Good Energy Confinement Research Articles

SAW velocity reduction and spurious modes suppression on the layer structure

This paper discusses the use of the layered structure for the SAW velocity reduction. First, it is shown that the insertion of SiO2 is crucial between the piezoelectric layer and the base substrate to keep a large electromechanical coupling factor even when the SAW velocity is reduced. It is also shown that the use of a high-velocity substrate such as Si is also crucial for bringing spurious resonances much higher than the main resonance. The impact of Si orientation is also discussed. Next, suppression of transverse mode resonances and lateral leakage is studied. It is shown that the double busbar structure offers Q enhancement through good lateral energy confinement, and its combination with the piston design also gives good transverse mode suppression. Furthermore, discussions are extended to a strong spurious resonance that appears near the anti-resonance.

Long-pulse H-mode operation with stored-energy monitoring for detachment feedback control with a new lower tungsten divertor in EAST

One of the key challenges for future fusion research is to mitigate the high steady-state heat load on the divertor target plates, and divertor detachment offers a promising solution. The Experimental Advanced Superconducting Tokamak (EAST) has developed several feedback control methods for divertor detachment. However, when an off-normal event momentarily disturbs the main plasma, impurity seeding may still be conducted by these methods for detachment, which probably drives the main plasma further away from its stable equilibrium or even causes the disruption. These off-normal events include excessive impurity seeding, loss of heating and dust droplets, which are not rare in current tokamak experiments, especially in long-pulse operation. To compensate for the drawbacks of these methods, we propose and develop a module of stored-energy monitoring to ensure stable plasmas in long-pulse operation. The stored energy usually decreases when the main plasma is away from its stable equilibrium, which is suitable for monitoring the state of the main plasma. Once the stored energy falls below a certain threshold, the module actively switches off the impurity seeding system. Without impurity seeding, the main plasma can recover with the increase in the stored energy. Only when the stored energy exceeds another threshold does the module switch on the impurity seeding to continue the detachment operation. The module function has been verified during the EAST radiative divertor experiments in the newly-upgraded lower tungsten divertor. A typical ∼20 s discharge in grassy-ELM H-mode regime with ∼5 MW source heating power is demonstrated with divertor partial detachment and good energy confinement by active impurity seeding (50% neon, 50% D2). The energy confinement factor is maintained at a high level, i.e. The electron temperature in the core region only has a slight change after the impurity seeding, while the electron density has a ∼10% increase. Furthermore, the ion temperature near the axis also has a remarkable increase. These achievements provide an important demonstration of the actively controlled radiative divertor mitigating the heat loads with good core confinement, which is an essential step toward steady-state operation of fusion reactors.

Mitigation of plasma–wall interactions with low-Z powders in DIII-D high confinement plasmas

Experiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in H-mode plasmas (I p = 1 MA, B t = 2 T, P NB = 6 MW, ⟨n e⟩ = 3.6–5.0 ⋅ 1019 m−3) into the upper small-angle slot divertor for 2 s intervals at constant rates of 3–204 mg s−1. The multi-species BN powders at a rate of 54 mg s−1 showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy W mhd by 5%. Rates of 204 mg s−1 of boron nitride powder further reduce edge localized mode-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an n = 2 tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation.

Design of All-Optical Directional Coupler Using Plasmonic MIM Waveguide for Switching Applications

In this paper, we have proposed, analyzed, and verified the performance of an optimized plasmonic 10-dB directional coupler and a 3-dB directional coupler in 2-D plasmonic waveguides using the finite-difference-time-domain (FDTD) method. A plasmonic 10-dB directional coupler and a 3-dB directional coupler are based on the metal–insulator-metal (MIM) slab waveguide and analyzed at the telecommunication wavelength (λ) of 1550 nm. Here, coupling and transmission characteristics are analyzed with the optimized separation distance between the two parallel waveguides. The developed approach ensures the minimization of the crosstalk and overall directional coupler length via simultaneous adjustment of the separation distance between the parallel waveguide and the length of the linear waveguide. Then, an optimized structure is acquired by trading off between coupling length and separation distance. The proposed 10-dB directional coupler and 3-dB directional coupler feature good energy confinement, ultra-compact, and low propagation loss, which has potential applications in photonic integrated devices, optical signal processors, and other all-optical switching devices.

Transverse energy confinement and resonance suppression in SAW resonators using low-cut lithium tantalate

This paper discusses the applicability of double busbar design to surface acoustic wave (SAW) devices employing low-cut lithium tantalate (LT) with a multi-layered structure. This design offers good energy confinement, scattering loss suppression and transverse mode suppression for a wide frequency range. In addition, the effectiveness of manipulating the slowness curve shape for transverse mode suppression is demonstrated. First, three different lateral edge designs are applied to the layered SAW configuration on low-cut LT, and their performances are compared using the periodic three-dimensional finite-element method powered by the hierarchical cascading technique. Then, the discussion is extended to the influence of the SAW slowness shape to the transverse mode suppression.

Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

Double Busbar Structure for Transverse Energy Leakage and Resonance Suppression in Surface Acoustic Wave Resonators Using 42°YX-Lithium Tantalate Thin Plate.

This article describes a new transverse edge structure with double busbar for surface acoustic wave (SAW) devices employing a 42°YX-lithium tantalate thin plate such as incredible high-performance (I.H.P.) SAW. This design offers good energy confinement and scattering loss suppression for a wide frequency range. First, preexisting transverse edge designs are reviewed, and their difficulties are pointed out using the dispersion relation for lateral SAW propagation. Then, numerical simulations are performed using the periodic 3-D finite-element method (FEM) powered by the hierarchical cascading technique, and effectiveness of the proposed structure is revealed. In addition, we also provide a possible solution to expand the frequency range giving well energy confinement and demonstrate effectiveness of manipulating the SAW slowness curve shape for transverse mode suppression.

Gyrokinetic simulation of turbulent transport for I-mode edge plasmas

I-mode is an attractive candidate among the confinement regimes proposed for burning operation with good energy confinement similar to H-mode but poor particle confinement similar to L-mode, resulting in no obvious impurity accumulation and free of large edge localized modes. Although I-mode has been recently achieved experimentally on several tokamaks, theory and simulation remain insufficient to explain the mechanisms behind the I-mode formation and its peculiar transport behaviour. In this work, an electrostatic gyrokinetic simulation using gyrokinetic toroidal code (GTC) is carried out for the typical I-mode plasma profiles. Linear simulations reveal that two competitive instabilities coexist within the range of short and long wavelengths during I-mode phase, respectively. It is found that the passing electron response cannot be treated adiabatically for those modes with mode numbers close to that of weekly coherent mode, which is considered to be a major player in the I-mode formation and responsible for the unusual transport behaviour exhibited in I-mode plasmas. Nonlinear simulations yield a turbulent heat transport level that is comparable to the experimental level, and about a half of the experimental particle transport level.

Approaching detachment in I-mode—response of core confinement and the edge pedestal in the ASDEX Upgrade tokamak

Experiments on nitrogen assisted divertor detachment in the improved energy confinement mode (I-mode) are reported from the ASDEX Upgrade tokamak. When nitrogen is introduced into the divertor and radiation losses cool the divertor plasma down, a loss of core confinement is observed, concomitant with an increase in low frequency edge fluctuation levels. The loss in confinement can be compensated and the I-mode can be maintained by additional heating power input. Detachment of the inner divertor leg has been observed for the first time in an I-mode discharge. The outer divertor leg remains attached in these experiments. Good energy confinement properties (H 98(y, 2) = 0.9) during the detachment of the inner divertor leg are reported.

Plasma Profiles Modifications by Upgraded Power of IBW Heating System in HT-7 Tokamak

The enhancement in local and globalized electron heating has been observed after upgrading of ion Bernstien waves (IBWs) heating system in HT-7 tokamak. The overall capability of injection power has been upgraded from 350 to 600 kW (15–30 MHz) for best and active control of pressure and current density profiles, and to discover the new phenomena of plasma heating for good energy and particle confinement. A new quadruple T-type IBWs coupling antenna has been utilized which has been mounted in the toroidal direction on low field side in the device. The electron heating on both on-axis and off-axis electrons not delete has been observed. Not delete The change in direct electron heating via electron Landau damping (ELD) from IBWs has been investigated, while the bulk electron temperature showed a large rise with a heating factor, ∆(Tene)/Prf, up to 9.3 × 1019 eV m–3 kW–1 with central electron density (ne) 3.7 × 1019 m–3. The particle confinement has also been achieved with IBW injection power of 550 kW at frequency of 27 MHz and with toroidal magnetic field of 1.92 T. The modifications observed in electron temperature and other plasma profiles are discussed under various plasma conditions. The improved energy confinement has also been observed in this scenario.

Divertor impurity seeding with a new feedback control scheme for maintaining good core confinement in grassy-ELM H-mode regime with tungsten monoblock divertor in EAST

Small perturbations and strong impurity exhaust capability associated with the small grassy ELMs render the grassy-ELM regime a suitable candidate for achieving steady-state H-mode operation with a radiative divertor, especially in a metal-wall device, such as the Experimental Advanced Superconducting Tokamak (EAST). As the degradation of pedestal performance with excessive divertor impurity seeding or accumulation tends to be accompanied with significantly increased radiation near the divertor X point, feedback control of the absolute extreme ultraviolet (AXUV) radiation near the X point has been employed to maintain the confinement property in EAST. However, the absolute value of the AXUV radiation at the outer target varies with plasma conditions as during the divertor detachment process. Thus, a new feedback-control scheme has been recently developed and applied to grassy-ELM H-mode plasmas in EAST to achieve stationary partial detachment while maintaining good global energy confinement with H 98,y2 >1. In this scheme, electron temperatures (T et) measured by divertor Langmuir probes are used to identify the onset of detachment, and then the plasma control system (PCS) switches to the feedback control of one channel of AXUV radiation near the X point, where a steep gradient in the radiation profile is present. The feedback is performed through pulse-width-modulated duty cycle of a piezo valve to seed impurities with mixed gas (50% Ne and 50% D2) from the outer target plate near the strike point in the upper tungsten monoblock divertor. T et near the strike point is maintained in the range of 5–8 eV, and peak surface temperature on the outer target plate (T IR,peak) is suppressed and maintained at ∼180 °C, based on infrared camera measurements. The plasma stored energy maintains nearly constant over the entire feedback-control period. It thus offers a highly promising plasma control scenario suitable for long-pulse high-performance H-mode operation in EAST, which is potentially applicable to future steady-state fusion reactors as an integrated solution for the control of both ELM-induced transient and steady-state divertor heat loads while maintaining good core confinement.

Stationary ELM-free H-mode in ASDEX Upgrade

A stationary H-mode without edge localized modes (ELMs) has been recently achieved in the ASDEX Upgrade tokamak by applying central electron cyclotron resonance heating above the L-H power threshold with adequate fueling. It is naturally obtained in favorable ∇B configuration and has several features which are desirable for future reactors, such as dominant electron heating, low input torque, possibility of access at low input power, good energy confinement (), high density (), and no impurity accumulation despite the absence of ELMs. This regime is similar to the EDA H-mode and always features an edge electromagnetic quasi-coherent mode which seems to be responsible for enhanced transport losses, as its appearance and disappearance are correlated with changes in edge and divertor parameters.

Integrated operation of steady-state long-pulse H-mode in Experimental Advanced Superconducting Tokamak

Recent Experimental Advanced Superconducting Tokamak (EAST) experiments have successfully demonstrated a long-pulse steady-state scenario with improved plasma performance through integrated operation since the last IAEA FEC in 2016. A discharge with a duration over 100 s using pure radio frequency (RF) power heating and current drive has been obtained with the required characteristics for future long-pulse tokamak reactors such as good energy confinement quality (H98y2 ~ 1.1) with electron internal transport barrier inside ρ < 0.4, small ELMs (frequency ~100–200 Hz), and good control of impurity and heat exhaust with the tungsten divertor. The optimization of X-point, plasma shape, the outer gap and local gas puffing near the low hybrid wave (LHW) antenna were integrated with global parameters of BT and line-averaged electron density for higher current drive efficiency of LHW and on-axis deposition of electron cyclotron heating in the long-pulse operation. More recently, a high βP RF-only discharge (βP ~ 1.9 and βN ~ 1.5, /nGW ~ 0.80, f bs ~ 45% at q95 ~ 6.8) was successfully maintained over 24 s with improved hardware capabilities, demonstrating performance levels needed for the China Fusion Engineering Test Reactor steady-state operation. A higher energy confinement is observed at higher βP and with favorable toroidal field direction. Towards the next goal (⩾400 s long-pulse H-mode operations with ~50% bootstrap current fraction) on EAST, an integrated control of the current density profile, pressure profile and radiated divertor will be addressed in the near future.

Observation of a fully non-inductive H-mode regime dominated by the sporadic-small edge-localized modes in EAST with a tungsten divertor

Here we report a fully non-inductive H-mode operation with a sporadic-small-edge-localized mode(ELM)-dominant pedestal realized in EAST with a tungsten divertor, where the normalized electron collisionality at the pedestal top is estimated below unity. This is regarded as the expansion and/or supplement of our previous research having been preliminarily reported (section 4.2 in (B N Wan et al 2017 Nucl. Fusion 57 102019)) in the EAST overview article contributed to the special issue on FEC 2016 summaries and overviews. In this operation, spontaneous avoidance of large ELMs is realized reproducibly, for the first time on EAST, at a lower collisionality (below unity) at the pedestal top, profiting from high heating power. Besides, this regime shows the well controlled bulk plasma density and light impurity concentration, good energy confinement with H98, (y2) ≃ 1, together with a loop voltage very close to zero. All of these features are considered critical to a steady-state operation with high performance, which EAST is on target to demonstrate in the near future in support to the International Thermonuclear Experimental Reactor (ITER). However, this operation regime suffers a continuous accumulation of medium-/high-Z impurities, which may seriously limit its sustainability. The resonant magnetic perturbation (RMP) with a dominant toroidal mode number n = 1 has been employed for assisting exhaust of heavy impurities, which is, unfortunately, not successful since the RMP with an effective poloidal spectrum always brings back large ELMs. This scenario transits to an ELM H-mode regime as plasma current is decreased, and elimination of large ELMs is well reproduced by ELITE simulation. Finally, we present a discussion on the potential mechanism eliminating large ELMs, which seems to highlight the LHW-induced profound change in edge magnetic topology.

Theoretical explanation of I-mode impurity removal and energy confinement

Improved (I) mode operation maintains the beneficial improved energy confinement of high (H) mode operation while also providing natural fueling to compensate for impurity removal. We show that outward collisional radial impurity flux occurs for I-mode, making the presence of turbulence compatible but unnecessary to explain I-mode impurity removal. We propose that the neoclassical radial impurity flux direction is a robust characteristic distinction between I-mode and H-mode, rather than the turbulent weakly coherent mode that is sometimes undetected, perhaps due to its strong spatial localization. In both regimes the observed E × B flow shear can break up eddies to reduce the turbulence level, leading to good bulk energy confinement.

Directional coupler based on an elliptic cylindrical nanowire hybrid plasmonic waveguide.

We present what we believe is a novel directional coupler based on an elliptic cylindrical nanowire hybrid plasmonic waveguide. Using the finite element method, the electric field distributions of y-polarized symmetric and antisymmetric modes of the coupler are compared, and the coupling and transmission characteristics are analyzed; then the optimized separation distance between the two parallel waveguides, 100nm, is obtained. This optimized architecture fits in the weak coupling regime. Furthermore, the energy transfer is studied, and the performances of the directional coupler are evaluated, including excess loss, coupling degree, and directionality. The results show that when the separation distance is set to 100nm, the coupling length reaches the shorter value of 1.646μm, and the propagation loss is as low as 0.076dB/μm, and the maximum energy transfer can reach 80%. The proposed directional coupler features good energy confinement, ultracompact and low propagation loss, which has potential application in dense photonic-integrated circuits and other photonic devices.

Analysis of performance degradation in an electron heating dominant H-mode plasma after ECRH termination in EAST

In recent EAST experiments, significant performance degradation accompanied by a decrease of internal inductance is observed in an electron heating dominant H-mode plasma after the electron cyclotron resonance heating termination. The lower hybrid wave (LHW) deposition and effective electron heat diffusivity are calculated to explain this phenomenon. Analysis shows that the changes of LHW heating deposition rather than the increase of transport are responsible for the significant decrease in energy confinement (). The reason why the confinement degradation occurred on a long time scale could be attributed to both good local energy confinement in the core and also the dependence of LHW deposition on the magnetic shear. The electron temperature profile shows weaker stiffness in near axis region where electron heating is dominant, compared to that in large radius region. Unstable electron modes from low to high k in the core plasma have been calculated in the linear GYRO simulations, which qualitatively agree with the experimental observation. This understanding of the plasma performance degradation mechanism will help to find ways of improving the global confinement in the radio-frequency dominant scenario in EAST.

Turbulence in high-beta ASDEX upgrade advanced scenarios

Recent experiments at ASDEX Upgrade achieve non-inductive operation in full tungsten wall conditions by applying electron cyclotron and neutral beam current drive. These discharges are characterised by a well-measured safety factor profile, which does not drop below one, and a good energy confinement. By reproducing the experimental heat fluxes, nonlinear gyrokinetic simulations suggest that the observed strong peaking of the ion temperature in the core is caused by the stabilising impact of a significant beam ion content, as well as strong electromagnetic effects on turbulent transport. Quasilinear transport models are not yet applicable in this interesting and reactor relevant parameter regime, but available simulation data may serve as a testbed for improvements. As the present plasma is close to the kinetic ballooning (KBM) threshold, elevating the safety factor profile under otherwise identical conditions is proposed to clarify, whether profiles are ultimately limited by KBM turbulence, or by global stability constraints.

A stationary long-pulse ELM-absent H-mode regime in EAST

A stationary edge-localized mode (ELM)-absent H-mode regime, with an electrostatic edge coherent mode (ECM) which resides in the pedestal region, has been achieved in the EAST tokamak recently. This regime allows the operation of a nearly fully noninductive long pulse (>15 s), exhibiting a relatively high pedestal and good global energy confinement with near 1.2, and excellent impurity control. Furthermore, this regime is mostly obtained with a 4.6 GHz lower hybrid current drive (LHCD) or counter-current neutral beam injection (NBI), plus electron cyclotron resonance heating, and an extensive lithium wall coating. This stationary ELM-absent H-mode regime transits to a stationary small ELM H-mode regime, and upon additional heating power from the 2.45 GHz LHCD, an ion cyclotron resonant frequency or co-current NBI is applied (under 4.6 GHz LHCD heating background). A slight change of the plasma configuration also makes the small ELMs reappear. The experimental observations suggest that a long-pulse ELM-absent regime can be induced by the ECM, which exhibits strong electrostatic fluctuations and may provide a channel for continuous particle (especially impurities) and heat exhaust across the pedestal. The ECM exists in the collisionality of = 2.5–4 and the pressure gradient = = 100–200 (kPa), which is in good agreement with the previous simulation of GYRO. This ELM-absent H-mode regime with ECM may offer a suitable candidate for high-performance, steady-state H-mode operation in future fusion reactors.

Gyrokinetic study of turbulence suppression in a JET-ILW power scan

For exploring tokamak operation regimes that deliver both high β and good energy confinement, power scans at JET with ITER-like wall have been performed. Relatively weak degradation of the confinement time coincides with increased core temperature of the ions at high power. The changes in core turbulence characteristics during a power scan with an optimized (broad) q profile are analyzed by means of nonlinear gyrokinetic simulations. The increase in β is crucial for stabilizing ion temperature gradient driven turbulence, accompanied by increased ion to electron temperature ratio, the presence of a dynamic fast ion species, as well as the geometric stabilization by increased thermal and suprathermal pressure. A sensitivity study with respect to the q profile reveals that electromagnetic effects are more pronounced at larger values of q. Further, it is confirmed that turbulence suppression due to rotation becomes less effective in such strongly electromagnetic systems. Electrostatic simplified models may thus perform well in present-day devices, in which high β is often correlated with high rotation, but provide poor extrapolation towards low rotation devices. Implications for ITER and reactor plasmas are discussed.