Plasma Current Density Research Articles

Predictive Fokker–Planck Modelling of Confined and Lost Fusion Alpha Particles in ITER

The paper represents results of predictive 3D Fokker–Planck modelling of phase space distributions of fusion alpha particles for basic ITER scenarios (Polevoi et al. in J Plasma Fusion Res Ser 5:82, 2002). We simulate the poloidal profiles of alpha induced current as well as of the fusion power deposition to bulk plasma electrons. It is demonstrated that anisotropy of velocity distributions of alphas results in a rather strong alpha driven current that makes up about 10–15 % of the equilibrium plasma current density in the 4th ITER scenario. We investigate the impact of the alpha driven current on the ITER magnetic configuration. In the 4th scenario fusion alphas are shown to result in ~15 % enhancement of the rotational transform and in ~11 % enlargement of the Shafranov shift of magnetic flux surfaces. Also we evaluate the capability of gamma diagnostics of high-energy alphas in ITER and examine the collisional losses of fusion alpha particles.

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.

Application of visible bremsstrahlung to Z(eff) measurement on the Experimental Advanced Superconducting Tokamak.

The multi-channel visible bremsstrahlung measurement system has been developed on Experimental Advanced Superconducting Tokamak (EAST). In addition to providing effective ion charge Zeff as a routine diagnostic, this diagnostic can also be used to estimate other parameters. With the assumption that Zeff can be seen as constant across the radius and does not change significantly during steady state discharges, central electron temperature, averaged electron density, electron density profile, and plasma current density profile have been obtained based on the scaling of Zeff with electron density and the relations between Zeff and these parameters. The estimated results are in good coincidence with measured values, providing an effective and convenient method to estimate other plasma parameters.

Tokamak equilibrium reconstruction code LIUQE and its real time implementation

Equilibrium reconstruction consists in identifying, from experimental measurements, a distribution of the plasma current density that satisfies the pressure balance constraint. The LIUQE code adopts a computationally efficient method to solve this problem, based on an iterative solution of the Poisson equation coupled with a linear parametrisation of the plasma current density. This algorithm is unstable against vertical gross motion of the plasma column for elongated shapes and its application to highly shaped plasmas on TCV requires a particular treatment of this instability. TCV's continuous vacuum vessel has a low resistance designed to enhance passive stabilisation of the vertical position. The eddy currents in the vacuum vessel have a sizeable influence on the equilibrium reconstruction and must be taken into account. A real time version of LIUQE has been implemented on TCV's distributed digital control system with a cycle time shorter than 200μs for a full spatial grid of 28 by 65, using all 133 experimental measurements and including the flux surface average of quantities necessary for the real time solution of 1.5 D transport equations. This performance was achieved through a thoughtful choice of numerical methods and code optimisation techniques at every step of the algorithm, and was coded in Matlab and Simulink for the off-line and real time version respectively.

Numerical Simulation of Non-Inductive Current Driven Scenario in EAST Using Neutral Beam Injection

For achieving the scientific mission of long pulse and high performance operation, experimental advanced superconducting tokamak (EAST) applies fully superconducting magnet technology and is equiped with high power auxiliary heating system. Besides RF (Radio Frequency) wave heating, neutral beam injection (NBI) is an effective heating and current drive method in fusion research. NBCD (Neutral Beam Current Drive) as a viable non-inductive current drive source plays an important role in quasi-steady state operating scenario for tokamak. The non-inductive current driven scenario in EAST only by NBI is predicted using the TSC/NUBEAM code. At the condition of low plasma current and moderate plasma density, neutral beam injection heats the plasma effectively and NBCD plus bootstrap current accounts for a large proportion among the total plasma current for the flattop time.

Research on Electromagnetic Engineering and Technology with Application of EFIT Code in J-TEXT Tokamak

The Grad-Shafranov equation is solved by EFIT code in J-TEXT tokamak. In this paper, it applies EFIT code to analyze the evolution of some plasma parameters during the discharge of J-TXET tokamak. Through plasmas balance calculation, the pressure of plasmas, internal magnetic surface and plasma current density profile are given. It provides important reference data for the operation and control of the J-TXET tokamak discharge.

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.

Nonlinear magnetosonic waves in dense plasmas with non-relativistic and ultra-relativistic degenerate electrons

Linear and nonlinear propagation of magnetosonic waves in the perpendicular direction to the ambient magnetic field is studied in dense plasmas for non-relativistic and ultra-relativistic degenerate electrons pressure. The sources of nonlinearities are the divergence of the ions and electrons fluxes, Lorentz forces on ions and electrons fluids and the plasma current density in the system. The Korteweg-de Vries equation for magnetosonic waves propagating in the perpendicular direction of the magnetic field is derived by employing reductive perturbation method for non-relativistic as well as ultra-relativistic degenerate electrons pressure cases in dense plasmas. The plots of the magnetosonic wave solitons are also shown using numerical values of the plasma parameters such a plasma density and magnetic field intensity of the white dwarfs from literature. The dependence of plasma density and magnetic field intensity on the magnetosonic wave propagation is also pointed out in dense plasmas for both non-relativistic and ultra-relativistic degenerate electrons pressure cases.

A signature for turbulence driven magnetic islands

We investigate the properties of magnetic islands arising from tearing instabilities that are driven by an interchange turbulence. We find that such islands possess a specific signature that permits an identification of their origin. We demonstrate that the persistence of a small scale turbulence maintains a mean pressure profile, whose characteristics makes it possible to discriminate between turbulence driven islands from those arising due to an unfavourable plasma current density gradient. We also find that the island poloidal turnover time, in the steady state, is independent of the levels of the interchange and tearing energy sources. Finally, we show that a mixing length approach is adequate to make theoretical predictions concerning island flattening in the island rotation frame.

Hall probe measurements of the poloidal magnetic field in Compact Toroidal Hybrid plasmas.

A linear array of 16 Hall effect sensors has been developed to directly measure the poloidal magnetic field inside the boundary of a non-axisymmetric hybrid torsatron/tokamak plasma. The array consists of miniature gallium arsenide Hall sensor elements mounted 8 mm apart on a narrow, rotatable printed circuit board inserted into a re-entrant stainless steel tube sheathed in boron nitride. The sensors are calibrated on the bench and in situ to provide accurate local measurements of the magnetic field to aid in reconstructing the equilibrium plasma current density profiles in fully three-dimensional plasmas. Calibrations show that the sensor sensitivities agree with the nominal manufacturers specifications of 1.46 V/T. Poloidal fields measured with the Hall sensor array are found to be within 5% of poloidal fields modeled with a Biot-Savart code.

Enhancement of the stability of electron field emission behavior and the related microplasma devices of carbon nanotubes by coating diamond films.

The enhanced lifetime stability for the carbon nanotubes (CNTs) by coating hybrid granular structured diamond (HiD) films on Au-decorated CNTs/Si using a two-step microwave plasma enhanced chemical vapor deposition process was reported. Electron field emission (EFE) properties of HiD/Au/CNTs emitters show a low turn-on field (E0) of 3.50 V/μm and a high emission current density (Je) of 0.64 mA/cm(2) at an applied field of 5.0 V/μm. There is no notable current degradation or fluctuation over a period of τ(HiD/Au/CNTs) = 360 min for HiD/CNTs EFE emitters tested under a constant current of 4.5 μA. The robustness of the HiD/CNTs EFE emitter is overwhelmingly superior to that of bare CNTs EFE emitters (τ(CNTs) = 30 min), even though the HiD/Au/CNTs do not show the same good EFE properties as CNTs, which are E0 = 0.73 V/μm and Je = 1.10 mA/cm(2) at 1.05 V/μm. Furthermore, the plasma illumination (PI) property of a parallel-plate microplasma device fabricated using the HiD/Au/CNTs as a cathode shows a high Ar plasma current density of 1.76 mA/cm(2) at an applied field of 5600 V/cm with a lifetime of plasma stability of about 209 min, which is markedly better than the devices utilizing bare CNTs as a cathode. The CNT emitters coated with diamond films possessing marvelous EFE and PI properties with improved lifetime stability have great potential for the applications as cathodes in flat-panel displays and microplasma display devices.

Investigation of the Plasma Current Density of a Pulsed Plasma Thruster

The pulsed plasma thruster of the magnetoplasmadynamic type represents a promising and cheap solution to today’s propulsion challenges in space. The work presented in this paper covers new information on the electromagnetic conditions during thrust generation by revealing the temporal and spatial distributions of the current density vectors between the thruster electrodes. This is achieved by building and calibrating miniaturized induction probes. The local magnetic field components of all three spatial directions are combined to derive the current density vector. Quantitative determination of the temporal behavior of vectors at 27 points evenly distributed in three planes delivers a comprehensive picture of the position and development of the current in a pulsed plasma thruster. Maximum current densities of and magnetic flux densities of up to 0.82 T have been observed close to the surface of the solid Teflon® propellant. It is concluded that no distinct movement of a current sheet was observed. The current density data are discussed and compared to high-speed camera photographs.

Predictive Simulations of Plasma Heating and Current Drive by Neutral Beam Injection on EAST

Long pulse and high performance steady-state operation is the main scientific mission of experimental advanced superconducting tokamak (EAST). In order to achieve this objective, high-power auxiliary heating systems are essential. Radio frequency (RF) wave heating and neutral beam injection (NBI) are two principal methods. NBI is an effective method of plasma heating and current drive, and it has been used in many magnetic confinement fusion devices. Based on the plasma equilibrium of EAST (Li et al., Plasma Phys Control Fusion 55:125008, 2013) plus previous EAST experimental data used as initial conditions, the NBI module (Polevoi et al., JAERI-Data, 1997) employed in automated system for transport analysis (ASTRA) code (Pereverzev et al., IPP-Report, 2002) is applied to predict the effects of plasma heating and current drive with different neutral beam injection power levels. At certain levels of plasma densities and plasma current densities, the simulation results show that the NBI heats plasma effectively, also increases the proportions of NB current and bootstrap current among total current significantly.

Physical characteristics of twin-wire indirect arc plasma

In this study, the behavior of arc in a twin-wire indirect arc welding is simulated for arc lengths of 5.2 mm, 6.2 mm and 7.2 mm. The plasma temperature is measured by spectrometric method. The results showed that increase in arc length increase the arc parameters (plasma temperature, current density, plasma velocity, heat flux and arc voltage) of twin-wire indirect arc (TWIA). The change in arc length was found to have little influence on the distribution and the variation in trend of arc parameters in the whole arc region. As the arc length increases, the arc parameters in cathode region increase at a slower rate than those of anode region. This probably is the reason for self-adjustment of TWIA and for the arc stability in a certain range of arc lengths. The temperature of anode region is slightly higher than that in the cathode region and other arc parameters have similar trends. The isotherms are displaced toward the anode side and the shape of TWIA is similar to an inclined and inverted bell. The displacement increases with increasing arc length. The experimental results of plasma temperatures are in good agreement with calculated results which verifies the accuracy of the calculated results.

Development of diamond cathode materials for enhancing the electron field emission and plasma characteristics using two-step microwave plasma enhanced chemical vapor deposition process

The enhancement on the plasma illumination characteristics of a cylindrical microplasma device due to the utilization of hybrid-diamond (HiD) films as cathode was systematically investigated. The improved plasma illumination behavior was closely related to the enhanced electron field emission (EFE) properties of the diamond films. The HiD films, which possessed better EFE properties, including lower turn-on field for inducing the EFE process [(E0)efe = 2.7 V/μm] and higher EFE current density [(Je)efe = 2.8 mA/cm2, at 10.6 V/μm], resulted in superior illumination performance for the microplasma devices. The plasma can be triggered at a low threshold field of (Eth)pl. = 0.166 V/μm, attaining a large plasma current density of (Je)pl. = 9.6 mA/cm2 at an applied field of 0.266 V/μm (plasma density of ne = 1.70 × 1015 cm−3). The better EFE for the HiD films is ascribed to the unique granular structure of the films. Transmission electron microscopic studies revealed that the HiD films contained large diamond aggregates evenly distributed among the ultrasmall grain matrix. There presents a-few-layer graphite, surrounding the large aggregates that formed electron transport networks and improved the EFE properties for the HiD films. The superior EFE properties for the HiD cathode materials are the prime factor for improving plasma illumination characteristics for the cylindrical microplasma devices.

Control of sawtooth periods by pulsed ECH/ECCD in the FTU tokamak

It is well known that fusion plasma operations can be limited in standard scenarios at high β by resistive instabilities, called neoclassical tearing modes (NTMs), which degrade the plasma confinement leading to a loss of plasma energy, and in some cases to disruption. The avoidance of their onset is a very important issue that has been largely investigated in many tokamaks. In particular, it has been shown that these modes, shaped as magnetic islands, can be triggered by finite seed perturbations associated with long sawtooth crashes. The control of the sawtooth periods is then a key step in the physics of plasma confinement in fusion devices: the shortening of these periods can reduce any triggered large seed island below the NTMs' growth threshold allowing maximum β values and high plasma performances to be achieved. A powerful tool for sawtooth control is the use of high localized electron cyclotron heating (ECH) and current drive (ECCD), capable of modifying the plasma current density and effecting the sawtooth period. Modulated ECH and ECCD have been used as triggers of sawtooth crashes to test conditions for an a priori constant sawtooth period. In the FTU high magnetic field compact tokamak (R0 = 0.93 m, a = 0.3 m, B0 = 4–8 T) similar experiments have been performed with an ECRH system of four gyrotrons operating at 140 GHz and delivering 0.5 MW each. Repetitive pulses of EC power from 1 to 2 gyrotrons up to 0.8 MW for 500 ms have been used to investigate the sensitivity of sawtooth periods during long and short EC switching on and off phases in view of a real time EC control system soon to be working in FTU. A new type of locking of the sawtooth periods to the EC modulation has been observed for deposition inside the q = 1 radius for EC on phase smaller than the ohmic period. In this paper, sawtooth period shortening and locking by ECH and co-ECCD inside the q = 1 radius is investigated and reproduced by numeric simulations.

Study of Plasma Current Density and Q-profiles for Circular Cross Section Tokamak

Study of Plasma Current Density and Q-profiles for Circular Cross Section Tokamak

Density limit experiments on FTU

One of the main problems in tokamak fusion devices concerns the capability to operate at a high plasma density, which is observed to be limited by the appearance of catastrophic events causing loss of plasma confinement. The commonly used empirical scaling law for the density limit is the Greenwald limit, predicting that the maximum achievable line-averaged density along a central chord depends only on the average plasma current density. However, the Greenwald density limit has been exceeded in tokamak experiments in the case of peaked density profiles, indicating that the edge density is the real parameter responsible for the density limit. Recently, it has been shown on the Frascati Tokamak Upgrade (FTU) that the Greenwald density limit is exceeded in gas-fuelled discharges with a high value of the edge safety factor. In order to understand this behaviour, dedicated density limit experiments were performed on FTU, in which the high density domain was explored in a wide range of values of plasma current (Ip = 500–900 kA) and toroidal magnetic field (BT = 4–8 T). These experiments confirm the edge nature of the density limit, as a Greenwald-like scaling holds for the maximum achievable line-averaged density along a peripheral chord passing at r/a ≃ 4/5. On the other hand, the maximum achievable line-averaged density along a central chord does not depend on the average plasma current density and essentially depends on the toroidal magnetic field only. This behaviour is explained in terms of density profile peaking in the high density domain, with a peaking factor at the disruption depending on the edge safety factor. The possibility that the MARFE (multifaced asymmetric radiation from the edge) phenomenon is the cause of the peaking has been considered, with the MARFE believed to form a channel for the penetration of the neutral particles into deeper layers of the plasma. Finally, the magnetohydrodynamic (MHD) analysis has shown that also the central line-averaged density at the onset of the MHD activity depends only on the toroidal magnetic field.

Development of atomic beam probe for tokamaks

The concept and development of a new detection method for light alkali ions stemming from diagnostic beams installed on medium size tokamak is described. The method allows us the simultaneous measurement of plasma density fluctuations and fast variations in poloidal magnetic field, therefore one can infer the fast changes in edge plasma current. The concept has been worked out and the whole design process has been done at Wigner RCP. The test detector with appropriate mechanics and electronics is already installed on COMPASS tokamak. General ion trajectory calculation code (ABPIons) has also been developed. Detailed calculations show the possibility of reconstruction of edge plasma current density profile changes with high temporal resolution, and the possibility of density profile reconstruction with better spatial resolution compared to standard Li-BES measurement, this is important for pedestal studies.

Low-recycling conditions and improved core confinement in steady-state operation scenarios in JET (Joint European Torus)

In this paper, we discuss the phenomena that link particle recycling from the vessel walls in the L-mode during discharge start-up and the core confinement in the H-mode during the subsequent main heating phase. We consider available data of JET experiments that aimed at approaching fully non-inductive ITER-relevant steady-state conditions and show that the high electron temperature produced at the edge by a low recycling during start-up tends to favour the build-up of high normalized β(βN) regimes in the H-mode, the confinement being improved in a large plasma volume. To provide an insight into this complex phenomenon we have modelled the relation between particle recycling in the scrape-off layer and the evolution of plasma transport, plasma current density and shear as well as the stability properties for such experimental conditions. The results confirm the existence of a link between the confinement in the H-mode phase and the values at the edge of electron temperature, bootstrap current density and local magnetic shear during start-up. Such a link could favour these regimes to be self-sustained in time.