Modification Of Current Profile Research Articles

Stability impacts from the current and pressure profile modifications within finite sized island

The stability (or instability) of finite sized magnetic island could play a significant role in disruption avoidance or disruption mitigation dynamics. Especially, various current and pressure profile modifications, such as the current drive and heating caused by electron cyclotron wave, or the radiative cooling and current expulsion caused by the shattered pellet injection could be applied within the island to modify its stability, thus changing the ensuing dynamics. In this study, we calculate the mode structure modification caused by such profile changes within the island using the perturbed equilibrium approach, thus obtain the change of stability criterion Δ′ and assess the corresponding quasi-linear island stability. The positive helical current perturbation is found to always stabilize the island, while the negative one is found to do the opposite, in agreement with previous results. The pressure bump or hole within the island has a more complicated stability impact. In the small island regime, its contribution is monotonic, with pressure bump that tends to stabilize the island while pressure hole destabilizes it. This effect is relatively weak, though, due to the cancellation of the pressure term’s odd parity contribution in the second derivatives of the mode structure. In the large island regime, such cancellation is broken due to the island asymmetry, and the pressure contribution to stability is manifested, which is non-monotonic. The stability analysis in this paper helps to more accurately clarify the expected island response in the presence of profile modifications caused by disruption avoidance or mitigation systems.

Correlation between the onset of parametric instability of lower hybrid waves and modification in edge plasma current profile on EAST

The plasma current profile modification in the edge region is observed in LHCD (lower hybrid current drive) plasmas for the first time in EAST, correlated with an increased level of PI (parametric instability) activities with the decrease in LH (lower hybrid) source frequency or the increase in plasma density. With the increase in the PI strength, an increase in the edge current is observed despite of a reduction in current drive capability. In these plasmas, strike-point splitting behaviors are also observed, implying the modification of the magnetic geometry in the scrape-off-layer. Results show the effects of LH frequency and plasma density on power dissipation in the edge region, suggesting a role of anomalous wave power damping on the modification of current profile at the plasma edge. These novel results are significant in that they give insight for the first time into how nonlinear wave-plasma interactions such as PI may directly impact the edge current profile, the control of which is critical in order to achieve optimized modes of operation in a steady state fusion reactor.

Hydraulic analysis of a GDI injector operation with close multi-injection strategies

The increase of injection pressure in GDI engines represents a proven development trend in the effort of limiting raw particulate emission, which is one of the most severe concerns for this kind of combustion system. Since increasing injection pressure level tends to cause significant issues in terms of spray penetration control, in parallel with ultra-high injection pressure multi-event strategies are being considered to improve the air-fuel mixture formation management. The adoption of these advanced injection strategies can potentially affect the injection system hydraulic behavior, hence an adequate control of each event fuel quantity is mandatory for a proper engine optimization.When the injector is operated according to a multi-event strategy, complex hydraulic and electro-magnetic phenomena may occur. Pressure waves triggered by each injection can eventually influence the subsequent events, modifying the injected volume compared to the expected, isolated event value. This hydraulic phenomenon is well-known for Diesel applications and is investigated in the present paper in a typical GDI configuration evidencing the effect of the dwell time value between consecutive events both in terms of injected volume and injection rate profile using a proprietary injection analyzer. Further, when consecutive injections are remarkably close, the succeeding injection event is likely to be influenced by a different, electro-magnetic phenomenon in addition to pressure waves propagation in the system pipes. With reduced dwell time strategies, the current profile driving the injector can be modified by a previous close-coupled actuation due to residual magnetization in the solenoid circuit. This phenomenon determines for the succeeding injection event a steeper transient of the solenoidal current, causing a faster injector needle rise and correspondingly a significantly increased injected quantity.In this paper, an innovative methodology is proposed to investigate in multi-injection strategies the effects of close injector actuations, evidencing the influence of hydraulic phenomena and of current profile modifications, respectively. The developed approach, named ETboost, consists in a double injection with a first dummy command. To this end, a short ETboost command is required not causing the injector needle opening and thus no triggering pressure waves that might influence the second injection. Using this analysis approach, the residual magnetization effects on a production GDI injector were experimentally investigated in terms of mean injected volume and instantaneous flow rate. The results obtained with reduced dwell-time were compared with the standard operation mode (single injection), evidencing a significant increment of the injected volume in particular in the ballistic operation where the actual injected quantity can be more than doubled if compared to standard actuation. The dependence of the injected volume on ETboost and dwell time was explored for different operating conditions, evidencing how reducing the dwell time and increasing the ETboost duration the injected volume is enlarged and the shot-to-shot dispersion is significantly reduced.

Role of infernal modes dynamics and plasma rotation on the onset of NTMs in ECH-ECCD TCV plasmas

Experiments in the TCV tokamak show that when high power central electron cyclotron heating (ECH) and current drive (ECCD) are deposited in the central plasma region, tearing-like instabilities can develop. In the present work we address, both analytically and numerically, the problem of tearing mode (TM) generation analysing extensively the behaviour of L-mode and low plasma current TCV discharges when high power heating is deposited in the plasma core region. Three possible mechanisms are discussed. The first is the modification of the equilibrium current density profile due to the power deposition leading to variation of the sign of the classical tearing stability index Δ', where finite β effects could also be taken into account. The second mechanism still the modification of the tearing stability parameter Δ', but this time due to the presence of an equilibrium sheared toroidal plasma rotation. Finally, the third mechanism considered is the role of infernal-type instabilities in driving tearing-like modes when the safety factor becomes sufficiently close to a rational in the core region.

An experimental study on near-orthogonal wave–current interaction over smooth and uniform fixed roughness beds

An experimental study of periodic waves interacting with near-orthogonal turbulent currents is presented in this paper. Mean velocity profiles for current-alone, wave-alone and combined wave–current flows are measured with Acoustic Doppler Velocimeters to resolve the changes in mean flow kinematics due to wave–current interaction. In this study, the near-bottom wave orbital velocity is approximately 1.5 times greater than the depth-averaged current velocity, and the log-profile method is used to determine the bottom roughness from the measured velocity profiles. The primary focus is on 90° wave–current interaction, while selected findings for 60° and 120° wave–current orientations are also presented. In the smooth bed experiment, the interaction is shown to be linear due to the relatively low Reynolds-number flows generated in the facility — a relatively common scenario in small-scale laboratory setups that has not received much attention in previous studies. The smooth turbulent flow equation, with modification of the input shear velocity, is found to accurately predict the mean flow roughness for linear interaction. The bottom roughness is subsequently increased with the introduction of a layer of uniform 12.5mm marbles to achieve a more realistic rough turbulent flow regime. The results agree qualitatively with previous experimental findings that showed a reduction in the near-bottom mean velocity due to a wave-enhanced (apparent) roughness. However, the Grant–Madsen (GM) model is found to over-estimate the apparent roughness when the angle of wave–current interaction is large, implying a lack of directional sensitivity of this model under “strong-wave, weak-current” conditions. A tentative explanation of this shortcoming is given in the paper. However, it is not possible to extend the GM model analysis to the present near-orthogonal wave–current flows, as results suggest that both 60° and 120° wave–current cases are sufficiently contaminated by the wave-induced mass transport component in the current direction to invalidate the use of the log-profile method to resolve the bottom roughness. In addition to the modification of current profiles by waves, the present study also shows a drastic transformation of wave-induced mass transport profiles by the external turbulent currents. The veering of mean flow over depth is consistent with the superposition of a wave-induced return flow on the external current, while additional veering in the near-bottom region may be attributed to turbulence asymmetry induced by the current component in the direction of the near-bottom wave orbital velocity which is shown to vary locally by ±10° due to parasitic waves emanating from the current inlet and outlet.

RETRACTED ARTICLE: Prevention of Plasma–Surface Interactions by Control of Plasma Equilibrium in Large Aspect Ratio Tokamaks

In tokamak plasma, impurities present a number of problems. One is the radiative power loss, principally due to line radiation from partially stripped ions. Another is fuel dilution. This arises because impurity atoms produce many electrons and, for a given plasma pressure, these electrons lake the place of fuel particles. At high concentrations impurities prevent the plasma being heated. This is particularly a problem during the plasma start-up phase since impurities radiate most strongly at low temperatures before they become highly ionized. Impurities can also lead to disruptions as a result of edge cooling and consequent current profile modification. On the other hand, these problems can be prevented by control of plasma equilibrium (which is defined by Grad–Shafranov (GS) equation). Numerous methods exist to solve the GS equation, describing the equilibrium of plasma confined by an axisymmetric magnetic field. In this paper, we have proposed a new numerical solution to the GS equation of an axisymmetric, transformed in cylindrical coordinates solved with the Chebyshev collocation method, when the source term (current density function) on the right hand side is linear. The Chebyshev collocation method is a method for computing highly accurate numerical solutions of deferential equations. We describe a circular cross section of tokamak and present numerical result of magnetic surfaces on the IR-T1 tokamak and then compare the results with an analytical solution.

Fast-ion transport and neutral beam current drive in ASDEX upgrade

The neutral beam current drive efficiency has been investigated in the ASDEX Upgrade tokamak by replacing on-axis neutral beams with tangential off-axis beams. A clear modification of the radial fast-ion profiles is observed with a fast-ion D-alpha diagnostic that measures centrally peaked profiles during on-axis injection and outwards shifted profiles during off-axis injection. Due to this change of the fast-ion population, a clear modification of the plasma current profile is predicted but not observed by a motional Stark effect diagnostic.The fast-ion transport caused by MHD activity has been studied in low collisionality discharges that exhibit strong modes. In particular due to sawtooth crashes, significant radial redistribution of co-rotating fast-ions is observed which can very well be described by the Kadomtsev model. In addition, first tomographic reconstructions of the central 2D fast-ion velocity space in the presence of sawtooth crashes allow the investigation of the pitch dependence of the mode-imposed redistribution: a stronger redistribution of mainly co-rotating fast ions is observed than of those with smaller pitch values.

Demonstration of sawtooth period control with EC waves in KSTAR plasma

The sawtooth period control in tokamak is important issue in recent years because the sawtooth crash can trigger TM/NTM instabilities and drive plasmas unstable. The control of sawtooth period by the modification of local current profile near the q =1 surface using ECCD has been demonstrated in a number of tokamaks [1, 2] including KSTAR. As a result, developing techniques to control the sawtooth period as a way of controlling the onset of NTM has been an important area of research in recent years [3]. In 2012 KSTAR plasma campaign, the sawtooth period control is carried out by the different deposition position of EC waves across the q =1 surface. The sawtooth period is shortened by on-axis co-ECCD (destabilization), and the stabilization of the sawtooth is also observed by off-axis co-ECCD at outside q =1 surface. In 2013 KSTAR plasma campaign, the sawtooth locking experiment with periodic forcing of 170 GHz EC wave is carried out to control the sawtooth period. The optimal target position which lengthens the sawtooth period is investigated by performing a scan of EC beam deposition position nearby q =1 surface at the toroidal magnetic field of 2.9 T and plasma current of 0.7 MA. The sawtooth locking by the modulated EC beam is successfully demonstrated as in [3-5] with the scan of modulation-frequency and duty-ratio at the low beta ( β N ~0.5) plasma. In this paper, the sawteeth behavior by the location of EC beam and the preliminary result of the sawtooth locking experiments in KSTAR will be presented.

Suppression of electric and magnetic fluctuations and improvement of confinement due to current profile modification by biased electrode in Saha Institute of Nuclear Physics tokamak

Improvement of plasma confinement is achieved in normal qa discharges of SINP-tokamak by introducing a biased electrode inside the last closed flux surface. All the important features of high confinement mode are observed biasing the electrode negatively with respect to the vacuum vessel. Arrays of electric and magnetic probes introduced in the edge plasma region reveal suppression of electric and magnetic fluctuations over distinct frequency ranges as well as modification of the toroidal current profile due to biasing. Further analysis identifies the electrostatic fluctuations to be due to drift mode and the magnetic fluctuations may be of slow compressional Alfven waves. Both get suppressed due to current profile modification during biasing, hence leading to the improvement of plasma confinement.

Full wave effects on the lower hybrid wave spectrum and driven current profile in tokamak plasmas

A numerical modeling of current profile modification by lower hybrid current drive (LHCD) using a fullwave/Fokker-Planck simulation code is presented. A MHD stable LHCD discharge on Alcator C-Mod was analyzed, and the current profile from full wave simulations was found to show better agreement with the experiment than a ray-tracing code. Comparison of full wave and ray-tracing simulation shows that, although ray-tracing can reproduce the stochastic wave spectrum broadening, the full wave calculation predicts even wider spectrum broadening, and the wave spectrum fills all of the kinematically allowed domain. This is the first demonstration of LHCD current profile modeling using a full wave simulation code in a multi-pass absorption regime, showing the clear impact of full wave effects on the LHCD driven current profile.

Investigation of Trigger Mechanism in the Explosive Nonlinear Growth of the Double Tearing Mode

Magnetic reconnection dynamics due to the nonlinearly destabilized double tearing mode (DTM) is simulated, focusing on the nonlinear growth phase in the framework of reduced resistive magnetohydrodynamics (MHD). The nonlinearly explosive growth of the DTM accompanying fast magnetic reconnection is found to result from a secondary instability, the mechanism of which consists of the sequential unstable modulation due to twodimensional distortion of magnetic islands and modification of the nonlinear current profile. The trigger dynamics of the nonlinear growth phase is illustrated via the investigation of the evolution of both the kinetic and magnetic energies of the secondary instability.

Overview of the Alcator C-Mod Research Program

This paper summarizes highlights of research results from the Alcator C-Mod tokamak covering the period 2006–2008. Active flow drive, using mode converted ion cyclotron waves, has been observed for the first time in a tokamak plasma, using a mix of D and 3He ion species; toroidal and poloidal flows are driven near the location of the mode conversion layer. ICRF induced edge sheaths are implicated in both the erosion of thin boron coatings and the generation of metallic impurities. Lower hybrid range of frequencies (LHRF) microwaves have been used for efficient current drive, current profile modification and toroidal flow drive. In addition, LHRF has been used to modify the H-mode pedestal, increasing temperature, decreasing density and lowering the pedestal collisionality. Studies of hydrogen isotope retention in solid metallic plasma facing components reveal significantly higher retention than expected from ex situ laboratory studies; a model to explain the results, based on plasma/neutral induced lattice damage, has been developed and tested. During gas-puff mitigation of disruptions, induced MHD instabilities cause the magnetic field to become stochastic, resulting in reduction of halo currents, spreading of plasma power loading and loss of runaway electrons before they cause damage. Detailed pedestal rotation profile measurements have been used to infer Er profiles, and correlation with global H-mode confinement. An improved L-mode regime, obtained at q95 ⩽ 3 with ion drift away from the active X-point, shows very good energy confinement with a strong temperature pedestal, a weak density pedestal, and no evidence of particle or impurity accumulation, without the need for ELMs or any additional edge density regulation mechanism.

Integrated scenario with type-III ELMy H-mode edge: extrapolation to ITER

One of the most severe problems for fusion reactors is the power load on the plasma facing components. The challenge is to develop operation scenarios, which combine sufficient energy confinement with benign heat loads to the plasma facing components. The radiative type-III ELMy H-mode seems a possible solution for such an integrated ITER scenario. Nitrogen seeded type-III ELMy H-modes for the standard inductive scenario and the high beta stationary hybrid scenario are investigated with respect to their transient and steady-state power fluxes to the divertor, confinement properties, edge operational space, core operational space, plasma purity and MHD behaviour. A large database of highly radiative type-III ELMy H-modes on JET is used for extrapolations to ITER. On this basis the transient heat load should be acceptable for ITER. It was found that the scaling of the confinement time with respect to the ion gyroradius is close to the gyro-Bohm scaling. Scalings with respect to the plasma collisionality suggest that the confinement will be good enough for an ITER scenario at 17 MA with a power amplification factor (Q) of 10 and might be marginally good enough for a Q = 10 scenario at 15 MA. Those extrapolations are supported by simulations with an integrated core/edge model COREDIV. In addition the hybrid scenario with type-III edge localized modes has been proven to have improved edge conditions without any modification of the central plasma current profile, indicating it is compatible with a high beta operation for a steady-state ITER Q = 5 scenario.

Simulating the interaction of seagrasses with their ambient flow

The interaction of seagrasses with the dynamics of an oscillatory wave induced flow is assessed with a new Lagrangian plant model. The plant model simulates moving plants in canopies and their dissipative effect on the ambient flow. Concomitantly the plant model is interactively coupled to a 3D hydrodynamic numerical model allowing for a bilateral feedback between moving plants and flow. Model results demonstrate that this interaction causes a modification of current profiles within and above a canopy as compared to an undisturbed flow. While the overall effect of submerged plant canopies is a dampening of dynamics, the flow may locally be intensified. The model predicted an intensification of the flow near the top of a canopy in concurrence with field and laboratory observations. Dissipation in the coupled model, due to the applied non-linear friction law, grows exponentially with increasing flow. As a result the permeability of a canopy to the ambient flow decreases with increasing dissipation. Consequently, at high flow velocities, while becoming increasingly impermeable, a canopy acts like an obstacle that deflects the flow above it, which causes the observed intensification. Results for canopies consisting of seagrasses with different leaf structure and plant geometry show remarkable differences in predicted plant motions, current profiles, drag forces, and velocity shear. Predictions for moving plants are compared with those for rigid, less flexible, structures and undisturbed flow.

Modification of edge current profile and improved confinement induced by biased electrode in the very low qa (VLQ) discharges of SINP – tokamak

A better energy-confined state with modified current density profile is achieved by fast biasing (applied voltage rise time ∼ the ion–ion collision time) an electrode placed inside the limiter in very low qa discharges of the Saha Institute of Nuclear Physics–tokamak (SINP–Tokamak) (Lahiri et al 1996 Nucl. Fusion 36 254). The plasma current increases and lasts for an extended period. Profile modification of the radial electric field (Er) in the edge region is also observed in the better-confined state. However, jΦ-profile modification as well as energy confinement time improvement occurs prior to Er-profile modification.

Identification of Models for Current Profile Modification in ASDEX Upgrade

Control of the shape of the current density profile is essential to improve the confinement and the stability in the plasma, particularly for advanced tokamak scenarios with internal transport barriers. For real-time control of the current density profile, it is necessary to identify a model that describes the time evolution of the current density profile when additional current is driven by external current drive tools. This paper focuses on the identification of such models in ASDEX Upgrade. Neutral beam injection is planned as a tool to control the current density profile in ASDEX Upgrade. The possibility of modifying the current density profile using neutral beam injection is investigated by the ASTRA code simulations using the Weiland transport model. It is difficult to derive a physics-based model for the current profile modification with neutral beam injection because it is nonlinear and multivariable. Therefore, a numerical model, a state-space model suited for systems with many input and output signals, is employed for the modeling. The matrices of the state-space model are estimated using a database by a standard prediction error method that minimizes the difference between the model output and the reference output. The database consists of a set of perturbed input signals and simulated output signals. The input signals are the variations of neutral beam power from different beam sources, and the output signals are the variations of the total plasma pressure and the current density profile. The ASTRA code with the Weiland transport model is used for the simulations to create the database since experimental data are currently not available at ASDEX Upgrade. A test of identified models is carried out using another database, also produced by ASTRA, applying a step response pattern to the input signals. It is found that the models obtained predict the output of this database with high accuracies. It is possible to apply the approach developed here to other actuators in a similar way for the current profile control in existing and future experiments.

Magneto-flow instability in symmetric field profiles

Since Rayleigh’s early work on shear-flow driven instabilities in fluids, it has been known that sheared flows are usually unstable only in the presence of an inflection point in the velocity profile. However, in magnetohydrodynamics, there are important instabilities for which no inflection point is required. In tokamak experiments, strongly sheared flows are associated with transport barriers. Instabilities that may limit the height and extent of transport barriers are of central importance. Here, we present linear and nonlinear simulations of an ideal magnetohydrodynamic instability that is driven by sheared flows without inflection points—instead, the instability mechanism requires reversed magnetic shear. Several symmetric field profiles are studied. In general, the instability leads to current profile modifications that push the local minimum value of the safety factor (qmin) upward. The possibility of causing disruption in a relatively slow time scale is pointed out when qmin crosses a rational (especially integral) value. The time scale of the instability is governed by the transit time of the shear flow, which is typically smaller than that of the Alfvén velocity. Characteristics of this instability are compared with recent experimental observations.

Observation of Instability-Induced Current Redistribution in a Spherical-Torus Plasma

A motional Stark effect diagnostic has been utilized to reconstruct the parallel current density profile in a spherical-torus plasma for the first time. The measured current profile compares favorably with neoclassical theory when no large-scale magnetohydrodynamic instabilities are present in the plasma. However, a current profile anomaly is observed during saturated interchange-type instability activity. This apparent anomaly can be explained by redistribution of neutral beam injection current drive and represents the first observation of interchange-type instabilities causing such redistribution. The associated current profile modifications contribute to sustaining the central safety factor above unity for over five resistive diffusion times, and similar processes may contribute to improved operational scenarios proposed for ITER.

Linear stability analysis of microinstabilities in electron internal transport barrier non-inductive discharges

The role of the current profile and ion and electron temperature ratio in Tokamak à Configuration Variable (TCV) electron internal transport barrier (eITB) non-inductive discharges is inspected by means of the linear gyrokinetic global code LORB5 (Bottino et al 2004 Phys. Plasmas 11 198) and the Gyro–Landau fluid transport model GLF23 (Waltz et al 1997 Phys. Plasmas 4 2482). In this work we have compared two different MHD equilibria for a TCV discharge with an eITB, corresponding to a monotonic and a reversed safety factor profile. The latter has been obtained using the calculated electron–cyclotron driven density profile, the bootstrap current and the experimental pressure. Global simulations show that the most unstable electrostatic instabilities in the spectrum are trapped electron modes (TEM) for both equilibria. In the vicinity of the reverse shear region, where the magnetic shear s is positive and small (s ≪ 1), modes tend to preserve the ballooning structure but their mixing length estimate for the thermal diffusion coefficient is significantly reduced as compared with the monotonic q equilibrium. This effect is further increased in the reverse shear region. Simulations performed with GLF23 show that the inclusion of the Shafranov shift in the model has an important stabilizing effect in all the plasma core and, in particular, in the barrier region. However, the Shafranov shift and reverse shear stabilization are not the only mechanisms through which the q profile affects the stability of the system. The local value of q in the barrier region also contributes to the stabilization. Global gyrokinetic and local Gyro–Landau fluid results suggest that the current profile modification alone, with the consequent negative shear and Shafranov shift stabilization on TEM modes, can explain the presence of the ITB in these TCV discharges.

The Electron Cyclotron Resonant Heating System on the DIII-D Tokamak

In the DIII-D electron heating and current drive installation, up to six gyrotron microwave generators in the 1-MW class at pulse lengths up to 5 s have been operated simultaneously. The frequency for all the gyrotrons is 110 GHz, corresponding to the second harmonic of the electron gyrofrequency at 2 T. The peak generated power has been >4 MW with peak injected power slightly greater than 3 MW. The radio frequency (rf) generators are located remotely and are connected to the tokamak by up to 100 m of evacuated circular corrugated waveguide carrying the HE1,1 mode with overall transmission efficiency, including coupling to the waveguide, of up to 75%. Ancillary equipment for polarization control, beam switching, power monitoring, control of launch direction, and system protection has been developed.The system has been used to support a wide variety of physics experiments, including control of magnetohydrodynamic modes, current density profile modifications, basic plasma heating and current drive, transport studies, and rf-assisted start-up. The gyrotron complex is being upgraded by the acquisition of additional tubes with 5- to 10-s pulse length capability.