Sawtooth Instability Research Articles

Evolution of the central safety factor during stabilized sawtooth instabilities at KSTAR

A motional Stark effect (MSE) diagnostic has recently been installed in the KSTAR tokamak. A difficulty faced at KSTAR and common to other MSE diagnostics is calibration of the system for absolute measurements. In this report we present our novel calibration routine and discuss first results, evaluating the evolution of the the central safety factor during sawtooth instabilities. The calibration scheme ensures that the bandpass filters typically used in MSE systems are aligned correctly and identifies and removes systematic offsets present in the measurement. This is verified by comparing the reconstructed safety factor profile against various discharges where the locations of rational q surfaces have been obtained from MHD markers. The calibration is applied to analyse the evolution of q0 in a shot where the sawteeth are stabilized by neutral beam injection. Within the analysed sawtooth periods q0 drops below unity during the quiescent phase and relaxes close to or slightly above unity at the sawtooth crash. This finding is in line with the classical Kadomtsev model of full magnetic reconnection and earlier findings at JET.

Interaction of the electron density fluctuations with electron cyclotron waves from the equatorial launcher in ITER

We present a numerical investigation of electron cyclotron beams interacting with electron density fluctuations in the ITER 15 MA H-mode scenario. In particular, here we study how the beam from the equatorial launcher, which shall be utilized to influence the sawtooth instability, is affected by the fluctuations. Moreover, we present the theory and first estimates of the power that is scattered from the injected O-mode to a secondary X-mode in the presence of the fluctuations. It is shown that for ITER parameters the scattered power stays within acceptable limits and broadening of the equatorial beams is less than those from the upper launcher.

Fast infrared thermography on the COMPASS tokamak

A new fast infrared camera was purchased on the COMPASS tokamak recently. It is equipped with a medium wavelength infrared (3–5mm) InSb detector and is capable of reaching framerate up to 1.9kHz in full frame acquisition mode (320×256px.) and up to 90kHz in sub-windowed mode (64×4px.).First experimental measurements of plasma heat flux to inner and outer wall limiters of the COMPASS tokamak using the new camera are presented. Time evolution of parallel heat flux during sawtooth instability is studied showing strong modulation of the heat flux absolute value whilst keeping constant its radial decay length. Fast modulation of outboard midplane limiter heat loading due to runaway electrons is also presented.The camera will be part of a new fast divertor thermographic system with exceptional spatial resolution (∼0.6–1.3mm/px. on the target plane, 0.04–0.14mm/px. mapped to the outer midplane) with a possibility of measurements of radial profiles on the divertor with 320×4px. with temporal resolution better than 20μs. A design of the foreseen optical divertor system is described together with a design of a special divertor graphite tile used for the IR thermography, that will allow in-situ surface emissivity calibration.

Fast Data Processing of a Polarimeter-Interferometer System on J-TEXT**supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2014GB106000, 2014GB106002, and 2014GB106003) and National Natural Science Foundation of China (Nos. 11275234, 11375237 and 11505238) and Scientific Research Grant of Hefei Science Center of CAS (No. 2015SRG-HSC010)

A method of fast data processing has been developed to rapidly obtain evolution of the electron density profile for a multichannel polarimeter-interferometer system (POLARIS) on J-TEXT. Compared with the Abel inversion method, evolution of the density profile analyzed by this method can quickly offer important information. This method has the advantage of fast calculation speed with the order of ten milliseconds per normal shot and it is capable of processing up to 1 MHz sampled data, which is helpful for studying density sawtooth instability and the disruption between shots. In the duration of a flat-top plasma current of usual ohmic discharges on J-TEXT, shape factor u is ranged from 4 to 5. When the disruption of discharge happens, the density profile becomes peaked and the shape factor u typically decreases to 1.

Measurement and simulation of passive fast-ion D-alpha emission from the DIII-D tokamak

Spectra of passive fast-ion D-alpha (FIDA) light from beam ions that charge exchange with background neutrals are measured and simulated. The fast ions come from three sources: ions that pass through the diagnostic sightlines on their first full orbit, an axisymmetric confined population, and ions that are expelled into the edge region by instabilities. A passive FIDA simulation (P-FIDASIM) is developed as a forward model for the spectra of the first-orbit fast ions and consists of an experimentally-validated beam deposition model, an ion orbit-following code, a collisional-radiative model, and a synthetic spectrometer. Model validation consists of the simulation of 86 experimental spectra that are obtained using 6 different neutral beam fast-ion sources and 13 different lines of sight. Calibrated spectra are used to estimate the neutral density throughout the cross-section of the tokamak. The resulting 2D neutral density shows the expected increase toward each X-point with average neutral densities of at the plasma boundary and near the wall. Fast ions that are on passing orbits are expelled by the sawtooth instability more readily than trapped ions. In a sample discharge, approximately 1% of the fast-ion population is ejected into the high neutral density region per sawtooth crash.

Interaction of external n = 1 magnetic fields with the sawtooth instability in low-q RFX-mod and DIII-D tokamaks

External n = 1 magnetic fields are applied in RFX-mod and DIII-D low safety factor Tokamak plasmas to investigate their interaction with the internal MHD dynamics and in particular with the sawtooth instability. In these experiments the applied magnetic fields cause a reduction of both the sawtooth amplitude and period, leading to an overall stabilizing effect on the oscillations. In RFX-mod sawteeth eventually disappear and are replaced by a stationary m = 1, n = 1 helical equilibrium without an increase in disruptivity. However toroidal rotation is significantly reduced in these plasmas, thus it is likely that the sawtooth mitigation in these experiments is due to the combination of the helically deformed core and the reduced rotation. The former effect is qualitatively well reproduced by nonlinear MHD simulations performed with the PIXIE3D code. The results obtained in these RFX-mod experiments motivated similar ones in DIII-D L-mode diverted Tokamak plasmas at low q95. These experiments succeeded in reproducing the sawtooth mitigation with the approach developed in RFX-mod. In DIII-D this effect is correlated with a clear increase of the n = 1 plasma response, that indicates an enhancement of the coupling to the marginally stable n = 1 external kink, as simulations with the linear MHD code IPEC suggest. A significant rotation braking in the plasma core is also observed in DIII-D. Numerical calculations of the neoclassical toroidal viscosity (NTV) carried out with PENT identify this torque as a possible contributor for this effect.

Collective Thomson scattering measurements of fast-ion transport due to sawtooth crashes in ASDEX Upgrade

Sawtooth instabilities can modify heating and current-drive profiles and potentially increase fast-ion losses. Understanding how sawteeth redistribute fast ions as a function of sawtooth parameters and of fast-ion energy and pitch is hence a subject of particular interest for future fusion devices. Here we present the first collective Thomson scattering (CTS) measurements of sawtooth-induced redistribution of fast ions at ASDEX Upgrade. These also represent the first localized fast-ion measurements on the high-field side of this device. The results indicate fast-ion losses in the phase-space measurement volume of about 50% across sawtooth crashes, in good agreement with values predicted with the Kadomtsev sawtooth model implemented in TRANSP and with the sawtooth model in the EBdyna_go code. In contrast to the case of sawteeth, we observe no fast-ion redistribution in the presence of fishbone modes. We highlight how CTS measurements can discriminate between different sawtooth models, in particular when aided by multi-diagnostic velocity-space tomography, and briefly discuss our results in light of existing measurements from other fast-ion diagnostics.

Change in Edge Fluctuations Synchronized with Heat Pulse in the JFT-2M Tokamak

The change in edge fluctuations synchronized with a heat pulse during Sawtooth oscillation has been investigated in the JFT-2M tokamak using a reciprocating Langmuir probe. Data window of a heat pulse is segmented into a number of time frames, and time traces of auto-power spectra at each frame are calculated. Conditional averaging over a number of heat pulse events in a single discharge is performed in order to reduce the variance of the fluctuation spectra. At each heat pulse event, a reduction in the geodesic acoustic mode fluctuation power and an increase in the turbulence fluctuation power are observed. The change in the fluctuation powers suggests that the edge heat pulse during Sawtooth instability could affect the edge turbulence structure.

Control of sawtooth via ECRH on EAST tokamak

Localized electron heating produced by electron cyclotron resonant heating (ECRH) system has been proven to be powerful tools for controlling sawtooth instabilities, because such system allows to directly modify the local plasma parameters that determine the evolution of sawtooth periods. In this paper, we present the experimental results carried out on experimental advanced superconducting tokamak (EAST) with regard to sawtooth period control via ECRH. The electron cyclotron heating system on EAST was capable of inject electron cyclotron wave toward certain locations inside or outside q = 1 magnetic surface on the poloidal cross section, which renders us able to investigate the evolution of sawtooth period against the ECRH deposition position. It is found that when ECRH deposition position is inside the q = 1 surface, the sawtooth oscillation is destabilized (characterized by reduced sawtooth period). So far, inside the q = 1 surface, there are not enough EAST experiment data that can reveal more detailed information about the relation between ECRH deposition position and sawtooth period. When ECRH deposition is outside the q = 1 surface, the sawtooth oscillation is stabilized (characterized by prolonged sawtooth period), and the sawtooth periods gradually decrease as ECRH deposition position sweeps away from q = 1 surface. The sawtooth periods reach maximum when ECRH deposition position falls around q = 1 surface. The magnetic shear at q = 1 surface is calculated to offer insights for the temporal evolution of sawtooth. The result has been found consistent with the Porcelli model.

Suprathermal electron dynamics and MHD instabilities in a tokamak

The dynamics of suprathermal electrons in the presence of magnetohydrodynamics (MHD) activity and the excitation of MHD modes by suprathermal electrons are studied experimentally to improve the understanding of the interaction of fast particles with MHD instabilities in a tokamak. The study focuses on three different aspects of the internal kink mode with poloidal/toroidal mode number : the sawtooth instability, electron fishbones and coupled bursts alternating with sawtooth crashes (CAS), all located where the safety factor profile approaches or takes the value . New quantitative results on suprathermal electron transport and an investigation of electron acceleration during sawtooth crashes are followed by the characterization of initial electron fishbone observations on the Tokamak à configuration variable (TCV). Finally, bursts associated with the sawtooth cycle, coupled to a persisting mode and alternating with sawtooth crashes, are discussed, in particular in view of the fast electron dynamics and their role in confinement degradation and mode excitation.

The high-βN hybrid scenario for ITER and FNSF steady-state missionsa)

New experiments on DIII-D have demonstrated the steady-state potential of the hybrid scenario, with 1 MA of plasma current driven fully non-inductively and βN up to 3.7 sustained for ∼3 s (∼1.5 current diffusion time, τR, in DIII-D), providing the basis for an attractive option for steady-state operation in ITER and FNSF. Excellent confinement is achieved (H98y2 ∼ 1.6) without performance limiting tearing modes. The hybrid regime overcomes the need for off-axis current drive efficiency, taking advantage of poloidal magnetic flux pumping that is believed to be the result of a saturated 3/2 tearing mode. This allows for efficient current drive close to the axis, without deleterious sawtooth instabilities. In these experiments, the edge surface loop voltage is driven down to zero for >1 τR when the poloidal β is increased above 1.9 at a plasma current of 1.0 MA and the ECH power is increased to 3.2 MW. Stationary operation of hybrid plasmas with all on-axis current drive is sustained at pressures slightly above the ideal no-wall limit, while the calculated ideal with-wall MHD limit is βN ∼ 4–4.5. Off-axis Neutral Beam Injection (NBI) power has been used to broaden the pressure and current profiles in this scenario, seeking to take advantage of higher predicted kink stability limits and lower values of the tearing stability index Δ′, as calculated by the DCON and PEST3 codes. Results based on measured profiles predict ideal limits at βN > 4.5, 10% higher than the cases with on-axis NBI. A 0-D model, based on the present confinement, βN and shape values of the DIII-D hybrid scenario, shows that these plasmas are consistent with the ITER 9 MA, Q = 5 mission and the FNSF 6.7 MA scenario with Q = 3.5. With collisionality and edge safety factor values comparable to those envisioned for ITER and FNSF, the high-βN hybrid represents an attractive high performance option for the steady-state missions of these devices.

Use of soft x-ray diagnostic on the COMPASS tokamak for investigations of sawteeth crash neighborhood and of plasma position using fast inversion methods.

The soft x-ray diagnostic is suitable for monitoring plasma activity in the tokamak core, e.g., sawtooth instability. Moreover, spatially resolved measurements can provide information about plasma position and shape, which can supplement magnetic measurements. In this contribution, fast algorithms with the potential for a real-time use are tested on the data from the COMPASS tokamak. In addition, the soft x-ray data are compared with data from other diagnostics in order to discuss possible connection between sawtooth instability on one side and the transition to higher confinement mode, edge localized modes and productions of runaway electrons on the other side.

Wave propagation in a 3D fully nonlinear NWT based on MTF coupled with DZ method for the downstream boundary

Wave propagation in a three-dimensional (3D) fully nonlinear numerical wave tank (NWT) is studied based on velocity potential theory. The governing Laplace equation with fully nonlinear boundary conditions on the moving free surface is solved using the indirect desingularized boundary integral equation method (DBIEM). The fourth-order predictor-corrector Adams-Bashforth-Moulton scheme (ABM4) and mixed Eulerian-Lagrangian (MEL) method are used for the time-stepping integration of the free surface boundary conditions. A smoothing algorithm, B-spline, is applied to eliminate the possible saw-tooth instabilities. The artificial wave speed employed in MTF (multi-transmitting formula) approach is investigated for fully nonlinear wave problem. The numerical results from incorporating the damping zone (DZ), MTF and MTF coupled DZ (MTF+DZ) methods as radiation condition are compared with analytical solution. An effective MTF+DZ method is finally adopted to simulate the 3D linear wave, second-order wave and irregular wave propagation. It is shown that the MTF+DZ method can be used for simulating fully nonlinear wave propagation very efficiently.

Shut-down dose rate analyses for the ITER electron cyclotron-heating upper launcher

The electron cyclotron resonance heating upper launcher (ECHUL) is going to be installed in the upper port of the ITER tokamak thermonuclear fusion reactor for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). The paper reports the latest neutronic modeling and analyses which have been performed for the ITER reference front steering launcher design. It focuses on the port accessibility after reactor shut-down for which dose rate (SDDR) distributions on a fine regular mesh grid were calculated. The results are compared to those obtained for the ITER Dummy Upper Port. The calculations showed that the heterogeneous ECHUL design gives rise to enhanced radiation streaming as compared to the homogenous dummy upper port. Therefore the used launcher geometry was upgraded to a more recent development stage. The inter-comparison shows a significant improvement of the launchers shielding properties but also the necessity to further upgrade the shielding performance. Furthermore, the analysis for the homogenous dummy upper port, which represents optimal shielding inside the launcher, demonstrates that the shielding upgrade also needs to include the launcher's environment.

On confinement characteristics of the newly developed sawtooth-free plasmas in KSTAR tokamak

Development of advanced scenarios, an important experimental goal of the Korea superconducting tokamak advanced research (KSTAR) project, has just begun. The safety factor (q) profile is a key to achieve these advanced scenarios. Particularly the hybrid scenario, one of the advanced scenarios, can be established generally with low magnetic shear (s) at the center with central q-value above unity so to avoid sawtooth instability. This q-profile was successfully produced using early divertor formation during a plasma current ramp-up phase in KSTAR. Auxiliary heating was also employed during the current ramp-up phase to delay the inductive current diffusion to the center of the plasma. In addition to the early divertor formation method, the target q-profile was attempted to be achieved by modifying the plasma current waveform using the so-called, ‘current-overshoot’ method and the timing of L-mode to H-mode transition. In this work, the confinement characteristics of these sawtooth-free regimes are investigated. The global energy confinement time is calculated and compared with that of conventional H-modes in KSTAR. The confinement enhancement factor reveals that the newly developed discharges are not improved over H-modes contrary to results of other tokamaks. To investigate the reason, transport modeling is performed self-consistently with an integrated simulation package incorporating plasma equilibrium, transport, and heating and current drive. The current ramp-up phase is simulated and impact of early divertor formation, current-overshoot, and early L–H transition on the target q-profile and s/q profile is addressed. The s/q profile is found to be not improved in these discharges compared with hybrid scenarios reported in other tokamaks. Based on these results, future experimental directions are addressed to access the hybrid regimes in KSTAR.

Study of advanced neutron emission spectrometry diagnostics for tokamak fusion experiments

The research progress of fusion neutron emission spectrometers in the EAST and HL-2A takamaks is reviewed. Considering the requirements for plasma physics research and neutron diagnostic development for the ITER device, two digital liquid scitillation spectrometers and a stilbene crystal spectrometer were set up for the discharges with the low neutron yield (10⁹-10¹¹ s^-¹) on EAST and HL-2A. We present some neutron spectrometry diagnostic results, such as the time traces of fusion neutron count rates during the EAST discharges, the evaluation of the heating effect in HL-2A neutral beam injection experiments and the neutron diagnostics of sawtooth instability in EAST plasmas. The ion temperature diagnostics of the core plasma were made by using the stilbene neutron spectrometer and the liquid scintillation neutron spectrometer in EAST, and the ion temperature measurement with the neutron emission spectrometer for the plasma with low temperature, which was less than 1 keV, was sfor the first time. A double ring neutron time-of-flight array spectrometer was designed and constructed on EAST for the fast ion physics and tokamak auxiliary heating research.

Preliminary design of the ITER ECH Upper Launcher

The design of the ITER electron cyclotron launchers recently reached the preliminary design level - the last major milestone before design finalization. The ITER ECH system contains 24 installed gyrotrons providing a maximum ECH injected power of 20MW through transmission lines towards the tokamak. There are two EC launcher types both using a front steering mirror; one equatorial launcher (EL) for plasma heating and four upper launchers (UL) for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). A wide steering angle range of the ULs allows focusing of the beam on magnetic islands which are expected on the rational magnetic flux surfaces q=1 (sawtooth instability), q=3/2 and q=2 (NTMs).In this paper the preliminary design of the ITER ECH UL is presented, including the optical system and the structural components. Highlights of the design include the torus CVD-diamond windows, the frictionless, front steering mechanism and the plasma facing blanket shield module (BSM). Numerical simulations as well as prototype tests are used to verify the design

Particle Flow during Sawtooth Reconnection: Numerical Simulations of Experimental Observations

The core plasma density in ohmic, sawtoothing regime in Tore Supra and JET tokamaks measured by fast-sweeping reflectometry develops a central peak. This peak is surrounded by a density plateau, which extends up to the q = 1 reconnection surface. Such mexican hat like profiles differ from the smooth dome of the temperature profile. Detailed tomographic reconstructions of the density in the poloidal plane show the development of crescent and ring structures during the sawtooth cycle. 3D full MHD nonlinear simulations with the XTOR-2F code recover these structures and show that they can be explained by the MHD activity related to the sawtooth instability. They show also that the electron density evolution is dominated by the perpendicular MHD flows, while the temperature, obtained with ECE measurements is dominated by parallel diffusion.

Micro-bubble morphologies following drop impacts onto a pool surface

AbstractWhen a drop impacts at low velocity onto a pool surface, a hemispheric air layer cushions and can delay direct contact. Herein we use ultra-high-speed video to study the rupture of this layer, to explain the resulting variety of observed distribution of bubbles. The size and distribution of micro-bubbles is determined by the number and location of the primary punctures. Isolated holes lead to the formation of bubble necklaces when the edges of two growing holes meet, whereas bubble nets are produced by regular shedding of micro-bubbles from a sawtooth edge instability. For the most viscous liquids the air film contracts more rapidly than the capillary–viscous velocity through repeated spontaneous ruptures of the edge. From the speed of hole opening and the total volume of micro-bubbles we conclude that the air sheet ruptures when its thickness approaches ${\ensuremath{\sim} }100~\mathrm{nm} $.

Sawtooth period control strategies and designs for improved performance

The sawtooth instability is associated with the triggering of neo-classical tearing modes, core fuelling, α-confinement and the exhaust of thermal helium. Sawtooth control is therefore important for optimal reactor performance in ELMy H-modes. Control schemes for the sawtooth period have been published in the literature, but the systematic design of high-performance controllers (yielding accurate and fast convergent responses) has not been addressed. In this work, three control strategies for high-performance sawtooth control are presented using electron cyclotron current drive (ECCD). Both degrees of freedom of the ECCD actuator will be explored and combined with advanced controller designs. First, the ECCD deposition location is used as a control variable, for which a gain-scheduled feedback controller and static feedforward control is derived. Second, the use of the driven current as a control variable is explored, and a simple controller is designed based on the identified dynamics. In the third approach both control variables are joined in an overall controller design, which enables the combination of high-performance control of the sawtooth period and control of the gyrotron power. Time-domain simulations with a combined Kadomtsev–Porcelli sawtooth model show that each strategy obtains a better closed-loop performance than standard linear feedback techniques on merely the deposition location.