Saturated Ohmic Confinement Regime Research Articles

Role of isotopes in microturbulence from linear to saturated Ohmic confinement regimes

Role of isotopes in microturbulence from linear to saturated Ohmic confinement regimes

Error-field penetration thresholds in ohmically heated ITER and SPARC plasmas

The critical n = 1, n = 2, and n = 3 error-field amplitudes needed to trigger error-field penetration in ITER and SPARC, steady-state, ohmically heated plasmas are calculated using a standard asymptotic matching approach. The calculation incorporates plasma impurities, trapped particles, the bootstrap current, and neoclassical poloidal flow-damping. The energy confinement time is specified by the neo-Alcator scaling law in the low-density linear Ohmic confinement (LOC) regime and by the ITER-89P L-mode scaling law in the high-density saturated Ohmic confinement (SOC) regime. The response of the plasma in the inner region is calculated using a linearized version of the four-field model. At the normal operating electron number density, diamagnetic levels of rotation are found to be sufficient to protect ITER and SPARC ohmically heated plasmas from m=2/n=1 error-field penetration. On the other hand, SPARC, and especially ITER, ohmically heated plasmas may be vulnerable to n > 1 error-field penetration. ITER and SPARC ohmically heated plasmas are also slightly more susceptible to error-field penetration when the electron fluid at the rational surface rotates in the ion diamagnetic direction, rather than the electron diamagnetic direction. At electron number densities that are sufficiently low that the plasma lies in the LOC confinement regime, the error-field penetration threshold increases with increasing density. However, as soon as the electron number density becomes large enough that the plasma enters the SOC regime, the increase in the error-field penetration threshold with increasing density levels off.

Full-f gyrokinetic simulations of Ohmic L-mode plasmas in linear and saturated Ohmic confinement regimes

Two time slices in linear and saturated Ohmic confinement (LOC and SOC) regimes in a Tore Supra Ohmic L-mode discharge are analyzed using nonlinear global full-f gyrokinetic simulations, and qualitative features of the LOC–SOC transition are reproduced. The exhaust of carbon impurity ions is caused by ion mixing, which is driven by the toroidal field stress. The intrinsic rotation develops in the opposite direction between the LOC and SOC phases, which is characterized by different features of the mode asymmetry between trapped electron modes (TEMs) in the LOC phase and ion temperature gradient-driven modes in the SOC phase, leading to the change of the profile shear stress. Here, the mode asymmetry or the ballooning angle depends not only on the profile shear and the Er shear but also on the radial electric field Er itself. The energy fluxes of electrons and deuterium ions are dominant in the LOC and SOC phases, respectively, and the ratio of the energy confinement time between the two phases agree with the experimental value. Turbulent frequency spectra are characterized by quasi-coherent modes (QCMs) and broad-band spectra in the LOC and SOC phases, respectively. The QCMs are produced by a split of the toroidal mode number spectra of TEMs induced by the Doppler shift of poloidal E × B rotation due to Er, which is sustained in the electron diamagnetic direction by the ripple induced counter-current rotation, and enhanced by higher temperature in the LOC phase.

Study of correlations between LOC/SOC transition, intrinsic toroidal rotation reversal and TEM/ITG bifurcation with different working gases in TCV

The effects of different working gases on the transition from linear ohmic confinement (LOC) regime to saturated ohmic confinement (SOC) regime and its relation to the intrinsic toroidal rotation reversal phenomenon were explored in the TCV tokamak. The energy confinement saturation was studied across D, H and He density ramps, and a range of ECRH injection power and through variations of ohmic plasma current. The occurrence of rotation reversal, concomitantly with the LOC–SOC transition, was observed only for certain cases, making us formally exclude a causal relation between the two phenomena. A strong correlation between the evolution of toroidal rotation profiles and electron density gradients was, however, observed, in agreement with previous works (Lebschy et al 2017 Nucl. Fusion 58 026013; Hornsby et al 2018 Nucl. Fusion 58 056008). Linear gyrokinetic simulations were performed to probe the turbulent regime of these discharges, showing a dominance of trapped electron mode (TEM) during the LOC phase and a mixture of TEM and ion temperature gradient (ITG) following the transition to SOC regime in D. Such a TEM/ITG bifurcation was less pronounced in H and He. MHD activity was monitored throughout the discharges and possible correlations between sawteeth instability activity, energy confinement time saturation and rotation reversal are highlighted.

Isotope effects on intrinsic rotation in hydrogen, deuterium and tritium plasmas

The isotope effect on intrinsic rotation was studied at the Joint European Torus (JET) tokamak. With the unique capability of JET to operate with tritium (T), for the first time, experiments in hydrogen (H), deuterium (D) and T in Ohmic plasmas were compared. Two rotation reversals per isotope type are observed in plasma density scans spanning the linear and the saturated Ohmic confinement regimes. A clear isotope mass dependence is observed at the higher densities. The magnitude of the core rotation was found to depend on isotope mass, with stronger co-current rotation observed in H. Change on intrinsic rotation characteristics coexist with a stronger thermal energy confinement in T.

Electron temperature fluctuations correlated with energy confinement degradation in the EAST Ohmic plasmas

In saturated Ohmic confinement regime of the experimental advanced superconducting tokamak, significant changes in the relative electron temperature fluctuations T̃e/T¯e, measured by a correlation electron cyclotron emission system, have been observed. It was found that T̃e/T¯e is strongly dependent on the normalized electron temperature and density gradient, R/LTe=R∇Te/Te and R/Lne=R∇ne/ne. T̃e/T¯e stay at a high level when R/LTe is within the range of 17−25 and R/Lne is within the range of 4−6. It has been also observed that the energy confinement degradation is correlated with the increase in electron temperature fluctuations. The linear stability analysis results show that the electron temperature fluctuations are mainly attributed to the ion temperature gradient modes.

Nonlinear gyrokinetic analysis of linear ohmic confinement to saturated ohmic confinement transition

This work presents a nonlinear gyrokinetic analysis, which addresses one of the perennial conundrums in ohmically heated fusion plasmas. The widely observed linear ohmic confinement (LOC) to saturated ohmic confinement (SOC) transition of the energy confinement time with increasing density is successfully reproduced from nonlinear gyrokinetic simulations for the first time. Suppression of trapped electron turbulence by collisional detrapping due to increasing density is found to be responsible for the transition. The microturbulence transition from trapped electron modes to ion temperature gradient modes is found to coincide, but is not a necessary condition for the LOC–SOC transition. Damping of nonlinearly generated zonal flow by increasing collisionality with density can be responsible for the energy confinement degradation in the SOC regime.

Experimental observation of turbulence transition and a critical gradient threshold for trapped electron mode in tokamak plasmas

In HL-2A and J-TEXT ohmic confinement regimes, an electrostatic turbulence with quasi-coherent characteristics in spectra of density fluctuations was observed by multi-channel microwave reflectometers. These quasi-coherent modes (QCMs) were detectable in a large plasma region (r/a∼0.3−0.8). The characteristic frequencies of QCMs were in the range of 30–140 kHz. The mode is rotated in the electron diamagnetic direction. In the plasmas with QCMs, trapped electron mode (TEM) was predicted to be unstable by gyrokinetic simulations. The combined experimental results show that the TEM is survived in the linear ohmic confinement regime of plasmas. The quasi-coherent TEM was replaced by broad-band fluctuations when the plasma transits from linear to saturated ohmic confinement regime. The observation was strongly related to the turbulence transition from TEM to ion temperature gradient mode. A critical gradient threshold for TEM excitation in electron temperature gradient was directly found. The effect of TEM on density profile peaking was presented.

The effects of dilution on turbulence and transport in C-Mod ohmic plasmas and comparisons with gyrokinetic simulations

Main ion dilution has been predicted by gyrokinetic simulations to have a significant effect on ion thermal transport in C-Mod ohmic plasmas. This effect was verified experimentally with a specific set of experiments on C-Mod in which ohmic deuterium plasmas across the linear ohmic confinement (LOC) through the saturated ohmic confinement (SOC) regimes were diluted by seeding with nitrogen gas (Z = 7) injection. The seeding was observed to increase the normalized ion temperature gradients (ITGs) by up to 30% without a corresponding increase in the gyrobohm normalized ion energy flux, indicating a change in either the stiffness or the critical ion temperature gradient associated with ITG turbulence. The seeding also reversed the direction of the intrinsic toroidal rotation in plasmas slightly above the normal intrinsic rotation reversal critical density. GYRO simulations of the seeded and unseeded plasmas show that the seeding affected both the critical gradient and the stiffness. For plasmas in the LOC regime, the dilution primarily increased the critical gradient, while for plasmas in the SOC regime the dilution primarily decreased the stiffness. At r/a = 0.8, where the experimental fluxes were above marginal stability, local GYRO predicted and experimental energy fluxes agreed, except for Qi in the SOC regime where GYRO under-predicted the experimental energy flux. At r/a = 0.6, where the experimental fluxes were close to marginally stable, local GYRO predicted ITG modes to be strongly unstable and are responsible for both Qi and Qe (with Qi > Qe), as opposed to the experiment where Qi < Qe. In contrast, global GYRO in this region predicted the ITG modes to be closer to marginal stability, and accurately predict the experimental Qi when the Ti profile is modified within experimental uncertainties. The fact that Qe is always less than Qi in the r/a = 0.6 simulations with kθρs≤1 indicates that high-k electron temperature gradient driven (ETG) modes must be included in future simulations and may be responsible for the electron energy transport in this case.

High density experiments in TCV ohmically heated and L-mode plasmas

Recent experiments have been performed on the Tokamak à configuration variable (TCV) to investigate the confinement properties of high density plasmas and the mechanism behind the density limit. In a limiter configuration with plasma elongation and triangularity the operational density range has been extended up to 0.65 of the Greenwald density at kA () and even to the Greenwald value at low plasma current kA (). A transition from the linear to the saturated ohmic confinement regime is observed at high density . A further density increase leads to sawtooth stabilization and is accompanied by a decrease of the energy and particle confinement times. The development of the disruption at the density limit was preceded by sawtooth stabilization. It is shown that electron cyclotron heating leads to the prevention of sawtooth stabilization and then to the increase of the density limit value.

Quasi-coherent modes and electron-driven turbulence

This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC modes have characteristics in between coherent and broad-band fluctuations as they oscillate around a given frequency but have a wide spectrum. They are ballooned at the LFS midplane and appear usually on a frequency ranging from 30 to 120 kHz. In ohmic plasmas from both tokamaks, QC modes are detected only in linear ohmic confinement (LOC) regime and disappear in saturated ohmic confinement (SOC) regime. Linear simulations from Tore Supra predict that the LOC and SOC regimes are dominated by electron and ion modes respectively. Measurements of the perpendicular velocity of density fluctuations have been made from the top of TEXTOR by poloidal correlation reflectometry. They suggest that QC modes have a phase velocity ∼400 m s−1 higher in the electron diamagnetic direction than lower frequency fluctuations. Additionally, the onset of QC modes during electron cyclotron resonance heating has been observed in a Tore Supra region where turbulence is suspected to be driven by electron modes. These experimental results and instability calculations show a correlation between onsets of QC modes and predictions of trapped electron modes.

Overview of experimental results and code validation activities at Alcator C-Mod

Recent research on the Alcator C-Mod tokamak has focused on a range of scientific issues with particular emphasis on ITER needs and on detailed comparisons between experimental measurements and predictive models. Research on ICRF (ion cyclotron range of frequencies) heating emphasized the origins and mitigation of metallic impurities while work on lower hybrid current drive experiments have focused on linear and nonlinear wave interactions that limit efficiency at high densities in regimes with low single pass absorption. Experiments in core turbulence and transport focused on quantitative, multi-field comparisons between nonlinear gyro-kinetics simulations and experimental measurements of profiles, fluxes and fluctuations. Experiments into self-generated rotation observed spontaneous flow reversal at a critical density identical to the transition density between linear ohmic confinement and saturated ohmic confinement regimes. H-mode studies have measured pedestal widths consistent with kinetic-ballooning-mode-like instabilities, while the pedestal heights quantitatively match the EPED code predictions. Experiments with I-mode have increased the operating window for this promising edge-localized-mode-free regime. Extrapolation of I-mode to ITER suggests that the fusion gain Q ∼ 10 could be possible in ITER. Investigations into the physics and scaling of the power exhaust channel width in attached enhanced D-alpha H-mode and L-mode plasma showed a direct connection between the midplane pressure-folding length and the outer divertor target footprint. The width was found to scale inversely with IP, while being independent of conducted power, BT or q95 and insensitive to the scrape-off layer connection length—a behaviour that suggests critical-gradient physics sets both pressure and heat-flux profiles.

Changes in core electron temperature fluctuations across the ohmic energy confinement transition in Alcator C-Mod plasmas

The first measurements of long wavelength (kyρs < 0.3) electron temperature fluctuations in Alcator C-Mod made with a new correlation electron cyclotron emission diagnostic support a long-standing hypothesis regarding the confinement transition from linear ohmic confinement (LOC) to saturated ohmic confinement (SOC). Electron temperature fluctuations decrease significantly (∼40%) crossing from LOC to SOC, consistent with a change from trapped electron mode (TEM) turbulence domination to ion temperature gradient (ITG) turbulence as the density is increased. Linear stability analysis performed with the GYRO code (Candy and Waltz 2003 J. Comput. Phys. 186 545) shows that TEMs are dominant for long wavelength turbulence in the LOC regime and ITG modes are dominant in the SOC regime at the radial location (ρ ∼ 0.8) where the changes in electron temperature fluctuations are measured. In contrast, deeper in the core (ρ < 0.8), linear stability analysis indicates that ITG modes remain dominant across the LOC/SOC transition. This radial variation suggests that the robust global changes in confinement of energy and momentum occurring across the LOC/SOC transition are correlated to local changes in the dominant turbulent mode near the edge.

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

We demonstrate that existing theories are insufficient to explain up/down asymmetries of argon x-ray emission in Alcator C-Mod ohmic plasmas. Instead of the poloidal variation, ñz/〈nz〉, being of order the inverse aspect ratio, ϵ, and scaling linearly with , it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behaviour between (T1020 m−3 MA−1). The transition from a poloidally symmetric to asymmetric impurity distribution is shown to occur at densities just below those that trigger a reversal of the core toroidal rotation direction, thought to be linked to the transition between the linear and saturated ohmic confinement regimes. A possible drive is discussed by which anomalous radial transport might sustain the impurity density asymmetry as the ratio of the perpendicular to parallel equilibration times, τ⊥,z/τ∥,z, approaches unity. This explanation requires a strong up/down asymmetry in radial flux which, while not observable on C-Mod, has been measured in TEXT and Tore Supra ohmic plasmas.

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.

Analysis of a global energy confinement database for JET ohmic plasmas

A database containing global energy confinement data for JET ohmic plasmas in the campaigns from 1984 to 1992 has been established. An analysis is presented of this database and the results are compared with data from other tokamaks, such as the Axially Symmetric Divertor Experiment (ASDEX), Frascati Tokamak Upgrade (FTU) and Tore Supra. The trends of JET ohmic confinement appear to be similar to those observed on other tokamaks: a linear dependence of the global energy confinement time on density is observed up to a density value where a saturation is attained; this density value defines the border between the linear and the saturated ohmic confinement regimes; this border is shifted towards higher density values if the q value of the discharge is decreased; the global confinement time in the saturated ohmic regime increases less than linearly with the value of the magnetic field