Understanding and controlling plasma rotation in tokamaks

Fusion energy is an attractive candidate for future energy production. At very high temperatures the nuclei of two hydrogen isotopes, deuterium and tritium, fuse, producing helium, a neutron and a very large amount of energy. The high temperatures that are needed to make fusion reactions possible can be reached in a plasma that is magnetically confined. The most successful concept for the magnetic confinement of a plasma is the tokamak. However, despite the success of the tokamak, several challenges still lie ahead. One of these challenges is the magnetic stability. If we want to assure the magnetic stability of a tokamak, it is necessary to know what causes magnetic instabilities. An important class of magnetohydrodynamic instabilities are so-called tearing modes or magnetic islands. These tearing modes can be triggered by an external magnetic error field, which can be due to misalignment of field coils, asymmetries in the tokamak vessel et cetera. It has been observed on several tokamaks that the excitation of these error field induced modes depends strongly on the plasma rotation. Also theories have been developed to describe this dependency. The TEXTOR tokamak is very well suited to investigate the relation between plasma rotation and the excitation of tearing modes. Two tangential neutral beam injectors give us complete control over the plasma rotation. A set of perturbation coils, called the dynamic ergodic divertor (DED), provide a fully adjustable perturbation field. With the tangential neutral beams and the DED, dedicated experiments were carried out to test the theory on mode excitation. When an external perturbation field is applied, the plasma rotation is expected change until the rotation velocity is so that tearing modes would be at rest in the frame of the perturbation field. Once this rotation velocity is reached, large tearing modes will develop in the plasma. This is called mode excitation or mode penetration. A too low perturbation field may not induce enough change in plasma rotation to reach the rotation velocity for which the tearing modes are at rest in the frame of the perturbation field. This means that the perturbation level needs to reach a certain threshold in order to excite tearing modes. The theory found in the literature predicts a monotonic change in the plasma rotation. It also predicts that the threshold for mode excitation increases with the plasma rotation velocity. The direction of the rotation, within the frame of the perturbation field, is not important, hence the threshold as a function of the rotation velocity is symmetric around the minimal threshold. The results of our experiments deviate from the predictions made by the theory. Instead of a monotonic change in the plasma rotation, we found that the plasma has an initial tendency to rotate in the co-current direction when the DED is applied. For a plasma that rotates faster in the co-current direction than the DED field, this means that the plasma first accelerates and only at a high perturbation strength starts to slow down. Not only the measured change in plasma rotation was different than expected. Also the relation between the plasma rotation and the threshold for mode excitation was not in agreement with the theory. Instead of the predicted symmetry around the minimal threshold, we measured that the threshold increases faster with increasing plasma corotation than with increasing counter-rotation. The reason for this discrepancy between theory and experiment was found in the plasma edge. An external perturbation field creates a stochastic zone in the plasma edge. Fast electron transport parallel to the stochastic field lines leads to a radial electron loss current. Ambipolarity requires this loss current to be balanced by a perpendicular return current, that in turn exerts a j × B-force onto the plasma. This stochastic torque is always in the co-current direction and saturates for a high perturbation strength. In most tokamaks the stochastic region will be very small and the stochastic force can be ignored. In TEXTOR however, the stochastic force is quite large, because one of the main objectives of the DED is to significantly perturb the plasma edge. The conventional theory of mode excitation does not take this stochastic force into account. The change in plasma rotation and the threshold as a function of plasma rotation is found by balancing an electromagnetic torque at the position of the mode with a viscous torque that opposes any change in the plasma rotation. Because the stochastic torque can not be ignored, in TEXTOR the balance of the EM torque, the viscous torque and the stochastic torque has to be taken. With the stochastic force included in the mode excitation model, the measurements of the plasma rotation and the rotation dependence of the mode excitation could be explained. For co-rotating plasmas the EM torque tries to slow the plasma down, while the stochastic torque tries to increase the plasma velocity. In an initial stage the stochastic torque is dominant and the plasma rotation increases. When the perturbation field is increased the EM torque takes over, the plasma slows down and mode excitation occurs. The stochastic torque opposes the EM torque, resulting in a high the excitation threshold. For counter-rotating plasmas, both the stochastic and the EM torque slow down the plasma. As a result the threshold for mode excitation is not as high for counter-rotating plasmas as it is for plasmas rotating with the same speed in the co-direction. Two conclusions can be drawn from this work: (a) In absence of a stochastic field the conventional mode excitation theory is valid. (b) The introduction of a stochastic edge field is beneficial for co-rotating plasmas: the threshold for mode excitation increases and – for moderate perturbations – the rotation itself also increases. ITER is expected to rotate in the co-direction. It may be therefore worthwhile to look into the possibility of a stochastic edge. Especially because, apart from a higher mode threshold and a faster rotation, stochasticity also suppresses unfavourable ELM’s in H-mode. Both with and without a stochastic edge region, the fact remains that a fast rotating plasma is less sensitive to tearing modes than a slowly rotating plasma. In present day tokamaks neutral beam injectors provide the necessary high rotation speed. For the next generation of fusion reactors the neutral beams will not be able to deliver enough momentum to a plasma. Other sources of momentum input are therefore needed. A promising source of momentum input is ion cyclotron resonance heating (ICRH). The effect of ICRH on plasma rotation is, however, still not very well understood. Depending on the plasma Understanding and controlling plasma rotation in tokamaks conditions ICRH can induce both co- or counter-rotation. In TEXTOR it was found that ICRH acts as a source of momentum in the counter-current direction. If ICRH changes the plasma rotation, we may assume that also electron cyclotron resonance heating (ECRH) has an influence on the plasma rotation. In TEXTOR it was found that the changes in rotation during ECRH were related to changes in the local transport. This opens perspectives for local shaping of the rotation profile. Especially the possibility to locally increase the velocity gradient is interesting, because a sheared velocity suppresses turbulence and thus increases the confinement

Status of PEM-based polarimetric MSE development at KSTAR

A multi-chord PEM (photo elastic modulator)-based polarimetric motional Stark effect (MSE) system is under development for the KSTAR tokamak. The conceptual design for the front optics was optimized to preserve not only the polarization state of the input light for the MSE measurements but also the signal intensity of the existing charge exchange spectroscopy (CES) system that will share the front optics with the MSE. The optics design incorporates how to determine the number of channels and the number of fibers for each channel. A dielectric coating will be applied on the mirror to minimize the relative reflectivity and the phase shift between the two orthogonal polarization components of the incident light. Lenses with low stress-birefringence constants will be adopted to minimize non-linear and random changes in the polarization through the lenses, which is a trade-off with the rather high Faraday rotation in the lenses because the latter effect is linear and can be relatively easily calibrated out. Intensive spectrum measurements and their comparisons with the simulated spectra are done to assist the design of the bandpass filter system that will also use tilting stages to remotely control the passband. Following the system installation in 2014, the MSE measurements are expected to be performed during the 2015 KSTAR campaign

Understanding and controlling plasma rotation in tokamaks

Fusion energy is an attractive candidate for future energy production. At very high temperatures the nuclei of two hydrogen isotopes, deuterium and tritium, fuse, producing helium, a neutron and a very large amount of energy. The high temperatures that are needed to make fusion reactions possible can be reached in a plasma that is magnetically confined. The most successful concept for the magnetic confinement of a plasma is the tokamak. However, despite the success of the tokamak, several challenges still lie ahead. One of these challenges is the magnetic stability. If we want to assure the magnetic stability of a tokamak, it is necessary to know what causes magnetic instabilities. An important class of magnetohydrodynamic instabilities are so-called tearing modes or magnetic islands. These tearing modes can be triggered by an external magnetic error field, which can be due to misalignment of field coils, asymmetries in the tokamak vessel et cetera. It has been observed on several tokamaks that the excitation of these error field induced modes depends strongly on the plasma rotation. Also theories have been developed to describe this dependency. The TEXTOR tokamak is very well suited to investigate the relation between plasma rotation and the excitation of tearing modes. Two tangential neutral beam injectors give us complete control over the plasma rotation. A set of perturbation coils, called the dynamic ergodic divertor (DED), provide a fully adjustable perturbation field. With the tangential neutral beams and the DED, dedicated experiments were carried out to test the theory on mode excitation. When an external perturbation field is applied, the plasma rotation is expected change until the rotation velocity is so that tearing modes would be at rest in the frame of the perturbation field. Once this rotation velocity is reached, large tearing modes will develop in the plasma. This is called mode excitation or mode penetration. A too low perturbation field may not induce enough change in plasma rotation to reach the rotation velocity for which the tearing modes are at rest in the frame of the perturbation field. This means that the perturbation level needs to reach a certain threshold in order to excite tearing modes. The theory found in the literature predicts a monotonic change in the plasma rotation. It also predicts that the threshold for mode excitation increases with the plasma rotation velocity. The direction of the rotation, within the frame of the perturbation field, is not important, hence the threshold as a function of the rotation velocity is symmetric around the minimal threshold. The results of our experiments deviate from the predictions made by the theory. Instead of a monotonic change in the plasma rotation, we found that the plasma has an initial tendency to rotate in the co-current direction when the DED is applied. For a plasma that rotates faster in the co-current direction than the DED field, this means that the plasma first accelerates and only at a high perturbation strength starts to slow down. Not only the measured change in plasma rotation was different than expected. Also the relation between the plasma rotation and the threshold for mode excitation was not in agreement with the theory. Instead of the predicted symmetry around the minimal threshold, we measured that the threshold increases faster with increasing plasma corotation than with increasing counter-rotation. The reason for this discrepancy between theory and experiment was found in the plasma edge. An external perturbation field creates a stochastic zone in the plasma edge. Fast electron transport parallel to the stochastic field lines leads to a radial electron loss current. Ambipolarity requires this loss current to be balanced by a perpendicular return current, that in turn exerts a j × B-force onto the plasma. This stochastic torque is always in the co-current direction and saturates for a high perturbation strength. In most tokamaks the stochastic region will be very small and the stochastic force can be ignored. In TEXTOR however, the stochastic force is quite large, because one of the main objectives of the DED is to significantly perturb the plasma edge. The conventional theory of mode excitation does not take this stochastic force into account. The change in plasma rotation and the threshold as a function of plasma rotation is found by balancing an electromagnetic torque at the position of the mode with a viscous torque that opposes any change in the plasma rotation. Because the stochastic torque can not be ignored, in TEXTOR the balance of the EM torque, the viscous torque and the stochastic torque has to be taken. With the stochastic force included in the mode excitation model, the measurements of the plasma rotation and the rotation dependence of the mode excitation could be explained. For co-rotating plasmas the EM torque tries to slow the plasma down, while the stochastic torque tries to increase the plasma velocity. In an initial stage the stochastic torque is dominant and the plasma rotation increases. When the perturbation field is increased the EM torque takes over, the plasma slows down and mode excitation occurs. The stochastic torque opposes the EM torque, resulting in a high the excitation threshold. For counter-rotating plasmas, both the stochastic and the EM torque slow down the plasma. As a result the threshold for mode excitation is not as high for counter-rotating plasmas as it is for plasmas rotating with the same speed in the co-direction. Two conclusions can be drawn from this work: (a) In absence of a stochastic field the conventional mode excitation theory is valid. (b) The introduction of a stochastic edge field is beneficial for co-rotating plasmas: the threshold for mode excitation increases and – for moderate perturbations – the rotation itself also increases. ITER is expected to rotate in the co-direction. It may be therefore worthwhile to look into the possibility of a stochastic edge. Especially because, apart from a higher mode threshold and a faster rotation, stochasticity also suppresses unfavourable ELM’s in H-mode. Both with and without a stochastic edge region, the fact remains that a fast rotating plasma is less sensitive to tearing modes than a slowly rotating plasma. In present day tokamaks neutral beam injectors provide the necessary high rotation speed. For the next generation of fusion reactors the neutral beams will not be able to deliver enough momentum to a plasma. Other sources of momentum input are therefore needed. A promising source of momentum input is ion cyclotron resonance heating (ICRH). The effect of ICRH on plasma rotation is, however, still not very well understood. Depending on the plasma Understanding and controlling plasma rotation in tokamaks conditions ICRH can induce both co- or counter-rotation. In TEXTOR it was found that ICRH acts as a source of momentum in the counter-current direction. If ICRH changes the plasma rotation, we may assume that also electron cyclotron resonance heating (ECRH) has an influence on the plasma rotation. In TEXTOR it was found that the changes in rotation during ECRH were related to changes in the local transport. This opens perspectives for local shaping of the rotation profile. Especially the possibility to locally increase the velocity gradient is interesting, because a sheared velocity suppresses turbulence and thus increases the confinement

Overview of physics results from MAST towards ITER/DEMO and the MAST Upgrade

New diagnostic, modelling and plant capability on the Mega Ampère Spherical Tokamak (MAST) have delivered important results in key areas for ITER/DEMO and the upcoming MAST Upgrade, a step towards future ST devices on the path to fusion currently under procurement. Micro-stability analysis of the pedestal highlights the potential roles of micro-tearing modes and kinetic ballooning modes for the pedestal formation. Mitigation of edge localized modes (ELM) using resonant magnetic perturbation has been demonstrated for toroidal mode numbers n = 3, 4, 6 with an ELM frequency increase by up to a factor of 9, compatible with pellet fuelling. The peak heat flux of mitigated and natural ELMs follows the same linear trend with ELM energy loss and the first ELM-resolved Ti measurements in the divertor region are shown. Measurements of flow shear and turbulence dynamics during L–H transitions show filaments erupting from the plasma edge whilst the full flow shear is still present. Off-axis neutral beam injection helps to strongly reduce the redistribution of fast-ions due to fishbone modes when compared to on-axis injection. Low-k ion-scale turbulence has been measured in L-mode and compared to global gyro-kinetic simulations. A statistical analysis of principal turbulence time scales shows them to be of comparable magnitude and reasonably correlated with turbulence decorrelation time. Te inside the island of a neoclassical tearing mode allow the analysis of the island evolution without assuming specific models for the heat flux. Other results include the discrepancy of the current profile evolution during the current ramp-up with solutions of the poloidal field diffusion equation, studies of the anomalous Doppler resonance compressional Alfvén eigenmodes, disruption mitigation studies and modelling of the new divertor design for MAST Upgrade. The novel 3D electron Bernstein synthetic imaging shows promising first data sensitive to the edge current profile and flows

Polarimetric spectra analysis for tokamak pitch angle measurements

Part of 16th International Conference on Laser Aided Plasma Diagnostics (16thLAPD) Measurements of the internal magnetic field structures using conventional polarimetric approaches are considered extremely challenging in fusion-reactor environments whereas the information on current density profiles is essential to establish steady-state and advance operation scenarios in such reactor-relevant devices. Therefore, on ITER a hybrid system is proposed for the current density measurements that uses both polarimetry and spectral measurements. The spectrum-based approaches have been tested in the Korea Superconducting Tokamak Advanced Research (KSTAR) during the past two plasma campaigns. As such, KSTAR is a test-bed for the proposed ITER hybrid system. Measurements in the plasma core are based on the motional Stark effect (MSE) spectrum of the neutral beam emission. For the edge profiles, the Zeeman effect (ZE) acting on the lithium emission spectrum of the newly installed (2013) Lithium-beam-diagnostic is exploited. The neutral beam emission spectra, complicated by the multi-ion-source beam injection, are successfully fitted making use of the data provided by the Atomic Data and Analysis Structure (ADAS) database package. This way pitch angle profiles could be retrieved from the beam emission spectra. With the same spectrometer/CCD hardware as on MSE, but with a different wavelength range and different lines of sight, the first ZE spectrum measurements have been made. The Zeeman splitting comparable to and greater than the instrumental broadening has been routinely detected at high toroidal field operations ( ~ 3 Tesla)

Instrumentation for a multichord motional Stark effect diagnostic in KSTAR

Contributed paper, published as part of the Proceedings of the 20th Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014 The motional Stark effect (MSE) diagnostic is used to measure the radial magnetic pitch angle profile in neutral beam heated plasmas. This information is used to calculate the safety factor, q, with magnetic equilibrium reconstruction codes such as EFIT. The MSE diagnostic is important during active shaping of the q profile to optimize confinement and stability, and it has become a key diagnostic in high performance tokamaks. A multichord photo-elastic modulator (PEM) based MSE system is being developed for a real-time plasma current profile control in Korea Superconducting Tokamak Advanced Research (KSTAR). The PEM-based approach is a standard method that measures the polarization direction of a single Stark line with narrow tunable bandpass filters. A tangential view of the heating beam provides good spatial resolution of 1–3 cm, which provides an opportunity to install 25 spatial channels spanning the major radius from 1.74 m to 2.84 m. Application of real-time control is a long-term technical goal after commissioning the diagnostic in KSTAR, which is expected in 2015. In this paper, we describe the design of this newly-constructed multichord MSE diagnostic in KSTAR. © 2014 AIP Publishing LLC

Validation of the OFIT technique for the detection of the plasma boundary at MAST : traineeship report

A plasma boundary reconstruction method based on optical images was introduced by Hommen et al.[1]. Consistency of the boundary with the boundary from magnetic measurements was demonstrated for a few cases, but systematic validation over a wide range of plasma parameters was lacking. Here we present a systematic comparison of the EFIT (magnetic) and OFIT (optical) boundaries for more than two hundred MAST (Mega Ampere Spherical Tokamak) discharges. For this comparison, the OFIT concept had to be implemented in the MAST data analysis chain and needed to be improved on a number of points. A dimensionless figure of merit has been introduced comparing the non-overlapping regions with the overlapping regions. With this criterion, overall discrepancies of about 6-8% were found between the optical and magnetic boundary. These dimensionless discrepancies translate to overall distances between the EFIT and OFIT boundaries of about 2-3 cm. Upper-limits on the error of OFIT, of 2:7 cm for the DND case and 4:1 cm for the SND case, were found. No (strong) correlation was found with plasma parameters such as normalized pressure ß or self induction li. We show that a significant part of the discrepancy is likely caused by faults in the EFIT reconstruction. This leads to the conclusion that the OFIT technique, if implemented and used correctly, is usable for reliable real-time detection of plasma position and shape, with an overall error less than 3-4 cm

Simulation of the motional stark effect on C-MOD using MSESIM and PERF

The Motional Stark Effect (MSE) is used to measure the internal topology of the magnetic field and radial electric field in a tokamak plasma. MSESIM, a MSE simulation code originally developed for MAST, was transformed to simulate the MSE effect on C-MOD. This code was extended to simulate the Paschen Back Effect and the non statistical population of the energy levels. MSESIM was benchmarked against PERF, a different MSE simulation code developed for JET and C-MOD, to gain more confidence in the results of both codes. MSESIM has been used to investigate the influence of the collection optics, beam divergence, collection volume, Paschen Back Effect, non statistical population of the energy levels and the narrow bandpass filter on the spectrum and polarisation angle of C-MOD. The MSESIM simulation results were compared with measurement to assess how well the code simulates reality

Real-time MSE measurements for current profile control on KSTAR

To step up from current day fusion experiments to power producing fusion reactors, it is necessary to control long pulse, burning plasmas. Stability and confinement properties of tokamak fusion reactors are determined by the current or q profile. In order to control the q profile, it is necessary to measure it in real-time. A real-time motional Stark effect diagnostic is being developed at Korean Superconducting Tokamak for Advanced Research for this purpose. This paper focuses on 3 topics important for real-time measurements: minimize the use of ad hoc parameters, minimize external influences and a robust and fast analysis algorithm. Specifically, we have looked into extracting the retardance of the photo-elastic modulators from the signal itself, minimizing the influence of overlapping beam spectra by optimizing the optical filter design and a multi-channel, multiharmonic phase locking algorithm

Two-dimensional studies of electron Bernstein Wave Emission in MAST

Angular scanning of electron Bernstein wave emission (EBE) has been conducted in MAST. From EBE measurements over a range of viewing angles, the angular position and orientation of the B-X-O mode conversion (MC) window can be estimated, giving the pitch angle of the magnetic field in the MC layer. The radial position of the corresponding MC layer is found from Thomson scattering measurements. Measurements at several frequencies can provide a pitch angle profile. Results of pitch angle profile reconstruction from EBE measurements are presented in comparison with motional Stark effect measurements. Microwave imaging of the B-X-O MC window is proposed as an alternative to angular scanning. The proposed scheme is based on an imaging phased array of antennas allowing the required angular resolution. Image acquisition time ismuch shorter than magnetohydrodynamic (MHD) time scales so the EBE imaging can be used for pitch angle measurements even in the presence of MHD activity