HL-2A Tokamak Research Articles

Turbulence regulation with radial wavenumber spectral shift caused by externally driven E × B velocity shear during edge-localized mode mitigation

Mitigation of edge-localized modes by lower hybrid current drive (LHCD) has been observed in H-mode plasmas of the HL-2A tokamak. It has been found that this mitigation is strongly desynchronized with the LHCD pulse but synchronized with the pedestal turbulence enhancement. This turbulence is enhanced by LHCD induced pedestal velocity shear resulting from the ion diamagnetic term . This velocity shear modifies the turbulence radial wavenumber (kr) spectrum by varying the averaged radial wavenumber from large negative value to zero, reducing the turbulence dissipation . A modified turbulence spectral shift model has been used to simulate the experimental results. A critical growth rate for the turbulence regulation is identified in this model and experimentally determined. The criterion for triggering the spectral shift and turbulence enhancement is established. It has been found that there exists a threshold Uc of the LHCD induced velocity shear rate,which determines the desynchronization time delay. This threshold value is directly related to , for the pedestal turbulence enhancement. Good agreement has been found between experiment and theory for the regulation of the turbulence intensity calculated with a selected poloidal wavenumber by its averaged radial wavenumber.

Verification of an energetic-electron-driven β -induced Alfvén eigenmode in the HL-2A tokamak

An energetic-electron-driven β-induced Alfvén eigenmode (e-BAE) in an HL-2A tokamak experiment (discharge #17461) was studied using a gyrokinetic particle simulation code. Investigations of e-BAEs excited by an initial perturbation, an external antenna, and an energetic-electron pressure gradient were performed, and the measured eigenfrequencies were found to be close to the experimental observations and the theoretical predictions. The damping mechanism is also discussed based on the simulation results. The e-BAE becomes unstable when the driving force of the energetic-electron pressure gradient exceeds the total background damping. Simulations show that the e-BAE propagates along the diamagnetic direction of the energetic electrons and that the most unstable mode number is n∕m = 1∕3; these findings are in good agreement with the experimental observations. The simulation results also show that the mode is radially localized near the rational surface. Both the mode width and growth rate increase as the background plasma density increases. As the density and temperature of the energetic electrons increase, the eigenfrequency increases slightly, and the growth rate increases considerably.

A novel magnetic island structure identification system based on Mirnov probes and DDR3 look-up table for HL-2A tokamak

A novel magnetic island structure identification (MISI) system based on Mirnov probes and DDR3 look-up table has been developed on the HL-2A device. The system is dedicated to identify the magnetic island by analyzing the measurement of Mirnov probes and equilibrium fitting (EFIT) code. The applications of high-precision sampling and PCI express (PCIe) platform in the system provides the convenience for Mirnov measurements and the EFIT results transmission. A high performance field programmable gate array (FPGA) is used to realize the real-time identification algorithm. By applying the DDR3 look-up table, 30 current filaments are arranged on a 129× 129 grid in the calculation of perturbation flux, simplifying the algorithm structure and improving the efficiency. For the advantage of parallel and pipeline processing abilities in the FPGA, the magnetic island structure can be identified with time resolution of 3 ms and spatial resolution of 1.37 cm and 1.02 cm.

Design, installation and operation of in-vessel resonant magnetic perturbation system on the HL-2A tokamak

H-mode discharges with edge localized modes (ELMs) have been achieved in the HL-2A tokamak with combined auxiliary heating of NBI and ECRH. In order to realize the mitigation or suppression of ELMs in HL-2A plasmas and also to explore the ELM suppression and other 3D magnetic field physics, a set of compact in-vessel resonant magnetic perturbation (RMP) coils (2×2 array) has been recently designed and installed on the HL-2A tokamak to meet the limited space requirement. There are two rows of coils (upper and lower separated poloidally 66.5°), with each row consisting of 2 coils separated from each other by 180° along the toroidal direction. Each coil spans about 11.4° toroidally. Meanwhile, an integral solid-state regulating power supply system with a high switching frequency and an AC–DC–AC converter configuration has been designed and fabricated for the RMP system. It can provide DC and AC with various waveforms. The maximum amplitude of DC is 5.0 kA and the frequency of AC can reach 3 kHz. These in-vessel coils can produce sufficient RMPs in the plasma edge during typical HL-2A H-mode discharges. The ELM mitigation by applying an n = 1 RMP has been observed in recent HL-2A experiments. The ELM frequency was increased by a factor of 2 and the amplitude of the Dαemission decreased, leading to a reduced heat load on the divertor plates.

Edge turbulence evolution and intermittency development near the density limit on the HL-2A tokamak

The development of intermittent non-Gaussian processes is studied in the edge turbulence of ohmically heated HL-2A discharges approaching the density limit. As the density limit is approached, the E×B shear flow at the last closed flux surface (LCFS) weakens, a strong positive skewness develops in the scrape-off layer (SOL), and negative skewness develops inside the LCFS of turbulent density fluctuations. A conditional averaging analysis confirms more frequent increased amplitude positive (negative) going density fluctuation activity in the SOL (inside the LCFS) as the density limit is approached. The measured turbulent stress across the edge, LCFS, and SOL region is decomposed into diffusive and residual stress components, and the nonlinear exchange of kinetic energy between the turbulence and the low-frequency E×B shear flow is determined. Residual stress acts to amplify the E×B flow at the LCFS, while the diffusive stress acts to dissipate the flow just inside this region, at the interface between the core plasma and the LCFS. The relative strength of the flow drive associated with the residual stress weakens as the density limit is approached, while the turbulent viscosity associated with the diffusive stress increases at high density. The adiabatic parameter, kz2vth2/ωνe, drops significantly to about 0.5 in the SOL when the density limit is approached, indicating a transition from the adiabatic regime to the hydrodynamic regime due to increased collisionality. Such changes enhance the particle transport through the nonadiabatic electron response and hence should result in a stronger edge cooling at fixed edge plasma heat flux.

Studies of Reynolds stress and the turbulent generation of edge poloidal flows on the HL-2A tokamak

Several new results on the physics linking edge poloidal flows to turbulent momentum transport are reported. These are based on experiments on the HL-2A tokamak. Significant deviation from the neoclassical prediction for mean poloidal flow in Ohmic and electron cyclotron resonance heating (ECRH) heated L-mode discharges is derived from direct measurement of the turbulent Reynolds stress. The deviation increases prominently with ECRH power. The turbulent poloidal viscosity is synthesized from fluctuation data, and is found to be comparable to the turbulent particle diffusivity. The intrinsic poloidal torque characterized by the divergence of the non-diffusive residual stress is deduced from synthesis for the first time in a tokamak plasma. Experimental evidence which demonstrates the dynamics of spectral symmetry breaking in drift wave turbulence is in good agreement with the development of the poloidal torque. Taken together, these results elucidate the connections between power injection, turbulence development, pressure gradient and residual stress from symmetry breaking.

Measurements of the fast electron bremsstrahlung during lower hybrid current drive in the HL-2A tokamak

Physics related to fast electrons in lower hybrid (LH) current drive (LHCD) plasma is a very important issue, since these particles will play an important role in runaway electron (RE) generation and lower hybrid wave (LHW)-related physics. Utilizing a new hard X-ray (HXR) pinhole camera, recent HL-2A tokamak experiments have devoted to enhancing the understanding of the physics on fast electrons and LHW. The fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV was measured by the HXR camera. To study the conversion of LHW-produced fast electrons into REs, a very short pulse of LHW, so-called “blip”, with duration of 5 ms was injected into the plasma during the current flattop phase. A strong enhancement of REs was induced by the blip injection. Measurements from the HXR camera show that the fast electrons generated by LHWs is mainly concentrated in 40-60 keV, which is well consistent with the calculated value based on Landau damping theory. The energy of these seed electrons is higher than the critical runaway energy. This phenomenon may be come from the synergetic effects of Dreicer and avalanche RE generation. Moreover, the measurements indicate that the spatial distribution of the fast electrons during LHCD has a peaked profile, implying that the fast electrons are mainly produced in the plasma core. It also suggests that the energy of the LHW mainly deposited in the plasma core region.

Current Profile Reconstruction Using Motional Stark Effect Polarimeter Data on HL-2A Tokamak

In order to reconstruct the plasma current density, the Current Profile Fitting (CPF) code has been successfully developed on the HL-2A tokamak. A seven-channel motional Stark effect (MSE) diagnostic based on dual photoelastic modulators is installed to measure the pitch angle of the magnetic field, which can be used as an internal magnetic field constraint for the CPF code. Recently, the MSE polarimeter was upgraded with a real-time wavelength matching system to improve the signal-to-noise ratio. The magnetic field angle (γpitch) with a temporal resolution of 10 ms can be provided. In the CPF code, the plasma current density is described as a polynomial, and the Least-Squares method is used to determine the coefficients of the polynomial. The Finite Difference method and the Strongly Implicit Procedure method are used to solve the Grad-Shafranov equation. The code operation is stable. With the improved-quality MSE data, the CPF calculation result of shot 30782 suggests that the safety factor q profile is monotonic. The minimum q value is less than 1 on-axis during sawtooth oscillations in shot 30782. And, the position of the q = 1 surface is consistent with the sawtooth inversion radius measured by electron cyclotron emission and soft X-ray diagnostics.

Design of a neutron camera for the HL-2A tokamak

Abstract A new neutron camera, comprising nine individual photo-multiplier tubes coupled with liquid scintillators through light guides, was designed, constructed and installed on the HL-2A tokamak. Using the HL-2A tokamak as an example, the basic principles for designing a camera system in low fusion power tokamaks were illustrated in this paper. The 2” × 2” BC501A liquid scintillation detectors coupled with the gain stabilization system, were developed for neutron measurements and applied on the tokamak. A magnetic shielding system was designed, using the finite element method (FEM), to ensure excellent operation of detectors under a strong magnetic field. Energy calibration has been conducted and the light output function was compared with the PTB’s values. The reasons for discrepancy and variations in the results are analyzed and discussed in detail in the paper.

High frequency mode generation by toroidal Alfvén eigenmodes

Nonlinear generation of high frequency modes (HFMs) by the toroidal Alfvén eigenmode (TAE) observed in the HL-2A tokamak is analyzed using nonlinear gyrokinetic theory. It is found that the HFM can be dominated by |nq−m|=1 perturbations with predominantly ideal magnetohydrodynamic polarization if the two primary TAEs are copropagating, while the HFM can be characterized by nq − m = 0 electrostatic perturbations if the two primary TAEs are counter-propagating. Here, n and m are the toroidal and poloidal mode numbers, respectively, and q is the safety factor. The nonlinear process is sensitive to the equilibrium magnetic geometry of the device.

Low-frequency fishbone driven by passing fast ions in tokamak plasmas

Low-frequency fishbone instability driven by passing fast ions via wave-particle resonance is studied within the framework of energetic particle mode (EPM), where and are toroidal transit frequency and poloidal transit frequency of passing fast ions respectively. The effects of finite orbit width of energetic particles are responsible for the low-frequency fishbone of EPM type. It is found that magnetic shear at the q = 1 radius plays an important role in the instability whereas the effect of background plasma beta is weak. In particular, for the case of co-injection of beam ions, there exists a critical magnetic shear below which the beam ion beta threshold for EPM excitation is very small. For moderate or higher magnetic shear, the beam ion beta threshold is much higher, on order of a few percent. These results are consistent with the observation of the low-frequency fishbone in the HL-2A tokamak.

Nonlinear mode coupling induced high frequency axisymmetric mode on the HL-2A tokamak

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.

Impact of impurity mixture gas seeded by supersonic molecular beam injection on edge-localized modes in the HL-2A tokamak

Impurity seeding experiments in ELMy H-mode plasmas by using supersonic molecular beam injection (SMBI) have been performed in the HL-2A tokamak. Besides pure impurity gas (neon, argon), impurities mixed with the main ion fuelling gas by different ratios were seeded into the plasma. It has been observed that the ELM behavior and plasma confinement are very sensitive to the impurity ratio. For 30% Ne impurity seeding, large ELMs are replaced by high frequency bursts (HFBs), which have higher frequency and smaller amplitude compared to the ELMs and the peak heat load on the divertor plate was considerably reduced. Analyses indicate that HFBs originate from the pedestal region and enhance the pedestal particle transport, resulting in a softened pedestal gradient after SMBI. However, for the pure Ne and Ar SMBI seeding, the ELM frequency is decreased and the plasma confinement is moderately improved. Experimental observations suggest that the impurity mixture gas plays a role in changing the pedestal dynamics and there should be an optimal impurity ratio for efficient heat load control in ELMy H-mode plasmas.

Observation of blobs using a gas puff imaging diagnostic on the HL-2A tokamak

A gas puff imaging (GPI) diagnostic has been developed and applied to measure edge plasma turbulence on the HL-2A tokamak. The principle and experimental setup of GPI are described. GPI is applied to investigate blobs in the edge and scrape-off layer. Statistical characterizations of GPI line emission intensity are calculated, including the probability density functions (PDFs), skewness, and kurtosis of the intensity, which are found to be consistent with measurements by Langmuir probes. Besides, the track of blob motions is recorded by time sequence of individual frames. The characteristics of the original images and the relatively high-frequency (>10 kHz)/low-frequency (1–10 kHz) component images are illustrated. The observation of the blob’s structures and high-speed motions proves the success and high performance of the GPI diagnostic.

An eight-channel correlation electron cyclotron emission diagnostic for turbulent electron temperature fluctuation measurement in HL-2A tokamak.

A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on the HL-2A tokamak in order to study electron temperature fluctuations. Eight radial locations are measured simultaneously through eight pairs of correlated channels. Multiplexers are employed in the intermediate frequency section instead of the conventional separated filter banks to meet strict cross-isolation specifications and lower insertion loss. Relative electron temperature fluctuations are observed by CECE for the first time on the HL-2A by using the spectral decorrelation method. The achieved minimum detectable fluctuation level is up to (T̃e/Te)min∼0.5%. When studying electron temperature fluctuations in the core region with gas puffing, the cross-power spectra show that the amplitude of the electron temperature fluctuation increases in a high temperature and low density plasma. Further analysis demonstrates that the electron temperature gradient ∇Te drives the electron temperature fluctuations together with electron heat transport.

Geant4 simulation study on detection efficiencies of the Compton suppression system at the HL-2A tokamak

A Compton suppression system (CSS), which plays an important role in γ-ray diagnostics, has been developed to acquire the γ-ray emission spectra generated in the HL-2A tokamak plasma. It takes advantages of the anti-coincidence method of two branches of γ-ray signals, one is from a high purity germanium (HPGe) detector, and the other from several bismuth germinate (BGO) detectors, to output the suppressed spectra. In the present paper, a Geant4 simulation, together with its subsequent data processing which took accidental coincidences into account, was performed to study the detection efficiencies of the CSS for γ-rays with energies between 0.2 and 13 MeV. Calculation results showed that there were small detection efficiency differences between the unsuppressed and suppressed spectra when HPGe count rates were smaller than 105 cps, and the area ratios of additional peaks to their corresponding full energy peaks were not associated with HPGe count rates. Based on the calculation results, the empirical formulas of detection efficiencies of the CSS were put forward. Finally, after the experimental verification of the empirical formulas, a suppressed γ-ray spectrum acquired by the CSS during 2018 HL-2A experiment campaign was unfolded using the detection efficiencies. This work makes it possible for the analyses of the γ-ray emission spectra generated from the HL-2A tokamak.

Progress of plasma density fluctuation measurements with phase contrast imaging on HL-2A tokamak

A CO2 laser-based phase contrast imaging (PCI) diagnostic has been developed on HL-2A tokamak. It can detect line integrated plasma density fluctuations by measuring the phase shift of laser beam after being scattered by the bulk plasma. The diagnosed radial region ranges from to 0.7. 32-channel HgCdTe detectors with alternative-current biased amplifiers are arranged in line at the imaging plane of the optical path. This PCI is able to diagnose density fluctuations with wavenumbers ranging from 2 to 15 cm−1 and the time resolution is better than 2 μs. The first experimental data were achieved in 2018 spring campaign of HL-2A tokamak. High performance is confirmed in different discharging configurations and makes it a keen tool in broadband turbulence investigations.

Localized modulation of turbulence by m/n = 1/1 magnetic islands in the HL-2A tokamak

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.

Beta induced Alfvén eigenmode driven by energetic ions on the HL-2A tokamak

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.

High-sensitivity far-forward collective scattering diagnostic on HL-2A tokamak.

The multichannel formic acid (HCOOH, λ = 432.5 µm) laser interferometer and Faraday-effect polarimeter on HL-2A tokamak have been developed to measure the far-forward collective scattering from electron density fluctuations. The far-forward collective scattering system provides eight channels of line-integrated electron density fluctuations, covering the wave-number range: k⊥ < 1.6 cm-1. With the new diagnostic, the density fluctuations caused by plasma energetic particles and turbulence have been routinely observed in HL-2A experiments.