Suppression Of Neoclassical Tearing Modes Research Articles

Effects of counter-current driven by electron cyclotron waves on neoclassical tearing mode suppression

Through theoretical analysis, we construct a physical model that includes the influence of counter-external driven current opposite to the plasma current direction in the neoclassical tearing mode (NTM). The equation is used with this model to obtain the modified Rutherford equation with co-current and counter-current contributions. Consistent with the reported experimental results, numerical simulations have shown that the localized counter external current can only partially suppress NTM when it is far from the resonant magnetic surface. Under some circumstances, the Ohkawa mechanism dominated current drive (OKCD) by electron cyclotron waves can concurrently create both co-current and counter-current. In this instance, the minimal electron cyclotron wave power that suppresses a particular NTM was calculated by the Rutherford equation. The result is marginally less than when taking co-current alone into consideration. As a result, to suppress NTM using OKCD, one only needs to align the co-current with a greater OKCD peak well with the resonant magnetic surface. The effect of its lower counter-current does not need to be considered because the location of the counter-current deviates greatly from the resonant magnetic surface.

Large Area Diamond Disk Growth Experiments and Thermomechanical Investigations for the Broadband Brewster Window in DEMO

An electron cyclotron system (ECS) is required in the European DEMO fusion reactor for plasma heating, magnetohydrodynamic control, especially the suppression of neoclassical tearing modes (NTMs), and thermal instability control. Microwave beams of 2 MW, generated by gyrotrons, shall propagate through corrugated HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> waveguides (WGs) of 63.5-mm inner diameter and finally reach specific locations in the plasma by a dedicated set of mirrors. The deployment of the beam may be done either by angular steering, i.e., tilting moveable mirrors, or by frequency steering, i.e., tuning the gyrotron frequency. In the last option, for long pulse operations, broadband optical grade diamond windows are required as confinement and/or vacuum barriers both at the torus and the gyrotron side. The Brewster-angle window represents a promising solution due to its intrinsic simplicity. However, the WG aperture imposes a minimum Ø180 mm for the disk at the Brewster-angle of 67.2° for diamond. This is an ambitious target, considering 140 mm as the state of the art. First, this article shows the very promising results of diamond growth experiments carried out by Diamond Materials GmbH with the first worldwide free-standing optical grade Ø180-mm disk having an average unpolished thickness of 1.3 mm. For the first time, one side of this large disk was polished and loss tangent measurements were performed at the center and over a large area. Then, the performance of the window featuring these large disks was characterized by a consistent set of thermomechanical analyses in view of the forthcoming window prototyping activity.

Experimental evidence of electron-cyclotron current drive-based neoclassical tearing mode suppression threshold reduction during mode locking on DIII-D

The repetitive locking-unlocking process of a large neoclassical tearing mode (NTM) is studied on the DIII-D tokamak. Cyclical periods of partial suppression are achieved when βp and the expected bootstrap current drop upon back-transitions to L-mode. Diminished L-mode bootstrap current is required to explain decreased island size as modeled by the generalized Rutherford equation. At EC power levels too low to fully suppress the NTM, partial suppression is enough to reduce the torque on the mode to the point where viscous drag from the background plasma overcomes the electromagnetic torque and restores fast plasma rotation and H-mode. Additional current drive is shown to further reduce the size of the mode until full suppression is achieved. These results suggest that the suppression of locked modes may be aided by the lower bootstrap current in L-mode plasmas, possibly improving chances of disruption avoidance in ITER.

Controlled neoclassical tearing mode (NTM) healing by fueling pellets and its impact on electron cyclotron current drive requirements for complete NTM stabilization

Controlled partial stabilization of core m/n = 2/1 neoclassical tearing modes (NTMs) by fueling deuterium pellets is reported in DIII-D and KSTAR H-mode plasmas (m/n are the poloidal/toroidal mode numbers). Analyses of DIII-D data exploring possible physics origins show that an explanation is offered by NTM-turbulence multi-scale interaction, triggered by a sudden increase of local gradients near q = 2 caused by the pellet. Pellet injection from the high-field side allows deep fueling which reaches the island region. In turn, low-k turbulent density fluctuations () increase by in the island region. This can drive transport across the island separatrices, reducing the pressure flat spot at the O-point and diminishing the NTM drive. The Mirnov probe array detects the reduction of the 2/1 magnetic amplitude by up to . Causality between elevated gradients outside of the island, turbulence spreading into the island and reduced NTM drive is qualitatively supported by non-linear gyrokinetic turbulence simulations. These show increased penetration of ion-scale from the background plasma to the O-point region when the background gradient is increased. This interaction has potentially far reaching consequences as it can lead to a reduction of the required electron cyclotron current density () for NTM suppression by 70%, as predicted by the modified Rutherford equation. This beneficial effect of fueling pellets can be important as is the anticipated active NTM control technique for ITER, but its efficiency will be lowered by third harmonic absorption in Pre-Fusion Power Operation-1 (PFPO-1) at half magnetic field.

Integrated current profile, normalized beta and NTM control in DIII-D

There is an increasing need for integrating individual plasma-control algorithms with the ultimate goal of simultaneously regulating more than one plasma property. Some of these integrated-control solutions should have the capability of arbitrating the authority of the individual plasma-control algorithms over the available actuators within the tokamak. Such decision-making process must run in real time since its outcome depends on the plasma state. Therefore, control architectures including supervisory and/or exception-handling algorithms will play an essential role in future fusion reactors like ITER. However, most plasma-control experiments in present devices have focused so far on demonstrating control solutions for isolated objectives. In this work, initial experimental results are reported for simultaneous current-profile control, normalized-beta control, and Neoclassical Tearing Mode (NTM) suppression in DIII-D. Neutral beam injection (NBI), electron-cyclotron (EC) heating & current drive (H&CD), and plasma current modulation are the actuation methods. The NBI power and plasma current are always modulated by the Profile Control category within the DIII-D Plasma Control System (PCS) in order to control both the current profile and the normalized beta. EC H&CD is utilized by either the Profile Control or the Gyrotron categories within the DIII-D PCS as dictated by the Off-Normal and Fault Response (ONFR) system, which monitors the occurrence of an NTM and regulates the authority over the gyrotrons. The total EC power and poloidal mirror angles are the gyrotron-related actuation variables. When no NTM suppression is required, the gyrotrons are used by the Profile Control category, but when NTM suppression is required, the ONFR transfers the authority over the gyrotrons to the NTM stabilization algorithm located in the Gyrotron category. Initial experimental results show that simultaneous control of different aspects of the plasma dynamics may improve the overall control and plasma performances. Also, the potential of the ONFR system to successfully integrate competing control algorithms is demonstrated.

Simultaneous detection of neoclassical tearing mode and electron cyclotron current drive locations using electron cyclotron emission in DIII-D

Accurate real-time measurements of magnetic island and electron cyclotron current drive (ECCD) locations are essential for efficient suppression of the neoclassical tearing mode (NTM). To determine these locations, many control systems rely on motional Stark effect constrained equilibria reconstruction and real-time Thomson scattering with TORBEAM current drive evaluation and therefore require time-intensive cross-calibration of at least two different diagnostics. Here we present a simpler, proof-of-concept analysis that uses only a single diagnostic (a radial array electron cyclotron emission radiometer) for the simultaneous determination of both the radial position of a magnetic island and the deposition location of ECCD. Measurements are compared with the modified Rutherford equation to demonstrate the effect of ECCD alignment on NTM suppression.

Modeling of the influences of multiple modulated electron cyclotron current drive on NTMs in rotating plasma

In this work, physical models of neoclassical tearing modes (NTMs) including bootstrap current and multiple modulated electron cyclotron current drive model are applied. Based on the specific physical problems during the suppression of NTMs by driven current, this work compares the efficiency of continuous and modulated driven currents, and simulates the physical processes of multiple modulated driven currents on suppressing rotating magnetic island. It is found that when island rotates along the poloidal direction, the suppression ability of continuous driven current can be massively reduced due to current deposition outside the island separatrix and reverse deposition direction at the X point, which can be avoided by current drive modulation. Multiple current drive has a better suppressing effect than single current drive. This work gives realistic numerical simulations by optimizing the model and parameters based on the experiments, which could provide references for successful suppression of NTMs in future advanced tokamak such as international thermonuclear experimental reactor.

Design and Analysis of Steerable ECRH Launcher for SST-1 Tokamak

In the tokamaks ECRH system is used for pre-ionization, start up, heating, current drive and suppression of NTMs (Neo Classical Tearing Modes). A Standard ECRH system consists of high power microwave source Gyrotron, circular corrugated waveguide based transmission line and launcher. The Focused ECH power is launched into plasma through launcher. The microwave beam emerges out from circular corrugated waveguide and propagates freely in air with finite divergence. So focusing and plane mirror combination is used to launch focused beam in plasma. Thus an ECRH launcher consists of metallic profiled and plane mirror, UHV compatible vacuum barrier window and a UHV gate valve. One 42 GHz gyrotron capable of delivering 500 kW of power for 500 ms and other 82 GHz gyrotron capable of delivering 200 kW of power for 1000s are used for SST-1 ECRH system. The launcher design consists of mirror design, design of supports and design of steering mechanism to provide suitable movements with minimum backless error. The whole assembly is UHV compatible. The launcher is capable of steering the beams by ±20° in both toroidal and poloidal directions. Mirrors are given motion by means of one rotary and one linear feedthrough. For 82 GHz launcher active cooling is provided, whereas for 42 GHz launcher no active cooling is provided.A detailed analysis is carried out for the mirrors of the high power launcher. The heat load for the 82 GHz launcher is 2 kW (~1% absorption) and for 42 GHz launcher it is 5 kW. For 82 GHz launcher, the maximum steady state surface temperatures of focusing and reflecting mirrors are 315K and 323K and von-mises stresses are within 10 MPa. Similarly for 42 GHz launcher maximum temperatures observed during 500 ms pulse are 301K and 303K for focusing and reflecting mirrors respectively. This paper explains the mechanical and thermal design and analysis of the launcher for the ECRH system.

Design of Control System for Suppression of Neoclassical Tearing Modes with Electron Cyclotron Heating and Current Drive on EAST

This paper discusses a control system for the Experimental Advanced Superconducting Tokamak (EAST) that suppresses neoclassical tearing modes (NTMs) (3/2 and 2/1) by applying electron cyclotron (EC) heating and current drive. It allows the magnetic island to be detected and localized in real time and the EC beam to be deposited exactly on the island position by stepwise tuning of the steerable launch mirror. The mode features are identified with temperature perturbation to develop the algorithm for real-time island location. The necessary parameters for the design of the control system are discussed and determined based on simulation of the nonlinear island growth of NTMs with application of EC current drive. Although similar work has been done on many other devices, this is the first attempt on EAST.

Real-time mirror steering for improved closed loop neoclassical tearing mode suppression by electron cyclotron current drive in DIII-D

The development and operation of the neoclassical tearing mode (NTM) avoidance and control system for DIII-D, which uses six sets of real-time steerable mirrors in order to move the electron cyclotron current drive (ECCD) deposition location in plasma, is described. The real-time DIII-D NTM control algorithm residing in the Plasma Control System (PCS) automatically detects an NTM by analysis of the Mirnov diagnostics, employs motional Stark effect (MSE) EFIT MHD equilibrium reconstruction to locate the rational q-surface where the NTM island can be found, then calculates the appropriate mirror position for alignment of the ECCD with the island using ray tracing. The control commands from PCS are sent to the electron cyclotron system to switch on and off or modulate the gyrotrons and to the steerable mirror system to move the steerable mirrors to the requested positions. Successful NTM suppression has been achieved in DIII-D using this control system to rapidly align the NTM island and the ECCD deposition location, and to actively maintain the alignment as plasma conditions change.

Fluid equations in the presence of electron cyclotron current drive

Two-fluid equations, which include the physics imparted by an externally applied radiofrequency source near electron cyclotron resonance, are derived in their extended magnetohydrodynamic forms using the formalism of Hegna and Callen [Phys. Plasmas 16, 112501 (2009)]. The equations are compatible with the closed fluid/drift-kinetic model developed by Ramos [Phys. Plasmas 17, 082502 (2010); 18, 102506 (2011)] for fusion-relevant regimes with low collisionality and slow dynamics, and they facilitate the development of advanced computational models for electron cyclotron current drive-induced suppression of neoclassical tearing modes.

Optimization simulations of electron cyclotron wave heating and current drive in experimental advanced superconducting tokamak

A large number of simulations of electron cyclotron heating and current drive (ECCD) were carried out using TORAY before the installation of an electron cyclotron wave system with a maximum of 4 MW power and 140 GHz frequency on the experimental advanced superconducting tokamak (EAST). The findings that the range of allowable launch angles is about 50° poloidal and about 60° toroidal, when all the plasma areas of interest are covered, determine the allowable steering angles of the launch mirror under a power deposition rate not less than 0.98. The variable range of the integrated driven current is larger at a radius of 0.5a–0.6a than at other radii. The best scheme for core localized heating is a launch from Z = 30 cm (the vertical mirror position). From an analysis of the optimum figures of merit of the ECCD, it is found that a launch from Z = 30 cm is the best for the suppression of neoclassical tearing modes.

Consequences of plasma rotation for neoclassical tearing mode suppression by electron cyclotron current drive

In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term ΔCD′. We investigate oscillations in ΔCD′ originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows that the net result of the oscillations in ΔCD′ is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island.

Integrated modelling of island growth, stabilization and mode locking: consequences for NTM control on ITER

Full suppression of neoclassical tearing modes (NTMs) using electron cyclotron current drive (ECCD) should be reached before mode locking (stop of rotation) makes suppression impossible. For an ITER scenario 2 plasma, the similar time scales for locking and island growth necessitate the combined modelling of the growth of the mode and its slow down due to wall induced drag. Using such a model, the maximum allowed latency between the seeding of the mode and the start of ECCD deposition and maximum deviation in the radial position are determined. The maximum allowed latency is determined for two limiting models for island growth; the polarization model with wmarg = 2 cm, representing the worst case, and the transport model with wmarg = 6 cm, representing the best case. NTMs with seed island widths up to 9.5 cm and 12 cm for the 2/1 and the 3/2 NTM, respectively, are suppressible. The maximum allowed latency is 1.05 s and 2.95 s for the 2/1 and 3/2 NTM, respectively, for the worst case model. Radial misalignment should not exceed 7–10 mm for the 2/1 NTM and 5–16 mm for the 3/2 NTM depending on the model for island growth. As long as the alignment suffices, it does not reduce the maximum allowed latency. Mode locking has serious implications for any real-time NTM control system on ITER that aims to suppress NTMs by ECCD.

Status of Resonant Diplexer Development for high-power ECRH Applications

Characteristics of ring resonator diplexers for high-power ECRH are briefly reviewed. Commissioning experiments performed on ASDEX Upgrade with the diplexer Mk IIa are presented, which demonstrate slow and fast switching of the power between two launchers, and thus the capability for efficient suppression of neoclassical tearing modes and simultaneous central heating of the plasma. The development of the compact diplexer Mk IIIb is discussed, and test results are presented. Finally, an evacuated design for 170 GHz is shown. © Owned by the authors, published by EDP Sciences, 2012.

Recent Progress of 2MW 140GHz ECRH System on HL-2A

In order to provide more capability of physics study for high-performance plasma, such as current profile control, neoclassical tearing modes suppression, transport study and so on, a new 2MW/140GHz/3s second-harmonic ECRH system with X-mode injection is on schedule on HL-2A. The total power of this system is 2MW and the pulse duration is 3 sec, which is generated by two gyrotrons manufactured by GYCOM. Two evacuated Φ63.5mm transmission lines were used to propagate these two 1MW/3s wave beams. A fast steerable launcher has been designed and installed on the HL-2A tokamak to inject four beams with narrow beam width and enable continuous beam scanning in toroidal and poloidal directions independently.

Dynamical modelling of neoclassical tearing mode suppression by ECCD

The rotation of a magnetic island through the EC power deposition region in combination with a finite collisional time scale on which the EC power is converted into current are shown to affect the NTM stabilization by ECCD. In particular, the stabilizing term from localized ECCD in the generalized Rutherford equation oscillates at the island rotation frequency. As a consequence, also the island width oscillates at the rotation frequency. When the rotation frequency is reduced, these oscillations increase in amplitude and result in a net increase of the time averaged stabilizing effect of ECCD.

ECE for NTM control on ITER

Control of Neoclassical Tearing Modes (NTMs) requires an accurate and low latency detection of the mode position. For a burning H-mode ITER plasma, simulations are conducted for both ECE detected via the equatorial port plug and along the line-of-sight of the ECCD launchers. Simulated ECE is detected using synthetic radiometers, with settings chosen to meet the required accuracy. A video bandwidth of 2 kHz is used which allows for an intermediate frequency bandwidth of BIF = 400 MHz for ECE detected via the equatorial port plug. For ECE detected via the ECCD line-of-sight, an intermediate fre- quency bandwidth of 1.5 GHz and 1 GHz for the 2/1 and 3/2 NTM respectively suces for accurate location detection. For both ECE systems, the latency re- quirements for NTM suppression are fullled.

Requirements on localized current drive for the suppression of neoclassical tearing modes

A heuristic criterion for the full suppression of an NTM was formulated as ηNTM ≡ jCD,max/jBS ⩾ 1.2 (Zohm et al 2005 J. Phys. Conf. Ser. 25 234), where jCD,max is the maximum in the driven current density profile applied to stabilize the mode and jBS is the local bootstrap current density. In this work we subject this criterion to a systematic theoretical analysis on the basis of the generalized Rutherford equation. Taking into account only the effect of jCD inside the island, a new criterion for full suppression by a minimum applied total current is obtained in the form of a maximum allowed value for the width of the driven current, wdep, combined with a required minimum for the total driven current in the form of wdepηNTM, where both limits depend on the marginal and saturated island sizes. These requirements can be relaxed when additional effects are taken into account, such as a change in the stability parameter Δ′ from the current driven outside the island, power modulation, the accompanying heating inside the island or when the current drive is applied preemptively. When applied to ITER scenario 2, the requirement for full suppression of either the 3/2 or 2/1 NTM becomes wdep ≲ 5 cm and wdepηNTM ≳ 5 cm in agreement with (Sauter et al 2010 Plasma Phys. Control. Fusion 52 025002). Optimization of the ITER ECRH Upper Port Launcher design towards minimum required power for full NTM suppression requires an increase in the toroidal injection angle of the lower steering mirror of several degrees compared with its present design value, while for the upper steering mirror the present design value is close to the optimum.

Modeling of Electron Cyclotron Heating in Large Tokamaks and ITER with the 3-D Full-Wave Code STELEC

We present modeling results of basic electron cyclotron heating (ECH) scenarios in several tokamaks and ITER performed with the most recent version of the three-dimensional (3-D) full-wave STELEC code (stellarator ECH, including tokamaks as a special case). This code includes all basic wave physics such as interference, diffraction, wave tunneling, mode conversion to electron Bernstein waves at the upper hybrid resonance (UHR), and appropriate boundary conditions. The code solves the wave equations in real 3-D magnetic geometry and thanks to the use of massive parallel teraflop computers, it is the first to provide full-wave solutions of the problem in toroidal plasmas. Several important new results are thus obtained that cannot be predicted with codes based on ray-tracing techniques, such as the influence of diffraction effects and the importance of the UHR for both X- and O-mode antenna excitation at fundamental harmonic. This last result also shows that the so-called “O and X” modes are coupled solutions. The coupling of these modes, partly supported by experiments in the DIII-D tokamak showing similar heating efficiencies for both radiated modes, leads to different power deposition profiles and spatial distribution, compared to results from ray-tracing codes. Coupling between the O-mode and the X-mode (launched at the low-field side) reveals the importance of electron Bernstein waves in ECH calculations for high-density ITER plasmas. These results not only could influence the predictions for neoclassical tearing mode suppression for ITER using electron cyclotron current drive but could also lead to important simplifications in ECH hardware (converters, polarization, etc.) and to a reduced cost of the ECH system on ITER. The code also allowed investigation of the urgent issue of the O-X-B ECH scenario for overdense tokamak/stellarator plasmas.