Mirnov Signals Research Articles

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.

Investigation of the J-TEXT plasma events by k-means clustering algorithm

Various types of plasma events emerge in specific parameter ranges and exhibit similar characteristics in diagnostic signals, which can be applied to identify these events. A semi-supervised machine learning algorithm, the k-means clustering algorithm, is utilized to investigate and identify plasma events in the J-TEXT plasma. This method can cluster diverse plasma events with homogeneous features, and then these events can be identified if given few manually labeled examples based on physical understanding. A survey of clustered events reveals that the k-means algorithm can make plasma events (rotating tearing mode, sawtooth oscillations, and locked mode) gathering in Euclidean space composed of multi-dimensional diagnostic data, like soft x-ray emission intensity, edge toroidal rotation velocity, the Mirnov signal amplitude and so on. Based on the cluster analysis results, an approximate analytical model is proposed to rapidly identify plasma events in the J-TEXT plasma. The cluster analysis method is conducive to data markers of massive diagnostic data.

L-H transition studies on MAST: power threshold and heat flux analysis

Spherical tokamaks are known to differ from conventional tokamaks in a number of physics areas, but our understanding is limited by a comparative lack of experimental data. A comprehensive study of the density dependence of the H-mode power threshold P LH on the mega-amp spherical tokamak (MAST) is presented, mapping out the low-density and high-density branches, and describing different types of L-H transitions and intermediate behaviours. Transitions at low densities are characterised by longer preceding 3–4 kHz I-phase oscillations or intermittent periods. Commonly used P LH scalings underestimate the experimental values in this data set by at least an order of magnitude, and a fit to the high-density branch gives . To investigate the possibility of a critical edge ion heat flux, transitions of different densities and neutral beam heating powers were analysed with the interpretative transport code TRANSP, revealing that the heat flux possesses a density dependence independent of the net power. The density dependence of the electron heat flux and ion heat flux is caused by a decrease in beam heating efficiency for lower densities, with the fraction of injected power heating the plasma decreasing from 80% to 20% at low densities. Low-density discharges have greater fast ion orbit and charge-exchange losses, which are seen in Mirnov signals as chirping 50–20 kHz modes and broadband MHD at 150–250 kHz. The captured beam power estimate by TRANSP, which only corrects for shine-through losses, is a significant overestimate for low-density plasmas on MAST. If the P LH study is adjusted to account for all losses, the scatter in H-mode points is reduced and the density exponent is increased to . A residual low-density branch remains due to the variation.

Beta-induced Alfvén eigenmodes with frequency chirping driven by energetic ions in the HL-2A Tokamak

The beta-induced Alfvén eigenmodes (BAEs) with frequency chirping, observed in the HL-2A Tokamak, are analysed by a MHD-kinetic hybrid code MEGA. Realistic parameters are applied to the code, such as equilibrium, electron density and temperature, ion temperature, which is different from the kinetic Berk-Breizman theory. The BAEs are observed by Mirnov probes and soft X-ray arrays. Toroidal and porloidal mode number are confirmed to be <i>n</i>/<i>m</i> = 2/3 by using the phase shift method with toroidal filtered Mirnov signal arrays. And the soft X-ray arrays’ signal shows that BAEs are located at the core of the plasma and they have a relatively broad mode structure. The BAEs with up- and down-chirping are reproduced with MEGA code. The simulation results of mode structure accord well with experimental observations. Compared with up-chirping BAEs, the down-chirping BAEs are excited with higher plasma parameters and beta value, thus the energetic ion distribution in pitch angle has a broader width, and the beta value of energetic ions in the core of plasma and diffusion value are higher in the down-chirping simulation. The simulation results show that the phase space distribution of energetic ions affects the wave chirping direction. The energetic ions parallel to the magnetic field drive the up-chirping behavior. When the down-chirping behavior dominates, the density of energetic ions perpendicular to the magnetic field increases significantly. It shows that the down-chirping BAEs require higher beta and energetic ion density, which is consistent with the previous simulation result.

Development of a real-time magnetic island reconstruction system based on PCIe platform for HL-2A tokamak

A real-time magnetic island reconstruction (MIR) system based on PCI express platform for HL-2A tokamak is introduced. The front-end analog circuit and high performance analog-to-digital converters complete high-precision synchronous sampling of 18 channel Mirnov signals, and the application of PCIe platform and direct memory access technology enables high speed data transmission between graphics processing unit and field programmable gate array (FPGA). FPGA, as a mainstream high speed parallelizable computing tool, was used to implement the MIR algorithm, while a parameter table is established in an external double data rate SDRAM to improve the computational efficiency. The software of the MIR system is developed with Compute Unified Device Architecture 8.0 in Centos 6 system, which mainly realizes driver development, data transmission, network communication, parameter calculation and system control. This system has been tested in HL-2A plasma discharge experiment, and the reconstructed magnetic island structure can achieve a spatial resolution of 1.02 cm while the time resolution can reach 2 ms.

MHD Mode Identification from Mirnov Coils Signals in Tokamak Via Combination of Singular Value Decomposition and Hilbert–Huang Transform Analysis Methods

In this work, we investigate how to study the MHD activities in Tokamak plasma via the combination of singular value decomposition (SVD) and Hilbert–Huang transform (HHT) methods. We apply this approach to the Mirnov coil signal fluctuations analysis without any filtering technique. First, the principal axes (PAs) of a pick-up Mirnov signals are extracted by SVD analysis. Next, the harmonics of dominants PAs is obtained by empirical mode decomposition (EMD) analysis. Moreover, the time–frequency behavior of Mirnov signals are extracted by HHT. The proposed technique is employed to analyze Mirnov coils signals for mode type and frequency identification, especially in multimode MHD activities. We obtained Spatial–temporal structures of the Mirnov coils fluctuations in terms of correlation functions to better identification of mode number and frequencies of dominant MHD modes. We also present the results of this method applied to IR-T1 and Golem Tokamaks Mirnov coils signals. Consequently, satisfying results from SVD + HHT analysis method and spatial–temporal structures for IR-T1 and Golem Tokamaks Mirnov data observed.

Experimental study of quasi-coherent mode using EAST tangential CO2 laser collective scattering diagnostic in far-forward mode

A quasi-coherent mode (QCM) was measured by the tangential CO2 laser collective scattering diagnostic at high plasma electron density during both enhanced Dα/small edge-localized mode (ELM) and ELM-free H mode phases in Experimental Advanced Superconducting Tokamak (EAST). Experimental results from only local oscillator CO2 laser scattering prove that the QCM is measured by the scattering diagnostic in the far-forward mode. The driven QCM density fluctuation (k⊥<3 cm−1) and magnetic fluctuation suggest that the QCM is an electromagnetic mode. The typical frequency of the QCM is f ≈ 26.5 kHz. A combination analysis of scattering signals and Mirnov signals suggests that the QCM has toroidal mode number n ≈ 17 and rotates along with the electron diamagnetic drift velocity direction in the lab frame. The analysis of Mirnov and reflectometer signals supports that the QCM locates in the edge pedestal region. The QCM power has been found to be related to both the Dα signal and the pedestal density gradient. A comparison of the EAST QCM and C-Mod quasi-coherent mode has been given in detail.

Design and Implementation of the Resonant Magnetic Perturbations Feedback Control System for Tearing Mode Suppression on J-TEXT

Tearing modes (TMs) degrade the performance of tokamak plasma, and can even lead to disruption. Using externally exerted resonant magnetic perturbations (RMPs) to suppress TM is a promising and effective way. In order to suppress the 2/1 TM, 2/1 RMP must be applied in given phase regions to stabilize magnetic island and accelerate island rotation. The RMP feedback control system acquires the multiple-channel poloidalMirnov signal, processes the acquired data and calculates the magnetic island phase in real time, and switches on the RMP power supply when the phase is in the designated region. The feedback control system is based on NI cRIO and developed mainly using LabVIEW field-programmable gate array. The typical 2/1 mode magnetic island on J-TEXT rotates at a frequency from 2 to 10 kHz. To ensure the control precision within 2°, the control period must be within 500 ns. Due to the noisy characteristic of the Mirnov signal, the feedback control system uses a series of error correction methods in real time to obtain accurate phase. Various control strategies have been adopted to exploit the RMP coil and power supply. Observable result is presented in this paper.

Effect of electron cyclotron beam width to neoclassical tearing mode stabilization by minimum seeking control in ITER

We report the effect of the electron cyclotron (EC) beam width on the full suppression time of neoclassical tearing mode (NTM) using the finite difference method (FDM) based minimum seeking controller in ITER. An integrated numerical system is setup for time-dependent simulations of the NTM evolution in ITER by solving the modified Rutherford equation together with the plasma equilibrium, transport, and EC heating and current drive. The calculated magnetic island width and growth rate is converted to the Mirnov diagnostic signal as an input to the controller to mimic the real experiment. In addition, 10% of the noise is enforced to this diagnostic signal to evaluate the robustness of the controller. To test the dependency of the NTM stabilization time on the EC beam width, the EC beam width scan is performed for a perfectly aligned case first, then for cases with the feedback control using the minimum seeking controller. When the EC beam is perfectly aligned, the narrower the EC beam width, the smaller the NTM stabilization time is observed. As the beam width increases, the required EC power increases exponentially. On the other hand, when the minimum seeking controller is applied, NTM stabilization sometimes fails as the EC beam width decreases. This is consistently observed in the simulation with various representations of the noise as well as without the noise in the Mirnov signal. The higher relative misalignment, misalignment divided by the beam width, is found to be the reason for the failure with the narrower beam widths. The EC stabilization effect can be lower for the narrower beam widths than the broader ones even at the same misalignment due to the smaller ECCD at the island O-point. On the other hand, if the EC beam is too wide, the NTM stabilization time takes too long. Accordingly, the optimal EC beam width range is revealed to exist in the feedback stabilization of NTM.

The effects of the cathode array on emitted hard x-ray from a small plasma focus device

In this study, the effects of the cathode array variations on emitted hard x-rays from a small Mather type plasma focus device (450 J) were investigated. The gradual elimination of the cathode rods inside the cathode array of the device lowered the quality and quantity of the emitted hard x-rays at different pressure values of argon gas. We theorized that the variations of the cathode array were able to change some discharge parameters that could vary the number of the energetic runaway electrons generated during the pinch phase which were responsible for the created features of the emitted hard x-rays. On the other hand, we hypothesized that the removal of the cathode rods could influence the current sheath dynamics during all the phases of a shot including its average axial velocity which was demonstrated by using two axial magnetic probes. We also theorized that cathode rod omission from the cathode array could also increase the initial inductance and the impedance of the system, and the impurities inside the plasma during the pinch phase which could lead to the growth of the instabilities. Moreover, by using the wavelet technique and studying the Mirnov signals, it was shown that the decrease of the cathode rod number increased the plasma magnetic field fluctuations or instabilities (MHD activities) that adversely affected the pinch quality, and reduced the emitted hard x-rays.

Observation of MHD phenomenon for SST-1 superconducting tokamak

Steady State Superconducting Tokamak (SST-1) is a medium size Tokamak (major radius = 1.1 m, minor radius = 0.2 m) and is operational at the Institute for Plasma Research (IPR), India. In the last few experimental campaigns SST-1 has successfully achieved plasma current in order of 60-70kA and plasma duration in excess of ∼ 500 ms at a central magnetic field of 1.5T. An attempt has made to study the behavior of the magneto-hydrodynamic (MHD) activity during different phases of plasma pulse which leads to major/minor disruptions, its present modes (poloidal/toroidal mode number i.e. m = 2, n = 1) impact on plasma confinement and signature of lock mode and its frequency in the SST-1 plasma using experimental data from Mirnov signals. Observed MHD phenomenon has also been correlated with other diagnostics (i.e. ECE, Density, Soft X-Ray etc.) and heating system (ECRH) for the recent campaigns of SST-1.

Excitation of edge plasma instabilities and their role in pedestal saturation in the HL-2A tokamak

In HL-2A, the characteristics of the edge plasma instabilities and their effects on the dynamical evolution of the pedestal in H-mode plasmas have been investigated. In the edge pedestal region with steep pressure gradient, a quasi-coherent mode (QCM) has been observed in density fluctuations with a frequency range of 50–100 kHz. It appears during the edge localized mode (ELM)-free period after the L–H transition and prior to the first ELM. A threshold in the pedestal density gradient has been identified for the excitation of this mode. The QCM can also be observed during inter-ELM periods. It is excited early in the inter-ELM period, and disappears when the ELM onset starts. The radial wave-number of the mode is estimated with two radially separated reflectometers. It shows that the mode is radially propagating inward. The poloidal wave number estimated with the Langmuir probes is kθ ~ 0.43 cm−1. The mode propagates poloidally in the electron diamagnetic direction in the plasma frame. The toroidal mode number, deduced from Mirnov signals, is n ~ 7. The corresponding poloidal mode number is m ~ 21 according to the local safety factor value. The analysis for the dynamical evolution of the pedestal during the ELM cycle clearly shows that the mode is excited before the ELM onset. During and after the ELM crash, the mode disappears. It suggests that the QCM is driven by the pedestal density gradient, and the mode in return regulates the pedestal density evolution.

Understanding the effect of resonant magnetic perturbations on tearing mode dynamics

Numerical understandings of the effect of resonant magnetic perturbations (RMPs) on 2/1 tearing mode (TM) dynamics observed on J-TEXT tokamak (Hu et al. Nucl. Fusion 52, 083011 (2012)) are presented in this paper. The non-uniform mode rotation frequency, modulated by electromagnetic force (Fem) and viscous force (Fvs), results in the applied RMPs contributing both net stabilizing and braking effect on TM. Numerical evaluation based on analytical theory shows the applied RMP contributes a dominant stabilizing effect, which is responsible for the suppression of TM. The dynamics of the first discovered small locked island are investigated. It is found that the island is locked at the stabilizing phase and the saturated island width is less than the linear layer width. The simulated Mirnov signal indicates the small locked island is likely to be the complete suppression case observed in experiments. Associated with the application of RMP, the shape of Mirnov signal deviates from sinusoidal before mode locking, which is explained by numerical modeling. The comparisons between the numerical and experimental results are in good agreement phenomenally.

Time–frequency analysis of nonstationary complex magneto-hydro-dynamics in fusion plasma signals using the Choi–Williams distribution

The Choi–Williams distribution is applied to the time–frequency analysis of signals describing rapid magneto-hydro-dynamic (MHD) modes and events in tokamak plasmas. A comparison is made with Soft X-ray (SXR) signals as well as Mirnov signal that shows the advantages of the Choi–Williams distribution over both continuous wavelets transform scalogram and the short-time Fourier transform spectrogram. Examples of MHD activities in HT-7 and EAST tokamak are shown, namely the onset of coupling tearing modes, high frequency precursors of sawtooth, and low frequency MHD instabilities in edge localized mode (ELM) free in H mode discharge.

Investigation of ring-like runaway electron beams in the EAST tokamak

In the EAST tokamak, asymmetrical ring-like runaway electron beams with energy more than 30 MeV and pitch angle about 0.1 were investigated. Those runaway beams carried about 46% of the plasma current and located around the q = 2 rational surface when m/n = 1/1 and m/n = 2/1 MHD modes existed in the plasma. Those runaway beams changed from a hollow to a filled structure during the slow oscillations in the discharge about every 0.2 s, which correlated with a large step-like jump in electron cyclotron emission (ECE) signals, a big spike-like perturbation in Mirnov signals and a large decrease in runaway energy. Between those slow oscillations with large magnitude, fast oscillations with small magnitude also existed about every 0.02 s, which correlated with a small step-like jump in ECE signals, a small spike-like perturbation in Mirnov signals, but no clear decrease in runaway energy and changes in the runaway beam structure. Resonant interactions occurred between runaway electrons and magnetohydrodynamic instabilities, which gave rise to fast pitch angle scattering processes of those resonant runaway electrons, and hence increased the synchrotron radiation. Theoretical calculations of the resonant interaction were given based on a test particle description. Synchrotron radiation of those resonant runaway electrons was increased by about 60% until the end of the resonant interaction.

NTM Localization by Correlation of Te and dB/dt

For electron cyclotron current drive-based stabilization of neoclassical tearing modes (NTMs), it is crucial that the current deposition occurs as close to the island as possible; hence, its location needs to be accurately known. An NTM, rotating in the laboratory frame, causes fluctuations of magnetic flux measurable by Mirnov coils (dB/dt). Temperature perturbations in the vicinity of an NTM are caused by displaced flux surfaces and thus have the same frequency as the Mirnov signal but show a constant phase difference, which depends on the mode topology (poloidal and toroidal periodicity), on the toroidal displacement of the Mirnov coil with respect to the temperature measurement, and on the sign of the temperature change between the X-point profile and the O-point profile, which inverts somewhere inside the island. The sign flip of ΔTe is equivalent to a change of the phase difference between Te and magnetic reference by π and therefore can be localized using the presented correlation method. Using the suggested algorithm, we can determine the rational surface that coincides with the radial island location with low latency and good reliability in real time from electron cyclotron emission temperature profiles when correlated with the appropriate magnetic fluctuations on a modern workstation computer.

Fast elaboration of diagnostic data for real time control in FTU tokamak

The automatic controller developed for MHD instability control on FTU via the real time (RT) EC launcher [1, 2] is based on the a-priori estimate of the instabilities location and on the fast elaboration of the stream of diagnostic data. A fast data acquisition system, based on existing standard FTU diagnostics, collects thermal and magnetic signals by a 12-channels ECE polychromator [3] and by a set of 22 Mirnov coils respectively. Moreover, the EC time waveform from directional couplers along the transmission line is acquired as well. This set of signals is processed in a timescale significantly shorter than the typical time step of the controller (1ms). RT elaboration algorithms aim at supplying the controller [4] with reliable information about the existence and the spatial location of the instability and about the actual ECRH deposition layer in plasma. The main algorithms blocks are ECE-ECE, ECE-Mirnov and ECE-ECRH cross-correlation, and SVD (Singular Value Decomposition) of Mirnov signals. The capabilities of effective detection, false positive resilience, and mode discrimination will be discussed through the application to actual plasma data.

Interpretation of tearing mode physics from ECE temperature fluctuations associated with magnetic islands and Mirnov signals

The experimental identification of the physical mechanisms behind the rotating magnetic helical modes with low numbers m,n which appear at magnetic surfaces where q = m/n is an elusive task. We discuss here a theoretical principle of a combined analysis of magnetic Mirnov signals and of electron cyclotron emission signals that can shed light on the underlying physics of these perturbations.

Study of central plasma structure using soft X-ray signals

A method for studying the central plasma structure using soft X-ray signals in the HT-7 tokamak is reported. Through analysing a typical discharge with MHD instability, the position and rotation direction of magnetic island are obtained from the soft X-ray signals. The magnetic island position and rotation direction are validated from the contour map of soft X-ray emissivity and the m=2 mode rotation direction observed on the Mirnov signals.

Numerical Simulation and Experimental Identification of Divertor Configuration in the HL-2A Tokamak

Single- and double-null divertor configurations in HL-2A are simulated by SWEQU equilibrium code. Lower divertor discharges in the first physics campaign have been achieved by two kinds of power supply method of multipole-field coils. Single-null divertor configuration has been identified by visible photography, target probe arrays and the reconstructed magnetic surface. Magnetic separatrix and minor radius of plasma column are obtained by a reconstructed code of multiple current filaments using 18 Mirnov signals.