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Mitigation of Edge localized mode(ELM) has been achieved with different external actuators such as lower hybrid wave (LHW), mixture supersonic molecular beam injection(SMBI), and laser blow-off(LBO) impurity seeding on HL-2A. During these experiments, the pedestal turbulence enhancement is commonly observed, which is found closely related to ELM mitigation. The turbulence enhancement is caused by the externally driven the velocity shear rate without change of the turbulence correlation length, but correlated to its radial wavenumber spectral shift. A common plausible mechanism for the ELM mitigation with different external source input seems to be involved. A modified theoretical model based on the turbulence radial wavenumber spectral shift is used and successfully interprets the experimental observations. The simulation suggests that a critical growth rate of the most unstable mode, also identified by the experimental results, survives in the competition of the velocity shear rate, enhancing the turbulence intensity. An example of the LHW case is used and good agreements have been found between the experimental results and simulation results.

Recent observations in HL-2A tokamak give new experimental evidences of energetic particle mode (EPM) avalanche. In a strong EPM burst, the mode structure propagates radially outward within two hundred Alfvén time, while the frequency of the dominant mode changes self-consistently to maximize wave-particle power exchange and mode growth. This suggests that significant energetic particle transport occurs in this avalanche phase, in agreement with theoretical framework of EPM convective amplification. A simplified relay runner model yields satisfactory interpretations of the measurements. The results can help understanding the nonlinear dynamics of energetic particle driven modes in future burning plasmas, such as ITER.

In this work, the coupling among several MHD modes across different spatial regions, including the neoclassical tearing mode (NTM) and two branches of Alfvén eigenmode (AE) in the core and the edge localized mode (ELM), has been investigated in the HL-2A high beta H-mode plasmas. The NTMs induce a saturated m/n = 1/1 helical core (m and n are the poloidal and toroidal mode numbers, respectively) through the ‘magnetic-flux pumping’ effect. The ELM crash results in a rapid (

Since the last IAEA Fusion Energy Conference in 2018, significant progress of the experimental program of HL-2A has been achieved on developing advanced plasma physics, edge localized mode (ELM) control physics and technology. Optimization of plasma confinement has been performed. In particular, high-β N H-mode plasmas exhibiting an internal transport barrier have been obtained (normalized plasma pressure β N reached up to 3). Injection of impurity improved the plasma confinement. ELM control using resonance magnetic perturbation or impurity injection has been achieved in a wide parameter regime, including types I and III. In addition, impurity seeding with supersonic molecular beam injection or laser blow-off techniques has been successfully applied to actively control the plasma confinement and instabilities, as well as plasma disruption with the aid of disruption prediction. Disruption prediction algorithms based on deep learning are developed. A prediction accuracy of 96.8% can be reached by assembling a convolutional neural network. Furthermore, transport resulting from a wide variety of phenomena such as energetic particles and magnetic islands has been investigated. In parallel with the HL-2A experiments, the HL-2M mega-ampere class tokamak was commissioned in 2020 with its first plasma. Key features and capabilities of HL-2M are briefly presented.

The impact of a low frequency MHD mode crash on triggering the H-mode has been studied in detail on the HL-2A tokamak. The mode manifests fishbone characteristics with a precession frequency f ≈ 14 – 19 kHz . The abrupt mode crash evokes substantial energy release from the core to the plasma boundary and hence increases the edge pressure gradient and E r × B flow shear, which further suppresses turbulence and leads to confinement improvement into the H-mode. Under the same NBI heating (∼1 MW), the I-phase plasma transits into H-mode with a rapid MHD mode crash while it returns to the L-mode without the presence of the mode in the I-phase. With increasing heating power by the ECRH added to the NBI, the MHD mode disappears. The statistical result shows that with the MHD mode crash the heating power for accessing the H-mode is significantly lower than that without the mode crash. All these facts reveal that the MHD mode crash in the I-phase plays a critical role in trigging the I → H transition at marginal heating power. In addition, it has been found that with the same NBI power heating, the magnitude of the mode (crash) increases with increasing plasma density, implying larger energy release being needed to access the H-mode for higher density plasmas.

The impact of supersonic molecular beam injection (SMBI) fuelling on plasma turbulence and shear flows, which forms a self-consistent state, has been studied in the HL-2A tokamak. The SMBI flattens the shear layer by collisional flow damping and increases the turbulence intensity. It is found that the SMBI can enhance the nonlinear regulation dynamics and acts as an external tool for turbulence quench via stimulating two dynamic processes. The SMBI-stimulated nonlinear interactions facilitate the L-H transition at the marginal heating power for accessing the H-mode. The effective reduction of the H-mode power threshold by SMBI is demonstrated. The result suggests that the SMBI could be a general tool for realization of controllable L-H transition via reducing the H-mode power threshold in future devices.

For the first time supersonic molecular beam injection (SMBI) and cluster jet injection (CJI) were applied to mitigate edge-localized modes (ELMs) in HL-2A successfully. The ELM frequency increased by a factor of 2–3 and the heat flux on the divertor target plates decreased by 50% on average after SMBI or CJI. Energetic particle induced modes were observed in different frequency ranges with high-power electron cyclotron resonance heating (ECRH). The high frequency (200–350 kHz) of the modes with a relatively small amplitude was close to the gap frequency of the toroidicity-induced Alfven eigenmode. The coexistent multi-mode magnetic structures in the high-temperature and low-collision plasma could affect the plasma transport dramatically. Long-lived saturated ideal magnetohydrodynamic instabilities during strong neutral beam injection heating could be suppressed by high-power ECRH. The absolute rate of nonlinear energy transfer between turbulence and zonal flows was measured and the secondary mode competition between low-frequency (LF) zonal flows (ZFs) and geodesic acoustic modes (GAMs) was identified, which demonstrated that ZFs played an important role in the L–H transition. The spontaneously generated E × B shear flow was identified to be responsible for the generation of a large-scale coherent structure (LSCS), which provided unambiguous experimental evidence for the LSCS generation mechanism. New meso-scale electric potential fluctuations (MSEFs) at frequency f ∼ 10.5 kHz with two components of n = 0 and m/n = 6/2 were also identified in the edge plasmas for the first time. The MSEFs coexisted and interacted with magnetic islands of m/n = 6/2, turbulence and LF ZFs.

New mesoscale electric fluctuations (MSEFs) are identified in the edge plasmas of the HL-2A tokamak using multiple Langmuir probe arrays. The MSEFs, resulting from the synchronization and having components of dominant geodesic acoustic mode (GAM) and m/n = 6/2 potential fluctuations, are found at the same frequency as that of the magnetic fluctuations of m/n = 6/2 (m and n are poloidal and toroidal mode numbers, respectively). The temporal evolutions of the MSEFs and the magnetic fluctuations clearly show the frequency entrainment and the phase lock between the GAM and the m/n = 6/2 magnetic fluctuations. The results indicate that GAMs and magnetic fluctuations can transfer energy through nonlinear synchronization. The nonlinear coupling analyses show that the MSEFs couple to turbulence and low frequency zonal flows (LFZFs). This suggests that the MSEFs may contribute to the LFZF formation, reduction of turbulence level, and thus confinement regime transitions. The analysis of the envelope modulation demonstrates that both the MSEFs and the LFZFs modulate the turbulence while the MSEFs are modulated by the LFZFs.

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 $\omega_{\phi}-2\omega_{\theta}-\omega=0$, where $\omega_{\phi}$, $\omega_{\theta}$ and $\omega$ are the toroidal, poloidal angular frequencies of energetic-ions and the mode frequency respectively. These findings can help to understand the tearing mode induced energetic-particle loss and particle acceleration during the tearing mode reconnection in laboratory and space plasmas.

Author(s): Xu, M; Duan, XR; Liu, Y; Song, XM; Liu, DQ; Wang, YQ; Lu, B; Yang, QW; Zheng, GY; Ding, XT; Dong, JQ; Hao, GZ; Zhong, WL; Shi, ZB; Yan, LW; Zou, XL; Liu, YQ; Chen, W; Xiao, GL; Zhang, YP; Jiang, M; Hou, YM; Cheng, J; Liang, AS; Bai, XY; Cao, JY; Cao, Z; Chen, HT; Cui, CH; Cui, ZY; Delpech, L; Diamond, PH; Ekedahl, A; Feng, BB; Giruzzi, G; Gong, SB; He, HM; Hoang, T; Huang, M; Huang, Y; Ida, K; Inagaki, S; Isobe, M; Itoh, K; Itoh, SI; Ji, XQ; Ke, R; Kobayashi, M; Lei, GJ; Li, B; Li, GS; Li, HJ; Li, JQ; Li, Q; Li, Q; Li, XD; Li, YG; Li, Y; Mao, WC; Mazon, D; McKee, GR; Morita, S; Nie, L; Peng, JF; Peysson, Y; Rao, J; Ren, Y; Tynan, GR; Wang, HX; Wang, QM; Wei, HL; Xia, F; Xiao, XP; Xuan, WM; Yan, Z; Yao, K; Yu, LM; Yu, Y; Yu, DL; Yuan, BD; Yuan, BS; Zhang, JH; Zhou, Y; Liu, Y | Abstract: 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.

Modulation of turbulent electron temperature fluctuations ($\tilde{T}_{e}$) and density fluctuations ($\tilde{n}_{e}$) 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 (2D) images of $\tilde{T}_{e}$ show the first evidence that the turbulence modulation occurs only when the island width exceeds a certain threshold value ($w_{c} \approx 10 \rho_{i} \approx 4 $ 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.

Divertor heat flux mitigation has been achieved using the Supersonic Molecular Beam Injection (SMBI) in type-III ELMy H-mode plasmas on the HL-2A tokamak. After an SMBI, ELM mitigation occurs and is characterized by an ELM frequency increase by a factor of 2–3 and an amplitude decrease by 30–40%. The corresponding confinement has a little degradation (stored energy drops is smaller than 7%). The burst rate of smaller-scale filament (density amplitude σ ) increases by 20–30%, and the divertor heat flux decreases by 40–50%, which is consistent with an increase in radiation power loss. This observation indicates that SMBI should be a good candidate for ELM mitigation.

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.

Interactions among pedestal shear flows, turbulence, and the formation of the edge transport barrier have been studied in H-mode plasmas of the HL-2A tokamak by multi-channel Doppler reflectometry with high spatiotemporal resolution. Geodesic acoustic mode (GAM) has been observed during the L-I-H transition. It has been observed that the plasma transits into the I-phase when the mean E × B shear flow reaches a critical value. The bi-spectrum analysis has shown that there is a strong interaction between GAM and limit cycle oscillation (LCO), and the energy transfer is from GAM to LCO, suggesting that GAM can assist the L-I transition. The regulation of the edge turbulence by LCOs helps to build the steep pedestal and initialize the confinement improvement of the plasma. It has been found that the mean E × B shear flow is further increased just before the I-H transition, accompanied by the turbulence suppression, leading to the edge transport reduction and the pedestal formation. It has been demonstrated th...

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 (T e 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 T e regime to ITG modes in the high T e regime.

A novel design of the three-step Langmuir probe (TSLP) array has been developed to investigate the zonal flow (ZF) physics in the HL-2A tokamak. Three TSLP arrays are applied to measure the three-dimensional (3D) features of ZFs. They are separated by 65 mm in the poloidal and 800 mm in the toroidal directions, respectively. The 3D properties of the geodesic acoustic mode (GAM) ZFs are presented. The poloidal and toroidal modes of the radial electric fields of the GAM perturbations are simultaneously determined in the HL-2A tokamak for the first time. The modes have narrow radial wave numbers (kr ρi = 0.03–0.07) and short radial scale lengths (2.4–4.2 cm). High coherence of both the GAM and the ambient turbulence separated by toroidal 22.5 ◦ along a magnetic field line is observed, which contrasts with the high coherence of the GAM and the low coherence of the ambient turbulence apart from the field line. The nonlinear three wave coupling between the turbulent fluctuations and the ZFs is a plausible mechanism for flow generation. The skewness and kurtosis spectra of the probability distribution function of the potential perturbations are contrasted with the corresponding bicoherence for the first time, which support the three wave coupling mechanism.

Recent experiment results from the HL-2A tokamak are presented in this paper. Supersonic molecular beam injection (SMBI) with liquid nitrogen temperature propellant is used. Low temperature SMBI can form hydrogen clusters that penetrate into the plasma more deeply and efficiently. Particle diffusion coefficient and convection velocity (D = 0.5?1.5?m2?s?1 and Vconv < 40?m?s?1, respectively) are obtained at the plasma periphery using modulated SMBI. Multi-probe measurements reveal the m = 0?1, n = 0 symmetries of directly measured low frequency (7?9?kHz) electric potential and field are simultaneously observed for the first time. Impurity transport is determined with the laser blow-off system and transport code. A disruption predictor has been derived based on MHD activity observations and statistical analysis. Sawtooth characteristics during ECRH are investigated and coupling between m = 1 and m/n = 2/1 modes is studied. Detachment features of HL-2A divertor are numerically and experimentally studied using the code SOLPS5.0 and measured data. The long divertor legs and thin divertor throats in HL-2A pose MHD shaping problems resulting in momentum losses even at low densities and strongly enhanced main chamber losses.

Wall conditioning with siliconization has been performed in the HL-2A tokamak to improve the plasma performance. It was based on the plasma chemical vapor deposition by using a four-anode glow discharge cleaning system and gas mixture of 90% He + 10% SiH 4 . It was found that the main components of the residual gas were CO, H 2 and H 2 O. The amount of H 2 O was reduced from 49.5% to 24.5%. After siliconization, the impurity fluxes released from the first wall were reduced, especially the oxygen level (till to 10%) and high z impurities like Cu were decreased significantly. The total radiated power measured by the bolometer was decreased from 70%) to 35%. A high density exceeding the Greenwald limit and energy confinement improvement have been achieved. The plasma operational region of HL-2A has been effectively broadened.

Most of routing algorithms for On-chip communication are neither application-aware nor routing packets using multiple paths. In addition, they hardly consider link bandwidth variation r esulting from widely applied global asynchronous local synchronous (GALS ) mechanism. In this paper, we propose a bandwidth-based application-aware multipath routing (BAMR) algorithm to assign multiple routing paths by leveraging the knowledge of application and network bandwidth features. With the increase of number of flows resulting from the split of flows, we present a new method named dynamic-amount fixed-number (DAFN) flow control mechanism to avoid deadlock. We compare our algorithm with XY, YX, an d O1TURN with synthetic traffic patterns and traffic trace of parallel implementation of H.264. Experiments demonstrate that BAMR achieves higher throughput with decrease in latency. Furthermore, the proposed algorithm achieves better workload balance by distributing traffic over multiple paths. To address the aforementioned three challenges, we propose a bandwidth-based application-aware mu ltipath routing (BAMR) algorith m. Our algorith m is flexible to different kinds of NoC topologies and traffic patterns. We first decide routing order of flows based on application graph and the bandwidth requirement. Then, considering the transmission capacity of network links, we split some flows into mu ltiple subflows to alleviate network congestion. Our algorithm can detect communication bottlenecks and make split based on link's residual bandwidth. For multipath routing, more flows may increase the possibility of deadlock. To this end, we propose a buffer management mechanism called dynamic- amount fixed-nu mber (DAFN), the number of Virtual Channels (VCs) for each individual router port and VC depth are predetermined at design phase based on the BAMR's results. Specifically, the number of VCs for a specified port at a specified router is depending on the number of flows passing through it. DAFN allocates each VC at a router with an identical global label with assigned flow's label. DAFN achieves deadlock freedom by restricting each flow to its exclusive VC, it takes full advantage of buffer space and avoids deadlock without additional hardware resources. Our work makes following contributions: We propose a mu ltipath routing algorithm named BAM R for GA LS No C. A flow splitting method aiming at to alleviate network congestion is presented, as part of BAMR.A static VC regulator named DAFN, with the co mbination of BAMR to make the full utilization of VC resources and avoid deadlock is also proposed.

In order to alleviate the problem raised in Gomez's method[1], where messages with different source-destination pairs have great probability choosing the overlapped paths therefore, new transmission bottleneck might be generated nearby, we present an intermediate node based deterministic fault-tolerant routing, named IDFT. Our method selects the qualified intermediate node among all the candidates by its residual bandwidth without over-stressing any particular node. In addition, deterministic routing is applied along both subpaths to guarantee deadlock-free and in-order message delivery. It thereby achieves traffic workload balance across the network and graceful performance degradation even in the presence of faults. Keywordsfault tolerance; bandwidth utilization; deterministic routing

Glass and copper mirrors were exposed inside the HL-2A vacuum chamber. Arc discharge mark has been found on the glass mirror. The optical transmission character of test mirrors has been measured. The main components of the deposited materials on the first mirror are carbon and iron analyzed by AES. The Monte-Carlo method has been used to simulate the defending effect of the shield for test mirror. After a 2 years tokamak experiment, one can find that the reflectivity of copper mirror decline slower than that of glass mirror. The first mirror is polluted easier during glow discharge and wall processing than omhic discharge. Using buffer and shutter can play down the pollution, especially during the glow discharge and wall processing.

Two experiment campaigns were conducted on the HL-2A tokamak in 2003 and 2004 after the first plasma was obtained at the end of 2002. Progresses in many aspects have been made, especially in the divertor discharge and feedback control of plasma configuration. Up to now, the following operation parameters have been achieved: Ip = 320 kA, Bt = 2.2 T and discharge duration Td = 1580 ms. With the feedback control of plasma current and horizontal position, an excellent repeatability of the discharge has been achieved. The tokamak has been operated at both limiter configuration and single null (SN) divertor configuration. The HL-2A SN divertor configuration is simulated with the MHD equilibrium code SWEQU. When the divertor configuration is formed, the impurity radiation in the main plasma decreases remarkably.

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project are presented in this paper. In HL-1M, strong fishbone instability was observed during off-axis electron cyclotron resonance heating (ECRH). This is the first observation of fishbone instability purely driven by energetic electrons produced by ECRH. The molecular beam injection (MBI) was first proposed and demonstrated in HL-1M. Recently, new results of the MBI experiment were obtained by increasing the pressure of the gas. A stair-shaped density increment was obtained with high-pressure multi-pulse MBI just like the density evolution behaviour during multi-pellet injection. It is shown that injected particles penetrated into the core region of the plasma. HL-2A is a divertor tokamak constructed at SWIP based on the original ASDEX main components. The mission of the HL-2A project is to explore the physics issues involved in an advanced tokamak. For the first phase, divertor (edge plasma) and confinement research will be emphasized. The major parameters of HL-2A are R = 1.65 m, a = 0.4 m, Bt = 2.8 T, IP = 0.48 MA. The main parameters and characteristics of the subsystems such as power supply, pumping, diagnostics and auxiliary heating are presented in this paper. The first plasma of HL-2A was obtained at the end of 2002.

Strong m = 1 MHD activities are observed in the HL-1M tokamak during off-axis electron cyclotron resonance heating (ECRH) when the cyclotron resonance location is placed just outside the q = 1 surface at the high-magnetic-field side of the magnetic surface. Addition of lower-hybrid waves to ECRH significantly enhances the MHD excitation, but lower-hybrid waves alone cannot excite or sustain the mode. This result is a clear demonstration of the suprathermal trapped electron effect on the instability because of the absence of energetic ions in the plasma.

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.

Edge localized mode (ELM) mitigation with lower hybrid current drive (LHCD) has been observed in the HL-2A tokamak. This mitigation effect is very sensitive to the parameters as the plasma density and the LHCD absorbed power, i.e., more easily to be observed for high density and large LHCD absorbed power. The divertor peak heat load released by ELM has been significantly reduced during the mitigation phase. The pedestal density gradient is slightly reduced during ELM mitigation, and the plasma rotation velocity and ion temperature are significantly reduced by LHCD. It has been found that the ELM mitigation is not synchronized with the LHCD, while it is synchronized with the increase of the pedestal turbulence, showing that the enhancement of the transport due to the pedestal turbulence can be the direct cause of the ELM mitigation.

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.

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.

Toroidal Alfven eigenmodes (TAEs) driven by fast ions during neutral beam injection heating have been observed on HL-2A. TAEs are found to be excited by the sideband resonance between fast ions and shear Alfven waves. Their frequencies range from 90 kHz to 200 kHz, and toroidal mode numbers of the most unstable TAEs are n = 1–3. Nonlinear behaviours of TAEs, such as frequency chirping, pitch-fork splitting, and nonlinear mode-mode coupling, are also excited. In the down-chirping case, TAEs show typical ballooning mode structures and their amplitudes are around δBr = 3.25 × 10−4T at the edge of the plasma. The mode frequency is proportional to the square of time, i.e., f(t) ∼ t1∕2, which agrees well with the theoretical prediction. Pitch-fork splitting of TAEs has also been excited on HL-2A, and their growth rate is about γl ∼ 0.22 × 104 s−1. TAE coupling with a tearing mode results in the appearances of series of Alfvenic modes (AMs). The two AMs, which have the same absolute mode number but rotate in dif...

Two m/n = 1/1 kink modes excited by energetic electrons (called double e-fishbone) have been observed near the q = 1 flux surfaces in the HL-2A discharges. The negative magnetic central shear configuration was achieved with localized electron cyclotron resonance heating and electron cyclotron current drive during plasma current ramp-up. The features of the modes have been first shown by advanced 2D electron cyclotron emission imaging (ECEI) system. From ECEI, two m/n = 1/1 modes propagating in the opposite directions poloidally have been clearly observed. These modes can be found only in low density discharge, and their frequencies are close to the precessional frequency of the trapped energetic electrons. More interestingly, the thermal energy transfer between the two modes was revealed by this new diagnostic, which is found to be related to the nonlinear interaction of the two modes and local electron thermal transport.

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 q min decreases during plasma current ramp-up. The value of q min 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 q min is larger than a certain value, and with the decreasing q min 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.

A new method of gas fuelling has been introduced in the HL-1M tokamak. The method consists of a pulsed high speed molecular beam formed by a Laval type nozzle. The velocity of the well collimated hydrogen beam is about 500 m/s. About 6 × 1019 molecules pass through the nozzle and into the vacuum chamber in each pulse. A series of helium pulses was injected into the HL-1M low density (e = 4 × 1018 m-3) hydrogen plasma. With penetration depth up to 12 cm, the ramp-up rate of the electron density, de/dt, was as high as 3.1 × 1020 m-3s-1 at steady state, and the resulting plasma density reached e = 5.6 × 1019 m-3. The profile peaking factor of the electron density, Qn = ne(0)/ne of about 100 ms after helium molecular beam injection (MBI) reached a maximum value of more than 1.51. The energy confinement time τE measured by diamagnetism is 26 ms, which is over 30% longer than that of the gas puffing (GP) results under the same operational conditions. The improvement of τE and increase of Qn for MBI were comparable to those of small pellet injection (PI) in HL-1M, as well as those of slow PI in ASDEX (Kaufmann, M., et al., Nucl. Fusion 28 (1988) 827). It is argued that the peaked density profile induced by the deepened particle injection is a factor essential for the confinement improvement apart from the isotope effect of helium particles, because the density peaking factor Qn is normally less than 1.4 for GP plasma in HL-1M. The particle confinement time with MBI increased sixfold in comparison with that before injection.

A 2D electron cyclotron emission imaging (ECEI) system has been developed for measurement of electron temperature fluctuations in the HL-2A tokamak. It is comprised of a front-end 24 channel heterodyne imaging array with a tunable RF range spanning 75–110 GHz, and a set of back-end ECEI electronics that together generate 24 × 8 = 192 channel images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The simulated performance of the local oscillator (LO) optics and radio frequency (RF) optics is presented, together with the laboratory characterization results. The Gaussian beams from the LO optics are observed to properly cover the entire detector array. The ECE signals from the plasma are mixed with the LO signal in the array box, then delivered to the electronics system by low-loss microwave cables, and finally to the digitizers. The ECEI system can achieve temporal resolutions of ∼μs, and spatial resolutions of 1 cm (radially) and 2 cm (poloidally).

Transitions between low (L-mode) and intermediate (I-phase) confinement regimes triggered by sawteeth are investigated using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak. The I-phase is characterized by limit-cycle oscillations (LCOs). Repeated L-I-L transitions induced by sawtooth heat pulses are also observed. Statistical analyses show that the delay time of the L-I transition relative to sawtooth crashes is less than ∼1 ms. The intensity of turbulence bursts in the I-phase is stronger than those observed in L-mode plasmas. Measurements are consistent with zonal flow reducing turbulence intensity in the frequency band ∼20-100 kHz, while the LCO turbulence of frequency higher than 100 kHz has more power than that in the L-mode. The analyses of time-resolved power transfers between turbulence and zonal flows suggest that power transfers exist between turbulence and zonal flows. The direction of the power transfers is from turbulence into geodesic acoustic modes in ohmic plasmas. © 2013 IAEA, Vienna.

The dynamic features of the low-intermediate--high-(L-I-H) confinement transitions on HL-2A tokamak are presented. Here we report the discovery of two types of limit cycles (dubbed type-$Y$ and type-$J$), which show opposite temporal ordering between the radial electric field and turbulence intensity. In type-$Y$, which appears first after an L-I transition, the turbulence grows first, followed by the localized electric field. In contrast, the electric field leads type-$J$. The turbulence-induced zonal flow and pressure-gradient-induced drift play essential roles in the two types of limit cycles, respectively. The condition of transition between types-$Y$ and -$J$ is studied in terms of the normalized radial electric field. An I-H transition is demonstrated to occur only from type-$J$.

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 (T i) and electron density are softened in the pedestal. It has been observed that the averaged gradient of the T i decreases around 44% and the well depth of radial electric field (E r) is reduced by the SMBI during ELM mitigation. Furthermore, at least 20% decrements of T i 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 T i (up to 40 ms or longer), indicating the importance of the T i in the ELM mitigation by SMBI. Finally, it has been observed that the T i 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.

In HL-2A, the characteristics of the edge plasma instabilities and their effects on the dynamical evolution of the pedestal in H-mode plasmas have been investigated. In the edge pedestal region with steep pressure gradient, a quasi-coherent mode (QCM) has been observed in density fluctuations with a frequency range of 50–100 kHz. It appears during the edge localized mode (ELM)-free period after the L–H transition and prior to the first ELM. A threshold in the pedestal density gradient has been identified for the excitation of this mode. The QCM can also be observed during inter-ELM periods. It is excited early in the inter-ELM period, and disappears when the ELM onset starts. The radial wave-number of the mode is estimated with two radially separated reflectometers. It shows that the mode is radially propagating inward. The poloidal wave number estimated with the Langmuir probes is k θ ~ 0.43 cm−1. The mode propagates poloidally in the electron diamagnetic direction in the plasma frame. The toroidal mode number, deduced from Mirnov signals, is n ~ 7. The corresponding poloidal mode number is m ~ 21 according to the local safety factor value. The analysis for the dynamical evolution of the pedestal during the ELM cycle clearly shows that the mode is excited before the ELM onset. During and after the ELM crash, the mode disappears. It suggests that the QCM is driven by the pedestal density gradient, and the mode in return regulates the pedestal density evolution.

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 (χ i) 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.

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, ΔTe/〈Te〉 ?> variations at different locations are restricted by the intensity of magnetic fluctuations. The ΔTe/〈Te〉 ?> and &lpar;δBθ&rpar;rms ?> 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 E×B ?> 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.

Recent experimental results that are associated with the core-localized (i.e. normalized radius ρ&equals;r/a<0.5 ?> ) 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 n = 3–7 are often observed with an increase in the edge safety factor, q a . Their measured frequency is more than that of the TAEs, and fmin∼fTAE ?> . 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 qmin ?> and q-profile. When the core plasma density is more than 3.0\times {{10}^{19}}$ ?> ne0>3.0×1019 ?> 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 n = 2–9 are observed through multiple diagnostic techniques, and f∗pi/2 , where f∗pi&equals;ω∗pi/2π ?> 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.

The ELM (edge localized mode) mitigation by SMBI (supersonic molecular beam injection) has been studied in the HL-2A H-mode plasmas. The ELM mitigation effect and its relationship with the deposition position of SMBI in the H-mode pedestal are reported for the first time experimentally. We found that when the deposition of SMBI is about 20% into the pedestal, the best ELM mitigation effect is achieved, which is identified by a significant increase of the ELM frequency and also a decrease of the ELM amplitude. The theoretical inference that no deep injection is needed is confirmed. The sand-pile model is used to simulate the ELM burst and explain the mitigation effect for different SMBI deposited positions. It is found that the gradient threshold is a key parameter in the process of the ELM mitigation, and there should be a local gradient threshold in the middle of the pedestal. When the deposition of SMBI is close to the edge of this region, the best ELM mitigation effect can be observed.

The β-induced Alfven eigenmode (BAE) excited by energetic electrons has been identified for the first time both in the Ohmic and electron cyclotron resonance heating plasma in HL-2A. The features of the instability, including its frequency, mode number, and propagation direction, can be observed by magnetic pickup probes. The mode frequency is comparable to that of the continuum accumulation point of the lowest frequency gap induced by the shear Alfven continuous spectrum due to finite β effect, and it is proportional to Alfven velocity at thermal ion β held constant. The experimental results show that the BAE is related not only with the population of the energetic electrons, but also their energy and pitch angles. The results indicate that the barely circulating and deeply trapped electrons play an important role in the mode excitation.

Using the profile analysis, the density perturbation transport analysis, and the Doppler reflectometry measurement, for the first time a spontaneous and steady-state particle-transport barrier has been evidenced in the Ohmic plasmas in the HL-2A tokamak with no externally applied momentum or particle input except the gas puffing. A threshold in density has been found for the observation of the barrier. The particle diffusivity is well-like, and the convection is found to be inward outside the well and outward inside the well. The formation of the barrier coincides with the transition between the trapped electron mode and the ion temperature gradient driven mode.