Plasma Current Distribution Research Articles

Synergistic current drive of helicon wave and lower hybrid wave in HL-2M

Non-inductive current drive plays a crucial role in tokamak, especially for its steady state operations. Recently, the helicon wave (HW) has been regarded as a promising tool for driving off-axis plasma current in reactor-grade machine. The lower-hybrid wave (LHW) is the most effective radio-frequency current drive method, however, it has the drawback, which is limited by the conditions of wave accessibility in the high parameter tokamak, making the wave power usually damped at the plasma edge. HW can spiral towards the plasma centre directly under a high electron density. To obtain a long pulse steady state operation of reactor tokamak, the complementarity of HW and LHW in the aspect of driven current distribution in the high parameter tokamak is considered. The synergy current drive of the HW and the LHW is studied numerically in the steady-state scenario of HL-2M. According to the fast wave dispersion relation of plasma, the HW parameters, including its wave frequency and launched parallel refractive index, are obtained firstly. Results of GENRAY code simulation show that a single pass wave power absorption of the HW can be obtained generally through the electron Landau damping and transit time magnetic pumping effects. On the other hand, the LHW parameters are adopted from the equipped system on the machine. Results of single pass wave absorption are also obtained in the case of LHW. And then, the synergy effects of HW and LHW are studied numerically based on the GENRAY/CQL3D models. The cooperation of these two waves results in a broad plasma current distribution along the radial direction (<inline-formula><tex-math id="M2">\begin{document}$\rho = $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231077_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231077_M2.png"/></alternatives></inline-formula>0.2－0.9) in the machine. Taking the electron distribution functions of these waves into account, it is clear that the electrons are accelerated by the HW in the parallel magnetic field direction, resulting in more electrons entering the region of LHW resonance area. As the consequence, a net plasma current appears. Furthermore, a fine-grained parametric scan is performed by changing the launched parallel refractive index of HW, and the results indicate that positive synergy effects can be generally observed once the related wave current drive profiles are overlapped. Finally, the synergy factor is shown to be proportional to this overlap and reaches its maximum value of 1.18 in HL-2M.

Bayesian inference of axisymmetric plasma equilibrium

We present a Bayesian method for inferring axisymmetric plasma equilibria from the magnetic field and plasma pressure measurements. The method calculates all possible solutions for plasma current and pressure distributions consistent with the measurements and magnetohydrodynamic (MHD) force balance. Toroidal plasma current and magnetic field coils are modelled as a set of axisymmetric current-carrying solid beams. The other parameters such as plasma pressure and poloidal current flux are given as a function of poloidal magnetic flux, which is determined given a 2D current distribution. Plasma pressure and poloidal current flux profiles are modelled as Gaussian processes whose smoothness is optimally chosen based on the principle of Occam’s razor. To find equilibrium solutions, we introduce an MHD force balance constraint at every plasma current beam as a part of the prior knowledge. Given all these physical quantities, predictions calculated by the predictive (forward) models for diagnostics are compared to the observations. The high dimensional complex posterior probability distribution is explored by a new algorithm based on the Gibbs sampling scheme.

Laser-driven, ion-scale magnetospheres in laboratory plasmas. II. Particle-in-cell simulations

Ion-scale magnetospheres have been observed around comets, weakly magnetized asteroids, and localized regions on the Moon and provide a unique environment to study kinetic-scale plasma physics, in particular in the collision-less regime. In this work, we present the results of particle-in-cell simulations that replicate recent experiments on the large plasma device at the University of California, Los Angeles. Using high-repetition rate lasers, ion-scale magnetospheres were created to drive a plasma flow into a dipolar magnetic field embedded in a uniform background magnetic field. The simulations are employed to evolve idealized 2D configurations of the experiments, study highly resolved, volumetric datasets, and determine the magnetospheric structure, magnetopause location, and kinetic-scale structures of the plasma current distribution. We show the formation of a magnetic cavity and a magnetic compression in the magnetospheric region, and two main current structures in the dayside of the magnetic obstacle: the diamagnetic current, supported by the driver plasma flow, and the current associated with the magnetopause, supported by both the background and driver plasmas with some time-dependence. From multiple parameter scans, we show a reflection of the magnetic compression, bounded by the length of the driver plasma, and a higher separation of the main current structures for lower dipolar magnetic moments.

Study of microscale surface morphology changing of the retroreflector in EAST by means of laser speckle interferometry technique

• The surface morphology of the retroreflector served in EAST is first time reconstructed through a 3D visual exhibition. • Three internal reflection planes of the retroreflector served in EAST appear severely different surface morphology features. • Laser speckle interferometry technique is a reliable surface morphology characterization approach for PFMs. The retroreflectors in EAST (Experimental Advanced Superconducting Tokamak) are important optical components of Polarimeter-Interferometer (POINT) system, working as the first mirrors to reflect the incident far-infrared laser. The surface morphology changing of the retroreflector has a direct impact on the optical reflection accuracy and it will generate a significant influence on EAST plasma density measurement and plasma current distribution diagnosis. Therefore, it is essential to research the surface morphology changing of the retroreflector systematically. Laser speckle interferometry technique is considered as a most promising method for real-time monitoring the PFMs surface morphology changing at plasma discharge intervals in the near future and its measurement accuracy and reliability have been adequately verified in the laboratory. One retroreflector served in 2019–2020 EAST Campaign is selected as a representative, compared with a brand-new retroreflector, to demonstrate that microscale surface morphology changing occurred on the internal reflection planes after bearing a whole EAST campaign plasma interaction. Meanwhile, the surface morphology of each internal reflection plane appears severely different features. Further research and analysis of the surface morphology are more contributed to evaluating the damage states of the retroreflector and providing more suggestions for the improvement and protection of the retroreflector. The surface morphology reconstruction results are given through a 3D visual exhibition and laser speckle interferometry technique is proposing a new research perspective for PWI research.

Calculation of Coil Currents for Equilibria with Reversed Plasma Current Density in J-Text Tokamak

To design the alternating current operation of the J-text tokamak, this paper studies the plasma equilibrium during the plasma toroidal current reversing. First, four alternative current reversal equilibrium configurations with circular cross section when the total toroidal current becomes zero were constructed. Then, the profile of the plasma current was used as the input of a fixed boundary equilibrium solver to calculate the poloidal field coil currents. According to the results, the relationship between the plasma current distribution and currents in ohmic heating coils and vertical field coils is discussed. Finally, the plasma equlibrium of a toroidal current reversal with total plasma current from 40 kA to –40 kA in 40 ms was designed.

Plasma current profile reconstruction for EAST based on Bayesian inference

Determining the distribution of plasma current in the equilibrium state is one of the most important steps to realize effective and safe operation of tokamak. In this study, a novel reconstruction code based on Bayesian inference is developed to infer the plasma current distribution for experiment analysis of EAST. Without iteratively solving Grad-Shafranov (G-S) equation to find an optimal fit for the external magnetic diagnostic measurements, the distribution of toroidal current density and poloidal flux can be rapidly derived in a probabilistic manner. The reconstructed results are consistent with the results based on equilibrium fitting (EFIT) code, and the execution time is less than 1 ms for each time slice, which indicates its potential for application in future real-time plasma feedback control.

Integrated simulation of plasma current profile in HL-2A high confinement mode(H mode)

The plasma current (<i>I</i><sub>p</sub><italic/>), magnetic field (<i>B</i>), and safety factor distribution (<i>q</i> profile) of the HL-2A tokamak device are crucial to monitoring the steady-state operational scenarios (in high confinement mode, H mode). Based on real experimental data and integrated modeling simulation method (OMFIT), the plasma parameters’ profiles such as magnetic field configuration and current density profiles in H mode were reconstructed. By building up an integrated simulation platform for dynamic equilibrium configuration, and combining the rapid workflow processing method and experimental data with integrated simulation models, the ion and electron temperature, density, and current density profiles were obtained. The integration simulation platform was established to reconstruct the internal magnetic surface configuration, the plasma boundary parameter distribution, the ion/electron temperature, current density, and the <i>q</i> profile. The Ohmic current, bootstrap current, and radio-frequency current profiles with its fractions were calculated. The width of the pedestal region was about 7.52 cm according to our simulation results. It was found that the pressure gradient changes its direction at radial coordinate <i>ρ</i>(<i>r/a</i>) = 0.1 and reaches its maximum value near <i>ρ</i> = 0.7, which may be the internal transport barrier (ITB) configuration caused by negative shear. The profile reconstruction and real-time monitoring of the physical parameters are conducive to evaluating the quality of H mode discharge experiment and can assist in the steady-state operation of advanced operating modes such as HL-2M high normalized beta (<i>β<sub>n</sub></i>) discharge.

Multi-channel analog lock-in system for real-time motional Stark effect measurements

The motional Stark estark effect (MSE) diagnostic system developed to measure the plasma current density distribution in the Korea Superconducting Tokamak Advanced Research performs data analysis by applying Fourier transform algorithm using the Interactive Data Language (IDL) software after measurement and digital archiving. However, in order to realize advanced plasma control aiming at the development of high performance tokamak operations, there has been a demand for the MSE diagnostic system capable of real-time pitch-angle measurement associated with a plasma control system (PCS). For real-time, multi-channel, and high-speed MSE data processing with high precision, an analog lock-in technique has been proposed and its feasibility has been demonstrated through the table-top tests using the photoelastic-modulator-base polarimetry. The prototype multi-channel analog lock-in system is ready for the plasma commissioning and the integration with the PCS.

Optimization of Magnetic Confinement for Quasi-Snowflake Divertor Configuration

To realize the commercialized operation, it is preferable that tokamak devices achieve the operating scenarios with steady-state, long-pulse, H-mode and high magnetic confinement plasma current. For enhancing the magnetic confinement capability and improving the poloidal beta βp in quasi-snowflake configuration, under the consideration of the effect on other performance parameters, the method of shifting magnetic axis is employed. According to the calculated method proposed in this paper, the plasma current density distribution is modified to realize the supposed radial movement of the magnetic axis. Then the value of βp, the coordinates of X2 point, the flux expansion of outside and inside strike points (fmout and fmin), poloidal field currents and safety factor q profile are formulated, the relations between these parameters and the horizontal displacement of the magnetic axis are analyzed. Finally, in lower single-null quasi-snowflake configuration, by shifting the magnetic axis from 1.9242 to 1.9412m, βp has an increase of 86.96 %, the confinement capability of the plasma is significantly enhanced, Meanwhile, the position of X2 point is varied from (2.6789 m, −1.6439 m) to (2.2955 m, −1.6039 m), which is closer to X1 point and the plasma. Therefore, fmout is enlarged from 6.646 to 7.706, which has an increase of 15.9%. And fmin has a decrease of 0.62%, which can be neglected. The safety factor at magnetic axis q0 is decreased by 1.75%, from 1.0897 to 1.0701, which is larger than 1 and satisfies the requirement of magnetohydrodynamic stability. In addition, EFIT code is used to verify the feasibility and the accuracy of the optimized results.

Equilibrium Configuration Reconstruction of Multipole Galatea Magnetic Trap Based on Magnetic Measurement

Equilibrium reconstruction is an important basic work for the design and operation of a multipole Galatea magnetic trap device, and it is also a fundamental theoretical work for plasma equilibrium control. Based on the static equilibrium model of the multipole Galatea magnetic trap, an equilibrium reconstruction scheme based on solving the Grad–Shafranov equilibrium equation is proposed. In the proposed scheme, a plasma current density distribution model is established, and the combination of the Green function and the finite difference method is investigated to realize the fast solution to the Grad–Shafranov equilibrium equation. At the same time, the least-squares fitting method is employed, along with the magnetic probe measurement data, which realizes correction and iteration of the parameters of the plasma current density distribution model. Through simulation, the magnetic field distribution, plasma current density distribution, and plasma pressure distribution of multipole Galatea magnetic trap can be obtained, which confirms the effectiveness of the equilibrium reconstruction when compares with other experimental results. At the end of this paper, the influence of the magnetic field produced by the solenoid coil on the equilibrium configuration of plasma is studied.

Micro-Faraday cup matrix detector for ion beam measurements in fusion plasmas.

Atomic beam probe is an extension of the routinely used beam emission spectroscopy diagnostic for the plasma edge current fluctuation measurement at magnetically confined plasmas. Beam atoms ionized by the plasma are directed to a curved trajectory by the magnetic field and may be detected close to the wall of the device. The arrival location and current distribution of the ions carry information about the plasma current distribution, the density profile, and the electric potential in the plasma edge. This paper describes a micro-Faraday cup matrix detector for the measurement of the few microampere ion current distribution close to the plasma edge. The device implements a shallow Faraday cup matrix, produced by printed-circuit board technology. Secondary electrons induced by the plasma radiation and the ion bombardment are basically confined into the cups by the tokamak magnetic field. Additionally, a double mask is installed in the front face to limit the ion influx into the cups and supplement secondary electron suppression. The setup was tested in detail using a lithium ion beam in the laboratory. Switching time, cross talk, and fluctuation sensitivity test results in the lab setup are presented along with the detector setup to be installed at the COMPASS tokamak.

Motion characteristics of coaxial gun discharge plasma current sheet

The coaxial gun discharge, used as plasma jet with high density and velocity, has a wide variety of applications such as plasma space propulsion, simulation experiment of thermal transient events in the International Thermonuclear Experimental Reactor, plasma refueling for fusion reactors and a laboratory scale platform for studying astrophysical phenomena. The plasma produced in the coaxial gun can be accelerated by self-induced Lorentz force, and the ionization in the transport process can be based on " snow-plow model” in which a coaxial current sheet moves forward and sweeps a large amount of the gas between two electrodes to cause the plasma dump. Based on the measurements of discharge current, voltage, photocurrent and magnetic signal, the experimental investigation on the characteristics of plasma motion and current sheet channel distribution in the gun operated under different discharge conditions and various pressures is carried out. In this paper, it is emphasized to explore the electrical and dynamic properties about plasma in the first half-cycle of current. The results show that the plasma velocity increases with the increase of the current amplitude, and that the transport distance is proportional to the axial kinetic energy of ions when the pressure is fixed at 10 Pa and discharge current is adjusted from 35.7 kA to 69.8 kA. Moreover, in the case of high current, the plasma jet from the nozzle tends to form a new current path at the bottom of the gun. However, when the discharge current is fixed at 49.8 kA and the gas pressures range from 5 Pa to 40 Pa, the plasma motion velocity and transport distance are continuously reduced. Meanwhile, it is not found that new current paths are generated at the bottom of the coaxial gun under high pressure. The generation of the new current path is relevant to the channel impedance formed by more charged particles left at the bottom of the gun and neutral particles leaking from current sheet during discharge. Besides, a multiple discharge phenomenon is presented in experiment and the secondary discharge breakdown position occurs at the head of the electrode when the current is reversed to a positive value. Therefore, this study provides a reasonable choice of electrical parameters to obtain optimal plasma characteristics during the discharge of the coaxial gun.

Modeling of the initial stages of the formation of heterogeneous plasma flows in the electric explosion of conductors

The experimental and theoretical studies on scattering of electric explosion products in the end-type plasma accelerator are carried out. Using the method of high-speed imaging (106 shots per second), it is revealed that there are three zones in a heterogeneous plasma flow. Zone 1 is a plasma focus, in Zone 2 there is a 90° turn of a flow, whereas a plasma flow is parallel to the dielectric disk in Zone 3. Following the concepts of plasma scattering when exposed to magnetic and gas-dynamic pressure, a mathematical model is proposed and provides an adequate explanation of plasma motion in Zone 3. It comprises equations of laws of mass and conservation of momentum, as well as the first and second Kirchhoff's laws. The outcomes of modeling are in compliance with the experimental data. A numerical single-fluid magnetohydrodynamic model is developed for general description of formation and evolution of a plasma flow. It is based on Navier-Stokes and Maxwell's equations. The obtained patterns of plasma current distribution agree satisfactory with the results of high-speed imaging. They point at a jet in the central part of the electrode, which splits out with the distance from its surface.

Inference of field reversed configuration topology and dynamics during Alfvenic transients

Active control of field reversed configuration (FRC) devices requires a method to determine the flux surface geometry and dynamic properties of the plasma during both transient and steady-state conditions. The current tomography (CT) method uses Bayesian inference to determine the plasma current density distribution using both the information from magnetic measurements and a physics model in the prior. Here we show that, from the inferred current sources, the FRC topology and its axial stability properties are readily obtained. When Gaussian process priors are used and the forward model is linear, the CT solution involves non-iterative matrix operations and is then ideally suited for deterministic real-time applications. Because no equilibrium assumptions are used in this case, inference of plasma topology and dynamics up to Alfvenic frequencies then becomes possible. Inference results for the C-2U device exhibit self-consistency of motions and forces during Alfvenic transients, as well as good agreement with plasma imaging diagnostics.

Acceleration and Pickup Ring of Energetic Electrons Observed in Relativistic Magnetic Reconnection Simulations

Abstract Pickup ring of energetic electrons found in relativistic magnetic reconnection (MR) driven by two relativistic intense femtosecond laser pulses is investigated by particle simulation in 3D geometry. Magnetic reconnection processes and configurations are characterized by plasma current density distributions at different axial positions. Two helical structures associated with the circular polarization of laser pulses break down in the reconnection processes to form a current sheet between them, where energetic electrons are found to pile up and the outflow relativistic electron jets are observed. In the field line diffusion region, electrons are accelerated to multi-MeV with a flatter power-law spectrum due to MR. The development of the pickup ring of energetic electrons is strongly dependent upon laser peak intensities.

First time observation of local current shrinkage during the MARFE behavior on the J-TEXT tokamak

Multifaceted asymmetric radiation as well as strong poloidal asymmetry of the electron density from the edge, dubbed as ‘MARFE’, has been observed in high electron density Ohmically heated plasmas on J-TEXT tokamak. Equilibrium reconstruction based on the measured data from the 17-channel FIR polarimeter–interferometer indicates that an asymmetric plasma current density distribution forms at the edge region and the plasma current shrinkage locates at the MARFE affected region. Furthermore, associated with the localized plasma current shrinkage, a locked mode MHD activity is excited, which then terminate the discharge with a major disruption. Localized plasma current shrinkage at the MARFE region is considered to be the direct cause for the density limit disruptions, and the proposed interpretation is consistent with the experimental observations.

RETRACTED: A new perspective on hydrogen plasma equilibrium calculation along with the MHD potential energy

In this contribution we have presented two techniques for determination of plasma equilibrium position in IR-T1 tokamak: Relaxation and optical techniques. An analysis method of tokamak plasma equilibrium by a relaxation method with specified magnetic axis was presented. The degrees of freedom due to designated positions of the magnetic axis are possible by using poloidal field coil currents. Stable steady state tokamak plasma equilibria are calculated along with the Magnetohydrodynamic (MHD) potential energy. The plasma generates a plasma current which partially or fully cancels the magnetic field from the poloidal field coils. For low-temperature plasmas, the plasma current distribution is centrally peaked; for high-temperature plasmas, the plasma current has a hole. A centrally peaked current distribution in a low-temperature plasma is evolved into a current distribution with a hole by increasing the plasma pressure by Ohmic heating, RF heating, or by neutral beam injection heating. In the second technique, an image processing technique is used for the output signal of CCD camera. Then, plasma emission intensity profile and plasma position were obtained. Results were compared and discussed.

The numerical code TOKSCEN for modelling plasma evolution in tokamaks

The code TOKSCEN (TOKamak SCENario) for the modelling of plasma evolution is described in this paper. The modelling is based on numerical solution of the Grad–Shafranov equation of plasma equilibrium and circuit equations for eddy currents at each time step. The circuit equations for eddy currents are solved in matrix form using the technique of matrix inversion. The plasma current distribution should be given. The code enables the calculation of an increment of vertical instability at each time step of the plasma evolution. The algorithms of the code were used for the modelling of processes in KTM and other tokamaks.

Numerical analysis of the coupled arc–weld pool–keyhole behaviors in stationary plasma arc welding

A unified fluid flow and heat transfer model is developed by taking into account the coupled transport mechanisms of the plasma arc, weld pool and keyhole in stationary plasma arc welding. The local thermodynamic equilibrium-diffusion approximation method is used to treat the interface between the arc plasma and the weld pool. The volume-of-fluid method is employed to track the keyhole boundary, and the underlying physics in the whole domain are self-consistent. The whole welding process from arc ignition to penetrated keyhole formation, including the weld pool surface depression, blind keyhole expansion, dynamic coupling between the arc plasma and keyhole, interaction of keyhole and weld pool, and the transient behaviors of temperature, fluid flow and electromagnetic fields in the arc column, keyhole and weld pool, are numerically simulated. The dynamic variations of the current density, plasma arc pressure and heat flux at the weld pool surface with the evolution of keyhole boundary are quantitatively analyzed. It is found that as the keyhole depth increases the plasma arc shape, temperature profile and current density distribution undergo dynamic evolution so that the heat flux on the keyhole wall (plasma arc–weld pool interface) changes continuously. However, the plasma jet velocity is insensitive to the keyhole depth so that the plasma arc pressure at the keyhole wall is almost stable. The stationary plasma arc welding experiments are performed, and the macrographs of welds are made to validate the model.

Effect of the Gas Pressure and the Charging Voltage on the Plasma Current Density Distribution in Conical Z Pinch Plasma Thruster

Plasma current density is measured inside the chamber of a conical z-pinch device using helium as the working gas. The gas pressure and the charging voltage are the major factors affecting the magnitude of the plasma current, pinching time, and the pinch duration. Calculations are carried out at different radial positions between the pin electrode and the chamber wall. The experimental results show that the current density reaches a maximum value of about 8.7 kA/cm2 above the pin electrode for 7 kV charging voltage and 1 Torr pressure, and this value decreases toward the chamber wall. The gas pressure has a significant effect on the current density at radial position of 0.5 cm from the pin indicating that the plasma could not be confined to a radius less than that. This result is confirmed by the recorded plasma current signal in which one peak only is recorded at the pin and two or three are recorded near the wall as a result of sequent compression and expansion. The experimental results show that a time of 2–2.5 μs is needed for the pinch to occur. Increasing the discharge voltage decreases both pinch time and the pinch duration due to the faster confinement process.