Edge Toroidal Rotation Research Articles

The effect of impurity seeding on edge toroidal rotation in the ADITYA-U tokamak

Intrinsic toroidal rotation velocity (Vφ ) has been measured from the Doppler shift of C5+ carbon spectral lines (at 529.05 nm) in the edge region of the ADITYA-U tokamak without any auxiliary torque input in an ohmically heated pure hydrogen (H2) plasma as well as in H2 plasmas seeded with medium-Z (neon and argon) impurities . The toroidal rotation in the edge region is observed to reverse its direction from the counter-current to the co-current direction with an increase in plasma current beyond I p ∼ 145–150 kA. Furthermore, a systematic decrease in the co-current Vφ has been observed with the edge density, which tends to decrease to almost zero velocity with an increase in the edge density. The injection of medium-Z (neon and argon) impurities is observed to influence the edge toroidal rotation significantly. In low I p discharges, argon injection leads to a reversal of edge intrinsic rotation from the counter-current to the co-current direction. In high I p discharges, both neon and argon seeding enhance the co-current rotation by about ∼5–10 km s−1, at a constant I p compared to pure H2 discharges. Simultaneous measurements of the edge radial electric field, E r, shows that the E r × Bθ flow seems to be driving the edge toroidal rotation in ADITYA-U. With impurity injection, the E r also gets modified, leading to an observed increase in the edge toroidal rotation.

Investigation of the J-TEXT plasma events by k-means clustering algorithm

Various types of plasma events emerge in specific parameter ranges and exhibit similar characteristics in diagnostic signals, which can be applied to identify these events. A semi-supervised machine learning algorithm, the k-means clustering algorithm, is utilized to investigate and identify plasma events in the J-TEXT plasma. This method can cluster diverse plasma events with homogeneous features, and then these events can be identified if given few manually labeled examples based on physical understanding. A survey of clustered events reveals that the k-means algorithm can make plasma events (rotating tearing mode, sawtooth oscillations, and locked mode) gathering in Euclidean space composed of multi-dimensional diagnostic data, like soft x-ray emission intensity, edge toroidal rotation velocity, the Mirnov signal amplitude and so on. Based on the cluster analysis results, an approximate analytical model is proposed to rapidly identify plasma events in the J-TEXT plasma. The cluster analysis method is conducive to data markers of massive diagnostic data.

Effects of collisional ion orbit loss on tokamak radial electric field and toroidal rotation in an L-mode plasma

Ion orbit loss has been used to model the formation of a strong negative radial electric field E r in the tokamak edge, as well as edge momentum transport and toroidal rotation. To quantitatively measure ion orbit loss, an orbit-flux formulation has been developed and numerically applied to the gyrokinetic particle-in-cell code XGC. We study collisional ion orbit loss in an axisymmetric DIII-D L-mode plasma using gyrokinetic ions and drift-kinetic electrons. Numerical simulations, where the plasma density and temperature profiles are maintained through neutral ionization and heating, show the formation of a quasisteady negative E r in the edge. We have measured a radially outgoing ion gyrocenter flux due to collisional scattering of ions into the loss orbits, which is balanced by the radially incoming ion gyrocenter flux from confined orbits on the collisional time scale. This suggests that collisional ion orbit loss can shift E r in the negative direction compared to that in plasmas without orbit loss. It is also found that collisional ion orbit loss can contribute to a radially outgoing (counter-current) toroidal-angular-momentum flux, which is not balanced by the toroidal-angular-momentum flux carried by ions on the confined orbits. Therefore, the edge toroidal rotation shifts in the co-current direction on the collisional time scale.

High-βp scenario realized by the integration of internal and external transport barriers in the HL-2A tokamak

High-βp scenario addresses needs for an attractive tokamak fusion reactor design, where βp is the normalized plasma poloidal beta. High-βp experiments have been performed during recent years in the HL-2A tokamak. The high-performance region is realized by the integration of internal and external transport barriers which are dubbed as ITBs and ETBs, namely, double transport barriers (DTBs), with the high-power NBI heating. Generally, the ITB forms and becomes strong after the NBI injection on HL-2A. Subsequently, the edge ion temperature and toroidal rotation increase, as a result that the ETB or pedestal creates and L-H transition occurs. A high-βp (βp∼3) scenario is obtained with Ip∼110 kA and q95∼5, and another high-βp (βp∼2.7) scenario is also realized with Ip∼160 kA and q95∼4.2. The two high-βp scenarios are both characterized by a large-radius ITB and an ETB. However, the two scenarios are different. For the former there are strong large-radius ITB and weak ETB on radial ion temperature profiles, but for the later there are weak large-radius ITB and strong ETB. Although two high-βp scenarios on HL-2A have been accessed, however they are both transient. In addition, MHD instabilities in high-βp scenarios are also present for a high fusion gain and reactor relevant high fusion-density plasma.

A lithium laser-ablation based time-of-flight (LILA-TOF) diagnostic for measuring plasma edge ion temperature and toroidal plasma rotation

A new method for studying the thermalization and transport of injected impurities at the edge of hot plasma, (considering the last closed magnetic surface, the free path is between 1 and 2 cm approx.) under no perturbative conditions, is presented. In the proposed technique, a Nd:YAG laser is used to ablate Li from the lithiated wall of the stellarator TJ-II. While the laser pulse allows for the analysis of the released species through laser induced breakdown spectroscopy (LIBS), its laser pulse also provides a time reference for the time-of-flight (TOF) measurements of the Li+ ions performed. This is done by positioning light detection systems sensitive to an intense Li II spectral line at different toroidal locations away from such a source. TOF times of tens to hundreds of microseconds are recorded. Then, by de-convolving the shape of the recorded light pulse, the velocity distribution of the lithium-ion during its thermalization with the background plasma can be extracted. From this velocity distribution, the ion temperature of the background ions and the toroidal rotation at the plasma periphery can be deduced. In contrast with conventional Doppler spectroscopy, this technique uses filter-scope detectors rather than high-spectral-resolution spectrometers thus a tradeoff between spectral and time or space resolution is required. Finally, preliminary results of the application of this novel technique are shown to validate this proof of principle.

Formation of edge pressure pedestal and reversed magnetic shear due to toroidal rotation in a tokamak equilibrium

Toroidal rotation is well known to play significant roles in the edge transport and L–H transition dynamics of tokamaks. Our recent calculation finds that a sufficiently strong localized toroidal rotation can directly bring out the formation of edge pressure pedestal with reversed magnetic shear that is reminiscent of an H-mode plasma, purely through the effects of toroidal rotation on the tokamak MHD equilibrium itself. In particular, the enhanced edge toroidal rotation enables a substantial peaking of the parallel current profile near edge in higher β regimes, which leads to the flattening or reversal of the local q (safety factor) profile. Here the formation of pressure pedestal along with the reversed magnetic shear region is shown to be the natural outcome of the MHD tokamak equilibrium in a self-consistent response to the presence of a localized toroidal rotation typically observed in high confinement-mode (H-mode) or quiescent H-mode.

Edge Toroidal Rotation Analysis by CXRS Diagnostic on EAST

Understanding the influence of edge toroidal rotation in confined plasmas on the L-H transition is important for improving the plasma performance of future fusion devices. We report the results of experiments on the Experimental Advanced Superconducting Tokamak (EAST) to study this relationship. We used edge toroidal charge exchange recombination spectroscopy (eCXRS) as a diagnostic to study edge toroidal rotation. By analyzing the contribution of each term in the radial electric field, our experimental results show how the L-H transition depends on the edge toroidal rotation. Generally, the power of the transition increases with increasing edge toroidal rotation. The observed reduction of injected power can be explained by the change of the edge radial electric field. This reduced power threshold at lower toroidal rotation could provide an important benefit for inherently low-rotation plasma devices such as ITER and the China Fusion Engineering Test Reactor (CFETR).

L–H transition trigger physics in ITER-similar plasmas with applied n = 3 magnetic perturbations

The L–H transition power threshold PLH is observed to increase with applied n = 3 resonant magnetic perturbations (RMP) in ITER-similar-shape plasmas with balanced neutral beam torque injection in DIII-D. The increase is most pronounced with added electron–cyclotron heating: PLH increases with decreasing edge plasma collisionality as PLH/PLH-08 ~ (ν*)−0.5, where PLH-08 is the 2008 ITPA multi-machine power threshold scaling. This result raises concerns for H-mode access at low edge collisionality in ITER, where RMP may have to be applied before the L–H transition to safely suppress the first edge-localized mode. Non-axisymmetric modifications with RMP include a simultaneous reduction of the radial electric field (Er) well depth and E × B shear. This can be attributed to increasing edge toroidal co-current rotation, and is consistent with substantially increased local long-wavelength turbulence (measured via beam emission spectroscopy). At high RMP perturbation strength the edge electric field Er reverses sign locally (becomes positive), with changes in dominant turbulence modes. Edge magnetic stochasticity provides an attractive explanation of the observed modifications, and the observed changes in toroidal rotation and Er are consistent with a simple fluid model describing radial electron current flow along stochastic fieldlines. The observed collisionality dependence of the L-mode edge electric field with applied RMP is also qualitatively consistent with this model. Reflectometry data indicate a significant reduction of the normalized L-mode radial density gradient a/Ln at high RMP field with simultaneous increase in radial particle flux and electron thermal flux from power balance analysis. We conjecture that the increase of PLH with RMP results from the combined effects of reduced E × B flow shear (increasing turbulent transport levels) and toroidal/poloidal flow modulation due to edge stochasticity. Initial experiments indicate that non-resonant n = 3 magnetic perturbations lead only to relatively small changes in Er, E × B shear and fluctuation characteristics, and have less impact on the L–H transition power threshold. This motivates further exploration of the RMP spectrum dependence of PLH for possible mitigation of the observed threshold increase.

Edge toroidal charge exchange spectra analysis in the EAST.

An edge toroidal charge exchange recombination spectroscopy (eCXRS) diagnostic has been deployed successfully on the Experimental Advanced Superconducting Tokamak (EAST) recently, providing edge plasma ion temperature and toroidal rotation. Edge CXRS on EAST typically monitors the carbon vi (529.059 nm, n = 8 → 7) spectral line. Spectral lines emitted from other different impurities that appeared in the wavelength ranges of 528 nm-530 nm illustrate the need to revise the edge change exchange spectra analysis in this region. Since the eCXRS sightlines end on the ion cyclotron range of frequencies (ICRF) stainless steel antenna, the passive Fe lines coming from the antenna could be recorded by eCXRS diagnostic when ICRF is used. A revised edge toroidal charge exchange spectra analysis including Fe spectral lines is presented. The accuracy of eCXRS measurement has been improved by the revised spectra analysis.

Toroidal magnetic field ripple and ion orbit loss effects on edge toroidal rotation in J-TEXT Tokamak plasmas

The edge toroidal rotation on J-TEXT is in the counter-current direction and sensitive to electron density and safety factor. During a density ramp-up phase, the initial toroidal rotation in the edge region (e.g. r/a = 0.7–1), having one speed with little radial dependence, evolves to develop a radial profile when the edge density is sufficiently large. The effects of ripple-induced thermal loss on plasma rotation are predicted by neoclassical theory for those discharges, and comparisons with the measurements are performed. The predictions agree well with the measurements over a range of plasma conditions, confirming that using the ripple effect to interpret the phenomenon is appropriate in certain parameter regimes, while in others the neoclassical effects are not sufficient to predict the experimental observations.

X-point position dependence of edge intrinsic toroidal rotation on the Tokamak à Configuration Variablea)

Recent theoretical work predicts intrinsic toroidal rotation in the tokamak edge to depend strongly on the normalized major radial position of the X-point. With this motivation, we conducted a series of Ohmic L-mode shots on the Tokamak a Configuration Variable, moving the X-point from the inboard to the outboard edge of the last closed flux surface in both lower and upper single null configurations. The edge toroidal rotation evolved from strongly co-current for an inboard X-point to either vanishing or counter-current for an outboard X-point, in agreement with the theoretical expectations. The whole rotation profile shifted roughly rigidly with the edge rotation, resulting in variation of the peak core rotation by more than a factor of two. Core rotation reversals had little effect on the edge rotation. Edge rotation was slightly more counter-current for unfavorable than favorable ∇B drift discharges.

Improvement of the edge rotation diagnostic spectrum analysis via simulation.

The edge rotation diagnostic (ERD) system has been developed on the Joint Texas Experimental Tokamak to measure the edge toroidal rotation velocity by observing the shifted wavelength of carbon V (C V 227.09 nm). Since the measured spectrum is an integrated result along the viewing line from the plasma core to the edge, a method via simulation has been developed to analyze the ERD spectrum. With the necessary parameters such as C V radiation profile and the ion temperature profile, a local rotation profile at the normalized minor radius of 0.5-1 is obtained.

L–H power threshold studies in JET with Be/W and C wall

A comparison of the L–H power threshold (Pthr) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and IP/BT pairs. The low density dependence of the L–H power threshold, namely an increase below a minimum density ne,min, which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above ne,min, Pthr is reduced by ∼30%, and by ∼40% when the radiation from the bulk plasma is subtracted (Psep), with JET-ILW compared to JET-C. At the L–H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, , while the power threshold at the minimum density scales as . The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (Psep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of Psep and Zeff or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Zeff and/or subdivertor neutral pressure in the L–H transition physics. Depending on the chosen divertor configuration, Pthr can be up to a factor of two lower than the ITPA scaling law for densities above ne,min. A shallow edge radial electric field well is observed at the L–H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, ⩽5 km s−1 ± 2–3 km s−1, at the L–H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative Er well and thus may not be correlated with the formation of the edge transport barrier in JET.

Plasma Response to Supersonic Molecular Beam Injection in J-TEXT

Recently, hydrogen fueling experiments with supersonic molecular beam injection (SMBI) system have been performed in the J-TEXT tokamak. To evaluate the effects of the injection amount of SMBI on plasma behaviors, moderate and intensive SMBs have been separately injected and compared with each other in Ohmic discharges. With moderate SMBs, electron density increases about twice as before, the size of magnetic island slightly decreases, and the edge toroidal rotation speed in a counter-current direction, measured by a high resolution spectrometer (Carbon V ion, 227.09 nm, r/a = 0.7–0.8), is accelerated from 8 km/s to 12 km/s. The state of higher electron density with moderate SMBI can be maintained for a long period, which indicates that plasma confinement is improved. However, with intensive SMBs, the accompanied magnetohydrodynamic (MHD) activities are triggered, and the electron density increases moderately. The edge toroidal velocity is decreased, in certain cases even reversed in the co-current direction. The statistical result of experimental data for moderate and intensive SMBs suggests a preferred fueling amount (less than 3.2×1019) to improve the SMBI fueling efficiency in experiments.

Effects of edge-localized mode-induced neoclassical toroidal viscosity torque on the toroidal intrinsic rotation in the EAST tokamak

Intrinsic rotation has been observed in lower hybrid current-driven (LHCD) H-mode plasmas with type-III edge-localized modes (ELMs) on Experimental Advanced Superconducting Tokamak (EAST), and it is found that the edge toroidal rotation accelerated before the onset of the ELM burst. Magnetic perturbation analysis shows there is a perturbation amplitude growth below 30 kHz corresponding to the edge rotation acceleration. Using the filament model, the neoclassical toroidal viscosity (NTV) code shows there is a co-current NTV torque at the edge, which may be responsible for the edge rotation acceleration. For maximum displacement ∼1 cm and toroidal mode number n=15, the calculated torque density is ∼0.44 N/m2, comparable with the average edge toroidal angular momentum change rate ∼1.24 N/m2. Here, the 1 cm maximum magnetic surface displacement estimated from the experimental observation corresponds to a maximum magnetic perturbation ∼ 10−3–10−2 T, in accordance with magnetic perturbation measurements during ELMs. By varying n from 10 to 20, the magnitude of the edge NTV torque density is mainly ∼0.1–1 N/m2. This significant co-current torque indicates that the NTV theory may be important in rotation problems during ELMs in H-mode plasmas. To better illuminate the problem, magnetic surface deformation obtained from other codes is desired for a more accurate calculation.

Internal Transport Barrier in Edge Plasma of Small Size Divertor Tokamak Using Neutral Beam Injection

We model the internal transport barrier “ITB” in edge plasma of small size divertor tokamak with B2SOLPS0.5.2D fluid transport code. The simulation results demonstrated the following: (1) we control the internal transport barrier by altering the edge particle transport through changes the edge toroidal rotation which agree with the result of Burrell et al. (Edge Pedestal control in quiescent H-mode discharges in DIII-D using co-plus counter-neutral beam injection, Nucl Fusion, 49, 085024 (9pp) in 2009). (2) The radial electric field has neoclassical nature near separatrix with discharge by co-injection NBI. (3) The toroidal plasma viscosity has strong influence on the toroidal velocity.

Edge pedestal characteristics in JET and JT-60U tokamaks under variable toroidal field ripple

The effects of toroidal field (TF) ripple on the edge pedestal characteristics were examined in the TF ripple scan experiments at the plasma current Ip of 1.1 MA in JET and JT-60U. The TF ripple amplitude δR was defined as a value averaged over the existing ripple wells at the separatrix on the outer midplane. By the installation of ferritic inserts (FIs), δR was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2 T) in JT-60U. In JET, δR was varied from 0.08% to 1% by feeding different currents to the odd and even set of coils out of 32 TF coils. The pedestal pressure pped was similar for the cases before and after the installation of FIs in JT-60U. Similarly, no clear difference in pped was also observed in the variation of δR in JET. The core and edge toroidal rotation clearly shifted in the counter-direction by increased δR. However, there were no changes in the spatial profiles of electron density, electron temperature and ion temperature. By the installation of FIs in JT-60U, the ELM frequency fELM decreased by ∼20%, while the ELM energy loss increased by 50–150%. The increased ELM loss power by 30% suggests a reduction of inter-ELM transport with the reduced δR. In JET, fELM increased only slightly with increased δR while the edge toroidal rotation frequency decreased as δR increased. From the inter-machine similarity experiment at 1.1 MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, in the single TF ripple scan at the higher Ip of 2.6 MA in JET, it clearly decreases with the increased δR, accompanying with a strong density pump out at large TF ripple. These results suggests that the effect of TF ripple on H-mode properties becomes stronger in the plasmas with higher Ip or lower edge collisionality of ripple diffusion.

Recent progress in RF heating and long-pulse experiments on EAST

New capabilities including radio-frequency (RF) powers and diagnostics have been developed since the last IAEA meeting. Divertor performance has been systematically assessed for both single null and double null configurations with normal and reversed BT directions to address asymmetries correlated with classic drift and the issues of heat load on divertor plates. 1 MW lower hybrid wave power injection has produced fully non-inductive diverted plasma discharges. Such discharges have been readily extended to well over 1 min duration. With the assistance of lower hybrid current drive (LHCD), EAST can be operated at 1 MA with both RTEFIT/Iso-flux or pre-programmed control algorithms. A later operation mode could produce an improved confinement plasma by increasing the ohmic power through regulating the loop voltage. Both direct electron and ion heating by ion cyclotron resonant frequency power have been observed for the first time on EAST. Intrinsic toroidal rotation and momentum study in ohmic and LHCD plasmas reveals the importance of the balance between neoclassical viscosity and neutral friction at the edge and finds clear linkage between edge and core toroidal rotation. The expected capabilities on EAST in the next two years will allow access to high-performance regimes, such as H-modes, and enable specific physics studies.

Advances in high-harmonic fast wave physics in the National Spherical Torus Experiment

Improved core high-harmonic fast wave (HHFW) heating at longer wavelengths and during start-up and plasma current ramp-up has now been obtained by lowering the edge density with lithium wall conditioning, thereby moving the critical density for perpendicular fast-wave propagation away from the vessel wall. Lithium conditioning allowed significant HHFW core electron heating of deuterium neutral beam injection (NBI) fuelled H-mode plasmas to be observed for the first time. Large edge localized modes were observed immediately after the termination of rf power. Visible and infrared camera images show that fast wave interactions can deposit considerable rf energy on the outboard divertor. HHFW-generated parametric decay instabilities were observed to heat ions in the plasma edge and may be the cause for a measured drag on edge toroidal rotation during HHFW heating. A significant enhancement in neutron rate and fast-ion profile was measured in NBI-fuelled plasmas when HHFW heating was applied.

Edge pedestal control in quiescent H-mode discharges in DIII-D using co-plus counter-neutral beam injection

We have made two significant discoveries in our recent studies of quiescent H-mode (QH-mode) plasmas in DIII-D. First, we have found that we can control the edge pedestal density and pressure by altering the edge particle transport through changes in the edge toroidal rotation. This allows us to adjust the edge operating point to be close to, but below the ELM stability boundary, maintaining the ELM-free state while allowing up to a factor of two increase in edge pressure. The ELM boundary is significantly higher in more strongly shaped plasmas, which broadens the operating space available for QH-mode and leads to improved core performance. Second, for the first time on any tokamak, we have created QH-mode plasmas with strong edge co-rotation; previous QH-modes in all tokamaks had edge counter-rotation. This result demonstrates that counter-NBI and edge-counter rotation are not essential conditions for QH-mode. Both these investigations benefited from the edge stability predictions based on peeling–ballooning mode theory. The broadening of the ELM-stable region with plasma shaping is predicted by that theory. The theory has also been extended to provide a model for the edge harmonic oscillation that enhances edge transport in the QH-mode. Many of the features of that theory agree with the experimental results reported either previously or in this paper. One notable example is the prediction that co-rotating QH-mode is possible provided sufficient shear in the edge rotation can be created.