Poloidal Rotation Research Articles

Response of the poloidal rotation to resonant magnetic perturbations in the EAST tokamak

Abstract In this paper the response of the plasma poloidal rotation to resonant magnetic perturbations (RMPs) is investigated in EAST Tokamak using the multi-channel Doppler backscattering (DBS) system. It shows that the poloidal rotation spins up towards the ion-diamagnetic drift direction with increasing external perturbation field, which will reduce the edge shear. In ohmically heated discharges, the n = 1 RMP can only affect the edge poloidal rotation when the RMP coil current is small, and the influence will gradually reach the inner regions with increasing RMP coil current. At the moment of the n = 1 RMP penetration, all the poloidal rotations measured by the DBS will increase significantly, and then they will keep almost unchanged with the increase of the RMP coil current. In H-mode discharge, the poloidal rotation is significantly influenced by the n = 2 RMP, and the edge velocity well even reverses, along with edge-localized modes (ELMs) mitigation. However, in the same shot, the n = 4 RMP with the same coil current amplitude can hardly affect the poloidal rotations and the behavior of ELMs.

X-ray imaging crystal spectrometer (XICS) diagnostic on the HL-3 tokamak

The construction of an X-ray imaging crystal spectrometer (XICS) diagnostic system on the HL-3 tokamak plays a crucial role in measuring core plasma parameter profiles, including ion temperature, electron temperature, rotational velocity, and impurity radiation profiles. This diagnostic system has been specifically designed to provide detailed and accurate data essential for understanding the behavior and characteristics of tokamak plasma. The resonance spectral line (w line of Ar XVII at 3.9494 Å) and its satellites of helium-like argon ions were chosen on the basis of the HL-3 parameter range. A spherically bent quartz crystal (1012) with a lattice constant of 2d = 4.562 Å, a curvature radius of Rc = 3.0 m, and a size of 10 cm (high) × 5 cm (wide) was mounted on an adjustable displacement platform in the XICS system. The position was adjusted in three dimensions (vertical, inclined, rotation) to effectively record the spectra of the helium-like argon ions. The XICS has a tangential angle of 49° for the toroidal direction in the magnetic axis. This layout accounts for 65.6 % of the toroidal rotation velocity component. The XICS system was set at an 11° poloidal angle in the mid-plane to cover a plasma range of 10 cm above to 50 cm below the mid-plane, which corresponds to the q = 1 surface of the HL-3 plasma (elongation κ > 1.8). This XICS layout produced a spectrum with poloidal and toroidal rotational contributions, requiring decoupling through data processing. In general, because the contribution of the poloidal rotation velocity is much smaller than that of the toroidal rotation velocity, it can be ignored. The XICS system offers a spatial resolution of ∼1.5 cm and a temporal resolution of 5–10 ms on the basis of a high-performance PILATUS3 × 900 K detector. The spectral resolution is λ/Δλ∼4×104, which satisfies the experimental measurement requirements. Preliminary results of ion and electron temperature profiles were obtained for the HL-3 tokamak.

Dynamics of JET runaway electron beams in D2-rich shattered pellet injection mitigation experiments

Abstract The publication provide further insights into the dynamics of JET runaway electron (RE) beams mitigated by D2-rich shattered pellet injection (SPI) (Reux et al 2022 Plasma Phys. Control. Fusion 64 034002). Multi-diagnostic analyses show that mechanisms causing continuous RE losses and energy transfer from hot electrons to cold background plasma can act before the SPI. After the SPI, measurements are compatible with a reduction of the maximum energy and pitch angle of the RE distribution while the population of supra-thermal electrons increases. The RE population growth is likely due to electron avalanche. Dark island-like pattern chains, characterised by an integer poloidal mode number and a certain minor radius, are identified in the JET RE beam synchrotron radiation videos. The synchrotron island dynamics is studied via a newly developed computer vision code (Sommariva and Silburn https://c4science.ch/source/pSpiPTV/). The radial motion of synchrotron island chains is found to be consistent with the most plausible time evolution of the radial current density profile compatible with both the RE synchrotron videos and the total RE current time trace. Similarly, correlations are identified between the temporal progression of the synchrotron islands poloidal rotation frequency and sudden MHD relaxation events. Loss-of-RE events probably caused by non-linear interactions between synchrotron islands are observed for the first time. Experimental evidences suggest that synchrotron islands are possibly related to the existence of magnetic islands which may lead to the development of new RE beam mitigation strategies.

First results of turbulence investigation on Globus-M2 using radial correlation Doppler reflectometry

The first results of investigation of the turbulence structure using Doppler backscattering (DBS) on the Globus-M2 tokamak are presented. A one-channel DBS system with a variable probing frequency within the 18–26 GHz range was installed to investigate the edge plasma at normalized minor radii 0.9–1.1. Radial correlation Doppler reflectometry was used to study the changes in turbulence eddies after the LH transition. Correlation analysis was applied to the phase derivative of complex in-phase and quadrature (IQ) signals of the DBS diagnostic as it contains information about the poloidal plasma rotation velocity. In L-mode, the radial correlation length L r is estimated to be 3 cm and after transition to H-mode reduces to approximately 2 cm. Gyrokinetic modelling in a linear local approximation using code GENE indicates that the instability with positive growth rate at the normalized minor radius 0.75 in L-mode and H-mode on Globus-M2 was the ion temperature gradient (ITG) mode.

Poloidal structure of the edge parallel flow in H-mode, L-mode and I-mode confinement regimes

Impurity profiles have been measured with the edge high field side (HFS) and low field side (LFS) charge exchange recombination spectroscopy suite at ASDEX Upgrade enabling the study of the poloidal structure of the edge parallel flow in H-mode, L-mode and I-mode. In H-mode, asymmetries in the impurity density, toroidal and poloidal rotations are found. In I-mode, only toroidal rotation asymmetries have been measured while in L-mode no asymmetries have been observed. The measured parallel flow can be divided in two components, the Pfirsch–Schlüter (PS) flow and the symmetric flow. Two different methods have been followed to determine both contributions to the parallel flow. The first method is based on the calculation of the PS flow at the HFS and LFS from the radial electric field. The second method directly provides the symmetric flow from the flux surface average (FSA) of the parallel flow. In H-mode, the methods provide different results, while in L-mode and I-mode they agree. The differences observed in H-mode between the two methods could be explained by the existence of asymmetries in the impurity density, by the non-negligible particle sources and radial losses, or by the approximations made in the calculation of the FSA of the parallel flow from measurements in two poloidal positions (midplane HFS and LFS) only.

A study of turbulent filaments in the edge plasma of the Wendelstein 7-X stellarator

Filaments are studied by examining fast camera images on the Wendelstein 7-X stellarator. Fast cameras offer a unique perspective, revealing the complex 3D structure of filaments in the entire poloidal cross-section of the plasma. By correlating individual pixels, their location, shape, and movement are analyzed in standard and high-ι configurations. The presence of filaments is not uniform poloidally around. The number of active areas matches the number of magnetic islands in both configurations. Filaments are found to extend to multiple toroidal turns in standard configuration. No time delay is observed between the different toroidal sections. Such behavior is not seen in high-ι configuration. Filaments are observed within and without the edge shear layer, indicated by the direction of their poloidal rotation. Inside the shear layer, their velocity scatters around 1.25 km s−1, accompanied by a lifetime between 80 and 120 µs. Outside, their velocity shows greater absolute values and variance, but still in a few km s−1 range. The similarities and differences between the two configurations are discussed and compared to previous results.

Studies of edge poloidal rotation and turbulence momentum transport during divertor detachment on HL-2A tokamak

In a magnetic confinement fusion device, the plasma undergoing nuclear fusion reaction must be maintained in a high-temperature and high-density confinement state for a long enough time to release high energy, while the heat loads on the divertor target plates need to be reduced to avoid damage to wall at the same time. The latter is one of the key challenges of ITER and commercial fusion reactors in future. Divertor detachment provides an effective solution to reduce the heat load on the target plate of tokamak. However, this may result in the change of plasma states at the boundary, thus affecting the plasma confinement. In this paper, edge plasma poloidal rotation and turbulence momentum transport are studied experimentally during the divertor detachment in the L-mode discharge of HL-2A tokamak. The detachment is achieved by injecting a mixture of gas (60% nitrogen+40% deuterium) into the divertor. The gas mixture is injected by pulsed injection, with pulse length being in a range of 5–20 ms. During the divertor detached phase, both the ion saturation current density and the heat flux to the outer target plate decrease considerably. The enhanced radiation is also observed in the divertor and <i>X</i>-point region. It is found that in the process of attachment-to-pre-detachement, the <inline-formula><tex-math id="M3">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M3.png"/></alternatives></inline-formula> poloidal flow velocity in the near scrape-off layer (SOL) changes from ion magnetic drift direction to electron magnetic drift direction. The turbulent driving force of poloidal flow, which is characterized by the negative radial gradient of momentum transfer flux (Reynolds stress), shows the same trend. In the detached phase, both the <inline-formula><tex-math id="M4">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M4.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M4.png"/></alternatives></inline-formula>flow and the Reynolds force become very small. Therefore, the dynamics of <inline-formula><tex-math id="M5">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M5.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M5.png"/></alternatives></inline-formula> poloidal flow velocity in the SOL is consistent with the evolution of rotation driving effect induced by the turbulent momentum transport. Combined with the <inline-formula><tex-math id="M6">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M6.png"/></alternatives></inline-formula> poloidal flow measured by the probe in the SOL and the beam emission spectrum inside the LCFS, the <inline-formula><tex-math id="M7">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M7.png"/></alternatives></inline-formula> poloidal velocity shearing rate near the LCFS can be inferred. Compared with the attached state, when the divertor is detached, the edge poloidal flow shearing rate decreases significantly, leading to the obviously enhanced turbulence level. Under the influence of both enhanced turbulent transport and radiation, the global confinement degrades moderately. The energy confinement time decreases about 15% and the confinement factor <inline-formula><tex-math id="M8">\begin{document}$ {H}_{89-P} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M8.png"/></alternatives></inline-formula> decreases about 10%. These results indicate that edge turbulent transport and plasma rotation dynamics play a role in the core-edge coupling process in which the divertor detachment affects the global confinement.

The investigation of edge-localized modes on the Globus-M2 tokamak using Doppler backscattering

The first results of investigation of edge localized modes (ELMs) in the Globus-M2 tokamak using the Doppler backscattering method are presented in this paper. Specifically, ELMs that are initiated by sawtooth crashes in the H-mode are discussed. The goal of this paper is study plasma turbulence behaviour during ELMs and to showcase what ELM characteristics can be obtained using Doppler backscattering (DBS). An increase of the poloidal rotation velocity during an ELM burst and a decrease in the inter-ELM periods was observed. The effect of ELMs on the plasma turbulence was investigated and estimated to span around 6 cm inside the separatrix. This is to do with the fact that the sawtooth crashes which are responsible for initiating the ELMs take place in the core plasma. Additional experiments with standard reflectometry indicate that ELMs develop 3 cm inside the separatrix where the pedestal region is believed to be in Globus-M2. The direction of the expansion of the ELMs from the inner plasma region to the edge was determined and the velocity was estimated to be around 8 km s−1. During a single ELM burst a series of filament structures were found in the peripheral DBS channels. In an attempt to understand the processes involved modelling of the reaction of the DBS signals to filaments was done using the BOUT ++ and IPF-FD3D full-wave codes, and the cases for both linear and nonlinear scattering were considered. The results show that the presence of nonlinear scattering during ELMs can lead to an overestimation of the measured velocity values in the region of filament existence near the separatrix.

Species dependence of the impurity injection induced poloidal flow and magnetic island rotation in a tokamak

Recent experiments have demonstrated the species dependence of the impurity poloidal drift direction along with the magnetic island rotation in the poloidal plane. Our resistive MHD simulations have reproduced such a dependence of the impurity poloidal flow, which is found mainly determined by a local plasmoid formation due to the impurity injection. The synchronized magnetic island rotation is dominantly driven by the electromagnetic torque produced by the impurity radiation primarily through the modification to the axisymmetric components of current density.

A Poloidal CO2 Laser Collective Scattering System for Measuring Electron-Scale Turbulence in the EAST

Anomalous electron thermal transport is an important issue that restricts the development of magnetic confinement thermonuclear fusion, and it is closely related to electron-scale turbulence. This paper introduces the poloidal CO2 laser collective scattering diagnostic system installed on the Experimental Advanced Superconducting Tokamak (EAST) for electron-scale turbulence measurement. The system can measure density fluctuations with four distinct wavenumbers simultaneously ranging from 10 to 30 cm−1 (correspondingly kθρs∼1to5) in two regions (the core region ρ≈0 to 0.4 and the outer region ρ≈0.4to0.8), which realizes the spatial resolution for turbulence measurement. And, the plasma poloidal rotation velocity in these two regions can be calculated using the measured density fluctuation frequency. In addition, the characteristics of small scattering angle and negligible wave refraction effects reduce the size of the ports required for this diagnostic system. These advantages make the diagnostic system an effective tool for measuring electron-scale turbulence and may play an important role in future burning plasma experiments.

Disruption halo current rotation scaling on Alcator C-Mod and HBT-EP

Asymmetric halo currents (HCs) can exert large net forces on the vacuum vessel and other components during disruptions on tokamaks. The displacements caused by these forces can then be amplified if these asymmetric forces rotate at frequencies resonant with the vessel. This paper reports on the investigation of a recently proposed scaling law for the disruption HC rotation frequency [Saperstein et al., “Halo current rotation scaling in post-disruption plasmas,” Nucl. Fusion 62, 026044 (2022)] that combines measurements on Alcator C-Mod with those on HBT-EP. We find that a new non-circular version of the scaling law [ ⟨ f rot ⟩ m / ⟨ m ⟩ ∝ 1 B T ( S / π )] takes into consideration the dependence of frot on the poloidal structure of the MHD instability (m) driving the asymmetry and describes the disruption-averaged rotation frequency on C-Mod. Disruption rotation is also found to be insensitive to the vertical position and impurity content of the plasma at the onset of the disruption. However, a stagnation in the time evolution of frot is occasionally observed. Observations are consistent with the dominance of poloidal rotation during the disruption, which is motivated by the poloidal drift nature of the scaling law.

Internal Kink Instabilities Driven by Poloidal Flow via Non-Resonant Excitation in Tokamaks

Non-resonant excitation due to plasma inertia may be dominant in inducing internal kink (IK) instabilities. Poloidal rotation can effectively modify plasma inertia and cause non-resonant excitation to occur. An extended dispersion relation including poloidal rotation is established to study the IK mode and the fishbone (FB) mode. It is found that in rotating plasmas, even for a stable IK mode (i.e., the perturbed potential energy of background plasma δWc is positive) and in the absence of energetic particles (EPs), poloidal rotation can drive the IK mode via non-resonant excitation. Moreover, the IK mode is easy to be driven by poloidal rotation in weak magnetic shear plasmas. Similar to toroidal rotation, when poloidal rotation frequency exceeds a threshold, the FB mode can transform into a branch of a non-resonant mode. The real frequency of the mode, being independent of the precessional frequency of EPs, is just equal to the poloidal rotation frequency. Thus, the non-resonant mode is characterized by the long-lived mode (LLM) observed in toroidal rotating plasmas. The critical gradient of the poloidal rotation profile plays a crucial role in causing the resonant mode to evolve into a non-resonant one; for instance, only for a very peaked poloidal rotation profile can the FB mode transform into the LLM. In addition, the diamagnetic drift frequency of thermal ions can stabilize the FB and the IK modes.

First Results of the Implementation of the Doppler Backscattering Diagnostic for the Investigation of the Transition to H-Mode in the Spherical Tokamak Globus-M2.

This paper presents the first results of a study of the LH transition on the new spherical Globus-M2 tokamak using the Doppler backscattering (DBS) diagnostic. New data characterizing the H-mode of discharges with higher values of the plasma parameters, such as magnetic field Bt up to 0.9 T and plasma current Ip up to 450 kA, were collected and analyzed. An upgraded neutral beam injection (NBI) system was used to initiate the LH transition. DBS allows the measurement of the poloidal rotation velocity and the turbulence amplitude of the plasma. The multi-frequency DBS system installed on Globus-M2 can simultaneously collect data in different areas spanning from the separatrix to the plasma core. This allowed for the radial profiles of the rotation velocity and electric field to be calculated before and after the LH transition. In addition, the values and temporal evolution of the velocity shear were obtained. The associated turbulence suppression after the transition to the H-mode was investigated using DBS.

Effect of radial electric field on ion-temperature gradient driven mode stability

To understand the effect of given radial electric field on ion-temperature gradient driven mode (ITG) stability in tokamak plasmas, we derive the eigenmode equation for ITG including the poloidal rotation and density modulation associated with radial electric field by using nonlinear gyrokinetic theory. The equation is solved for the eigenfrequency, growth rate and parallel mode structure of ITG both in short- and long-wavelength limit with energetic-particle-induced geodesic acoustic mode (EGAM) as a specific form. The eigenmode equation is not only solved analytically, but also solved numerically to validate the analytic solutions. It is found that, the radial electric field induced poloidal rotation can significantly stabilize ITG, while the density perturbation of the radial electric field may slightly distort the ITG parallel mode structure, but has little effect on ITG stability. The result is consistent with the common picture of turbulence suppression by poloidal shear flow. The general model is also applicable to the investigation of the indirect interaction of ITG and energetic particle driven Alfvén instabilities via zonal structures generation, by introducing poloidal rotation and density modulation associated with zonal structures spontaneously excited by Alfvén instabilities. The indirect channel is supplement to the direct interaction of microturbulences and energetic particle driven Alfvén instabilities.

Electric Properties of Chiral Plasma Plumes Without External Magnetic Field

Helical and chiral plasma structures are ubiquitous, especially in nuclear fusion devices and space plasmas. However, the spontaneous formation of helical structures has rarely been noticed in non-equilibrium cold plasma jets that have wide applications in biomedicine and material science. Recently the chiral plasma plume without any notable asymmetry has been discovered. Under the specific parameters of pulsed discharges, the plasma bullets that at the beginning propagated in the straight lines spontaneously turn into 3-D helical discharge trajectories. Here, the electromagnetic mechanism for the chiral plasma plumes is clarified by the numerical investigation of the 3-D electromagnetic field. According to the specific standing-wave mode with the periodic poloidal component in simulation, a type of plasma plume which shows the apparent poloidal rotation only at the periodic position of the tube and moves nearly in a straight line at other positions is revealed. These findings are relevant to the development of high-voltage active electromagnetic media for artificial 3-D optical meta-material devices and precise 3-D materials processing involving helical and chiral patterns in diverse fields.

Isotope effects under the influence of global radial electric fields in a helical configuration

Isotope effects under the influence of a radial electric field are examined in a helical magnetic field configuration. We perform global gyrokinetic simulations with additional poloidal rotations to estimate quasi-linear heat flux due to ion temperature gradient mode under the mixing length model. In single-ion-species plasmas, the mass number dependency of heat flux agrees with gyro-Bohm scaling in the absence of a radial electric field. Favorable mass number dependencies violating gyro-Bohm scaling are observed in the presence of a global radial electric field or a heavy hydrogen component in multi-ion-species plasmas. The radial electric field and the heavy hydrogen component affect the heat flux through an increase of wavelength as well as mode stabilization. Poloidal Mach number characterizes the transition from unfavorable to favorable mass number dependency under radial electric fields. While the heat flux is independent of mass number for a given poloidal Mach number, the heat flux decreases for higher mass numbers in a given radial electric field. The heat flux is also independent of average mass number in multi-ion-species plasmas because the heavy hydrogen component effectively enhances the light hydrogen heat flux. The present results are potentially relevant to the violation of gyro-Bohm scaling observed in the recent deuterium experiments in the Large Helical Device.

Theory of mean E B shear in a stochastic magnetic field: ambipolarity breaking and radial current

The mean E B shear in a stochastic magnetic field is calculated, using the radial force balance relation and transport equations. This analysis is relevant to the L → H transition with resonant magnetic perturbations, and special focus is placed upon the physics of non-ambipolar transport and radial current. The key physical process is the flow of fluctuating currents along wandering magnetic fields. The increments in poloidal and toroidal rotation, density and ion pressure are calculated. The radial envelope of the magnetic perturbations inside the plasma defines a new scale , which is the characteristic scale of the magnetic fluctuation intensity profile. The net particle outflow due to stochastic magnetic fields is calculated and is determined by the net radial current through the separatrix. Implications for the L → H transition are discussed.

Momentum transfer and plasma rotation caused by destabilized eigenmodes in tokamaks

The influence of magnetohydrodynamic eigenmodes destabilized by energetic ions on the momentum of these ions and concomitant sheared plasma rotation are studied. Two mechanisms affecting rotation are revealed: (i) spatial channelling (SC) – radially separated emission and absorption of the momentum; (ii) mode induced redistribution (MIR) across the magnetic field of the momentum of energetic ions. Forces arising during SC and MIR produce both toroidal and poloidal rotations. In addition, the momentum emission during SC leads to a radial flux of fast ions and generation of a radial electric field. Using the developed theory, estimates were made for the ITER (International Thermonuclear Experimental Reactor) 15 MA baseline scenario. They show that a global toroidicity-induced Alfvén eigenmode destabilized by alpha particles and neutral beam injection can result in significant radial electric field and forces applied to plasma. However, available data are not sufficient for a reliable prediction of the effects of SC and MIR in ITER. In general, one can expect that sheared rotation arising after destabilization of Alfvén modes and fast magnetoacoustic modes by energetic ions will tend to suppress the turbulence and improve plasma performance. The importance of plasma rotation is supported, in particular, by the fact that during the JET DTE1 experimental campaign the best parameters were achieved in a deuterium–tritium discharge where the rotation frequency was largest.

Prevention of electron cyclotron current drive triggering explosive bursts in reversed magnetic shear tokamak plasmas for disruption avoidance

An explosive burst excited by a neoclassical tearing mode (NTM) is one of the possible candidates for disruptive terminations in reversed magnetic shear (RMS) tokamak plasmas. For the purpose of disruption avoidance, numerical investigations have been implemented on the prevention of explosive bursts triggered by the ill-advised application of electron cyclotron current drive (ECCD) in RMS configuration. Under the situation of controlling NTMs by ECCD in RMS tokamak plasmas, a threshold in electron cyclotron driven current has been found. Below the threshold, not only are the NTM islands not effectively suppressed but a deleterious explosive burst could also be triggered, which might contribute to major disruption to tokamak plasmas. In order to prevent this ECCD from triggering explosive bursts, three control strategies have been attempted in this work and two of them have been recognized to be effective. One is to apply differential poloidal plasma rotation in the proximity of outer rational surface during the ECCD control process; the other is to apply two ECCDs to control NTM islands on both rational surfaces at the same time. In the former strategy, the threshold is diminished due to the modification of the classical tearing mode index. In the latter strategy, the prevention is accomplished as a consequence of the reduction of the coupling strength between the two rational surfaces via the stabilization of inner islands. Moreover, the physical mechanism behind the excitation of the explosive burst and the control processes by different control strategies have all been discussed in detail.

Physics studies of ADITYA & ADITYA-U tokamak plasmas using spectroscopic diagnostics

Several spectroscopic diagnostics encompassing the spectral emission range from the x-ray to near infrared (NIR) have been developed, installed and operated for diagnosing and physics studies in the ADITYA and ADITYA-U tokamaks. The recycling and impurity influxes and plasma Z eff after lithium (Li) coating have been studied using a PMT (photomultiplier tube)-filter based system by capturing H α , O1+, C2+, and visible continuum emissions. Significant reduction in the Z eff values has been observed in the discharge with the Li coated walls. The measured radial profile of H α emission using a filter-PMT array, has been modelled using a neutral transport code. The results show substantial contributions from the molecular hydrogen and molecular hydrogen ion dissociation (∼56%) and charge-exchange (∼30%) processes in the measured H α emission. Furthermore, a high-resolution, 1 m spectrometer with charge coupled device detector capable of multi-track measurements, has been used to study impurity transport, neutral and ion temperature and intrinsic plasma rotation. By modelling the measured radial profile of O4+ spectral line emission using an impurity transport code, substantial contribution of edge fluctuations on the oxygen transport has been observed. The toroidal ( ∼ 20 km s−1 in core) and poloidal ( ∼ 4.5 km s−1 at edge) rotation velocities are measured using C5+ (529 nm) and C2+ (464.7 nm) passive line emissions respectively. The measurement of radial profile of toroidal plasma rotation revealed a reversal of rotation direction depending on the electron density content of the ADITYA-U plasmas. The neutral temperature measurements showed a poloidal asymmetry indicating a presence of asymmetrical source of neutral heating. Moreover, the modelling of measured Fe14+ and Fe15+ vacuum ultraviolet spectral lines has revealed the neo-classical nature of iron transport in ADITYA core. Fast visible camera images captured the formation of filament structures triggered by interchange instabilities during plasma disruptions.