Transition Power Threshold Research Articles

Helium plasma operations on ASDEX Upgrade and JET in support of the non-nuclear phases of ITER

For its initial operational phase, ITER has until recently considered using non-nuclear hydrogen (H) or helium (He) plasmas to keep nuclear activation at low levels. To this end, the Tokamak Exploitation Task Force of the EUROfusion Consortium carried out dedicated experimental campaigns in He on the ASDEX Upgrade (AUG) and JET tokamaks in 2022, with particular emphasis put on the ELMy H-mode operation and plasma-wall interaction processes as well as comparison to H or deuterium (D) plasmas. Both in pure He and mixed He + H plasmas, H-mode operation could be reached but more effort was needed to obtain a stable plasma scenario than in H or D. Even if the power threshold for the LH transition was lower in He, entering the type-I ELMy regime appeared to require equally much or even more heating power than in H. Suppression of ELMs by resonant magnetic perturbations was studied on AUG but was only possible in plasmas with a He content below 19%; the reason for this unexpected behaviour remains still unclear and various theoretical approaches are being pursued to properly understand the physics behind ELM suppression. The erosion rates of tungsten (W) plasma-facing components were an order of magnitude larger than what has been reported in hydrogenic plasmas, which can be attributed to the prominent role of He2+ ions in the plasma. For the first time, the formation of nanoscale structures (W fuzz) was unambiguously demonstrated in H-mode He plasmas on AUG. However, no direct evidence of fuzz creation on JET was obtained despite the main conditions for its occurrence being met. The reason could be a delicate balance between W erosion by ELMs, competition between the growth and annealing of the fuzz, and coverage of the surface with co-deposits.

Determining the access to H–mode in the ITER pre–fusion and fusion power operation phases at low plasma current with full–radius TGLF–SAT2 simulations of L–mode plasmas

The pre–fusion power operation 1 phase of ITER is planned to be characterized by electron cyclotron resonance heating only. Under the assumption that the access to H–mode is determined by a critical ion heat flux at the plasma edge, full–radius ASTRA simulations with the TGLF–SAT2 transport model are performed in order to compute the ion heat flux produced by the thermal exchange between electrons and ions in different operational conditions. Both hydrogen and deuterium plasmas at 5 MA are considered, respectively at 1.8 T and 2.65 T, corresponding to one third and half of the nominal maximum magnetic field. Different levels of electron cyclotron heating power are considered in sets of simulations with increasing values of the electron line averaged density. The predictions are compared with the currently available scaling of the critical ion heat flux. In hydrogen, 20 MW of electron heating power are predicted to allow H–mode access in a vanishingly small density window, whereas 30 MW and 40 MW would allow more substantial H–mode operational windows. Despite the fact that in deuterium plasmas the thermal exchange between electrons and ions is smaller by the hydrogen to deuterium mass ratio compared to hydrogen plasmas, the lower H–mode power threshold in deuterium leads to the prediction that an even broader and more robust domain to access H–mode is obtained at half field at 40 MW in deuterium as compared to operation in hydrogen at one third of the maximum magnetic field, even at the same power.

Turbulence characteristics and flow dynamics impacts on the H-mode transition in favourable magnetic geometry with lower power threshold.

The L-H transition power threshold (PLH) in favourable magnetic geometry (ion ∇B drift pointing towards X-point) is much lower than in the unfavourable magnetic geometry (ion ∇B drift pointing away from X-point) on multiple tokamaks. In a systematic experiment on DIII-D, the ion ∇B drift direction was changed continuously from the unfavourable to favourable configuration during plasma discharges. During such process, the input neutral beam power was kept constant at a value that was above PLH for favourable configuration, but lower than PLH for unfavourable configuration. Toroidal field and plasma current were also kept constant and there was little change in the edge electron density ne and electron temperature Te profiles. The density fluctuation amplitude was reduced approaching the transition, while a large increase of turbulence Reynolds stress and flow shear were simultaneously observed. The turbulence decorrelation rate was found to increase as the ion ∇B drift direction was moving towards the favourable configuration, but the flow shear also increased and exceeded the turbulence decorrelation rate. These measurements demonstrate an important correlation between turbulence and turbulence-driven flow and a lowering of PLH, provide insights into the underlyingphysics behind the hidden parameters and inform a more complete physics-based model of the L-H transition power threshold. This article is part of a discussion meeting issue 'H-mode transition and pedestal studies in fusion plasmas'.

H-mode dithering phase studies on ST40.

The dithering H-mode phase, characterized by oscillations, is generally observed at input power values close to the L-H transition power threshold and low plasma collisionalities (low electron density and/or high plasma temperature). Measurements to characterize the dithering phase are presented for the low aspect ratio, high magnetic field tokamak, ST40. The dithering phase oscillation frequency is observed between 400 and 800 Hz and demonstrates an inverse relationship with core plasma density. Dithering phase H-modes are documented across a nonlinear, low-density power threshold operational space, with signature low- and high-density branches. The minimum power threshold for dithering H-mode access is measured at a core, line average electron density of 4.7(±0.5) × 1019 m-3, close to a predicted value of 4.1(±0.4) × 1019 m-3 from multi-machine studies. ASTRA calculated values of power coupled to the ion species, at the dithering H-mode transition, exhibit a similar nonlinear dependence on density. This analysis points to the important contribution of the ion thermal channel to the L-H phase transition. The low-frequency plasma density and D-alpha dithers appear to be accompanied by sudden bursts of magnetohydrodynamic (MHD) activity. A simple model is tested to demonstrate a possible scenario of self-regulation among turbulence, zonal flows, pressure (density) gradient and MHD activities. This article is part of a discussion meeting issue 'H-mode transition and pedestal studies in fusion plasmas'.

Reducing the L-H transition power threshold in ITER-similar-shape DIII-D hydrogen plasmas

Recent dedicated DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (at a plasma current I p ∼ 1.5 MA and ITER-similar edge safety factor q 95 ∼ 3.6) show that the L-H transition power threshold P LH can be reduced substantially (∼30%) with L-mode helium admixtures n He/n e ⩽ 25%. In the ensuing H-mode, helium ion fractions n He/n H remain below 25%. H-mode normalized pressure and confinement quality are only slightly affected by helium seeding, and Z eff ⩽ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures T e(0)/T i(0) ⩾ 1 and edge heat flux ratio Q e/Q i(ρ = 0.95) ∼ 1.2–1.5. Without mitigation, P LH is higher by a factor of 2–3 in comparison to similar ISS deuterium plasmas. ISS hydrogen plasmas with lower plasma current I p ∼ 1 MA (increased edge safety factor q 95 ∼ 5.1) exhibit a substantially lower power threshold. This plasma current dependence, also observed previously on ASDEX-U and in JET, is not accounted for by the commonly used 2008 ITPA multi-machine threshold scaling, but could potentially allow H-mode access at marginal heating power during the initial plasma current ramp-up. Attempts to reduce P LH with low-field- and high-field-side hydrogen pellet injection, using 1.7 mm diameter pellets, have not demonstrated a robust threshold reduction, in contrast to successful earlier experiments with larger 2.7 mm pellets. Techniques for reducing P LH are very important for ITER, in particular for accessing H-mode in hydrogen plasmas during the Pre-Fusion Power Operation-1 (PFPO-1) campaign with marginal auxiliary heating power (20–30 MW of ECH).

Power transfer efficiency and the power threshold for E–H mode transition in inductively coupled plasmas

The effects of gas pressure and gas component on the power transfer efficiency [Formula: see text] and transition power threshold [Formula: see text] during the E–H mode transition in inductively coupled plasmas are studied. The evolutions of [Formula: see text] during the E–H mode transition in pure Ar and Ar/O2 discharges are similar, i.e., in E-mode discharge, [Formula: see text] slightly increases with raising the applied power ([Formula: see text] is below 30%), whereas, an abruptly upward jump of nearly two to four times for [Formula: see text] occurs when the discharge transits to H-mode, and then [Formula: see text] monotonously increases with increasing the applied power. In addition, as the pressure rises, [Formula: see text] increases rapidly first and then slowly in pure Ar discharge. However, [Formula: see text] in Ar/O2 mixture gases discharge shows a non-monotonic behavior. It reveals a tendency of first increase at low pressures and then decreases at high pressures, which is because the resistive component of the plasma varies with the electron density [Formula: see text] and the electron effective collision frequency [Formula: see text]. Furthermore, a non-monotonic behavior of transition power threshold [Formula: see text] with the pressure is found, i.e., with increasing the pressure, [Formula: see text] first decreases and then increases. It may be caused by the threshold electron density, which is constant at low pressures [Formula: see text] but increases at high pressures [Formula: see text].

I-mode operation in helium plasma with pure radio frequency wave heating and ITER-like tungsten divertor on EAST

The characteristics of I-mode operation in helium (He) plasma with radio-frequency (RF) wave heating and ITER-like tungsten divertor have been investigated for the first time on EAST, under the favorable B T configuration. Stationary I-mode in He plasma lasting for ∼5.4 s was achieved at I p = 0.5 MA, B T = 2.4 T with a total injected RF power of 3.5 MW, along with the appearance of the 30–90 kHz weakly coherent mode and the ∼13 kHz edge temperature ring oscillation. The pulse length was only limited by the heating power duration. He concentration was found to play a critical role in I-mode operation, as L–I transition threshold power was increased and the energy confinement was degraded with increasing He concentration. The experimental investigation result on energy confinement and divertor heat flux between I- and H-mode plasma was also given at a similar He concentration.

Magnetic and Pressure Effects on E–H Mode Transition Power and Electron Energy Distribution in Helical Antenna-Coupled RF Plasma

We used helical antenna-coupled radio frequency (RF) plasmas to study the evolution of the mode transition power and electron kinetics under different applied magnetic fields and argon pressures. Langmuir probe and emission spectrum data indicate that the discharge mode changes from the capacitive coupling E mode to the inductive coupling H mode through a strong mixed mode (E + H). One reason for this strongly mixed ionization is the antenna structure, which increases the capacitive coupling. Furthermore, the $\text{E}\to \text{H}$ transition threshold power decreases with increasing pressure regardless of whether there is a magnetic field, and the presence of the magnetic field significantly increases the transition power. The electron energy distribution function (EEDF) curve has a Druyvesteyn shape at low RF power (E mode). The presence of a magnetic field increases the electron energy range, and the binding of the electrons by the magnetic field causes the EEDF to deviate significantly from the Maxwellian shape. As RF power increases, the EEDF tends toward the Maxwellian shape during the transition from the E mode to the H mode. The magnetic field appears to act like a drag on this process, particularly at very low pressures.

On the role of the hydrogen concentration in the L-H transition power threshold in EAST

A decrease in the L-H transition power threshold under the lithium wall conditioning has been observed since 2010 in EAST. The physical mechanism responsible for the reduced is found to be related to three phases of wall recycling as determined by the accumulation of lithium in the vessel. In phase I (a few rounds of wall conditioning) the wall recycling is low, while no clear variation of L-H transition power threshold is observed. In phase II (after another few rounds of wall conditioning) a clear increase in the wall recycling is observed, leading to decreased L-H transition power threshold. In phase III the wall recycling is higher and the hydrogen concentration is lower compared to phases I and II. In this third phase the L-H transition power threshold is mainly reduced by the decreased . During phase III of the wall recycling, the quantitative relationship between the hydrogen content and normalized L-H transition power threshold is studied for campaigns in 2010, 2012, 2015 and 2016. Data scaling reveals that in the hydrogen concentration range of 3–45%, i.e. increases significantly for , while it increases mildly for .

Particle source and radial electric field shear as the factors affecting the LH-transition possibility and dynamics in a tokamak

Numerous theoretical and experimental studies have proved the important role of radial electric field inhomogeneity, or shear, in LH-transition initiation, and established heating power threshold for transition, although some experiments provide the observation of LH-transition dependency on particle source. It is necessary to apply a concerted approach to describe LH-transition initiation possibility and dynamics, considering Er shear, particle source and turbulence properties as the main factors responsible for LH-transition initiation.

H-mode operation in helium plasma with tungsten divertor and low input torque in EAST

To support the ITER early non-nuclear operational phase, recent helium experiments have been investigated intensively in the superconducting tokamak EAST equipped with ITER-like Tungsten divertor, under the condition of low injected torque. This paper demonstrates experimental observations from these experiments. In particular, we report the impact of helium concentration on the L-H transition power threshold, energy confinement, and the ELM behavior. Dependences of L-H threshold on electron density, plasma current and magnetic field are also discussed.

Numerical analyses of impurity behaviors for CFETR advanced scenarios by core-edge integrated simulations

Abstract The impurity behaviors for the advanced scenarios of CFETR with full tungsten divertor have been analyzed. Core plasma profiles for steady state and hybrid scenarios are simulated by consistently iterative transport calculations within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate the effects of different seeding impurities (Ne, Ar, Kr) with the OMFIT modelled core plasma parameters as input. The impurity behaviors for the two modelled scenarios are similar. Seeding by all the considered impurities can effectively reduce the power to the divertor below the acceptable level. For Ne and Ar seeding, it is possible to achieve H-mode plasma operation with power from the core plasma to the SOL higher than L-H transition power threshold. However, higher Ne seeding rate would strongly reduce the fusion power due to the dilution effect. Kr seeding seems to be infeasible because the power crossing the separatrix would be lower than the L-H transition threshold, when the seeding rate is high enough to meet the divertor requirements. Higher separatrix electron density or SOL diffusion can slightly reduce the core radiation. Although normal COREDIV simulations are performed with a fixed H98 factor, simulations with fixed transport indicate that higher impurity seeding rate leads to a decrease of confinement and fusion gain, due to the increase of impurity radiation loss in the core plasma.

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.

Overview of KSTAR research progress and future plans toward ITER and K-DEMO

A decade-long operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) has contributed significantly to the operation of superconducting tokamak devices and the advancement of tokamak physics which will be beneficial for the ITER and K-DEMO programs. Even with limited heating capability, various conventional as well as new operating regimes have been explored and have achieved improved performance. As examples, a long pulse high-confinement mode operation with and without an edge-localized mode (ELM) crash was well over 70 and 30 s, respectively. The unique capabilities of KSTAR allowed it to improve the capability of controlling harmful instabilities, and they have been instrumental in uncovering much new physics. The highlights are that the L/H transition threshold power is sensitive to the resonant magnetic perturbation (RMP) and insensitive to non-resonant magnetic perturbation. Co-Ip offset rotation dominated by an electron channel predicted by general neoclassical toroidal viscosity theory was confirmed. Improved heat dispersal in a divertor system using three rows of rotating RMP was demonstrated and predictive control of the ELM-crash with a priori modeling was successfully tested. In magnetohydrodynamic physics, validation of the full reconnection model (i.e. q0 > 1 right after the sawtooth crash) and self-consistent validation of the anisotropic distribution of turbulence amplitude and flow in the presence of the 2/1 island with theoretical models were achieved. The turbulence amplitude induced by RMP was linearly increased with the slow RMP coil current ramp-up time (i.e. the magnetic diffusion time scale). The Dα spikes (i.e. ELM-crash amplitude) was linearly decreased with the turbulence amplitude and not correlated with the perpendicular electron flow. In the turbulence area, a non-diffusive ‘avalanche’ transport event and the role of a quiescent coherent mode in confinement were studied. To accommodate the anticipation of a higher performance of the KSTAR plasmas with the increased heating powers, a new divertor/internal interface with a full active cooling system will be implemented after a full test of the new heating (neutral beam injection II and electron cyclotron heating) and current drive (CD) (Helicon and lower hybrid CD) systems. An upgrade plan for the internal hardware, heating systems and efficient CD system may allow for a long pulse operation of higher performance plasmas at βN > 3.0 with f bs ~ 0.5 and Ti > 10 keV.

Safety factor and turbulence dynamics dependence of the L-H power threshold on DIII-D

The L-H transition power threshold (PLH) is found to have a significant dependence on q95, at ne ∼ 3.2 × 1019 m−3 on DIII-D. Comprehensive 2D turbulence and flow measurements in the plasma edge reveal the co-existence of two frequency bands of broadband modes across the L-H transition with higher flow shear at higher q95, which can help explain the linear decrease in PLH as q95 is increased from 3.5 to 4.9. Density fluctuation measurements by beam emission spectroscopy at higher q95 show that the lower frequency band (<20 kHz) of the broadband modes propagates in the ion diamagnetic direction in the lab frame (identified as an ion mode), whereas the higher frequency band (>20 kHz) propagates in the electron diamagnetic direction (identified as an electron mode). The turbulence amplitude at the plasma edge is higher at higher q95, implying a higher drive for a Reynolds stress driven zonal flow. At lower density, ne ∼ 1.5 × 1019 m−3, there is little dependence of PLH on q95. Linear CGYRO kinetic simulation has shown a switch from one mode to two modes at higher q95 at ne ∼ 3.2 × 1019 m−3 compared to lower q95. The ion temperature profile has been shown to strongly impact the dual mode characteristics with the increasing growth rate of the ion mode at a higher edge Ti gradient. The observations suggest that it may be possible to reduce the required input power to trigger the L-H transition for ITER if both the ion and electron modes are similarly driven in the plasma edge.

A new model of the L–H transition and H-mode power threshold

In order to understand the mechanism of the confinement bifurcation and H-mode power threshold in magnetically confined plasma, a new dynamical model of the L–H transition based on edge instability phase transition (EIPT) has been developed. With the typical plasma parameters of the EAST tokamak, the self-consistent turbulence growth rate is analyzed using the simplest case of pressure-driven ballooning-type instability, which indicates that the L–H transition can be caused by the stabilization of the edge instability through EIPT. The weak E × B flow shear in L-mode is able to increase the ion inertia of the electrostatic motion by increasing the radial wave number of the tilted turbulence structures, which play an important role for accelerating the trigger process of EIPT rather than directly to suppress the turbulent transport. With the acceleration mechanism of E × B flow shear, fast L–H and H–L transitions are demonstrated under the control of the input heating power. Due to the simplified scrape-off-layer boundary condition applied, the ratio between the heating powers at the H–L and L–H transition respectively differs from the ratio by Nusselt number. The results of the modeling reveal a scaling of the power threshold of the L–H transition, PL−H ∝ n0.76B0.8 for deuterium plasma. It is found finite Larmor radius induces an isotope effect of the H-mode power threshold.

Effect of poloidal density asymmetries on shear flows and radial electric field at the plasma edge

Mass flows and radial electric field driven by edge poloidal density asymmetries can be used as a highly effective control mechanism for the edge and thus the global confinement in tokamaks. The underlying physics can be demonstrated entirely within a simple magnetohydrodynamic equilibrium model that addresses the poloidal pressure asymmetry self-consistently. As an example, strong dependence of the low to high transition power threshold on the magnetic topology, an experimental observation still poorly understood, can be easily explained within this framework. Similar arguments indicate that the ITER fueling ports above the midplane may lead to higher input power requirements.

3D field induced collisionless zonal flow decay in tokamak plasmas

We perform a gyrokinetic study of 3D resonant magnetic perturbation (RMP) field effect on zonal flows in tokamak plasmas. The result indicates that energy-dependent secular radial drift of particles due to the tangential component () of the RMP field induces a further long term algebraic decay of the Rosenbluth–Hinton (1998 Phys. Rev. Lett. 80 724) residual flow expected in an axisymmetric tokamak. This mechanism could be related to an increase of H-mode transition power threshold in the presence of RMP observed in many tokamak experiments.

Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on DIII-D

Measurements of long wavelength ( < 1) density fluctuation characteristics in the edge of both Deuterium (D) and Hydrogen (H) plasmas across the L-H transition on DIII-D demonstrate the existence of single or double bands of low-wavenumber turbulence observed near the edge of H and D plasmas. These are strongly correlated with the L to H-mode transition power threshold (PLH) and can help explain the isotopic and density dependence of PLH, and how the PLH difference is reduced at higher density. Understanding and accurately predicting the L-H power threshold is critical to accessing to H-mode, and operating and achieving high confinement in burning plasmas such as ITER. Above about ne ~ 4 × 1019 m−3, PLH is seen to converge for H and D, and increases for both with higher density. Surprisingly, the PLH increases significantly at low density in H but not in D plasmas. Two distinct frequency bands of density fluctuations are observed in the D plasmas at low density, ne ~ 1.2–1.5 × 1019 m−3, but not in H plasmas with similar density, which appears to be correlated to the much lower power threshold in D at low density. Consistently, E × B shear in the region of r/a ~ 0.95–1.0 is larger in D plasmas than in H plasmas at low density; as the PLH increases with increasing density, the dual mode structure disappears while E × B shear becomes similar and small for both D and H plasmas at higher density, ne ~ 5 × 1019 m−3, where PLH is similar for both D and H plasmas. The increased edge fluctuations, increased flow shear, and the dual-band nature of edge turbulence correlating with lower PLH may account for the strong isotope and density dependencies of PLH and support current L-H transition theories but suggest a complex behavior that can inform a more complete model of the L-H transition threshold.

Effects of low-Z and high-Z impurities on divertor detachment and plasma confinement

The impurity-seeded detached divertor is essential for heat exhaust in ITER and other reactor-relevant devices. Dedicated experiments with injection of N2, Ne and Ar have been performed in DIII-D to assess the impact of the different impurities on divertor detachment and confinement. Seeding with N2, Ne and Ar all promote divertor detachment, greatly reducing heat flux near the strike point. The upstream plasma density at the onset of detachment decreases with increasing impurity-puffing flow rates. For all injected impurity species, the confinement and pedestal pressure are correlated with the impurity content and the ratio of separatrix loss power to the l-H transition threshold power. As the divertor plasma approaches detachment, the high-Z impurity seeding tends to degrade the core confinement owing to the increased core radiation. In particular, Ar injection with up to 50% of the injected power radiating in the core cools the pedestal and core plasmas, thus significantly degrading the confinement. As for Ne seeding, medium confinement with H98∼0.8 can be maintained during the detachment phase with the pedestal temperature being reduced by about 50%. In contrast, in the N2 seeded plasmas, radiation is predominately confined in the boundary plasma, which leads to less effect on the confinement and pedestal. In the case of strong N2 gas puffing, the confinement recovers during the detachment, from ∼20% reduction at the onset of the detachment to greater than unity comparable to that before the seeding. The core and pedestal temperatures feature a reduction of 30% from the initial attached phase and remain nearly constant during the detachment phase. The improvement in confinement appears to arise from the increase in pedestal and core density despite the temperature reduction.