Poloidal rotation induced by injecting lower hybrid waves in tokamak plasma edge

The poloidal rotation of the magnetized edge plasma in tokamak driven by the ponderomotive force which is generated by injecting lower hybrid wave(LHW) electric field has been studied. The LHW is launched from a waveguide in the plasma edge, and by Brambilla's grill theory, analytic expressions for the wave electric field in the slab model of an inhomogeneous cold plasma have been derived. It is shown that a strong wave electric field will be generated in the plasma edge by injecting LH wave of the power in MW magnitude, and this electric field will induce a poloidal rotation with a sheared poloidal velocity.

Magnetic configuration effects on the edge heat flux in the limiter plasma on W7-X measured using the infrared camera and the combined probe

Controlling the heat and particle fluxes in the plasma edge and on the plasma facing components is important for the safe and effective operation of every magnetically confined fusion device.This was attempted on Wendelstein 7-X in the first operational campaign,with the modification of the magnetic configuration by use of the trim coils and tuning the field coil currents,commonly named iota scan.Ideally,the heat loads on the five limiters are equal.However,they differ between each limiter and are non-uniform,due to the（relatively small） error fields caused by the misalignment of components.It is therefore necessary to study the influence of the configuration changes on the transport of heat and particles in the plasma edge caused by the application of error fields and the change of the magnetic configuration.In this paper the upstream measurements conducted with the combined probe are compared to the downstream measurements with the DIAS infrared camera on the limiter.

Plasma Blobs and Turbulence Impacting Magnetic Confinement(A Short Review)

Space-based plasma(i.e.,a highly ionized gas or the fourth state of matter)blobs are isolated pockets of this highly ionized gas made up of charged particles.These blobs are believed to have a substantial impact on the structure and dynamics of the cosmos and can be seen in a variety of astronomical objects,including stars,galaxies,and the intergalactic medium.Some plasma blobs are connected to intense phenomena like magnetic reconnection,shock waves,and supernovae,while others may be the result of more passive processes like cooling and gravitational collapse.In both astrophysics and plasma physics,there is ongoing research on the characteristics and behavior of plasma blobs.This phenomenon has a very adverse effect on tokamak-based MCF(magnetic confinement fusion),which is the subject of this short review paper.

Investigation of Edge Plasma Features in the HL-IM Tokamak

Edge plasma features in typical HL- 1M discharges were presented. Particle confinement and plasma rotation have been investigated in the discharges with lower hybrid current drive (LHCD), molecular beam injection (MBI) and pellet fuelling. LHCD can make particle confinement increase a factor of 2-3 for low-density discharge. Particle confinement time and poloidal rotation can be at least doubled after pellet injection, while MBI can make confinement time increase about one order of magnitude with higher performance.

Experiments and Studies of Edge Plasma Radial Transport and Confinement Property on the HL-1M Tokamak

This paper describes a Mach/Langmuir probe array with five pins and six pins, which can measure not only parallel flows and the flow perpendicular to the magnetic field but also the radial and the poloidal electric field E. arid E as well. Experimental measurements of the edge fluctuations, velocities of the toroidal, the poloidal flow and electric field have been carried out on both of SOL and the boundary region of HL-1M for Ohmic, biased H-mode, Lower Hybrid Current Drive (LHCD), Supersonic Molecular Beam Injection (MBI), Multi-shot Pellet Injection (MPI), Neutral Beam Injection (NBI), Ion Cyclotron Resonance Heating (ICRH) and Electric Cyclotron Resonance Heating (ECRH) discharges. The results show that the suppressions of the fluctuations are related to poloidal rotations produced by different discharge modes in the improved particle confinement property, simultaneously the change of the radial and poloidal electric field is generated and becomes more negative at the Tokamak plasma edge, and the sheared poloidal flow is related to the reduction in fluctuation level, and the poloidal velocity is mainly dominated by the E × B drift.

Observation of Edge Plasma Fluctuations with a Fast Camera in Heliotron J

A fast video camera is verified to be a powerful t0ol for observation of filaments/blobs near the last closed flux surface （LCFS）. In order to extract the fluctuation component from the raw data of the fast camera, a pre-processing technique, sliding time window averaging subtraction （STWAS） has been developed to remove the background of slowly varying emission from the bulk plasma. By using this pre-processing technique, the fast camera data are analyzed. A method to identify the filaments in the pre-processed image is also discussed.

Edge Plasma behavior during Improved Confinement by Lower Hybrid Wave Heating in HT-6M Tokamak

Lower hybrid heating (LHH) has been successfully carried out in the HT-6M toka-mak. The H-mode has been obtained with a power threshold of 50 kW under a boronized wall condition. Both energy and particle confinements have been improved along with a dropped edge plasma density and an increase electron temperature during the LHH phase. A negative Er well plays a key role of triggering and sustaining the good confinement. Both electrostatic fluctuation of the plasma potential and the density fluctuations dropped to an ultra-low level. The observation of an enhanced Er shear before the reduction in turbulence level is consistent with an increased Er shear as the cause of turbulence suppression.

Estimation of Neutral Density in Edge Plasma with Double Null Configuration in EAST

In this work, population coefficients of hydrogen＇s n = 3 excited state from the hydrogen collisional-radiative （CR） model, from the data file of DEGAS 2, are used to calculate the photon emissivity coefficients （PECs） of hydrogen Balmer-α （n = 3 →n = 2） （Hα）. The results are compared with the PECs from Atomic Data and Analysis Structure （ADAS） database, and a good agreement is found. A magnetic surface-averaged neutral density profile of typical double-null （DN） plasma in EAST is obtained by using FRANTIC, the 1.5-D fluid transport code. It is found that the sum of integral Dα and Hα emission intensity calculated via the neutral density agrees with the measured results obtained by using the absolutely calibrated multi-channel poloidal photodiode array systems viewing the lower divertor at the last closed flux surface （LCFS）. It is revealed that the typical magnetic surface-averaged neutral density at LCFS is about 3.5×10^16 m^-3 .

Influence of the X-point location on edge plasma transport in the J-TEXT tokamak with a high-field-side single-null divertor

High-density experiments in the high-field-side mid-plane single-null divertor configuration have been performed for the first time on J-TEXT.The experiments show an increase in the highest central channel line-averaged density from 2.73×10^(19)m^(-3) to 6.49×10^(19)m^(-3),while the X-point moves away from the target by increasing the divertor coil current.The corresponding Greenwald fraction rises from 0.50 to 0.79.For the impurity transport,the density normalized radiation intensity(absolute extreme ultraviolet and soft x-ray)of the central channel density decreased significantly(>50%)with an increase in the plasma density.To better understand the underlying physics mechanisms,the 3 D edge Monte Carlo code coupled with EIRENE(EMC3-EIRENE)has been implemented for the first time on J-TEXT.The simulation results show good agreement with the experimental findings.As the X-point moves away from the target,the divertor power decay length drops and the scrape-off layer impurity screening effect is enhanced.

Dynamical evolution of cross phase of edge fluctuations and transport bifurcation

The dynamical evolution of edge turbulence during a transport bifurcation is explored using a flux-driven nonlinear fluid model with a geometry relevant to the plasma edge region.The simulations show that the self-generated mean shear flows can dramatically modify the phase angle between turbulent fluctuations.The changes in phase differences and amplitudes of edge fluctuations give rise to the modifications of turbulent edge transport.The statistical properties of flux and fluctuations are also investigated before and after edge shear flow generation.

Numerical accuracy and convergence with EMC3-EIRENE

The iterative Monte Carlo(MC)code EMC3-EIRENE is frequently used for plasma edge simulations in 3 D applications.So far,a quantitative evaluation of the numerical quality of the code results remains an open issue.In this paper,we demonstrate a framework for the practical assessment of accuracy and convergence with EMC3-EIRENE.Moreover,we provide a first accuracy analysis with EMC3-EIRENE for a DIII-D divertor edge plasma case.First,we introduce post-processing averaging to efficiently reduce the variance of the statistical error.Then,we estimate the deterministic error contributions based on their theoretical reduction rates by comparing solutions with a different grid resolution,time step,or number of MC particles per iteration.Finally,using parameterized expressions for the error and the computational time,suitable numerical parameters are determined to achieve faster and/or more accurate results.We found that simulations can be more than twice as fast without losing accuracy by making use of post-processing averaging and choosing optimal parameters.In addition,we conclude that the discretization error is the dominant error contribution for the case selected in this paper,which demonstrates the importance of constructing an adequate mesh.

Review of Divertor Studies in LHD

In the Large Helical Device （LHD）, two different divertor configurations, i.e. helical divertor （HD） and local island divertor （LID）, are utilized to control the edge plasma. The HD with two X-points is an intrinsic divertor for heliotron devices, accompanied with a relatively thick ergodic layer outside the confinement region. Edge and divertor plasma behavior from low density to high density regimes is presented, referring to the divertor detachment. The effect of the ergodic layer on the edge transport is also discussed. On the other hand, the LID is an advanced divertor concept which realizes a high pumping efficiency by the combination of an externally induced magnetic island and a closed pumping system. Experimental results to confirm the fundamental divertor performance of the LID are presented.

Study of Carbon Impurity Transport at SOL in EAST

Transport of carbon in the edge plasma of EAST with a heating power Pin of 8 MW is studied using DIVIMP code. The background plasmas for DIVIMP are taken from the results by using B2.5-Eirene code. For different plasma densities at the core-SOL interface and the different divertor operational regimes, namely low recycling, high recycling and detachment, the simulated results show that the impurity density in SOL is higher for the high recycling regime than that for the low recycling regime, while impurity density in SOL is lower for the detachment regime than that for both the low and high recycling regimes.