The Principle of Plasma Boundary Identification on HL-2A Tokamak

Plasma boundary identification is a basic task for studies on equilibrium and confinement in a divertor tokamak. With the progress on the experiments after engineering experiments, the boundary identification becomes an important issue for HL-2A. In order to satisfy the requirements of preciseness, simplified measurements and quickness, the filament current method instead of solving the equilibrium equations is used to identify plasma boundary on HL-2A. The involved principle, mathematics and the progresses, which have been made with this method, are given.

Plasma Boundary Position Measurements Using Frequency Sweep Microwave Reflectometer in LHD

A broadband frequency tunable microwave reflectometer system, which has the abil- ity of fast and stable frequency sweeping operation, is applied in the large helical device （LHD） to measure the boundary of the high-temperature plasma. When the microwave is launched, with extraordinary polarization, from the low field side of the magnetic field with its frequency swept from low to high, the microwave of minimum right-hand cut-off frequency is reflected most out- side. We can then estimate the plasma boundary position by measuring the change of the reflected power from the cut-off layer.

Toroidal Multipolar Expansion for Fast L-Mode Plasma Boundary Reconstruction in EAST

A new method for plasma boundary reconstruction, based on the toroidal multipolar expansion （TME） scheme, is applied successfully in EAST. TME applies a limited number of toroidal multipolar moments based on toroidal coordinates to treat a two-dimensional problem of axisymmetric plasma equilibrium. The plasma boundary reconstructed by TME is consistent with the results by using EFIT. The method is sufficiently reliable and fast for real time shape control.

The Plasma Boundary in Low-Parameter Divertor Discharges on the HL-2A Tokamak

Because the last closed flux-surface （LCFS） finally determines the shape of plasma cross-section, and the shaping of plasma is a key issue directly related with lots of the hot subjects of an advanced tokamak（AT） and of a reactor, e. g., the control on the current density, equilibrium, scrapeoff layer（SOL）, aspects of edge particle and energy, high plasma kinetic energy, and of course, the MHD instability suppression, the further studies on the LCFS related subjects, e. g., its determination, flux loss, etc., are still tokamak research. It equilibrium, poloidal needed for present is also important to get precise LCFS-related equilibrium or configuration properties for the controls on an AT and for the understandings on the underlying basic physical issues in a discharge.

Detailed analysis and simulation verification for reconstruction of plasma optical boundary with spectrometric technique on HL-2M tokamak

The plasma optical boundary reconstruction technique based on Hommen's theory is promising for future tokamaks with high parameters. In this work, we conduct detailed analysis and simulation verification to estimate the ‘logic loophole' of this technique. The finite-width effect and unpredictable errors reduce the technique's reliability, which leads to this loophole. Based on imaging theory, the photos of a virtual camera are simulated by integrating the assumed luminous intensity of plasma. Based on Hommen's theory, the plasma optical boundary is reconstructed from the photos. Comparing the reconstructed boundary with the one assumed, the logic loophole and its two effects are quantitatively estimated. The finite-width effect is related to the equivalent thickness of the luminous layer, which is generally about 2-4 cm but sometimes larger. The level of unpredictable errors is around 0.65 cm. The technique based on Hommen's theory is generally reliable, but finite-width effect and unpredictable errors have to be taken into consideration in some scenarios. The parameters of HL-2M are applied in this work.

Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

Geomagnetic storms can result in large magnetic field disturbances and intense currents in the magnetosphere and even on the ground.As an important medium of momentum and energy transport among the solar wind,magnetosphere,and ionosphere,field-aligned currents(FACs)can also be strengthened in storm times.This study shows the responses of FACs in the plasma sheet boundary layer(PSBL)observed by the Magnetospheric Multiscale(MMS)spacecraft in different phases of a large storm that lasted from May 27,2017,to May 29,2017.Most of the FACs were carried by electrons,and several FACs in the storm time also contained sufficient ion FACs.The FAC magnitudes were larger in the storm than in the quiet period,and those in the main phase were the strongest.In this case,the direction of the FACs in the main phase showed no preference for tailward or earthward,whereas the direction of the FACs in the recovery phase was mostly tailward.The results suggest that the FACs in the PSBL are closely related to the storm and could be driven by activities in the tail region,where the energy transported from the solar wind to the magnetosphere is stored and released as the storm is evolving.Thus,the FACs are an important medium of energy transport between the tail and the ionosphere,and the PSBL is a significant magnetosphere–ionosphere coupling region in the nightside.

Turbulent boundary layer control with a spanwise array of DBD plasma actuators

The turbulent boundary layer control on NACA 0012 airfoil with Mach number ranging from 0.3 to 0.5 by a spanwise array of dielectric barrier discharge(DBD)plasma actuators by hot-film sensor technology is investigated.Due to temperature change mainly caused through heat produced along with plasma will lead to measurement error of shear stress measured by hot-film sensor,the correction method that takes account of the change measured by another sensor is used and works well.In order to achieve the value of shear stress change,we combine computational fluid dynamics computation with experiment to calibrate the hot-film sensor.To test the stability of the hot-film sensor,seven repeated measurements of shear stress at Ma=0.3 are conducted and show that confidence interval of hot-film sensor measurement is from−0.18 to 0.18 Pa and the root mean square is 0.11 Pa giving a relative error 0.5%over all Mach numbers in this experiment.The research on the turbulent boundary layer control with DBD plasma actuators demonstrates that the control makes shear stress increase by about 6%over the three Mach numbers,which is thought to be reliable through comparing it with the relative error 0.5%,and the value is hardly affected by burst frequency and excitation voltage.

Observations of kinetic Alfvén waves and associated electron acceleration in the plasma sheet boundary layer

Kinetic Alfvén waves(KAWs),with a strong parallel disturbed electric field,play an important role in energy transport and particle acceleration in the magnetotail.On the basis of high-resolution observations of the Magnetospheric Multiscale(MMS)Mission,we present a detailed description of the acceleration process of electrons by KAWs in the plasma sheet boundary layer(PSBL).The MMS observed strong electromagnetic disturbances carrying a parallel disturbed electric field with an amplitude of up to 8 mV/m.The measured ratio of the electric to magnetic field perturbations was larger than the local Alfvén speed and was enhanced as the frequency increased,consistent with the theoretical predictions for KAWs.This evidence indicates that the electromagnetic disturbances should be identified as KAWs.During the KAWs,the energy flux of electrons at energies above 1 keV in the parallel and anti-parallel directions are significantly enhanced,implying occurrences of electron beams at higher energies.Additionally,the KAWs became more electrostaticlike and filled with high-frequency ion acoustic waves.The energy enhancement of electron beams is in accordance with the derived work done with the observed parallel disturbed electric field of KAWs,indicating electron acceleration caused by KAWs.Therefore,these results provide direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.

The relations between density of FACs in the Plasma Sheet Boundary Layers and Kp index

We have studied 172 field-aligned currents （FACs） cases observed by the ClusterlI satellites when they crossed the plasma sheet boundary layer （PSBL） in the magnetotail from July to October 2001. We mainly analyzed the relationship between the characteristic of FACs at the PSBL in magnetotail and the Kp index. The main results indicated the followings： 1） In the different geomagnetic activity levels, the relative occurrence of FACs in PSBL increased monotonically with geomagnetic activity. 2） The density of FACs in PSBL increased monotonically with Kp index. In the storm main phase, the density of FACs increased dramatically, the maximum FACs approximately equaled 19.05 nA m-2 while Kp equaled 5.3） The variation of FACs density in PSBL was consistent with the variation of the Kp index. However, when AE〈800 nT, FACs density in PSBL increased with increasing AE, and when AE〉800 nT, it decreased with increasing AE. Therefore, our results suggested that the FACs density in PSBL had a closer correlation with Kp index.

Probability of Field-Aligned Currents Observed by the Satellite Cluster in the Magnetotail

Field-aligned current （FAC） density distribution at the plasma sheet boundary layers is statistically studied. The FAC is calculated by the so-called curlometer technique with the data from FGM onboard the four Cluster spacecraft in 2001. By calculation we obtain a large number of FAC samples. In the samples, most of calculated FAC densities were very small and around zero caused by some errors or noise. In order to get the real FAC density distribution in the magnetotail, we use a three-Gaussian distribution to fit the errors, then subtract the estimated error contribution from the full distribution and obtain the FAC density distribution. The result shows that the FAC occurrence versus its density has a distribution consisting of a Gauss/an distribution with an additional decreasing exponential distribution. The most probable value of the FAC density is 3.45 pT/km.

Effect of edge magnetic island on carbon screening in the J-TEXT tokamak

The effect of externally applied resonant magnetic perturbation(RMP)on carbon impurity behavior is investigated in the J-TEXT tokamak.It is found that the m/n=3/1 islands have an impurity screening effect,which becomes obvious while the edge magnetic island is generated via RMP field penetration.The impurity screening effect shows a dependence on the RMP phase with the field penetration,which is strongest if the O point of the magnetic island is near the low-field-side(LFS)limiter plate.By combining a methane injection experimental study and STRAHL impurity transport analysis,we found that the variation of the impurity transport dominates the impurity screening effect.The impurity diffusion at the inner plasma region(r/a<0.8)is enhanced with a significant increase in outward convection velocity at the edge region in the case of the magnetic island’s O point near the LFS limiter plate.The impurity transport coefficient varies by a much lower level for the case with the magnetic island’s X point near the LFS limiter plate.The interaction of the magnetic island and the LFS limiter plate is thought to contribute to the impurity transport variation with the dependence on the RMP phase.A possible reason is the interaction between the magnetic island and the LFS limiter.

Reconstruction of plasmoid and traveling compression region in the near-Earth magnetotail

Cluster spacecraft observed an earthward flowing plasmoid along with a travelling compression region （TCR） structure in southern plasma sheet boundary layer （PSBL） at 21：09 UT of September 19, 2001. We have reconstructed the two-dimensional topology of the magnetic field structure observed by C1 using Grad-Shafranov reconstruction method. Results show that CI passed through part of a plasmoid, which compressed the lobe magnetic field and formed a TCR. The size of the whole plas- moid structure in X direction is estimated to be about 3 Re. Furthermore, using multi-spacecraft observations, we have found some detailed information about this structure. First, C1 observed bi-streaming electron components, which supports our sug- gestion that the spacecraft passed through closed field lines. Second, a small magnetic field perturbation within this plasmoid accompanied by slight decrease in electron flux suggests that a flux rope core might exist at the center of the plasmoid.