Enhancement Of Energy Confinement Research Articles

Development of high-performance long-pulse discharge in KSTAR

High-performance long-pulse plasma operation is essential for producing economically viable fusion energy in tokamak devices. To achieve such discharges in KSTAR, firstly, the rapid increase in the temperature of plasma-facing components was mitigated. The temperature increase of the poloidal limiter, especially, was associated with beam-driven fast ion orbit loss and the discrepancy of the equilibrium reconstructed with heated magnetic probes of signal drift. The fast ions lost to the poloidal limiter were reduced by optimizing the plasma shape and the composition of neutral beam injection (NBI). This nonlinear signal drift was successfully reduced by a new thermal shielding protector on the magnetic probes. Secondly, a lower loop voltage approach was implemented to reduce a poloidal flux consumption rate. A plasma current of 400 kA and a line-averaged electron density of ∼2.0 × 1019 m−3 were chosen by considering the L–H power threshold, fast ion orbit loss, and beam shine-through power loss for low loop voltage in KSTAR. In addition, the application of electron cyclotron heating also helped maintain the plasma with low loop voltage (∼25 mV) by enhancing the NBI-driven current and achieving a high poloidal beta (β P) state. KSTAR has achieved a long pulse (∼90 s) operation with the high performance of β P ⩽ 2.7, thermal energy confinement enhancement factor (H98y2) ∼ 1.1, and fraction of non-inductive current (f NI) ⩽ 0.96. Still, gradual degradation of the plasma performance has been observed over time in the discharges. In one of the long-pulse discharges, β P reduced by ∼18% over the time of ∼8τ R (current relaxation time, τ R ∼ 5 s) and ∼1067τ E,th (thermal energy confinement time, τ E,th ∼ 45 ms). The degradation may be closely associated with weak, yet growing, and persistent toroidal Alfvén eigenmodes and their effect on fast ion confinement.

Dual-hydrophilic Janus evaporator for Long-term and efficient Bimode solar evaporation

Solar steam generation has emerged as a promising and sustainable technology to solve the global water scarcity. However, the fabrication of low-cost and high-efficiency evaporator with long-term stability remains a great challenge. Herein, a dual-hydrophilic Janus evaporator coupled with inclined isolating structure is fabricated to realize long-term efficient solar evaporation and flexible salt manipulating, where the upper electrospun Mo2C-C@PAN membrane serves as the evaporation layer and the bottom nonwoven fabric functions is used as mass transporting layer. The encapsulating feature of the Mo2C-C@PAN enables better energy confinement and reduces the usage of solar absorber by ∼50 % without deteriorating the evaporation performance. The hydraulic conductivity difference prevents the Marangoni effect inner the Janus evaporator, while the inclined isolating structure enables unidirectional ions transporting. With features of adequate water transport, enhancement of energy confinement, and salt manipulation, the constructed inclined Janus evaporators can not only work stably for 200 h in 10 wt% NaCl solution without salt accumulation, but also achieve efficient salt harvesting with rate of ∼260 g m−2 d−1 under one sun. This study provides new possibilities for a dual-hydrophilic Janus evaporator for a long-term, efficient, and synergistic extraction of fresh water and mineral salt resources.

A study of the heating and current drive options and confinement requirements to access steady-state plasmas at Q ∼ 5 in ITER and associated operational scenario development

Access to steady-state (SS) plasma scenarios that can be potentially achieved in ITER by the exploitation of the neutral beam (NB) and electron cyclotron (EC) heating and current drive capabilities, including their foreseen upgrades, has been assessed and development of the Q ∼ 5 ITER SS operation (SSO) scenario has been carried out by combining all the necessary physics and engineering components. A set of modelling codes including 0.5D/1.5D transport and source modelling suites (METIS, ASTRA and CORSICA) and ideal MHD stability analysis codes (KINX and MISHKA) are applied to combined analysis and scenario development. Operating the plasma with a moderate density peaking factor (n e0/<n e> ∼ 1.3) for sufficient fusion gain (Q up to 5) at a low density (f GW = 0.6–0.8) and the required current drive efficiency produced an internal inductance (l i(3) ∼ 0.8–0 9) which improves the stability margin for low-n external ideal MHD modes. Based on this first observation, a combined operational space and ideal MHD stability analysis has recently identified Q ∼ 5 ITER SS target plasmas (Polevoi et al 2020 Nucl. Fusion 60 096024), and then the CORSICA scenario modelling suite has been applied to study access to the target plasmas as well as to develop a candidate operation scenario. Ideal MHD stability analysis on the SS plasmas has been performed using DCON embedded in the CORSICA scenario modelling suite. The use of off-axis electron cyclotron current drive (ECCD) with a power level of 20–30 MW from the equatorial and upper launchers was essential for the tailoring of the current profile to maintain ideal MHD stability of SS plasmas in ITER. Upgrading the NBI power from the baseline 33 MW to 49.5 MW provided the necessary current drive capability to allow fully non-inductive Q ∼ 5 operation with an energy confinement enhancement requirement (H 98 ≲ 1.6) similar to that achieved in some experiments in present tokamaks (Petty et al 2020 62nd Annual Meeting of the APS Division of Plasma Physics, Virtual Meeting (USA, 9–13 November 2020); Snyder et al 2019 Nucl. Fusion 59 086017; Solomon et al 2013 Nucl. Fusion 53 093033). The CORSICA simulations of the access to the Q ∼ 5 ITER SS target plasmas show that, provided that the enhancement of the energy confinement (H 98 ∼ 1.5–1.6) is achieved, it is possible for ITER to demonstrate the Q ∼ 5 SSO with NB and EC heating and current drive within the limits of the ITER Central Solenoid (CS)/Poloidal Field (PF) coil systems.

Tenfold increase in the fusion triple product caused by doubling of toroidal magnetic field in the spherical tokamak Globus-M2

The current work reports on the significant rise of the fusion triple product in experiments carried out on the compact spherical tokamak (ST) Globus-M2 with a twofold increase in the toroidal magnetic field. A tenfold rise in the n . T . τ E product was recorded during an increase in the magnetic field from 0.4 to 0.8 T and the plasma current from 0.25 to 0.4 MA at an unchanged auxiliary heating power value. Limited reasons may affect this positive trend, among which are energy confinement improvement and an increase in the efficiency of neutral beam heating. Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. It was experimentally confirmed that strong dependence of the energy confinement time on the magnetic field value is conserved at a higher magnetic field approaching 0.8 T. Enhancement of energy confinement is connected to a collisionality (ν *) decrease. While for conventional tokamaks the confinement dependence on collisionality becomes weaker with decreasing ν * dependence, in the ST, in contrast, we observe its strengthening.

I-mode pedestal relaxation events at ASDEX Upgrade

The I-mode confinement regime can feature small edge temperature drops that can lead to an increase in the energy deposited onto the divertor targets. In this work, we show that these events are associated with a relaxation of both electron temperature and density edge profiles, with the largest drop found at the pedestal top position. The relative energy loss is about 1 %, and is thus lower than that of type-I ELMs for the same pedestal top collisionality. Stability analysis of edge profiles reveals that the operational points are far from the ideal peeling-ballooning boundary. Also, we show that these events appear close to the H-mode transition in the typical I-mode operational space in ASDEX Upgrade, and that no further enhancement of energy confinement is found when they occur. Moreover, scrape-off layer transport during these events is found to be very similar to type-I ELMs, with regard to timescales (≈ 800 µs), filament propagation, toroidally asymmetric energy effluxes at the midplane and asymmetry between inner and outer divertor deposited energy. In particular, the latter reveals that more energy reaches the outer divertor target. Lastly, first measurements of the divertor peak energy fluence are reported, and projections to ARC—a reactor that could potentially operate in I-mode—are drawn.

Favourable scenarios established by SMBI for the realization of the ELMy H-mode at HL-2A**Project supported by the National Natural Science Foundation of China (Grant Nos. 11375057 and 11175061) and the National Magnetic Confinement Fusion Science Program, China (Grant Nos. 2010GB102003 and 2014GB108003).

The ELMy H-mode plasmas realized with the supersonic molecular beam injection (SMBI) are studied in relation to the energy confinement and the heating power for the L–H transition ( in the HL-2A tokamak. A database is assembled for this study based on the ELMy H-mode discharges during the experimental campaigns in the period 2009–2013 at the HL-2A tokamak. The statistical results show that the SMBI is favourable for reaching the H-mode by reducing the heating power at the L–H transition and for the H-mode performance by improving the energy confinement compared with the ordinary gas puffing (GP). The reduction of is about 20% when the density is low, and the energy confinement enhancement factor of 1.5 is achieved with the SMBI. Note that in the database the density dependence of is non-monotonic with the at which the is minimum. Most of data are on the low density branch where the increases with the decrease in density. The minimum of the in HL-2A is comparable to the ITPA multi-machine threshold power scaling . The physics behind the reduction of the with the SMBI is also investigated in relation to the change of the density gradient at the plasma edge, the gas fuelling efficiency, and the recycling.

Control of 3D edge radiation structure with resonant magnetic perturbation fields applied to the stochastic layer and stabilization of radiative divertor plasma in LHD

It is found that resonant magnetic perturbation (RMP) fields have a stabilizing effect on the radiating edge plasma, realizing stable sustainment of radiative divertor (RD) operation in the Large Helical Device (LHD). Without RMP, thermal instability leads to radiative collapse. Divertor power load is reduced by a factor of 3–10 during the RMP-assisted RD phase, while maintaining relatively good core plasma confinement with confinement enhancement factor . It has also been demonstrated that the RMP field itself can initiate transition to RD operation by increasing perturbation strength, while keeping constant density and injection power. The results show a possibility of a new control knob for divertor power load in a 3D magnetic field configuration. It is also found that after the transition to RD, the energy confinement enhancement factor based on ISS04 scaling increases by a factor of 1.4 compared with the attached phase. The operation range of the RMP-assisted RD is identified in terms of RMP strength and radial location of the resonance layer of the RMP. A 3D edge radiation structure is analysed using the edge transport code EMC3-EIRENE and the results are compared with experiments. The comparison indicates that the application of RMP modulates the 3D edge radiation structure such that an intense radiation appears around the X-point of the m/n = 1/1 island in the case with RMP, while it is located at the inboard side without RMP.

STRUCTURE DEPENDENT VARIATIONS OF GROUP VELOCITY, ENERGY LOSS AND CONFINEMENT IN A REGULAR GRATED WAVEGUIDE

The Green's function method in the Dyson's formulation has been employed for the ab initio numerical study of structural effects on the device performances in terms of the minimum group velocity, energy loss and energy confinement in a quasi two-dimensional grated waveguide device model. The structure parameters to be varied in this work consist of the number of the grating teeth (N) and the grating groove depth (g). It is found that those parameters exhibit roughly linear and mostly monotonous variations at the lower resonance. The reduction of the group velocity and enhancement of energy confinement are also shown to be effectively attained by increasing N, while leaving the loss parameter relatively unaffected. On the other hand, the calculated results for the upper resonance wavelength are shown to exhibit consistently non-monotonous responses to increasing g. Similarly distinct behaviors are also found in the relationship between the minimum group velocity (vg, min) and the energy loss (L) as well as that between vg, min and the confined energy (W) for various N and g at the upper resonances. This peculiarly different behaviors are shown to be related to the variations of the associated local density of states with respect to N and g calculated for the upper resonance.

Comparison between dominant NB and dominant IC heated ELMy H-mode discharges in JET

The experiment described in this paper is aimed at characterization of ELMy H-mode discharges with varying momentum input, rotation, power deposition profiles and ion to electron heating ratio obtained by varying the proportion between ion cyclotron (IC) and neutral beam (NB) heating. The motivation for the experiment was to verify if the basic confinement and transport properties of the baseline ITER H-mode are robust to these changes, and similar to those derived mostly from dominant NB heated H-modes. No significant difference in the density and temperature profiles or in the global confinement were found. Although ion temperature profiles were seen to be globally stiff, some variation of stiffness was obtained in the experiment by varying the deposition profiles, but not one that could significantly affect the profiles in terms of global confinement. This analysis shows the thermal plasma energy confinement enhancement factor to be independent of the heating mix, for the range of conditions explored. Moreover, the response of the global confinement to changes in density and power were also independent of heating mix, reflecting the changes in the pedestal, which is in agreement with globally stiff profiles. Consistently, the pedestal characteristics (pressure and width) and their dependences on global parameters such as density and power were the same during NB only or with predominant IC heating.

Dimensionless pedestal identity experiments in JT-60U and JET in ELMy H-mode plasmas

This paper reports the first results of collaborative experiments in JT-60U and JET to compare ELMy H-mode characteristics in the two machines. The JT-60U/JET identity experiments were aimed explicitly at matching edge pedestal parameters in plasmas of the same geometry. The similar size of the two devices allows plasma configurations and conditions to be established for which the match of the dimensionless parameters (ρ*, ν*, β and q95) and of the equilibrium geometry is carried out in plasmas that are also almost identical in dimensional terms.The results of the power and density scans at fixed (and matched) Ip − BT show that, in a common range of ρ*, the pedestal electron collisionality, , and normalized pressure, βp,ped do not match in the two machines, against expectations. In particular, the range of βp,ped of JT-60U ELMy H-modes with Type I ELMs is accessible to JET plasmas only with a Type III ELM edge, and in JT-60U is consistently higher by ≈2 than in JET, for the same ρ*. A comparison of ne and Te pedestal profiles indicates that ∇Te is comparable in the two devices, while JET ne profiles are steeper than JT-60U. The global energy confinement enhancement factor (H98y − 2) is higher in JET than in JT-60U, and part of this difference is attributed to the reduced pedestal pressure in JT-60U compared with JET.The two experiments are analysed in detail to try to identify ‘hidden’ physics that may be the cause of the failure to achieve the pedestal identity as predicted. Although a full explanation for the different behaviour of the edge pedestal in the two devices has not yet been found, the possible influence of aspect ratio effects on MHD stability, differences in the plasma current profile and ripple loss (and rotation) effects are discussed.

Overview of the Large Helical Device

The Large Helical Device (LHD) experiments have started after aconstruction period of eight years, and two experimental campaigns wereperformed in 1998. The magnetic field was raised up to 2.75 T at a magneticaxis position of 3.6 m at the end of the second campaign. In the thirdcampaign, started in July in 1999, the plasma production with ECH of 0.9 MWand auxiliary heating with NBI of 3.5 MW have achieved an electrontemperature of 3.5 keV and an ion temperature of 2.4 keV. The maximumstored energy has reached 0.75 MJ with an averaged electron density of7.7×1019 m-3 by hydrogen pellet injection. The ICRF heatinghas sustained the plasma for longer than 2 s and the initial stored energyof the NBI target plasma has increased from 0.27 MJ to 0.335 MJ. The majorcharacteristic of the LHD plasma is the formation of the temperaturepedestal, which leads to some enhancement of energy confinement over theISS95 scaling law. The confinement characteristic is gyro-Bohm and themaximum energy confinement has reached 0.28 s. The LHD has also shown itshigh potentiality for steady-state operation by realizing a 22 s dischargein the second campaign.

H-mode confinement and fusion performance in JET

JET has conducted extensive experiments in H-mode plasmas focusing on two broad aims. The first, involving the characterization of reactor-relevant aspects of the regime, encompasses the study of steady-state H-modes at plasma currents of up to 6 MA. In these plasmas, energy confinement is typically a factor of 1.8 - 2 greater than predicted by L-mode scalings and local transport varies with the ion Larmor radius, at least for input powers well above the H-mode threshold power. Using nitrogen impurity seeding, a regime has been developed in which good core confinement, with an enhancement of 1.5 relative to L-mode scaling, is combined with radiated power fractions of up to 80%. The second major goal is to optimize fusion power production by exploiting high-confinement ELM-free H-modes, which exhibit energy confinement enhancement factors of three or more relative to L-mode plasmas. In this regime, fusion power quotients have been achieved, though the highest fusion performance is transient.

Recent VH-mode results on DIII-D

A regime of improved H-mode energy confinement, VH-mode, is obtained in the DIII-D tokamak with adequate vessel conditioning. The improved confinement in VH-mode is consistent with the extension of the region of high E*B velocity shear turbulence suppression zone further in from the plasma boundary. The energy confinement enhancement in VH-mode can be limited by ELMs, localized momentum transfer events, or operation at high heating power or low q. Energy confinement enhancement improves with increasing triangularity of the plasma cross section and is independent of elongation. The termination of VH-mode is associated with an edge localized kink mode which is destabilized by both the large edge pressure gradient and edge current density.

Subignited, ITER-Like Designs—A Question of Confinement Margin

If the next-step International Thermonuclear Experimental Reactor (ITER) is designed to operate at finite energy multiplication (Q [approximately] 10 to 20), as opposed to ignition (Q [approximately] [infinity]), appreciable reductions in size and cost will result. Ignition will be attainable in such a [open quotes]high-Q targeted[close quotes] device under slightly enhanced confinement conditions. For example, with the nominal design guidelines, from the ITER Conceptual Design Activity (CDA), designing for Q = 15 instead of ignition results in [approximately] 20% savings in size and cost. Ignition would still be achievable in such a reduced-size device if the L-mode energy confinement enhancement factor (i.e., H factor) is [approximately] 15% higher than the assumed nominal value of 2.0. This size/cost impact is large compared to other sensitivities, and the range of H-factor improvement needed to recoup ignition is small compared to the uncertainty in the confinement scalings themselves. 12 refs., 2 figs., 1 tab.