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This conference report gives a summary on the contributed papers and discussions presented at the 8th Asia-Pacific Transport Working Group Meeting held at the Engineering & Technical College of Chengdu University of Technology, Leshan, China, from 12 to 15 June 2018. The objective of the meeting is to develop a predictive understanding of the physical mechanisms responsible for particle, energy and momentum transport in magnetically confined plasmas. In order to achieve this objective, five technical working groups were organized under the headings: (A) edge/SOL turbulence and L-H/H-L transition, (B) core-edge turbulence coupling, and transport in high beta plasmas, (C) transport and turbulence in 3D magnetic topology, (D) interaction between energetic particles, MHD instabilities and turbulence, and (E) model reduction and experiments for validation. A summary of the selected significant progresses from the presentations in the five technical working groups is given. The Young Researchers Forum which was held in this meeting is described. [ABSTRACT FROM AUTHOR]

The oscillations of poloidal plasma flows induced by radially sheared zonal flows are investigated by newly developed correlation Doppler reflectometers in the HL-2A tokamak. The non-disturbing diagnostic allows one to routinely measure the rotation velocity of turbulence, and hence the radial electric field fluctuations. With correlation Doppler reflectometers, a three-dimensional spatial structure of geodesic acoustic mode (GAM) is surveyed, including the symmetric feature of poloidal and toroidal Er fluctuations, the dependence of GAM frequency on radial temperature and the radial propagation of GAMs. The co-existence of low-frequency zonal flow and GAM is presented. The temporal behaviors of GAM during ramp-up experiments of plasma current and electron density are studied, which reveal the underlying damping mechanisms for the GAM oscillation level. [ABSTRACT FROM AUTHOR]

Research on beta induced Alfvén eigenmodes (BAEs) driven by energetic ions has been carried out on the HL-2A tokamak. The BAEs can be observed in the cases of being beyond or below unity. The mode frequencies are around 60–95 kHz. The radial mode structures have been detected by the multi-channel microwave reflectometer and soft x-ray arrays. The results suggest that the BAEs are highly localized at the normalized radius of –0.26. Theoretical analyses based on the general fishbone-like dispersion relation and Alfvén mode code support the experimental measurements in both mode frequencies and locations. The BAEs are found to be excited more easily in low electron density discharges. The coexistence of BAEs and reversed shear Alfvén eigenmodes enables the estimation of directly from experiments. [ABSTRACT FROM AUTHOR]

Two different directions of scale-to-scale energy transfer that corresponds to forward and inverse energy cascades, respectively, were clarified in toroidal plasma turbulence by means of experimental analysis. In the present work we first report the evidence that the turbulence dominated by broad wavenumber spectrum was developed under the action of forward (direct) energy cascade in a reversed field pinch (RFP) plasma, while the generation of GAM zonal flow (ZF) was related with the energy concentrating into low frequency region from higher frequency region due to inverse energy cascade in a tokamak plasma. The two evidences are compared. The results indicate a modulation mechanism in turbulence which is relative to the generation or development of specific turbulent flows. This hints a correlation between energy transfer and the development of toroidal plasma turbulence. [ABSTRACT FROM AUTHOR]

The dynamic features of triggering of type-III edge localized modes (ELMs) by macro-scale radially elongated flow (REF) in magnetically confined toroidal plasmas are presented. The REF, localized near the mid-plane and with a long radial correlation length at the low field side, was observed prior to the collapse of pedestal in series. It is demonstrated that the REF gets energy from and modulates ambient turbulence via nonlinear three wave interaction. The formation of REF leads to the onset of type-III ELMs, where the collapse of plasma energy occurs in the outer region ~0.3 of the plasma column within a few tens of microseconds without global MHD instabilities. The path of the onset is identified and a clue to the trigger mechanism of violent events in high temperature plasmas is discussed. [ABSTRACT FROM AUTHOR]

The mission of HL-2A is to explore the key physical topics relevant to ITER and to the advanced tokamak operation (e.g. the operation of future HL-2M), such as the access of H-mode, energetic particle physics, edge-localized mode (ELM) mitigation/suppression and disruption mitigation. Since the 2016 Fusion Energy Conference, the HL-2A team has focused on the investigations on the following areas: (i) pedestal dynamics and L–H transition, (ii) techniques of ELM control, (iii) turbulence and transport, (iv) energetic particle physics. The HL-2A results demonstrated that the increase of mean shear flow plays a key role in triggering L–I and I–H transitions, while the change of flow is mainly induced by the ion pressure gradient. Both mitigation and suppression of ELMs were realized by laser blow-off seeded impurity (Al, Fe, W). The 30% Ne mixture supersonic molecular beam injection seeding also robustly induced ELM mitigation. The ELMs were mitigated by low-hybrid current drive. The stabilization of m/n  =  1/1 ion fishbone activities by electron cyclotron resonance heating was found on the HL-2A. A new m/n  =  2/1 ion fishbone activity was observed recently, and the modelling indicated that passing fast ions dominantly contribute to the driving of 2/1 fishbone. The non-linear coupling between the toroidal Alfven eigenmode and tearing mode (TM) led to the generation of a high frequency mode with the toroidal mode number n  =  0. The turbulence was modulated by TM when the island width exceeds a threshold and the modulation is localized merely in the inner area of the islands. Meanwhile, turbulence radial spread took place across the island region. [ABSTRACT FROM AUTHOR]

The resonant interaction between energetic-particles and tearing mode is an unresolved physics issue at present. It is found for the first time in tokamaks that an unstable tearing mode with slowly rotating m/n  =  2/1 helicity, where m/n represent poloidal/toroidal mode-numbers, interacts with energetic-ions and results in amplitude-bursting/frequency-chirping fishbone-like activities. Nonlinear hybrid kinetic-MHD simulations with M3D-K code prove that the co-passing energetic-ions are responsible for the drive of tearing modes, and the wave-particle resonance condition is satisfied at , where , and are the toroidal, poloidal angular frequencies of energetic-ions and the mode frequency respectively. These findings can help the understanding of tearing mode induced energetic-particle loss and particle acceleration during the tearing mode reconnection in laboratory and space plasmas. [ABSTRACT FROM AUTHOR]

Experimental observations and theoretical interpretations are presented in detail for the high frequency axisymmetric mode driven by nonlinear mode coupling. In the neutral beam injection heating plasma, toroidal Alfvén eigenmodes (TAEs) are found to nonlinearly interact with the tearing mode and result in the generation of other Alfvénic sidebands (Chen et al 2014 Europhys. Lett. 107 25001). Then two TAEs couple together and contribute to the excitation of an axisymmetric mode with frequency in the ellipticity-induced Alfvén eigenmode (EAE) frequency region. Direct evidence suggests the axisymmetric modes play an energy transfer channel role during the nonlinear procedure and cause the growth of magnetohydrodynamic instability with finite toroidal mode number via beating with tearing mode. Finally, the nonlinear process leading to high frequency nE  =  0 mode generation is demonstrated using nonlinear gyrokinetic theory. Here nE is toroidal mode number and the subscript E represents the mode frequencies lying in the EAE frequency range. [ABSTRACT FROM AUTHOR]

Modulation of turbulent electron temperature fluctuations () and density fluctuations () by an m/n  =  1/1 tearing mode island was observed in the core plasma region of the HL-2A tokamak. High spatiotemporal resolution two-dimensional images of show the first evidence that the turbulence modulation occurs only when the island width exceeds a certain threshold value ( cm) and the modulation is localized merely in the inner area of the island due to significant alteration of local profiles and turbulence drives. Evidence also reveals that for large islands turbulence spreading takes place across the island region. The results are generally consistent with theories and simulations. [ABSTRACT FROM AUTHOR]

Dedicated experiments have been carried out at the J-TEXT tokamak for investigating plasma responses to the externally applied resonant magnetic perturbation (RMP) field with various RMP currents under four different toroidal phases. After its penetration, the dominant component of the RMP resonated at q  =  2 surface and excited an m/n  =  2/1 magnetic island, which rotated poloidally when changing the toroidal phase of the RMP. The experimental results show significant influence of the RMP on plasma toroidal and perpendicular rotations, density fluctuations and geodesic acoustic modes (GAMs) as well as nonlinear interaction of ambient turbulence. Whereas the plasma rotations do not show big difference at the RMP-induced island O- and X-point, the turbulence level is substantially lower at the island O-point due to reduced local temperature gradient. The GAM zonal flows outside the island area are profoundly affected by the RMP in various RMP phases. Moreover, it was found that under certain RMP currents the nonlinear coupling of ambient turbulence was considerably enhanced, no matter of at the island O- or X-point. The results suggest a possible controlling role of the RMP in regulating the nonlinear interplay of electrostatic turbulence. [ABSTRACT FROM AUTHOR]

The tungsten spectrum in the extreme ultraviolet (EUV) wavelength range has been investigated in tokamak HL-2A to find line emissions from low-ionized tungsten ions which can be used for tungsten transport study in the plasma edge. For this purpose, tungsten is injected in the HL-2A plasma using the laser blow-off (LBO) technique with a newly developed double-layer tungsten target. The total number of injected tungsten particles can be accurately controlled by adjusting the distance from the focal lens to the tungsten target. A quasi-continuum spectrum called an unresolved transition array (UTA), consisting of many line emissions from several ionization stages of tungsten ions, is observed at around 50 Å as a typical indication of the tungsten spectrum. In the present HL-2A experiment the quasi-continuum of tungsten is mainly composed of line emissions from W14+ to W23+ ions. Analyzing the observed tungsten spectrum, two isolated line emissions of W VII (5p–5d: 216.219 Å) and W VII (5p–5d: 261.387 Å) from W6+ ions are successfully identified for the first time in tokamaks. The intensity ratio and the wavelengths of the two lines determined in the present experiment show good agreement with the intensity ratio calculated with a collisional-radiative model developed for the W6+ ion and a previous result by Sugar and Kaufman (1975 Phys. Rev. A 12 994), respectively. On the other hand, the W VII line emission occasionally reappears after the LBO injection, suggesting a re-entry of the tungsten deposited onto divertor plates and a movement of the strike point during the discharge to a certain area of the divertor plate where tungsten is deposited. Based on the W VII emission observed during the re-entry event of tungsten ions, an influx of the W6+ ion, , to the edge plasma is evaluated using the inverse photon efficiency calculated with the ADAS code and using an average-configuration option. As a result, it is found that is typically within the range (0.1–2.0)  ×  1013 cm−2 s−1. A brief discussion is also made on the influx evaluated in the present study. [ABSTRACT FROM AUTHOR]

Isotope effects on instabilities driven by ion temperature gradient (ITG) and impurities in tokamak plasmas in the presence of tungsten ions are numerically studied. It is revealed that the tungsten ions significantly modify the isotope scaling of the maximum growth rates of the instabilities with respect to the main or effective ion mass number or Meff [=(1  −  fz)Mi  +  fzMz] with (=Znz/ne) being impurity charge concentration. The most reasonable scaling is deduced as with , for ITG driven modes, whilst holds with , for tungsten impurity modes, in significant contrast with the case of light or intermediate impurities where the scaling is and for ITG and impurity modes, respectively. These results suggest that existence of tungsten impurity would enhance (weaken) the isotope effect of instability driven by ITG (tungsten impurity ions), which is beneficial (harmful) for improvement of confinement when hydrogen isotopes are used in plasmas. The results might also provide hints on studies of particle and energy transports and discharge performance, particularly, in ITER-like wall machines. [ABSTRACT FROM AUTHOR]

In HL-2A, an inverse sawtooth oscillation is observed with a long-lasting m/n  =  1/1 mode during ECRH phase with power deposition inside sawtooth inversion radius (inner-deposited ECRH), while a normal sawtooth instead appears when the ECRH power is deposited outside sawtooth inversion radius (outer-deposited ECRH). Aluminum is then injected as a trace impurity with laser blow-off (LBO) method into the inner- and outer-deposited ECRH phases of HL-2A discharges to investigate the effect of ECRH on impurity transport. Temporal behavior of soft x-ray (SXR) array signals is analyzed with a 1D impurity transport code, and radial structures of impurity transport coefficients are obtained. The result shows that the radial transport of Al ions is strongly enhanced during the inner-deposited ECRH phase. In particular, an outward convection velocity is developed with positive values of 0  ⩽  V(ρ)  ⩽  3.8 m s−1 in ρ  ⩽  0.5, while the convection velocity is inward in ρ  ⩾  0.6. In the outer-deposited ECRH discharge, on the other hand, the convection velocity takes a big negative value in ρ  ⩽  0.4 and close to zero at ρ ~ 0.6. In ohmic discharges, an inward V(ρ) always appears in the whole plasma radii and gradually increases toward the plasma edge (−3.2 m s−1 at ρ  =  1). The simulation result also indicates that centrally-peaked Al ion density profiles presented in the outer-deposited ECRH discharge can be flattened by the inner-deposited ECRH. Modification of impurity transport is discussed in the presence of long-lasting m/n  =  1/1 MHD mode. [ABSTRACT FROM AUTHOR]

Kinetic Alfvén and pressure gradient driven instabilities are very common in magnetized plasmas, both in space and the laboratory. These instabilities will be easily excited by energetic particles (EPs) and/or pressure gradients in present-day fusion and future burning plasmas. This will not only cause the loss and redistribution of the EPs, but also affect plasma confinement and transport. Alfvénic ion temperature gradient (AITG) instabilities with the frequency and the toroidal mode numbers are found to be unstable in NBI internal transport barrier plasmas with weak shear and low pressure gradients, where and are the frequencies of the beta- and toroidicity-induced Alfvén eigenmodes, respectively. The measured results are consistent with the general fishbone-like dispersion relation and kinetic ballooning mode equation, and the modes become more unstable the smaller the magnetic shear is in low pressure gradient regions. The interaction between AITG activity and EPs also needs to be investigated with greater attention in fusion plasmas, such as ITER (Tomabechi and The ITER Team 1991 Nucl. Fusion31 1135), since these fluctuations can be enhanced by weak magnetic shear and EPs. [ABSTRACT FROM AUTHOR]

Effect of the pedestal deposited impurity on the edge-localized mode (ELM) behaviour has been observed and intensively investigated in the HL-2A tokamak. Impurities have been externally seeded by a newly developed laser blow-off (LBO) system. Both mitigation and suppression of ELMs have been realized by LBO-seeded impurity. Measurements have shown that the LBO-seeded impurity particles are mainly deposited in the pedestal region. During the ELM mitigation phase, the pedestal density fluctuation is significantly increased, indicating that the ELM mitigation may be achieved by the enhancement of the pedestal transport. The transition from ELM mitigation to ELM suppression was triggered when the number of the LBO-seeded impurity exceeds a threshold value. During the ELM suppression phase, a harmonic coherent mode (HCM) is excited by the LBO-seeded impurity, and the pedestal density fluctuation is significantly decreased, the electron density is continuously increased, implying that HCM may reduce the pedestal turbulence, suppress ELMs, increase the pedestal pressure, thus extending the Peeling–Ballooning instability limit. It has been found that the occurance of the ELM mitigation and ELM suppression closely depends on the LBO laser spot diameter. [ABSTRACT FROM AUTHOR]

The type of edge turbulence transition from resistive ballooning modes (RBM) to drift-type ion temperature gradient (ITG) modes has been observed in the gas puffing modulation experiments and also simulations at the HL-2A tokamak. The experimental evidence on RMB and ITG characteristics is given by Langmuir probe and Doppler reflectometry data. In the low temperature region (Te  <  80 eV), the variation of turbulence intensity is in phase with that of the parallel resistivity, consistent with RBM features. However, in the high temperature region (Te  >  80 eV), the turbulence intensity is enhanced with increasing temperature and reduction of the parallel resistivity, implying a drive by drift wave type ITG modes. By numerical simulations with drift resistive inertial ballooning model, the dependence of the growth rate on the electron temperature for the most unstable mode has been investigated based on experimental parameters. Both experiments and simulations have suggested that the dominating instability transits from RBMs in the low Te regime to ITG modes in the high Te regime. [ABSTRACT FROM AUTHOR]

The radial profiles of perpendicular flows in the presence of the magnetic island were firstly measured in the HL-2A tokamak by hopping the work frequency of the Doppler backward scattering reflectometer system along with a two-dimensional electron cyclotron emission imaging diagnostic identifying the island locations. It has been observed that across the O-point cut the perpendicular flow is quite small at the center of the island and strongly enhanced around the boundary of the island, resulting in a large increase of the flow shear in the outer half island, while across the X-point cut the flow is almost flat in the whole island region. Meanwhile it was found that the density fluctuations are generally weakened inside the island. The results indicate that both the perpendicular flow and the density fluctuation level are modulated by the naturally rotating tearing mode near the island boundary. The cross-correlation between the perpendicular flows and the oscillating electron temperature further reveals that the modulation of the perpendicular flow occurs mainly inside and in the vicinity of the island. [ABSTRACT FROM AUTHOR]

We report an experimental result on the stabilization of the energetic–ion driven internal kink mode (ion fishbone) by electron cyclotron resonance heating (ECRH), observed for the first time in a toroidal plasma. The mode asserts itself a resistive branch close to the marginal stability point. The resulting fishbone mode depends not only on the injected power but also on the radial deposition location of ECRH, and the instability can be completely suppressed when the injected ECRH power exceeds certain threshold. Analysis by the fishbone dispersion relation, including the resistive effect, suggests that the magnetic Reynolds number plays a key role in the mode stabilization—it weakens the mode growth-rate and enhances the critical energetic–ion beta without changing the energetic–ion population. This ion fishbone stabilization mechanism can be important for future devices such as ITER, which has significant ECRH capability. [ABSTRACT FROM AUTHOR]

Multi-scale interactions have been observed recently in the HL-2A core NBI plasmas, including the synchronous coupling between kink mode and tearing mode, nonlinear couplings of TAE/BAE and TM near surface, AITG/KBM/BAE and kink mode near surface, and between kink mode and high-frequency turbulence. Experimental results suggest that several couplings can exist simultaneously, Alfvenic fluctuations have an important contribution to the high-frequency turbulence spectra, and the couplings reveal the electromagnetic character. Multi-scale interactions via the nonlinear modulation process maybe enhance plasma transport and trigger sawtooth-crash onset. [ABSTRACT FROM AUTHOR]

Strong resonant and non-resonant internal kink modes (abbreviated as RKs and NRKs, respectively), which are also called resonant and non-resonant fishbones, are observed on HL-2A tokamak with high-power ECRH  +  ECCD− (or ECRH) and ECRH  +  ECCD+, respectively. (‘Resonant’ derives from the existence of q  =  1 surface (the resonant surface), and ‘non-resonant’ originates from the absence of q  =  1 surface (). ECCD+ and ECCD− mean the driving direction of energetic electrons is the same and opposite to plasma current, respectively.) RK has features of periodic strong bursting amplitude and rapid chirping-down frequency, but NRK usually has the saturated amplitude, slow changed or constant frequency and long-lasting time. The NRK excited by energetic electrons is found for the first time. The reversed q-profiles are formed, and qmin decreases during plasma current ramp-up. The value of qmin is slightly smaller and a bit bigger than unity for RK and NRK conditions, respectively. The internal kink mode (IKM) structures of RKs and NRKs are confirmed by the ECEI system. Although there are different current drive directions of ECCD for excitation of RK and NRK, they all propagate in electron diamagnetic directions in poloidal. The radial mode structures, frequency and growth rate for IKMs are obtained by solving the dispersion relationship. The NRK is stable when qmin is larger than a certain value, and with the decreasing qmin the frequency drops, but the growth rate almost keeps constant when . This result is in agreement with experimental observation. Studying IKMs excited by energetic electrons can provide important experimental experiences for ITER, because the NRKs may be excited by high-power non-inductive drive of ECCD or ECRH in the operation of hybrid scenarios. [ABSTRACT FROM AUTHOR]

Edge localized mode (ELM) is successfully mitigated by helium and deuterium supersonic molecular beam injections (SMBIs) on HL-2A. During the ELM mitigation by SMBIs, gradients of ion temperature (Ti) and electron density are softened in the pedestal. It has been observed that the averaged gradient of the Ti decreases around 44% and the well depth of radial electric field (Er) is reduced by the SMBI during ELM mitigation. Furthermore, at least 20% decrements of Ti have to be attained to achieve a noticeable increase (decrease) of the ELM frequency (amplitude). In addition, the duration of ELM mitigation with helium SMBI is much longer than that with deuterium, likely due to the higher level of recycling neutral gas compared to that of deuterium; in the case of ELM mitigation by helium SMBI, the recovery duration of the density gradient is much shorter (10–20 ms) than that of Ti (up to 40 ms or longer), indicating the importance of the Ti in the ELM mitigation by SMBI. Finally, it has been observed that the Ti is reduced before the beginning of the ELM mitigation, suggesting that the mechanism of the ELM mitigation by SMBI is closely related to the cooling effect. [ABSTRACT FROM AUTHOR]

Ion internal transport barriers (iITBs) are first observed in neutral beam injection (NBI) heated plasmas at the HL-2A tokamak. The position of the barrier foot, in the stationary state, coincides with the q  =  1 surface within its uncertainty of measurement. iITBs can develop more easily at the beginning of NBI heating. Also, iITBs are unstable for the sawtooth plasma. Simulations reveal that the thermal diffusivity of ions (χi) inside the barrier can be as low as the neoclassical level. It is observed that the flow shear in the stationary iITB state reaches the level required for suppressing the ion temperature gradient mode instability, which indicates the important role of flow shear in sustaining the iITB. [ABSTRACT FROM AUTHOR]

Understanding of nonlocal electron heat transport is of key importance for current magnetic confinement fusion research. Global nonlocal response presents a fundamental challenge to the standard anomalous transport model based on local microinstabilities and turbulence. Here, we present for the first time a new nonlocal phenomenon triggered by the fishbone instability in HL-2A neutral beam injection plasmas. Rapid core heating leads to a simultaneous decrease in temperature at the plasma edge. The effect reveals fast anomalous transport of core heat pulses to the plasma edge, not compatible with diffusive time scales. More importantly, variations at different locations are restricted by the intensity of magnetic fluctuations. The and form two types of hysteresis loops at two sides of the inversion radius. The ECEIs show that the 2D mode structure of the fishbone is intensive shearing/spiraling during the nonlocal transport. Experimental results suggest that magnetic perturbation, long-range correlation, mesoscale structure and flow play crucial roles in the nonlocal response. The Hurst exponent and auto-correlation coefficient indicate that the nonlocal transport is potentially linked to the self-organized critical (SOC) dynamics. This work will be beneficial for understanding of the plasma dynamics in future fusion reactors. [ABSTRACT FROM AUTHOR]

Recent experimental results that are associated with the core-localized (i.e. normalized radius ) Alfvénic modes in HL-2A neutral beam injection (NBI) plasmas with weak magnetic shears are reported. In the different plasma parameter regions, the energetic ions produced by the NBI drive multiple Alfvénic instabilities, such as the toroidal Alfvén eigenmode (TAE), beta-induced Alfvén eigenmode, reversed shear Alfvén eigenmode (RSAE) and fishbone and energetic particle mode (EPM). Here, we focus on the high-frequency RSAE (HFRSAE) and resonant kinetic ballooning mode (rKBM). A group of downward-sweeping frequency coherent modes (HFRSAEs) with 100  <  f  <  500 kHz and n  =  3–7 are often observed with an increase in the edge safety factor, qa. Their measured frequency is more than that of the TAEs, and . The analysis suggests that these modes localize inside the high-order Alfvén eigenmode (AE) gap of the Alfvénic continuum, and their eigenfrequency and eigenfunction depend on the and q-profile. When the core plasma density is more than m−3 and the impurity or supersonic molecular beam enters the bulk plasma, the profiles of the plasma density/pressure peak, and the magnetic shear is weak or negative. In this case, a group of multi-harmonic coherent modes (rKBMs) with 30  <  f  <  150 kHz and n  =  2–9 are observed through multiple diagnostic techniques, and , where is the diamagnetic drift frequency of the thermal ion. It is found that the HFRSAEs can transit into the rKBMs when the density profile suddenly peaks. Neutron monitoring outside the vacuum chamber demonstrates that the HFRSAE and rKBM both degrade the confinement of the energetic ions. The rKBM instabilities also affect the bulk plasma performance. [ABSTRACT FROM AUTHOR]

Self-organized criticality (SOC) dynamics have been investigated in non-local transport regimes induced by the supersonic molecular beam injection in the HL-2A tokamak. Experimental evidence shows that the SOC or avalanche behaviors, such as the Hurst parameter, self-similarity and large-scale radial correlations in turbulence, are remarkably enhanced during the prompt non-local phase, together with an increase of inward propagation of turbulent events. These results highlight the importance of SOC paradigm during the transient non-local thermal transport in magnetically confined plasmas. [ABSTRACT FROM AUTHOR]

Impurity transport in the scrape-off layer (SOL) has been studied in ohmically heated discharges of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy. The vertical profile (from the plasma center to the lower X-point) of carbon emissions of CIII (977 Å: 2s2 1S0-2s2p ) and CIV (1548 Å: 2s 2S-2p 2P) as well as the ratio of CIV to CIII were measured to investigate the edge impurity transport with relation to impurity source locations and sputtering characteristics. The experimental result shows that the impurity profile in the SOL has been clearly changed against different source locations. The emission of CIII and CIV from the mid-plane is stronger than that from the X-point when the impurity source is located at the divertor plate. The profile becomes flat as a result. When the impurity source changes to the dome source, the profile clearly changes to a slightly peaked one, indicating the edge carbon emission at the X-point is stronger than the mid-plane. The change to the limiter source makes the profile further peaked by increasing the carbon emission at the X-point. In the case of the dome impurity source, the intensity of CIII/ne and CIV/ne, normalized to line-averaged electron density, ne, decreases with ne at low ne (ne  ⩽  2.6   ×   1019 m−3) and becomes saturated at high ne (ne  >  2.6   ×   1019 m−3). In contrast, the ratio of CIV to CIII increases with ne at low ne and starts to decrease at high ne. A numerical simulation with 3D edge plasma transport code, EMC3-EIRENE, suggests that a poloidal asymmetry in the impurity flow profile and an enhanced physical sputtering play an important role in the edge impurity distribution, particularly in the screening efficiency of C2+ and C3+ ions in the SOL region of the HL-2A tokamak. [ABSTRACT FROM AUTHOR]

A class of evident MHD activities prior to major disruption has been observed during recent radiation induced disruptions of the HL-2A tokamak discharges. It can be named SOD, synchronous oscillations prior to disruption, characterized by synchronous oscillation of electron cyclotron emission (ECE), core soft x-ray, Mirnov coil, and radiation signals at the divertor plate. The SOD activity is mostly observed in a parametric regime where the poloidal beta is low enough before disruption, typically corresponding to those radiation-induced disruptions. It has been found that the m/n = 2/1 mode is dominant during the SODs, and consequently it is the drop of the mode frequency and the final mode locking that lead to thermal quench. The mode frequency before the mode locking corresponds to the toroidal rotation frequency of the edge plasma. It is also found that during SODs, the location of the q = 2 surface is moving outward, and most of the plasma current is enclosed within the surface. This demonstrates that the current channel lies inside the rational surface during SOD, and thus the resistive kink mode is unstable. Further analysis of the electron temperature perturbation structure shows that the plasma is indeed dominated by the resistive kink mode, with kink-like perturbation in the core plasma region. It suggests that it is the nonlinear growth of the m/n = 2/1 resistive kink mode and its higher order harmonics, rather than the spontaneous overlapping of multiple neighboring islands, that ultimately triggered the disruption. [ABSTRACT FROM AUTHOR]

Two groups of frequency sweeping modes are observed and interpreted in the HL-2 A plasmas with qmin ∼ 1. The tokamak simulation code calculations indicate the presence of a reversed shear q-profile during the existence of these modes. The mode frequencies lie in between TAE and BAE frequencies, i.e. ωBAE < ω < ωTAE, and these modes are highly localized near qmin, i.e. r/a ∼ 0.25. A group of modes characterized by down-sweeping frequency with qmin decrease due to qmin > 1 and nqmin − m > 0, and another group of modes characterized by up-sweeping frequency with qmin drop, owing to qmin < 1 and nqmin − m < 0 before sawtooth crash. The kinetic Alfvén eigenmode code analysis supports that the down-sweeping modes are kinetic reverse shear Alfvén eigenmodes (KRSAEs), and the up-sweeping modes are RSAEs, which exist in the ideal or kinetic MHD limit. In addition, the down- and up-sweeping RSAEs both have fast nonlinear frequency behaviour in the process of slow frequency sweeping, i.e. producing pitch-fork phenomena. These studies provide valuable constraint conditions for the q-profile measurements. [ABSTRACT FROM AUTHOR]