigger the mode transition. The transition is accompanied with a great suppression of microturbulence and significant change in the cross-correlation spectrum. A possible mechanism for the suppression could be ascribed to the increasing shear flow. These results could shed light on the understanding of the nonlinear mechanism of confinement mode transition on EAST.

Underlying competition mechanisms in the dynamic profile formation of high-density helicon plasma

The formation mechanism of the density profile of helicon discharge, which has been a dispute for a long time, is investigated by using a careful self-consistent model. A detailed investigation of the local balance between the source and the loss fluxes reveals how the centrally peaked density profile is generated, despite the strong surface power absorption by the mode-converted Trivelpiece-Gould (TG) wave from the helicon wave, without any assumption of anomalous diffusion. Our results suggest that the flux transport toward the wall balances out the surface source flux by the TG wave, while the plasma core grows by the power of helicon wave deposition, resulting in the centrally peaked density profile. It is also found that the density profile can be controlled successfully to produce centrally peaked, flat, or hollow profiles by adjusting the contribution of the higher axial mode number of the TG wave.

Light impurity transport in JET ILW L-mode plasmas

A series of experimental observations of light impurity profiles was carried out in JET (Joint European Torus) ITER-like wall (ILW) L-mode plasmas in order to investigate their transport mechanisms. These discharges feature the presence of 3He, Be, C, N, Ne, whose profiles measured by active Charge Exchange diagnostics are compared with quasi-linear and non-linear gyro-kinetic simulations. The peaking of 3He density follows the electron density peaking, Be and Ne are also peaked, while the density profiles of C and N are flat in the mid plasma region. Gyro-kinetic simulations predict peaked density profiles for all the light impurities studied and at all the radial positions considered, and fail predicting the flat or hollow profiles observed for C and N at mid radius in our cases.