Plasma Shape Research Articles

Finite magnetic well effects on resistive and drift-resistive ballooning modes in a shaped tokamak

The impact of plasma shaping through magnetic well modifications on the stability of resistive ballooning modes (RBMs) in tokamaks is analysed, also including finite diamagnetic flows. Various limiting cases of the dispersion relation, obtained by matching the ballooning equation across the ideal and resistive layers, are analysed. It is found that stability is generally improved by the combination of vertical elongation and positive triangularity, although, in some cases, the growth rate of the unstable mode can be enhanced by these effects. Usually, vertically elongated plasmas with no triangularity are prone to exhibit worse stability properties. A value for the critical β above which RBMs are driven unstable is identified, and a connection with type-III ELM activity is established.

Techniques and Applications of KSTAR Plasma Equilibrium Reconstruction Analysis

Plasma gathered in a space tries to disperse under its own pressure. In the Tokamak, it is prevented using the force of the magnetic field and the plasma current. Therefore, when the force is balanced, the current, magnetic field, and pressure of the plasma are connected by a relationship. The equation describing this plasma equilibrium is called the Grad-Shafranov equation, and the plasma equilibrium analysis basically interprets this equation to find the magnetic field structure in the plasma and the spatial distribution of macroscopic physical quantities of the plasma. All Tokamak devices in the world, as well as KSTAR, essentially perform the plasma equilibrium analysis. The plasma equilibrium analysis is used to obtain the macroscopic physical quantities including the magnetic field structure of the experiment firstly and for in-depth analysis such as modeling and simulation. In addition, plasma equilibrium analysis is used to control plasma shape in real time.

Predicted optimization of β N limit for steady-state scenario with double transport barriers in HL-2M tokamak

The equilibria of the HL-2M tokamak, designed for steady-state operation with reversed magnetic shear and double transport barriers, are calculated using TOQ code, and the linear ideal magnetohydrodynamic (MHD) stability of the equilibria is investigated using GATO code. To improve the beta () limit, the MHD stability of the equilibria with different heights of internal transport barrier (ITB) and edge transport barrier (ETB) is simulated. The simulation results indicate that the ITB drives low globalized modes while the ETB drives high edge localized modes, and thus a moderate barrier height ratio is beneficial to MHD stability. By adjusting the heights of both barriers with different plasma shapes, as well as different locations and widths of ITBs, optimal equilibria with improved MHD stability are achieved, which always have a moderate barrier height ratio unless the ITB is extremely wide or very close to the edge. In ideal-wall conditions, when the ITB is wide enough or very close to the edge, limits could exceed six, or even reach seven in some special cases. Increasing elongation and triangularity, broadening the ITB, as well as moving the ITB to a large minor radius, can contribute to the optimization of the limit, especially when combined with the stabilizing effect from an ideal wall.

Linear simulation of magnetohydrodynamic plasma response to three-dimensional magnetic perturbations in high-β P plasmas

We report the numerical analyses of the linear magnetohydrodynamics (MHD) plasma response to applied three-dimensional magnetic perturbations (MPs) in a joint DIII-D/EAST collaboration on high-β P (poloidal beta) plasmas, utilizing the extended-MHD code M3D-C1, with the purpose of gaining a better understanding of the existing experiment in which n = 3 MPs were applied to such high-β P plasmas attempting to control large-amplitude type-I edge-localized modes (ELMs). These high-β P plasmas obtained at the DIII-D tokamak feature an upper-biased double-null configuration, a high edge safety factor q 95 ∼ 6.4, and a stable internal transport barrier (ITB), leading to relatively high core pressures. Single-fluid simulations show that the plasma response to n = 3 MPs, including both non-resonant/kinking and resonant components, is significantly weaker than that to n = 1 or 2 MPs. To survey the impact of q 95 on the plasma response to applied MPs, the self-consistent equilibrium-generating workflow for analysis module, developed in the OMFIT integrated modeling framework, is employed to generate a series of equilibria with a wide range of q 95, while other key parameters, including the normalized beta, electron density at the pedestal top, and plasma shape, are kept fixed. Compared to the vacuum response, single-fluid M3D-C1 simulations predict a much more significant decrease in resonant plasma response to the applied n = 3 MPs at the maximum penetration radii as q 95 increases. In contrast to single-fluid simulation results, showing that resonant penetration occurs only near the pedestal top where the E × B toroidal rotation frequency is zero, two-fluid simulations show two comparable resonant penetrations located near the pedestal top and the ITB foot, where the perpendicular electron rotation frequency is zero. Such resonant field penetration near the ITB foot may be responsible for the observed formation of a staircase structure in both the electron density and temperature profiles, and thereby a considerable deterioration in the global plasma performance, when MPs are applied in high-β P plasmas. Motivated by this numerical work, we provide some ideas for future research, with the purpose of realizing effective ELM control in such high-β P plasmas in the DIII-D and EAST devices.

Inter-machine plasma perturbation studies in EU-DEMO relevant scenarios: lessons learnt for EM forces prediction during VDEs

Abstract To support the deployment of DEMO wall protection strategy, the development of comprehensive analyses is essential to understand the implications of transient perturbations on the plasma shape control and on the vertical stability, some of the most critical aspects to be considered in elongated plasmas. Therefore, the design activities of the DEMO limiter structures need the deep understanding of the effects induced by transient plasma perturbations coupled with one of the most severe load conditions occurring in tokamaks, the Vertical Displacement Event (VDE). Since Electromagnetic (EM) loads during VDE phases are among the DEMO limiters design drivers, this study focuses on predictive simulations of the final plasma position and of EM loads following a VDE. For this purpose, a multi-tokamaks study supported by the construction of an inter-machine database containing experimental transient plasma perturbations and VDEs from JET and ASDEX Upgrade (AUG) has been carried out. It aims at the characterization of some transient plasma perturbations that may lead to high control efforts by the vertical stability system in terms of variations of the plasma internal parameters and vertical displacements. Consequently, such experimental transient plasma perturbations have been properly scaled to DEMO reference geometries with different magnetic configurations, to be simulated in terms of plasma dynamical behaviour by means of MAXFEA code. Finally, initial predictive EM loads on DEMO limiter structures will be discussed in the case of VDEs following plasma perturbations.

Investigation on property of electron beam plasma with diamond window

The properties of Ar plasma generated by electron beam with initial energy of 45 keV passing through a 5 μm-thick diamond film window was investigated by experimental diagnostic and Monte Carlo simulation. It is found that the plasma light emission intensity enhances with increasing the electron beam current, while the plasma shape has no significant change. When the gas pressure increases, the plasma shrinks and becomes brighter, and its shape gradually changes from cone-shape to semi prolate spheroid. The electron density increases with increasing gas pressure and electron current. When the gas pressure is higher than 10 kPa, the electron density can reach the order of 1010 cm−3 at an electron current of 0.3 mA. Under high-pressure conditions, the plasma range with respect to gas pressure satisfies well a simple inverse relationship. The electron energy deposition distribution obtained by Monte Carlo simulation is consistent with the measured plasma light emission intensity distribution. Optical emission spectroscopy was used to analyze collision process in the electron beam plasma. The line intensities of the 2p 2, 2p 6 and 2p 10 levels grow relatively with increasing gas pressure, indicating that the atom-atom collisional processes are enhanced.

Application of local Monte Carlo method in neutronics calculation of EAST radial neutron camera

The Local Monte Carlo (LMC) method is used to solve the problems of deep penetration and long time in the neutronics calculation of the radial neutron camera (RNC) diagnostic system on the experimental advanced superconducting tokamak (EAST), and the radiation distribution of the RNC and the neutron flux at the detector positions of each channel are obtained. Compared with the results calculated by the global variance reduction method, it is shown that the LMC calculation is reliable within the reasonable error range. The calculation process of LMC is analyzed in detail, and the transport process of radiation particles is simulated in two steps. In the first step, an integrated neutronics model considering the complex window environment and a neutron source model based on EAST plasma shape are used to support the calculation. The particle information on the equivalent surface is analyzed to evaluate the rationality of settings of equivalent surface source and boundary. Based on the characteristic that only a local geometric model is needed in the second step, it is shown that the LMC is an advantageous calculation method for the nuclear shielding design of tokamak diagnostic systems.

The 7-in. freestanding diamond thermal conductive film fabricated by DC arc Plasma Jet CVD with multi-stage magnetic fields

Diamond has attracted increasing attention because it has high thermal conductive. Here, a freestanding diamond thermal conductive film with a diameter of about 7 in. and an average thickness of 1.54 mm was prepared by DC arc plasma jet CVD with multi-stage magnetic fields. Specifically, the thickness uniformity was ensured by adding the magnetic field during the deposition of the freestanding diamond film. The morphology, structure and thermal conductivity of the diamond film were studied. The results showed that the expansion area of the DC arc plasma approached the surface of the substrate increasing duo to add the bottom magnetic field. Structural characterization revealed that the grain of the diamond film in the arc main were coarse, affected by the shape of DC arc plasma, while in the arc center and edge were fine. The main orientation of polycrystalline structure of the diamond film was the crystalline plane of (220). The Raman peaks of diamond film in different areas moved to a higher wavelength, indicating the existence of compressive stress inside the diamond film. The coarse columnar grain and a few grain boundaries were in the arc main. Therefore, the average value of the thermal conductivity (k⊥) of diamond film measured in the arc main was 1728.9 ± 4.9 W/(m·k). Meanwhile, the thermal conductivity of the arc center and edge area was lower than that of the arc main.

Modelling of sawtooth-induced fast ion transport in positive and negative triangularity in TCV

Internal kinks are a common magneto hydro-dynamic (MHD) instability observed in tokamak operation when the q profile in the plasma core is close to unity. This MHD instability impacts both the transport of the bulk plasma (current, particle and energy transport) and minority species, such as fast ions. In tokamak à configuration variable (TCV) (R 0/a = 0.88 m/0.25 m) the fast ion population is generated in the plasma by neutral beam tangential injection of energies up to 28 keV. TCV features 16 active shaping coils permitting a great flexibility in plasma shape, including negative triangularity (δ) configurations that show surprisingly high confinement. This study focuses on the transport of fast ions induced by sawteeth, by comparing two triangularity cases and simulation results with experimental data. Comparison of two equilibria with opposite δ shows that the fast ion drifts are larger for δ < 0. Furthermore, the sawtooth-induced transport in this case is larger than δ > 0 in similar conditions. Comparison with experimental data confirms the dominance of the modification of thermal kinetic profiles following the sawtooth crash in explaining drops in the neutron rates and fast ion D-α signals. Additional fast ion diffusion, however, improves the interpretation of the experimental data. For δ < 0, the amplitude of the perturbation better representing the experimental data is larger. Finally, an exploratory study for 50 keV particles (soon available in TCV) shows that the situation does not worsen for such particles.

Helium injection plasmas in FTU

In order to extend observations on the increase of electron density peaking in neon doped plasmas, already reported in FTU Mazzotta et al (2015 Nucl. Fusion 55 073027), some sessions have been performed by injecting helium gas on the L-mode plasmas during the last two experimental campaigns. This favorable scenario, which can fall within so called ‘highly radiative’ or ‘plasma detachment’ themes, is investigated in this paper. The description of the impact of the helium injection on plasma behavior, by varying plasma parameters and shape, is exposed, especially with respect to the density peaking and edge conditioning. It has been recorded that, not only the total amount of puffed helium, but also the injection rate intervenes in triggering a particle inflow. Finally, by using VUV spectroscopy measurements, a model to estimate concentrations of impurities is outlined here for the first time, in this way the helium amount is deduced.

Design of a new reflectometric system for real time plasma position control on the RFX-mod2 device

RFX-mod2 (R = 2.0 m, a = 0.49 m), the upgraded version of the previous RFXmod fusion device, will be equipped with a new reflectometry system specifically designed for plasma position control purposes. Featuring a high temporal and spatial resolution and being suitable for harsh fusion reactor experimental conditions (long pulses, high neutron fluxes), reflectometry has been proposed as a good candidate for this task. On RFX-mod2 the diagnostic system will consist of four bistatic ultrafast independent reflectometric units working in the frequency range (16–26.5 GHz) and installed in four different poloidal locations at the same toroidal angle: two on the equatorial plane (high field side/low field side) and two at the vertical top/bottom ports. Standard pyramidal horns will be installed in the external midplane and in the vertical ports, while parabolic hoghorn reflectors have been designed for the internal midplane. Different technical solutions for the integration in the machine, as the additive manufacturing for the antenna production, are presented. Despite the differences with respect to the application on large Tokamaks like DEMO, the RFX-mod2 plasma position reflectometer can contribute to test on a simple machine some of the issues related to the development of a reflectometry based plasma position and shape control.

Benchmarking 2D against 3D FDTD codes for the assessment of the measurement performance of a low field side plasma position reflectometer applicable to IDTT

O-mode reflectometry, a technique to diagnose fusion plasmas, is foreseen as a source of real-time (RT) plasma position and shape measurements for control purposes in the coming generation of machines such as DEMO. It is, thus, of paramount importance to predict the behavior and capabilities of these new reflectometry systems using synthetic diagnostics. Finite-difference time-domain (FDTD) time-dependent codes allow for a comprehensive description of reflectometry but are computationally demanding, especially when it comes to three-dimensional (3D) simulations, which requires access to High Performance Computing (HPC) facilities, making the use of two-dimensional (2D) codes much more common. It is important to understand the compromises made when using a 2D model in order to decide if it is applicable or if a 3D approach is required. This work attempts to answer this question by comparing simulations of a potential plasma position reflectometer (PPR) at the Low Field-Side (LFS) on the Italian Divertor Tokamak Test facility (IDTT) carried out using two full-wave FDTD codes, REFMULF (2D) and REFMUL3 (3D). In particular, the simulations consider one of IDTT’s foreseen plasma scenarios, namely, a Single Null (SN) configuration, at the Start Of Flat-top (SOF) of the plasma current.

First GEM measurements at WEST and perspectives for fast electrons and heavy impurities transport studies in tokamaks

Soft X-ray (SXR) radiation emitted from tokamak plasmas contains very useful information about plasma stability, shape and impurity content, all key parameters to improve plasma performance. In the deuterium-tritium phase of ITER, the high neutron fluxes, gamma and hard X-ray emission will constitute too harsh an environment to permit the use of classical semiconductor detectors. New SXR detector technologies, more robust to such environments, should thus be investigated. First GEM (Gas Electron Multiplier) measurements performed at WEST were successful and showed that both spatially and spectrally resolved calibrated data could be acquired. Strategies to reconstruct tungsten (W) impurity radiation synthetic diagnostics, modelling and real measurements based on multiple diagnostics are proposed.

New Identification Approach and Methods for Plasma Equilibrium Reconstruction in D-Shaped Tokamaks

The paper deals with the identification of plasma equilibrium reconstruction in D-shaped tokamaks on the base of plasma external magnetic measurements. The methods of such identification are directed to increase their speed of response when plasma discharges are relatively short, like in the spherical Globus-M2 tokamak (Ioffe Inst., St. Petersburg, Russia). The new approach is first to apply to the plasma discharges data the off-line equilibrium reconstruction algorithm based on the Picard iterations, and obtain the gaps between the plasma boundary and the first wall, and the second is to apply new identification methods to the gap values, producing plasma shape models operating in real time. The inputs for on-line robust identification algorithms are the measurements of magnetic fluxes on magnetic loops, plasma current, and currents in the poloidal field coils measured by the Rogowski loops. The novel on-line high-performance identification algorithms are designed on the base of (i) full-order observer synthesized by linear matrix inequality (LMI) methodology, (ii) static matrix obtained by the least square technique, and (iii) deep neural network. The robust observer is constructed on the base of the LPV plant models which have the novelty that the state vector contains the gaps which are estimated by the observer, using input and output signals. The results of the simulation of the identification systems on the base of experimental data of the Globus-M2 tokamak are presented.

Popping a Picture of Plasma Bubbles at Low Density

Faraday rotation illuminates plasma bubble size and shape at densities 40 times lower than previously studied

Optical Boundary Reconstruction With Visible Camera in the EXL-50 Spherical Tokamak

Plasma position and shape parameters are essential for equilibrium control and physics analysis in a tokamak. Such information is often obtained via optical boundary analysis. In this study, a new analysis of the visible image histogram of a high-speed high-resolution camera is presented which can effectively distinguish between the foreground information (showing the plasma of interest) and the background information (showing the inside views of the vacuum vessel wall and flanges, and so on). Using this method, we can obtain a reasonable approximation to the plasma boundary close to the last closed flux surface (LCFS) where plasma brightness contrast is greatest. The reliability and accuracy of this method are checked by comparing it favorably with other algorithms for different plasma shapes. The method is successfully tested also when the outboard plasma boundary oscillates in time with a magnitude of 10%.

Effect of shaping on turbulent dynamics in reversed-field pinch simulations

We study the influence of the shape of the plasma container on the dynamics of the reversed-field pinch (RFP). The geometries we consider are periodic cylinders with elliptical and circular-shaped cross-sections. Numerical simulations of fully nonlinear viscoresistive magnetohydrodynamics are carried out to illustrate how the plasma dynamics is affected by shaping. It is shown that independent of the plasma shape, the quantity $\beta$ , comparing the hydrodynamic pressure to the magnetic pressure, decreases for increasing values of the Lundquist number, but the pressure gradient fluctuations remain roughly constant, when compared to the Lorentz force. Different elliptical shapes of the cross-section of the domain lead to the excitation of different toroidal (or axial) magnetic and dynamic modes. Furthermore, it is found that in a geometry with circular cross-section, a significant local poloidal angular momentum is observed, absent in the geometries with elliptical cross-section. Because the confinement is dominantly determined by plasma movement, and the dynamics of the velocity and magnetic field are modified by the modification of the geometry, shaping can thus affect the performance of RFP devices.

Effects of negative triangularity shaping on energetic particle driven Alfvén eigenmodes in DIII-D * *Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish

Shape variations from positive to negative triangularity may lead to improved performance regimes in a tokamak with a reduction in microturbulence as well as improved thermal confinement. The motivation of this investigation is to model and compare the neutral beam driven Alfvén eigenmode (AE) activity in two DIII-D discharges with positive and negative triangularity shaping of plasma. The simulations are performed using the linear version of the FAR3d code, which solves the reduced MHD equations for thermal plasma with addition of moment equations for the energetic ion density and parallel velocity with appropriate Landau closure relations. Our numerical results indicate that for similar physical parameters, the unstable AEs observed in the negative triangularity case have lower growth rates as compared to the positive triangularity regime. Our findings may be useful to analyze the influence of the reverse-D like geometry on the AE instabilities in DIII-D and may lead to better configurations for minimizing fast ion losses in a tokamak device.

A brief history of negative triangularity tokamak plasmas

This review is devoted to tokamak plasmas with a cross sectional shape featuring negative triangularity, which appear to hold great promise as a candidate reactor configuration owing to their improved confinement. A brief historical perspective of its role in the worldwide magnetic fusion program is offered before reviewing theoretical predictions and experimental results on both magneto-hydrodynamic stability and turbulent transport. The material covers more prominently the confined plasma region, while limited work in the published literature is devoted to the scrape-off layer and plasma-wall interactions. A discussion on the suitability of this plasma shape in future reactors concludes the paper.

Design and integration studies of a diagnostics slim cassette concept for DEMO

The diagnostics slim cassette (DSC) is a concept currently proposed for DEMO to host the microwave reflectometry diagnostic in a dedicated poloidal section, for the feedback control of plasma position and shape, also providing the radial edge density profile at several poloidal angles. This DSC, which is expected to house up to 100 antennas and waveguides, is to be integrated with the breeding blanket (BB) segments. Moreover, it is being designed with remote handling compatibility in mind to facilitate a ‘fast’ exchange by remote maintenance (RM) when needed, fulfiling one of the aims of the work package diagnostic and control. In this approach the pre-assembled (banana-shaped) DSC modules are inserted/removed from the upper port (UP) of the vacuum vessel (VV), at least when the BB segments are replaced. Here the main constraints related to the integration of the DSC with the BB, the UP and the equatorial port are identified and discussed in detail, with a strong focus on the required RM operations inside the VV (in-vessel), providing solutions that can be adapted to present and future blanket and UP designs.