Favourable scenarios established by SMBI for the realization of the ELMy H-mode at HL-2A

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（P_（L-H） ） 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 2009–2013at 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 P_（L-H） is about 20% when the density is low, and the energy confinement enhancement factor of H_（H98y2）= τ_E/τ_（th,98y2） ≈ 1.5 is achieved with the SMBI. Note that in the database the density dependence of P_（L-H） is non-monotonic with the ˉne,min≈ 3×10^（19） m^（-3） at which the P_（L-H） is minimum. Most of P_（L-H） data are on the low density branch where the P_（L-H） increases with the decrease in density. The minimum of the P_（L-H） in HL-2A is comparable to the ITPA multi-machine threshold power scaling P_（thr_scal08）. The physics behind the reduction of the P_（L-H） 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.

Tungsten control in type-I ELMy H-mode plasmas on EAST

The first experimental investigation of the tungsten behavior in ELMy H-mode plasmas with co-/counter neutral beam injection(NBI)and unfavor-able/favorable B t was performed on EAST.Tungsten was found to accumulate easily in ELMy H-mode plasma with co-NBI heating and unfavorable B t.Thus,in this case the tungsten concentration can exceed 10^(-4),resulting in degradation of the plasma confinement and periodic H–L transitions.To reduce the tungsten concentration in steady-state type-I ELMy H-mode operation,counter-NBI is applied to modify the density and temperature and brake the plasma toroidal rotation.The applied counter-NBI decreases the PHZ+E_(r) inward pinch velocity and rever-ses the direction of neoclassical inward convection,thus decreasing the tungsten concentration from-7×10^(-5) to-2×10^(-5) in type-I ELMy H-mode plasma with favorable B_(t).A comparison of the effects of different B_(t) directions on the tungsten behavior also shows that favor-able B_(t) is beneficial for reducing the tungsten concentration in the core plasma.These results imply that counter-NBI with favorable B_(t) can effectively prevent tungsten accu-mulation and expand the operating window for exploring steady-state type-I ELMy H-mode operation of EAST.

Experimental study of density gradient-driven micro-instabilities and the confinement degradation during H-mode in EAST

A broadband(BB)mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time.This BB mode usually grows during L–H transitions,featuring a BB quasi-coherent mode with increasing frequency.During H-mode operations,it is characterized by steady-state BB in the high-frequency range(f~200–2000 k Hz),at the electron scale(k_(θ)ρ_(s)=1–2),mainly driven by the density gradient,and is sensitive to the value ofηein the region of interest(ρ=0.4–0.8),wherehe=(R/L_(Te))/(R/L_(ne))is the ratio of the normalized electron temperature gradient and density gradient,and the regionρ=0.4–0.8 usually has a relatively low collisionality(v_(eff)<5).The frequency of BB is found to be dependent on the electron temperature and density gradient,which is a typical feature of electron-driven turbulence.A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found,which indicates a strong electron thermal transport induced by the BB turbulence.The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest(ρ=0.4–0.8),thus makingηedecrease and the BB destabilize further.These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode.It should be noted that this mode is not observed by other diagnostics in EAST,and shows very different features to the coherent modes in the edge.

Blob properties in I-mode and ELM-free H-mode compared to L-mode on EAST

The blob properties in I-mode and ELM-free H-mode plasmas compared to L-mode have been investigated on the EAST tokamak,including the blob detection rate Nb,sizeδb,lifetimeτb and radial velocity vr,b.The blob properties in L-mode and I-mode are similar,and those in ELM-free H-mode are different to them.The blob Nbis smaller whileτbis larger in H-mode.The experimental blob sizeδband velocity scalings vr,b-δb show a good agreement with the theoretical models.The variation in blob properties during the L-I and H-L transitions,and their relations to the scrape-off layer(SOL)density,edge and SOL turbulence,and SOL collisionality are discussed.The suppression of the edge(inside the last closed flux surface)turbulence is not reflected in the blob behavior,while the blob detection rate shows a correlation with the SOL density and its low-frequency(3–50 kH z)fluctuations.In addition,the blob detection rate is found to increase with the divertor collisionalityΛdiv,indicating a dependence of blob behavior onΛdiv.The differences in blob detection rates among the three operating regimes might be due to their different SOL densities and collisionalities.The investigation contributes to understanding the influences of edge and SOL plasma parameters on the blob behavior.

Measurement of molybdenum ion density for L-mode and H-mode plasma discharges in the EAST tokamak

We report the measurement of total molybdenum ion density for L-mode and H-mode plasmas on EAST using spectral lines observation and calculation based on an impurity transport code.A flat-filed extreme ultraviolet spectrometer with some spatial resolution is used to obtain the radial profiles of molybdenum spectral line emissions.The absolute calibration for the extreme ultraviolet spectrometer is finished by comparing the calculated bremsstrahlung intensity with the readings of CCD detector.Molybdenum ion transport study is performed using the radial ion density profiles and one-dimensional impurity transport code STRAHL.The total molybdenum density profiles are determined from the transport analysis.The molybdenum density during L-mode and H-mode phases are obtained,which are about 3 and 4 orders of magnitude smaller than the electron density,respectively.An inward pinch is found during the H-mode phase that leads to the peaked profile of molybdenum density.

Transport Simulation of ECRH H-Mode Experiments on HL-2A Tokamak

Transport simulation of ECRH H-mode experiments on HL-2A tokamak is carried out using ONETWO code, the GLF23 and PEDESTAL models, along with TORAY code for ECRH. It is found that the initial electron and ion temperature profiles affect L-H transition significantly, and larger initial temperature gradient at the edge plasma benefits the transition. The simulation results show that it is possible to achieve ECRH H-mode with appropriate initial electron and ion temperature profiles under present discharge conditions on HL-2A tokamak. In addition, the pedestal density, electron temperature and pedestal width are predicted, and the evolutions of electron and ion temperature profile are calculated.

Realization of minute-long steady-state H-mode discharges on EAST

In the 2016 EAST experimental campaign,a steady-state long-pulse H-mode discharge with an ITER-like tungsten divertor lasting longer than one minute has been obtained using only RF heating and current drive,through an integrated control of the wall conditioning,plasma configuration,divertor heat flux,particle exhaust,impurity management,and effective coupling of multiple RF heating and current drive sources at high injected power.The plasma current（Ip - 0.45 MA） was fully-noninductively driven（Vloop 〈 0.0 V） by a combination of-2.5 MW LHW,-0.4 MW ECH and -0.8 MW ICRF.This result demonstrates the progress of physics and technology studies on EAST,and will benefit the physics basis for steady state operation of ITER and CFETR.

Long Pulse H-Mode Scenarios Sustained by RF Heating on EAST

Abstract EAST has demonstrated its capability of long pulse operation using RF heating （LHCD and ICRF） in past experiments. One key issue to realize the long pulse H-mode expert- meats is to sustain the plasma current for steady state operation. Based on the calculations of the transport code ONETWO and its coupled RF code GENRAY, two scenarios have been proposed to achieve the 10 s H-mode plasma with Ip=400 kA, 〈 ne 〉=4.5×1019 m-a, βN=2, and the 100 s H-mode plasma with Ip=280 kA, 〈 ne 〉=3.5×1019 m-a, βN=1.8 recently. The current drive of lower hybrid wave is an important issue in the two scenarios. An experimental result on lower hybrid current drive in H-mode plasmas on EAST is also presented.

Combined Langmuir-magnetic probe measurements of type-Ⅰ ELMy filaments in the EAST tokamak

Detailed investigations on the filamentary structures associated with the type-I edge-localized modes（ELMs） should be helpful for protecting the materials of a plasma-facing wall on a future large device.Related experiments have been carefully conducted in the Experimental Advanced Superconducting Tokamak（EAST） using combined Langmuir-magnetic probes.The experimental results indicate that the radially outward velocity of type-I ELMy filaments can be up to 1.7 kms^（-1） in the far scrape-off layer（SOL） region.It is remarkable that the electron temperature of these filaments is detected to be ~50eV,corresponding to a fraction of 1/6 to the temperature near the pedestal top,while the density ~3×10^（19）m^（-3） of these filaments could be approximate to the line-averaged density.In addition,associated magnetic fluctuations have been clearly observed at the same time,which show good agreement with the density perturbations.A localized current on the order of ~100kA could be estimated within the filaments.

Effect of Wave Accessibility on Lower Hybrid Wave Current Drive in Experimental Advanced Superconductor Tokamak with H-Mode Operation

The effect of the wave accessibility condition on the lower hybrid cm＇rent drive in the experimental advanced superconductor Tokamak （EAST） plasma with H-mode operation is studied. Based on a simplified model, a mode conversion layer of the lower hybrid wave between the fast wave branch and the slow wave branch is proved to exist in the plasma periphery for typical EAST H-mode parameters. Under the framework of the lower hybrid wave simulation code （LSC）, the wave ray trajectory and the associated current drive are calculated numerically. The results show that the wave accessibility condition plays an important role on the lower hybrid current drive in EAST plasma. For wave rays with parallel refractive index n｜｜= 2.1 or n｜｜ = 2.5 launched from the outside midplane, the wave rays may penetrate the core plasma due to the toroida] geometry effect, while numerous reflections of the wave ray trajectories in the plasma periphery occur. However, low current drive efficiency is obtained. Meanwhile, the wave accessibility condition is improved if a higher confined magnetic field is applied. The simulation results show that for plasma parameters under present EAST H-mode operation, a significant lower hybrid wave current drive could be obtained for the wave spectrum with peak value n｜｜ = 2.1 if a toroidal magnetic field BT =2.5 T is applied.

Simulations of H-Mode Plasmas in Tokamak Using a Complete Core-Edge Modeling in the BALDUR Code

A theory-based model for predicting the pedestal formation in both ion and electron temperatures, and hydrogenic and impurity density is developed and implemented in the 1.5D BALDUR codes for self-consistently simulating H-mode plasma in tokamak. In the simulation, the transports around pedestal, including the electron and ion thermal, hydrogenie and impurity particle transports are calculated using an （Mixed B/gB） model, which is modified to anomalous semi-empirical mixed Bohm/gyro-Bohm include the effects of ωE×B flow shear and magnetic shear. Because of the reduction of transport, the pedestal can be formed. For a preliminary test, this core-edge model is used to simulate the temporal evolution of plasma current, temperature, and density profiles for DIII-D discharges. It is found that the simulations successfully reproduce the experimental results. A statistical analysis, including RMSE and offset, is used to quantify the agreement between the prediction and the corresponding experimental results. The simulation results show an agreement with average RMSE of 11.87%, 14.53%, 7.59% and 12.21% for electron temperature, ion temperature, electron density, and deuterium density profiles, respectively. In addition, it is found that the suppression function developed is effective only in the edge region.

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive （LHCD） alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in Da emission and a spontaneous rise in main plasma density. Good lower hybrid （LH） coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D2 gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive （CD） efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray trueing code GENRAY are reported.

Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode （GAM） and limit cycle oscillations （LCOs）, which have been measured by Doppler reflectometry prior to the high confinement mode （H-mode） in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase （I-phase）. Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E x B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L-H transition through providing a positive feedback for the increase of the mean E × B flow strength.

Heat Flux on EAST Divertor Plate in H-mode with LHCD/LHCD+NBI

Based on the surface temperature measured by the infrared camera on the experimental advanced superconducting tokamak （EAST）, the heat fluxes on the lower outer divertor target plate during H-mode with the lower-hybrid wave current drive （LHCD） only and with the LHCD combined with the neutral beam injection （NBI） are calculated by the DFLUX code and compared. The analyzed discharges are lower single null divertor configuration discharges. In the case with the LHCD only （Ip ~ 400kA, PLHCD ~ 2 MW）, ELM-free appears after L-H transition with the peak heat flux on the lower outer target plate less than 1 MW/m2. However, there is no ELM- free appearing after the L-H transition in the case with the LHCD＋NBI （Ip ~ 300 kA, PLHCD ＋ PNBI ~ 2 MW）. The results show that the peak heat fluxes on the lower outer target plate in the LHCD＋NBI H-mode cases are larger than those in the LHCD H-mode under the similar auxiliary heating power. This is because the heat flux profiles of the lower outer target plate as a function of plate location in ELMing with the LHCD＋NBI are narrower than those with the LHCD only. The results are consistent with the results in terms of the scrape-off layer width observed in the EAST.

Experimental observations of naturally occurring dust using a high-speed vacuum ultraviolet imaging system in EAST

In the ELMy H-mode experiment,naturally occurring dust originating at the high-field side is clearly observed using the high-speed vacuum ultraviolet imaging system developed on the Experimental Advanced Superconducting Tokamak(EAST).The main ablation cloud shape is similar to the classical shape observed in pellet fueling experiments.However,during the dust penetration,an erupted secondary cloudlet with a bent‘cigar’shape is observed and moves upwards along the direction perpendicular to the magnetic field line,which is different to the obviation in the pellet fueling experiments.This may be due to the ion diamagnetic drift effect.The velocities of the secondary cloudlet are estimated to be 50–80 m s^(-1).In addition,a significant degradation of the plasma confinement is observed during the dust penetration.

JET and the Physics Basis of ITER

JET has made unique contributions to the physics basis of ITER by virtue ofits ITER-like geometry, large plasma size and D-T capability. The paper discusses recent JET resultsand their implications for ITER in the areas of standard ELMy H-mode, D-T operation and advancedtokamak modes. In ELMy H-mode the separation of plasma energy into core and pedestal contributionsshows that core confinement scales like gyroBohm transport. High triangularity has a beneficialeffect on confinement and leads to an integrated plasma performance exceeding the ITER Q =10reference case. A revised type I ELM scaling predicts acceptable ELM energy losses for ITER, whileprogress in physics understanding of NTMs shows how to control them in ITER. The D-T experiments of1997 have validated ICRF scenarios for heating ITER/a reactor and identified ion minority schemes(e.g. (~3He)DT) with strong ion heating. They also show that the slowing down of alpha particles isclassical so that the self-heating by fusion alphas should cause no unexpected problems. With thePellet Enhanced Performance mode of 1988, JET has produced the first advanced tokamak mode, withpeaked pressure profiles sustained by reversed magnetic shear and strongly reduced transport. Morerecently, LHCD has provided easy tuning of reversed shear and reliable access to ITBs. Improvedphysics understanding shows that rational g-surfaces play a key role in the formation anddevelopment of ITBs. The demonstration of real time feedback control of plasma current and pressureprofiles opens the path towards fully controlled steady-state tokamak plasmas.

RF design and study of a 325 MHz 7 Me V APF IH-DTL for an injector of a proton medical accelerator

A compact interdigital H-mode drift-tube linac (IH-DTL) with the alternating-phase-focusing (APF) method, working at 325 MHz was designed for an injector of a proton medical accelerator. When fed in with a proper RF (radio frequency) power, the DTL cavity could establish the corresponding electromagnetic field to accelerate the ‘‘proton bunches’’ from an input energy of 3 MeV to an output energy of 7 MeV successfully, without any additional radial focusing elements. The gap-voltage distribution which was obtained from the CST■ Microwave Studio software simulations of the axial electric field was compared with that from the beam dynamics, and the errors met the requirements within ± 5%. In this paper, the RF design procedure and key results of the APF IH-DTL, which include the main RF characteristics of the cavity, frequency sensitivities of the tuners, and coupling factor of the RF power input coupler are presented.

Tangential and Vertical Compact Torus Injection Experiments on the STOR-M Tokamak

This paper describes the setup and results of compact torus (CT) injection experiments on the STOR-M tokamak. Tangential CT injection into STOR-M induced H-mode-like phenomena including doubling the electron density, reduction in the Ha radiation level, suppression of the floating potential fluctuations, suppression of the m = 2 Mirnov oscillations, and increase in the global energy confinement time. Experimental setup, bench-test results, and some preliminary injection data for vertical CT injection experiments on STOR-M will be shown. In addition, numerical simulations of the CT trajectories in tokamak discharges for both tangential and vertical injection geometries will be discussed.

Observation of Pedestal Plasma Turbulence on EAST Tokamak

Pedestal plasma turbulence was experimentally studied by microwave reflectometry on EAST tokamak. The characteristics of edge pedestal turbulence during dithering L-H transition, ELM-free H-mode phase and inter-ELM phase have recently been studied on EAST. An edge spatial structure of density fluctuation and its dithering temporal evolution is observed for the first time on the EAST tokamak during the L-H transition phase. A coherent mode usually appears during the ELM-free phase prior to the first ELM on EAST tokamak. The mode frequency gradually decreases as the pedestal evolves. Analysis shows that the coherent mode is in the pedestal region inside the separatrix. In plasma with type-III ELMs, a precursor mode before ELM is usually observed. The frequency of the precursor was initially about 150 kHz and gradually decreased till the next ELM. The mode amplitude increases or shows saturation before ELM. In the plasma with compound ELMs composed of high and low frequency ELMs, the precursor was also observed before the high frequency ELM while the harmonic oscillations with frequencies of 20 kHz, 40 kHz and 60 kHz appear before the low frequency ELM.