Plasma Cross Section Research Articles

An Optimal Real-Time Controller for Vertical Plasma Stabilization

Modern Tokamaks have evolved from the initial axisymmetric circular plasma shape to an elongated axisymmetric plasma shape that improves the energy confinement time and the triple product, which is a generally used figure of merit for the conditions needed for fusion reactor performance. However, the elongated plasma cross section introduces a vertical instability that demands a real-time feedback control loop to stabilize the plasma vertical position and velocity. At the Tokamak \`a Configuration Variable (TCV) in-vessel poloidal field coils driven by fast switching power supplies are used to stabilize highly elongated plasmas. TCV plasma experiments have used a PID algorithm based controller to correct the plasma vertical position. In late 2013 experiments a new optimal real-time controller was tested improving the stability of the plasma. This contribution describes the new optimal real-time controller developed. The choice of the model that describes the plasma response to the actuators is discussed. The high order model that is initially implemented demands the application of a mathematical order reduction and the validation of the new reduced model. The lower order model is used to derive the time optimal control law. A new method for the construction of the switching curves of a bang-bang controller is presented that is based on the state-space trajectories that optimize the time to target of the system. A closed loop controller simulation tool was developed to test different possible algorithms and the results were used to improve the controller parameters. The final control algorithm and its implementation are described and preliminary experimental results are discussed.

Analogue of bead lightning in a pulse discharge initiated by runaway electrons in atmospheric pressure air

A breakdown initiated by runaway electrons in an inhomogeneous electric field in atmospheric pressure air was studied. Bright plasma channels (spark filaments) in the center of the discharge gap were observed on the background of a diffuse discharge by applying high-voltage pulses with an amplitude of about 200 kV across the discharge gap with an interelectrode distance of 18 mm. Their occurrence probability is no more than 10%. The glow of the bright plasma channels is similar to that of a spark channel. It was found that the bright plasma channels appear with a decrease in the discharge current. Its radiation intensity is an order of magnitude lower than that of the diffuse discharge. Such bright plasma channels were observed in pulseperiodic discharges, where nanosecond voltage pulses with amplitudes of 10–15 kV were applied across the discharge gap with an interelectrode distance of 6 mm. The pulse repetition rate was about 400 Hz. Occurrence of the bright plasma channels on the background of diffuse discharge is caused by generation of runaway electrons and the discharge current redistribution over the discharge plasma cross-section during a decrease in the discharge current. It is assumed that the discharge recorded is an analogue of bead lightning observed in the Earth’s atmosphere.

Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics

For a comprehensive comparison with theoretical models and advanced numerical turbulence simulations, a large spectrum of fluctuation parameters was measured on the devices ASDEX Upgrade, TCV, and Tore-Supra. Radial profiles of scale-resolved turbulence levels in H-mode discharges are measured and compared with GENE simulations in the transition range from ion-temperature-gradient to trapped-electron-mode turbulence. Correlation reflectometry is used to study the microscopic structure of turbulence and GAMs in discharges where poloidal flow damping was varied by means of variations of the shape of the poloidal plasma cross-section and collisionality. Full-wave codes and synthetic diagnostics are applied for the interpretation of the data.

The quiescent H-mode regime for high performance edge localized mode-stable operation in future burning plasmasa)

For the first time, DIII-D experiments have achieved stationary quiescent H-mode (QH-mode) operation for many energy confinement times at simultaneous ITER-relevant values of beta, confinement, and safety factor, in an ITER-like shape. QH-mode provides excellent energy confinement, even at very low plasma rotation, while operating without edge localized modes (ELMs) and with strong impurity transport via the benign edge harmonic oscillation (EHO). By tailoring the plasma shape to improve the edge stability, the QH-mode operating space has also been extended to densities exceeding 80% of the Greenwald limit, overcoming the long-standing low-density limit of QH-mode operation. In the theory, the density range over which the plasma encounters the kink-peeling boundary widens as the plasma cross-section shaping is increased, thus increasing the QH-mode density threshold. The DIII-D results are in excellent agreement with these predictions, and nonlinear magnetohydrodynamic analysis of reconstructed QH-mode equilibria shows unstable low n kink-peeling modes growing to a saturated level, consistent with the theoretical picture of the EHO. Furthermore, high density operation in the QH-mode regime has opened a path to a new, previously predicted region of parameter space, named “Super H-mode” because it is characterized by very high pedestals that can be more than a factor of two above the peeling-ballooning stability limit for similar ELMing H-mode discharges at the same density.

Progress of high power and long pulse ECRH system in EAST

In accordance with the long pulse objectives of the Experimental Advanced Superconducting Tokamak (EAST), an electron cyclotron resonance heating (ECRH) system with the feature of 4MW power for a pulse length up to 1000s at 140GHz, using second harmonic of the extraordinary mode (X2) is presently under construction at the institute of plasma physics, Chinese academy of sciences (ASIPP). The missions of the system are to provide central heating, current drive, plasma profile tailoring and control of magneto-hydrodynamic (MHD) instabilities. The continuous wave (CW) power is transmitted from the gyrotrons to EAST via low-loss evacuated waveguide transmission lines. Considering the diverse applications of the EC system, the front steering launcher is designed to inject four individually steered beams across nearly the entire plasma cross section. The beam's launch angles can be continuously varied with the optimized scanning range of over 30° in poloidal direction and ±25° in toroidal, as well as the polarization will be adjusted during the discharge by the orientations of a pair of polarizers in the transmission line to maintain the highest absorption for different operational scenarios. The commissioning of the first 2MW system will be commenced in the end of 2014.

STUDY OF THE EFFECTS OF ECCENTRIC PLASMA COATING OVER METAMATERIAL CYLINDER

A plasma sheath can significantly alter the electromagnetic properties of an object, which leads to many practical applications. In this article, the electromagnetic scattering properties of a DB metamaterial cylinder coated with unmagnetized plasma are presented. The effects of layer thickness, non-uniform cladding (eccentric coating), electron number density, electron-neutral collision frequency and the frequency of incident wave on radar cross-section (RCS) of the object are discussed. It is found that the RCS of the DB metamaterial objects can be reduced or enhanced by appropriate values of plasma parameters, thickness or eccentricity. The anomalous behavior of backscattering cross-section of plasma coated DB cylinder has been observed at frequencies near plasma frequency. The results may serve as a noteworthy reference for experimentalists working in plasma stealth technology for metamaterials.

Theoretical and experimental investigation of plasma antenna characteristics on the basis of gaseous collisionality and electron density

This paper reports plasma antenna characteristics investigated theoretically and experimentally, on the basis of the plasma parameters: gaseous collisionality and electron density. The antenna structure is a basic quarter-wavelength monopole antenna in the UHF band. The dependence of the antenna gain on the plasma parameters is obtained by analytical equations from plasma and antenna theory, and by numerical simulations. In the plasma antenna, the ratio of the electron elastic collision frequency to the total number of electrons at the plasma cross section determines the antenna’s internal loss and the electrical equivalent antenna length, whereas the ratio of the radio wave frequency to the total number of electrons at the plasma cross section determines the antenna’s resonant frequency. These results are confirmed by experimental results of the antenna’s impedance and radiation patterns.

Design of Controller for New EAST Fast Control Power Supply

The effectiveness of the magnetic confinement of plasma can be improved by elongating the plasma cross-section in tokamak devices. But elongated plasma has vertical displacement instability, so a feedback control system is needed to restrain the plasma's vertical displacement. A fast control power supply is needed to excite the active feedback coils, which produces a magnetic field to control the plasma's displacement. With the development of EAST, the fast control power supply needs to keep on enhancing the fast response and output current. The structure of a new power supply is introduced in this paper. The method of multiple inverters paralleled with the current sharing reactor is presented to meet the need for large current and fast control. According to the design demands of the EAST fast control power supply, the adjuster of the current close loop is applied to the inverter, which can advance its ability to restrain the loop current in low frequency and DC output. The result of the experiment confirms the validity of the proposed scheme and control strategy.

Localized Intermittent Electron Flux in an ECR Plasma

Intermittent enhancement of local electron flux has recently been observed in a linear electron cyclotron resonance plasma. The aim of this paper is to measure the 2-D distribution of the electron flux, which appears at random positions in the plasma cross section, using intensified charge-coupled device imaging. A series of images of the emission from the Ne 2p 1 state is presented; it clearly demonstrates a circular region of enhanced excitation due to increase in the electron temperature.

Two-dimensional study of edge impurity transport in the Large Helical Device

Edge impurity transport has been studied in the stochastic magnetic field layer called the ergodic layer of the Large Helical Device. Chord-integrated full vertical profiles of C IV (312.4 Å: 1s23p–1s22s) near edge X-points are observed at horizontally elongated plasma cross sections. Measured C IV profiles are almost flat at low density (⩽2 × 1013 cm−3), while two peaks called the ‘X-point peak’ newly begin to appear near edge separatrix X-points with increase in density in addition to ordinary edge peaks. The X-point peaks become very clear at high-density range (⩾6 × 1013 cm−3). The C3+ ions analyzed with three-dimensional edge plasma transport code, EMC3–EIRENE, move upstream at low density due to large thermal force. The widely expanded C3+ distribution makes the C IV profile flat. With increasing the density, the friction force becomes dominant and the impurity ions start to move downstream. The C3+ ions stay in the vicinity of the edge X-point where magnetic field lines are very short. The X-point peaks are then clearly formed with increase in the C3+ density. Two-dimensional C IV distribution is also observed for different plasma axis positions. It is found that the C IV emission becomes strong along the poloidal trajectory of X-points and the C IV poloidal trace moves from inboard to outboard X-point trajectories when the plasma axis is changed from 3.60 to 3.75 m. The three-dimensional simulation can well reproduce the enhancement of C IV trajectory along the X-point. However, a discrepancy with the experiment is still seen in the C IV intensity between inboard and outboard X-points.

Tomographic inversion techniques incorporating physical constraints for line integrated spectroscopy in stellarators and tokamaks.

Accurate tomographic inversion is important for diagnostic systems on stellarators and tokamaks which rely on measurements of line integrated emission spectra. A tomographic inversion technique based on spline optimization with enforcement of constraints is described that can produce unique and physically relevant inversions even in situations with noisy or incomplete input data. This inversion technique is routinely used in the analysis of data from the x-ray imaging crystal spectrometer (XICS) installed at the Large Helical Device. The XICS diagnostic records a 1D image of line integrated emission spectra from impurities in the plasma. Through the use of Doppler spectroscopy and tomographic inversion, XICS can provide profile measurements of the local emissivity, temperature, and plasma flow. Tomographic inversion requires the assumption that these measured quantities are flux surface functions, and that a known plasma equilibrium reconstruction is available. In the case of low signal levels or partial spatial coverage of the plasma cross-section, standard inversion techniques utilizing matrix inversion and linear-regularization often cannot produce unique and physically relevant solutions. The addition of physical constraints, such as parameter ranges, derivative directions, and boundary conditions, allow for unique solutions to be reliably found. The constrained inversion technique described here utilizes a modified Levenberg-Marquardt optimization scheme, which introduces a condition avoidance mechanism by selective reduction of search directions. The constrained inversion technique also allows for the addition of more complicated parameter dependencies, for example, geometrical dependence of the emissivity due to asymmetries in the plasma density arising from fast rotation. The accuracy of this constrained inversion technique is discussed, with an emphasis on its applicability to systems with limited plasma coverage.

Method for estimating the propagation direction of a coherent plasma structure using a one-dimensional diagnostic array.

This article proposes a new method to evaluate basic characteristics of the dynamics of a coherent plasma structure (blob). With this method, one can evaluate the propagation angle of a blob in a two-dimensional plasma cross section as well as the blob velocity, size, and amplitude from one-dimensional data. The method is applied to blob measurements from the Lithium beam emission spectroscopy system in ASDEX-Upgrade. Statistical features of the observed blob velocities, angles of propagation, blob sizes, and amplitudes are discussed. The validity of the method is examined by comparing two values of the propagation angle that are evaluated in an independent manner.

On interlayer stability and high-cycle simulator performance of diamond-like carbon layers for articulating joint replacements.

Diamond like carbon (DLC) coatings have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the orthopaedic field, layer performance, stability and adhesion in physiologically relevant setups are crucial and not consistently investigated. In vitro wear testing as well as adequate corrosion tests of interfaces and interlayers are of great importance to verify the long term stability of DLC coated load bearing implants in the human body. DLC coatings were deposited on articulating lumbar spinal disks made of CoCr28Mo6 biomedical implant alloy using a plasma-activated chemical vapor deposition (PACVD) process. As an adhesion promoting interlayer, tantalum films were deposited by magnetron sputtering. Wear tests of coated and uncoated implants were performed in physiological solution up to a maximum of 101 million articulation cycles with an amplitude of ±2° and −3/+6° in successive intervals at a preload of 1200 N. The implants were characterized by gravimetry, inductively coupled plasma optical emission spectrometry (ICP-OES) and cross section scanning electron microscopy (SEM) analysis. It is shown that DLC coated surfaces with uncontaminated tantalum interlayers perform very well and no corrosive or mechanical failure could be observed. This also holds true in tests featuring overload and third-body wear by cortical bone chips present in the bearing pairs. Regarding the interlayer tolerance towards interlayer contamination (oxygen), limits for initiation of potential failure modes were established. It was found that mechanical failure is the most critical aspect and this mode is hypothetically linked to the α-β tantalum phase switch induced by increasing oxygen levels as observed by X-ray diffraction (XRD). It is concluded that DLC coatings are a feasible candidate for near zero wear articulations on implants, potentially even surpassing the performance of ceramic vs. ceramic.

Validation of x-ray line ratios for electron temperature determination in tokamak plasmas

X-ray imaging crystal spectroscopy (XICS) has been implemented on magnetic confinement fusion devices as a novel means of measuring local plasma temperature, impurity density, and flow profiles. At Alcator C-Mod, XICS allows for spatially-resolved, high spectral resolution measurements between 0.3 and 0.4 nm, enabling detailed analysis of He-like argon x-ray emission. Electron temperatures in the range of 0.5 keV ⩽Te ⩽3.0 keV are determined from He-like argon emissivity ratios of the n = 3 dielectronic satellites to the w-line and its surrounding n ⩾ 3 satellites, specifically the wavelength range of 3.9440 Å ⩽ λ ⩽ 3.9607 Å. These data are validated against measurements of Te from existing electron cyclotron emission and Thomson scattering diagnostics. Line ratio data are analysed via a tomographic inversion procedure, overcoming the traditional issue of spectra being averaged over the plasma cross-section. The implications of utilizing x-ray line ratios as a valid local temperature diagnostic are not limited to Alcator C-Mod; properties of plasma in future experiments as well as in astrophysical settings can also be investigated. The results of this experiment confirm that x-ray line ratios can be used as an accurate electron temperature diagnostic. The electron temperature can be determined from the relation to the line ratio, x, as Te[keV] = 0.1552x−0.7781 with 0.0223 < x < 0.2449.

Development of New Fast Control Power Supply for EAST

Experimental Advanced Superconducting Tokamak (EAST), a National Mage project of Science Research of China, is a very important fusion experiment facility. The ability of magnetic confinement to plasma can be improved by elongating plasma cross section in EAST. However, elongated plasma has vertical displacement instability; without control, plasma will dash against a wall of vacuum vessel and disrupt, which will cause failure of plasma discharge. Therefore, a feedback control system is needed to restrain plasma vertical displacement. Fast control power supply excites active feedback coils, which produces magnetic field to control plasma's displacement. With the development of EAST, the power supply needs to keep on enhancing the ability of fast response and output current. The structure and control method of the new power supply are introduced in this paper. The technique of carrier-wave phase-shift pulsewidth modulation is applied to raise equal switch frequency of converter and improve performance of output wave. The analysis of harmonic and the simulation are carried out, the validity of the proposed scheme and control strategy are confirmed by experiments, and the power supply system can supply large current and has the ability of fast response.

First results of the J-TEXT high-resolution 3-wave polarimeter-interferometer system

A far-infrared laser polarimeter-interferometer systembased on Three-wave technique has been established on the J-TEXT tokamak.The system determines Faraday angle by measuring phase difference betweentwo collinear, counter-rotating, circularly polarized laser beams, andacquires line-integrated electron density simultaneously by phase comparisonbetween the two beams and a third local oscilate (LO) beam. Three seperatelypumped HCOOH lasers at 432 μm are adopted as sources, suppling more than100 mW power output in sum. Parabolic mirrors are used to expand probe beamsto 450 mm wide, covering ∼ 80% of plasma cross section, which allowsprofile measurement with high spatial resolution. First experimental resultsof the polarimeter-interferometer have been obtained. 12 chords (3 cm chordspacing) simultaneous polarimetric and interferometric measurements areachieved, with phase resolution up to 0.1° at bandwidth of 50 kHz. Withthe high temporal and phase resolution, perturbations associated with thesawtooth cycle and MHD activity have been observed.

Observation of visible forbidden lines from highly charged tungsten ions at the large helical device

Visible line emission from highly charged tungsten ions has been observed at the large helical device (LHD) using a tracer encapsulated solid pellet. One of the measured lines is assigned to a magnetic-dipole (M1) line of the ground-term fine-structure transition of W26+. The other line is unidentified but probably due to a highly charged tungsten ion. Photon emission was observed at 40 lines of sight divided along the vertical direction of a horizontally elongated poloidal cross section of the LHD plasma. The line-integrated intensity of the M1 line along each line of sight indicates a peaked profile at the plasma center where the electron temperatures are high enough so that tungsten ions are highly ionized.

Direct extraction of coherent mode properties from imaging measurements in a linear plasma column

Spectral properties of coherent waves in an argon plasma column are examined using fluctuation data from fast imaging. Visible light from ArII line emission is collected at high frame rates using a high-speed digital camera. A cross-spectral phase technique allows direct visualization of dominant phase structures as a function of frequency, as well as identification of azimuthal asymmetries present in the system. Experimental dispersion estimates are constructed from imaging data alone. Drift-like waves are identified by comparison with theoretical dispersion curves, and a tentative match of a low-frequency spectral feature to Kelvin-Helmholtz-driven waves is presented. Imaging measurements are consistent with previous results, and provide non-invasive, single-shot measurements across the entire plasma cross-section. Implications of the measured spectral properties for imaging measurements of mode dynamics are explored.

Active Feedback Control Used in Plasma Vertical Unstable Displacement for EAST

EAST (Experimental Advanced Superconducting Tokamak), a National Mage-project of Science Research of China, is a very important fusion experiment facility. The ability of magnetic confinement to plasma can be improved by elongating plasma cross-section to form divertor configuration in EAST. But elongated plasma has vertical displacement instability, without control, plasma will dash against wall of vacuum vessel and disrupt, that will cause failure of plasma discharge. So feedback control system is needed to restrain plasma vertical displacement. Fast control power supply excites active feedback coils, which produces magnetic field to control plasmas displacement. The validity of proposed scheme was confirmed by experiments, the active feedback system can reach design demand.

A fast, high-order solver for the Grad–Shafranov equation

We present a new fast solver to calculate fixed-boundary plasma equilibria in toroidally axisymmetric geometries. By combining conformal mapping with Fourier and integral equation methods on the unit disk, we show that high-order accuracy can be achieved for the solution of the equilibrium equation and its first and second derivatives. Smooth arbitrary plasma cross-sections as well as arbitrary pressure and poloidal current profiles are used as initial data for the solver. Equilibria with large Shafranov shifts can be computed without difficulty. Spectral convergence is demonstrated by comparing the numerical solution with a known exact analytic solution. A fusion-relevant example of an equilibrium with a pressure pedestal is also presented.