MHD Instabilities Research Articles

Investigation of H-mode density limit in mixed protium–deuterium plasmas at JET with ITER-like wall

Analysis of comparable discharges fuelled by either deuterium or protium reveals a clear relationship between the isotope mass and the H-mode density limit. Notably, the density limit is significantly lower in protium, showing a reduction of up to 35 % compared to identical deuterium plasma conditions. Within mixed H-mode density limit (HDL) plasmas, the maximum achievable density, or H-mode density limit, decreases with increasing protium concentration, denoted as cH. For instance, the highest corresponding maximum Greenwald fraction (fGW) of about 1.02 was observed in the pulse with the lowest cH value of 4.4 %. This fGW decreases to 0.96 at cH = 48 %. The average atomic mass, A¯, of the plasma species decreases in these pulses from the value of 1.96 (cH = 4.4 %) down to 1.52(cH = 48 %). Interestingly, the maximum achievable density appears to be largely unaffected by the applied power value, regardless of whether deuterium or protium is used, as well as under mixed H/D fuelling conditions.Additionally, the measured Greenwald fractions are agreed with a heuristic model based on the SOL pressure threshold of an MHD instability, as proposed by Goldston. This comparison, especially concerning the model’s dependence on isotopic mass, shows full consistency between the measured and predicted Greenwald fractions.

Study on MHD Pulses, Waves, and Instability in Trigging multiple renewables noise on various spectral information scales

Major concepts covered in this research include: State of being free vibration and method start changing; MHD waveguides like the magnetic properties dorsal fin, radicular groups, and radicular streaming sites; processes for frequency supplied in imperativeness overflows throughout sub hurricanes and solar activity; the possibility of Heat diffusion resonant frequencies along empty field lines; potential motorists of MHD waves; diagnostic tools of plasma screens with MHD waves; the collaboration of MHD waves mostly with positive ions limits (ionosphere and chromosphere). This review is directed largely at experts in the subjects of magnetosphere material science and solar theoretical physics, while it does delve into the intricacies of the surround investigation fields to those who are concerned. Though there are several noise contributors, aerodynamics is the most significant. When it concerns to aerodynamics noise, the distal end of blades of wind turbines is where the majority of the action occurs. A subject of this research is on blade tip noise caused by wind generators, and an evaluation and comparison of the many methods being used and researched to minimize this difficulty is the paper's primary focus. Subjective assessment outcomes are also compared to quantifiable measurements. Several methods are proposed, and the risks and benefits are compared. Among the most intriguing instances of plasma in reality that we're able to examine close quarters are really the Solar energy, Atmosphere's geomagnetic, and stratosphere. Were very could be used to model such plasmas effectively in principle (MHD). In MHD, the statistics and dynamics of liquids that conduct electricity are hypothetically characterized.

Implementation of matrix compression in the coupling of JOREK to realistic 3D conducting wall structures

Abstract JOREK is an advanced non-linear simulation code for studying MHD instabilities in magnetically confined fusion plasmas and their control and/or mitigation. A free-boundary and resistive wall extension was introduced via coupling to the STARWALL and CARIDDI codes, both able to provide dense response matrices describing the electromagnetic interactions between plasma and conducting structures. For detailed CAD representations of the conducting structures and high resolutions for the plasma region, memory and computing time limitations restrict the possibility of simulating the ITER tokamak. In the present work, the Singular Value Decomposition provided by routines from the ScaLAPACK library has been successfully applied to compress some of the dense response matrices and thus optimize memory usage. This is demonstrated for simulations of Tearing Mode and Vertical Displacement Event instabilities. An outlook to future applications on large production cases and further extensions of the method are discussed.

The Phenomena of Flares and CMEs in the Sun and Stars: Why Are the Energy Upper Limits in Both Phenomena Comparable?

Solar/stellar flares which release large energies occur less frequently than those with small energies. Flare energies on a given star have not (yet) been observed to exceed E fm . Similarly, coronal mass ejections (CME’s) with large kinetic energies (KE) on a given star also occur less frequently than those with small energies. The KE's of CME’s on a given star have not (yet) been observed to exceed KE m . Interestingly, in any given star, E fm is found to be comparable in magnitude to KE m . This is puzzling since a one-to-one correlation between flares and CME’s does not exist: some flares occur without any accompanying CME, and some CME’s have no accompanying flare. Moreover, different types of MHD instabilities are believed to give rise to flares and CME’s. The result E fm ≈ KE m could indicate support for Bemporad’s conclusion: magnetic energy in the corona is in global equipartition with gravitational potential energy.

Integrated analysis of plasma rotation effect on HL-3 hybrid scenario

Abstract The hybrid scenario, which has good confinement and moderate MHD instabilities, is a proposed operation scenario for international thermonuclear experimental reactor (ITER). In this work, the effect of plasma rotation on the HL-3 hybrid scenario is analyzed with the integrated modeling framework OMFIT. The results show that toroidal rotation has no obvious effect on confinement with a high line averaged density of n bar ∼ 7 × 1019 m−3. In this case, the ion temperature only changes from 4.7 keV to 4.4 keV with the rotation decreasing from 105 rad/s to 103 rad/s, which means that the turbulent heat transport is not dominant. While in the scenarios characterized by lower densities, such as n bar ∼ 4 × 1019 m−3, turbulent transport becomes dominant in determining heat transport. The ion temperature rises from 3.8 keV to 6.1 keV in the core as the rotation velocity increases from 103 rad/s to 105 rad/s. Despite the ion temperature rising, the rotation velocity does not obviously affect electron temperature or density. Additionally, it is noteworthy that the variation in rotation velocity does not significantly affect the global confinement of plasma in scenarios with low density or with high density.

Experimental study of core MHD behavior and a novel algorithm for rational surface detection based on profile reflectometry in EAST

Microwave reflectometry is a powerful diagnostic that can measure the density profile and localized turbulence with high spatial and temporal resolution and will be used in ITER, so understanding the influence of plasma perturbations on the reflect signal is important. The characteristics of the reflect signal from profile reflectometry, the time-of-flight (TOF) signal associated with the MHD instabilities, are investigated in EAST. Using a 1D full-wave simulation code by the Finite-DifferenceTime-Domain (FDTD) method, it is well validated that the local density flattening could induce the discontinuity of the simulated TOF signal and an obvious change of reflect amplitude. Experimental TOF signals under different types of MHD instabilities (sawtooth, sawtooth precursors and tearing mode) are studied in detail and show agreement with the simulation. Two new improved algorithms for detecting and localizing the radial positions of the low-order rational surface, the cross-correlation and gradient threshold (CGT) method and the 2D convolutional neural network approach (CNN) are presented for the first time. It is concluded that TOF signal analysis from profile reflectometry can provide a straightforward and localized measurement of the plasma perturbation from the edge to the core simultaneously and may be a complement or correction to the q-profile control, which will be beneficial for the advanced tokamak operation.

Suppression of “Edge MHD Instability of LHD” by External RMP

Suppression of “Edge MHD Instability of LHD” by External RMP

Benign termination of runaway electron beams on ASDEX Upgrade and TCV

This paper discusses the development of a benign termination scenario for runaway electron (RE) beams on ASDEX Upgrade and TCV. A systematic study revealed that a low electron density (n e) companion plasma was required to achieve a large MHD instability, which expelled the confined REs over a large wetted area and allowed for the conversion of magnetic energy to radiation. Control of the companion plasma ne was achieved via neutral pressure regulation and was agnostic to material injection method. The neutral pressure required for recombination was found to be dependent on impurity species, quantity and RE current. On TCV, n e increased at neutral pressures above 1 Pa, indicating that higher collisionality between the REs and neutrals may lead to an upper pressure limit. The conversion of magnetic energy to radiated energy was measured on both machines and a decrease in efficiency was observed at high neutral pressure on TCV. The benign termination technique was able to prevent any significant increase in maximum heat flux on AUG from 200 to 600 kA of RE current, highlighting the ability of this approach to handle fully formed RE beams.

Synthetic Diagnostic of Spectra of Charge-Exchange Atoms for Analysis of Influence of the MHD Instability on Fast-Particle Confinement in Spherical Tokamaks Globus-M/M2

Absorbed power of the neutral-injection beam in spherical tokamaks Globus-M/M2 is estimatednumerically. Deceleration of fast particles is simulated by means of the NUBEAM code. The signal of analyzerof charge-exchange atoms is simulated by means of the FIDASIM code using the distribution functionof fast ions calculated by means of the NUBEAM code. Comparison of calculated and experimental signalsallowed determining the degree of influence of instabilities on confinement of fast particles along withabsorbed beam power.

Synthetic Diagnostic of Spectra of Charge-Exchange Atoms for Analysis of Influence of the MHD Instability on Fast-Particle Confinement in Spherical Tokamaks Globus-M/M2

Synthetic Diagnostic of Spectra of Charge-Exchange Atoms for Analysis of Influence of the MHD Instability on Fast-Particle Confinement in Spherical Tokamaks Globus-M/M2

Design of line array detection scheme for Polarimeter-Interferometer system on EAST

A multi-channel POlarimeter-INTerferometer (POINT) system has been constructed in the core region of Experimental Advanced Superconducting Tokamak (EAST) plasma for electron density profile and current density profile measurements, with a spatial resolution of 8.5 cm. To explore the small spatial scale density fluctuation phenomena in the core plasma induced by MHD instabilities effects, a higher spatial resolution and higher responsivity detection is required.A novel line array detection scheme of POINT is proposed. By replacing the single Schottky mixer with a highly integrated line array HEMT detector at the central channel position, the existing single channel can be extended to 5 channels for spatial continuous measurement in the core plasma, which has a significantly simplified structure design compared with the discrete upgrade scheme. Related optical design, component selection, and bench tests have been carried out to validate the feasibility and reliability of the novel scheme.The IF signals are detected by an AlGaN/GaN line array high electron mobility transistors (HEMT) detector based on five silicon lenses integrated. The average responsivity of the line array detector elements is 40 kV/W. Compared with the existing planar diode Schottky mixer with a sensitivity of 750 V/W, the line array detector can also meet the essential requirements of coherent signal measurement. Benefiting from the highly integrated array detector, the spatial resolution will be improved to 4.5 mm in the novel scheme. Additionally, a circular Plano cylindrical lens is designed as a beam-shaping component to compress the Gaussian beam into a stripe. Bench test results displayed the excellent performance of the cylindrical lens, which is consistent with the simulation results. Furthermore, normalized responsivity coefficients of array elements are determined by test results.

The JET hybrid scenario in Deuterium, Tritium and Deuterium-Tritium

The JET hybrid scenario has been developed from low plasma current carbon wall discharges to the record-breaking Deuterium-Tritium plasmas obtained in 2021 with the ITER-like Be/W wall. The development started in pure Deuterium with refinement of the plasma current, and toroidal magnetic field choices and succeeded in solving the heat load challenges arising from 37 MW of injected power in the ITER like wall environment, keeping the radiation in the edge and core controlled, avoiding MHD instabilities and reaching high neutron rates. The Deuterium hybrid plasmas have been re-run in Tritium and methods have been found to keep the radiation controlled but not at high fusion performance probably due to time constraints. For the first time this scenario has been run in Deuterium-Tritium (50:50). These plasmas were re-optimised to have a radiation-stable H-mode entry phase, good impurity control through edge Ti gradient screening and optimised performance with fusion power exceeding 10 MW for longer than three alpha particle slow down times, 8.3 MW averaged over 5 s and fusion energy of 45.8 MJ.

A model of failed solar eruption initiated and destructed by magnetic reconnection

ABSTRACT Solar eruptions are explosive disruption of coronal magnetic fields, and often launch coronal mass ejections into the interplanetary space. Intriguingly, many solar eruptions fail to escape from the Sun, and the prevailing theory for such failed eruption is based on ideal magnetohydrodynamic (MHD) instabilities of magnetic flux rope (MFR); that is, an MFR runs into kink instability and erupts but cannot reach the height for torus instability. Here, based on numerical MHD simulation, we present a new model of failed eruption in which magnetic reconnection plays a leading role in the initiation and failure of the eruption. Initially, a core bipolar potential field is embedded in a background bipolar field, and by applying shearing and converging motions to the core field, a current sheet is formed within the core field. Then, tether-cutting reconnection is triggered at the current sheet, first slow for a while and becoming fast, driving an erupting MFR. Eventually, the rise of MFR is halted by the downward magnetic tension force of the overlying field, although the MFR apex has well exceeded the critical height of torus instability. More importantly, during the rise of the MFR, it experiences a significant rotation around the vertical axis (with a direction contrary to that predicted by kink instability), rendering the field direction at the rope apex almost inverse to the overlying field. As a result, a strong current sheet is formed between the MFR and the overlying flux, and reconnection occurring in this current sheet ruins completely the MFR.

Design of a scintillator-based fast ion loss detector system on the HL-2M tokamak

The fast ion loss detector provides information on the pitch angle and gyroradius of the lost fast ions when energetic ion-driven MHD instability occurs. Based on the device's operating parameters, a fast ion loss detector system based on a scintillator detector was designed and developed on the HL-2M tokamak. In this paper, the trajectory of fast ions on HL-2M is simulated by a self-developed program. The detector used in this system is a ZnS: Ag (23×58 mm2) scintillation detector, which can measure the loss of fast ions at large angles. The light signal emitted by the scintillator will be divided into two paths through the spectroscope. One path transmits light signals to a high-speed camera to capture real-time lossless fast ion evolution images. The other path reaches the photomultiplier tube array to measure the loss rate of fast ions.

Locked mode detection during error field identification studies

At the beginning of a machine operation, an assessment of the intrinsic error fields, spurious magnetic field perturbations which can affect plasma dynamics, is often carried out by executing the compass scan method [Scoville J.T. et al. Nucl. Fusion 43 250 (2003)]. This method relies on the application of 3D magnetic fields with various phases, induced by EF correction coils, to trigger a locked mode. The instant of locked mode onset allows the identification of the amplitude and phase of the intrinsic error field, from which the empirical correction currents for its minimization can be deduced. The presence of a locked mode needs to be carefully monitored during this study because of the potential disruptive mode behavior, especially in devices which can tolerate a maximum number of disruptions, as in SPARC and in ITER. A novel method, the so-called non-disruptive compass scan method [Paz-Soldan C. et al., Nuclear Fusion 54 (2014) 073013], avoids the disruption risk, as the name recalls, via magnetic island healing, i.e. stabilizing the locked mode. The magnetic island healing is achieved by switching off the error field correction coil current during the execution of the compass scan and asynchronously by increasing the plasma density. The crucial point of this new method is the detection of the locked mode to initiate the EFCC-density control actions. In this work, the locked mode detector adopted during non-disruptive compass scan test at JET is presented, together with brand-new locked mode metrics, which take into account the actual poloidal deformation due to a locked mode and a class of MHD instabilities, named Beta Alfvén Eigenmodes, that appear in the Mirnov signal in concomitance to the locked mode. The use of multiple metrics for locked mode detection during the execution of the non-disruptive compass scan increases the fidelity of the real-time control system to pinpoint the event, compensating possible magnetic probe failure, and initiate the control sequences to heal the magnetic island.

A Fully Implicit Parallel Solver for MHD Instabilities in a Tokamak

A Fully Implicit Parallel Solver for MHD Instabilities in a Tokamak

Helical mode localization and mode locking of ideal MHD instabilities in magnetically perturbed tokamak plasmas

In H-mode tokamak plasmas, the achievable pressure-gradient is limited by type-I Edge Localized Modes (ELMs), which are projected to cause severe damage to future fusion devices. There are several approaches aiming to mitigate/suppress the occurrence of ELMs, such as the application of an external non-axisymmetric magnetic perturbation field, which breaks the axisymmetry of the tokamak plasma. In this work we use the CASTOR3D code to investigate helical localization and mode locking of edge-localized ideal MHD instabilities in rotating and flow-free magnetically perturbed tokamak plasmas with periodicity. Helically localized instabilities are separated into two classes: quasi-locked and strictly locked. In a non-rotating plasma, the localization of quasi-locked modes is determined by an envelope while their precise location under the envelope is arbitrary, whereas strictly locked modes can only occur at a single helical position. Strictly locked modes only rotate if the toroidal plasma rotation exceeds a critical threshold; above the threshold the forced rotation of the strictly locked modes is non-uniform. For quasi-locked modes, no such critical threshold exists; they rotate uniformly beneath their envelope in the case of finite plasma rotation. The helical localization of both quasi-locked and strictly locked instabilities is determined by the energetic decomposition of the instabilities close to the most unstable flux-surface; for example, strongly current-density driven instabilities are aligned with regions of augmented parallel equilibrium current-density. Finally, we compare the computationally determined localization of MHD instabilities to experimental observations. The determined MHD instability is located at the same position as the experimentally measured modes with respect to the equilibrium corrugation, verifying that ideal MHD can describe the experimentally observed instabilities.

Investigation of MHD instabilities and related dynamic interactions during the major disruption in the HL-2A tokamak

Avoiding the major disruption is of paramount importance in future reactor-level devices, for which understanding the disruption mechanism is essential. In this work, MHD instabilities and related dynamic interactions during the major disruption have been investigated in the ohmically heated plasmas in the HL-2A tokamak. It is reported that the interaction between a kind of edge-oscillating-mode (EOM) perturbation and a rotating m/n = 2/1 tearing mode (TM) inside the plasma plays an important role in inducing the mode locking and the subsequent disruption. The EOM perturbation is oscillating in the laboratory frame, which is proposed to be originally generated by the penetrated error field at the plasma edge and is modulated by the rotating 2/1 TM mode. Before mode locking, the 2D electron cyclotron emission imaging shows that the momentary coupling of the EOM and the 2/1 mode can be decoupled each other and the mode structure does not alter significantly. After the mode locking, the EOM and the 2/1 mode expand and couple each other and induce the heat transfer from the core to the edge. The influence of the TM instability and the EOM perturbation on surrounding plasmas prior to the mode locking has also been presented. The results deepen our understanding of the disruption dynamics related to the external field perturbations, especially in the presence of TMs inside the plasma.

Progress on the conceptual design of the antennas for the DTT ECRH system

One of the main goals of the Divertor Tokamak Test (DTT) facility is to reach a ratio of power crossing the separatrix over the major radius of about 15 MW m − 1, as the one expected in DEMO. For this purpose, up to 45 MW of external additional heating power shall be coupled to the plasma, provided by Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating and Neutral Beam Injection. The foreseen total ECRH power installed at full power shall be 32 MW, generated using 1 MW/170 GHz gyrotrons, for 100 s. The present ECRH system foresees two launching antennas per DTT sector, based on the front-steering concept. The equatorial antenna is dedicated to plasma heating and current drive and the upper antenna to the stabilization of MHD instabilities. This paper focuses on the latest design concept for these two antennas and on the definition of the ex-vessel matching optics unit of the last section of the evacuated Transmission Line (TL). The design has been updated to be compatible with the insertion of CVD diamond windows, to separate the vacuum environment of DTT from the one of the TL. This choice requires adding corrugated waveguide sections between the last mirror of the TL and the first mirror inside the port, requiring some adaptation of the in-vessel optics and of the supporting structure. The possibility to modify the steering range for the launching mirror has been also investigated to be compatible with the new design of the first wall and for the upper antenna, to reach the q = 2 surface in the new plasma scenario.

Suppression of resistive interchange instability by external RMP

We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on resistive interchange MHD instability which is typically observed in the Large Helical Device. We apply the m/n = 1/1 external RMP to discharges with the m/n = 1/1 interchange instability. We find that the electron density fluctuation as well as the magnetic field one is clearly reduced without a reduction of the pressure gradient by imposing the external RMP. Moreover, the achieved beta value as well as the beta gradient at the resonant surface is a little improved under certain conditions. Next, we investigate the responses of interchange instability to the external RMP under some different operational conditions. We find that the amplitude of the external RMP to completely suppress the instability with the shielding of the external RMP has a higher correlation with the volume-averaged beta value than other plasma parameters.