MHD Modes Research Articles

The Tore Supra Front Steering Long Pulse ECRH Antenna. Past Experience - Present Evolution - Future Experiments

The long pulse capability of Tore Supra and its ECRH system makes it an ideal machine to prove steady state feedback control as required in ITER. Although Neoclassical Tearing Modes (NTMs) have not yet been observed on Tore Supra, the control of other MHD modes represents a very similar task from a control point of view and the stabilisation of such modes for long periods using ECRH will provide essential experience for the implementation of such control schemes on ITER. For this work to progress on Tore Supra, it must be possible to vary the injection angles in real time under feedback control from measured plasma parameters. At Tore Supra the front mirror position - and hence the injection angles - is adjusted using stepper motors controlled through a serial link. The use of a serial link limiting the sampling time for the control system to 50-100 ms and the dynamic response of the stepper motors results in a system frequency response <5 Hz. For following the evolution of the current profile this seems fully acceptable though it could prove somewhat slow for reacting to fast beta changes. In any case the time constants typically associated with growth rate of NTMs on ITER would not require a significantly faster system. Initial commissioning of such real time control was performed in 2004. Unfortunately, following a limited number of cycles and prior to using the system for plasma experiments, one water bellows in the antenna ruptured indicating a major weakness in the design of the water-cooling system for the antenna. As a consequence the water-cooling of the mobile mirror has been redesigned. Careful calculations and subsequent tests have been used to optimise the trajectories of the flexible water connections and a more robust layout is being implemented, ready for operation in the summer 2005.

Observation of Internal Structure of Edge MHD Modes in High Beta Plasmas on the Large Helical Device

Observation of Internal Structure of Edge MHD Modes in High Beta Plasmas on the Large Helical Device

Detection of ultra-long-period oscillations in an EUV filament

We report the first detection of long-period (8-27 h) oscillatory intensity variations in a coronal filament. The filament is observed continuously as it crosses the solar disk in a 12-min-cadence SoHO/EIT 195 A uninterrupted data set. Cyclic intensity variations are found to be correlated along the filament, while the most pronounced oscillations are detected at its southern end for nearly 6 days. The dominant period of these oscillations is 12.1 h and the amplitude of the intensity variations reaches approximately 10% of the background intensity. The ultra-long-period oscillations may be interpreted in terms of slow string MHD modes or may be connected with thermal over-stability associated with peculiarities of the cooling/heating function and with the effect of neutrals. These theoretical predictions however do not explain the spatial structure of the oscillations along the filament.

On the use of MHD mode analysis as a technique for determination of q-profiles in JET plasmas

JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis, and determination of mode radial position has the potential as a diagnostic for the plasma q profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pickup signals. Cross correlation of magnetic and multiarray signals from ECE, SXR, and reflectometer systems provide the mode localization. The present status of the diagnostic systems used for fluctuation analysis is summarized, with particular emphasis on the fast magnetic pickup system. A new data collection system allows the magnetic signals to be recorded at 2 MHz for up to 32 s in each pulse. An example of q-profile validation from the observation of Alfven eigenmode cascades and a snake is given. At JET, Alfven cascades are used to determine qmin in optimized shear discharges, while a variety of low frequency modes (such as the snake) allow the determination of the radial position of q integer values in monotonic and reversed shear q profiles.

Application of the two-dimensional Newcomb problem to compute the stability matrix of external MHD modes in a tokamak

The theory of the Newcomb equation has been applied to low-n external modes in a tokamak and a method has been developed for computing the stability matrix that gives the change of plasma potential energy due to external modes in terms of the surface values of the perturbations. By using this method, the spectral properties of the ideal external modes have been elucidated, such as the coupling between external modes and internal modes, and the difference in the stability properties between a normal shear tokamak and a reversed shear tokamak. These results will also be useful in the stability analysis of resistive wall modes.

Fast MHD oscillations in line-tied homogeneous coronal loops

Loop oscillations have been abundantly reported in recent years. Earlier analytical studies of loop oscillations con- sider freely propagating waves, allowing for line-tying by a quantization of the wavenumber. Here we consider the rich spectrum of fast MHD modes (both standing and leaky) in coronal loops, allowing for line-tying and performing some comparisons with observational data. We point out that in a straight and homogeneous cylindrical flux tube there should be observational signa- tures of the excitation of higher order harmonics. Our results indicate that these modes become leaky with the addition of the chromospheric structure at the base of the loop. Leakage can be quite efficient in damping the oscillations for many of these high frequency (compared to fundamental) modes.

The Singular Value Decomposition as a Tool of Investigating Central MHD Instabilities in the HL-1M Tokamak

A variety of strong MHD instabilities are always resulted from MHD activity of Tokamak plasmas. Central MHD instabilities can be observed with pinhole cameras to record soft x-ray (SXR) emission from the plasma along many chords with a high temporal resolution. The investigation of MHD instabilities often necessitates an analysis on spatial-temporal signals. The method of Singular Value Decomposition (SVD) can split such signals into orthogonal spatial and temporal vectors. By this means, the repetition time and the characteristic radius of various MHD phenomena such as sawteeth and snake-like perturbation can be obtained. Moreover, the (1,1) MHD mode is analyzed in great detail by SVD and used to determine the radius of the q = 1 surface.

Progress in real-time feedback control systems in RFX

Major modifications of the RFX load assembly and power supplies are in progress to allow extensive active control schemes, such as equilibrium and plasma position control and innovative control of the MHD modes. The digital control system is implemented in VME64 using a distributed architecture. The use of a ‘stable’ operating system that is likely to survive some generations of processors can help coping with evolution of technology. PowerPC and Pentium processors were thus considered as candidates and tested and the first one has been selected due to the better performance in floating point computation. Wind River VxWorks has been chosen as real-time operating system. 100 Mbit switched Ethernet has been evaluated for real-time communication by using the user datagram protocol (UDP). Measurements have been executed on a prototype system to assess data transfer latency, jitter and reliability and the results confirm that the solution is suitable for the application. The paper describes in detail the reasons for the choice in the hardware components. Results from several tests comparing the performance of different solutions are also provided.

MHD Instabilities and Their Effects on Plasma Confinement in Large Helical Device Plasmas with Intense Neutral Beam Injection

MHD stability of the Large Helical Device (LHD) plasmas produced with intense neutral beam injection is experimentally studied. When the steep pressure gradient near the edge is produced through L-H transition or linear density ramp experiment, interchange-like MHD modes whose rational surface is located very close to the last closed flux surface are strongly excited in a certain discharge condition and affect the plasma transport appreciably. In NBI-heated plasmas produced at low toroidal field, various Alfvén eigenmodes are often excited. Bursting toroidal Alfvén egenmodes excited by the presence of energetic ions induce appreciable amount of energetic ion loss, but also trigger the formation of internal and edge transport barriers.

High frequency MHD activity related to type I ELMs in ASDEX Upgrade

High frequency coherent magnetic fluctuations related to the type I edge localized mode (ELM) cycle have been found in ASDEX Upgrade. Typical frequencies are in the range 300–500 kHz, and the poloidal mode number, m, measured at the outboard midplane, is estimated to be 10–15. These MHD modes are present in many low density (low collisionality) discharges, while they have not been found in high density ones yet. New findings on already known low frequency type I ELM precursors confirm that in ASDEX Upgrade type I ELM mode precursors are normally phase locked to the wall in discharges with NBI heating parallel to and in the direction of the plasma current (co-injection) and can be seen with a non-zero rotation frequency only in shots with anti-parallel NBI (counter-injection). Additional information is given by ECE data. The existence of low and high frequency MHD events suggests an interpretation in the framework of the peeling–ballooning model. The high frequency magnetic activity seems to be related strictly to the increase in (and eventual saturation of) edge electron temperature, while low frequency modes are related directly to the ELM crash itself.

The frequently interrupted regime of neoclassical tearing modes (FIR-NTMs): required plasma parameters and possibilities for its active control

The plasma parameters required for the occurrence of (3, 2) FIR-NTMs and thus good confinement properties at high normalized plasma pressure (βN) are investigated. Both on ASDEX Upgrade and on JET it is found that, with remarkable consistency between the devices, for a variety of plasma parameters in conventional current profile scenarios, this regime establishes itself for discharges with sufficient high normalized plasma pressure at mode onset, βN,onset > 2.3. An active triggering of this regime has been achieved by lowering the magnetic shear at the q = 4/3 rational surface. That way an ideal (4, 3) MHD mode is being destabilized, forcing the required three-wave coupling between the (3, 2) NTM, (1, 1) and (4, 3) mode activity.

Advances of Reflectometry on Tore-Supra: From Edge Density Profile to Core Density Fluctuations

A new generation of broadband reflectometers based on solid state components has been installed on Tore-Supra. With reflectometers covering the range 50 to 155 GHz, the whole plasma can be scanned. Two X-mode reflectometers (V and W band) are dedicated to electron density profile measurements. Diagnostics are operated routinely with an automatic algorithm to reconstruct the density profile from shot to shot. A fast acquisition mode is available to study short time plasma evolution. For turbulence and transport studies, a third reflectometer operating between 105 and 155 GHz measures density fluctuations in the plasma centre. Sensitive to large scales, it can retrieve density perturbation due to MHD modes or broadband turbulence. Lastly, a Doppler reflectometer, based on back scattering, is being installed for measuring the poloidal rotation and fluctuations amplitude at higher wave numbers.

MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L–H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ⩽ 0.75 T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the rational surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L–H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfvén eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ⟨βb∥⟩, and the ratio of energetic ion velocity to the Alfvén velocity, Vb∥/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

Twofold effect of Alfvén waves on the transverse gravitational instability

This paper is devoted to the study of the gravitational instability of a medium permeated by a uniform magnetic field along which a circularly polarized Alfven wave propagates. We concentrate on the case of perturbations purely transverse to the ambient field by means of direct numerical simulations of the MHD equations and of a linear stability analysis performed on a moderate amplitude asymptotic model. The Alfven wave provides an extra stabilizing pressure when the scale of perturbations is sufficiently large or small compared with the Jeans length $L_J$. However, there is a band of scales around $L_J$ for which the Alfven wave is found to have a destabilizing effect. In particular, when the medium is stable in absence of waves, the gravitational instability can develop when the wave amplitude lies in an appropriate range. This effect appears to be a consequence of the coupling between Alfven and magnetosonic waves. The prediction based on a WKB approach that the Alfven wave pressure tensor is isotropic and thus opposes gravity in all directions is only recovered for large amplitude waves for which the coupling between the different MHD modes is negligible.

Preferential Heating of Particles by H-Cyclotron Waves Generated by a Global Magnetohydrodynamic Mode in Solar Coronal Holes

Preferential heating of particles by H-cyclotron waves is investigated to explain the recent measurements of anisotropic heating of ions in solar coronal holes. First, the dispersion relation of H-cyclotron waves generated by a global MHD mode in the solar coronal hole is numerically studied. Then, heating of H, O+5, and Mg+9 due to the resonant and nonresonant dissipations of H-cyclotron waves is quantitatively studied. It is shown that H-cyclotron waves can be excited at a frequency ωRe, H ≃ (2.3-2.4)Ω, which corresponds to ~(3.68-3.84)Ω or ~(3.10-3.24)Ω, and thus can heat O+5 through the fourth harmonic resonance and Mg+9 through the third harmonic resonance. Here Ωσ is the cyclotron frequency of particle species σ. The H-cyclotron waves are efficient at heating O+5 if ωRe, H ≃ 2.4Ω and Mg+9 if ωRe, H ≃ 2.3Ω. Protons can only be weakly heated by the H-cyclotron waves through the nonresonant dissipation. The amount of heating for H, O+5, and Mg+9 can reach the order of measurements. Furthermore, the harmonic resonances of O and Mg with other charge states and of other heavy ions with proper charge states are discussed.

Identification of the structure of large-scale MHD perturbations in a tokamak from Mirnov signals

An algorithm is proposed that allows one to identify the MHD mode structure in toroidal plasmas by processing signals from Mirnov probes measuring plasma MHD activity. The algorithm differs fundamentally from the diagnostic methods presently used in tokamaks, being simpler and more efficient. The algorithm is based on constructing an analytic signal using the Hilbert transformation of the Mirnov signals at a given instant. The phase and amplitude dependences obtained take into account the toroidal effects and allow one to determine the number and amplitude of the excited MHD mode. The algorithm was approbated with both test signals and actual signals from MHD diagnostics in the T-10 tokamak. It is demonstrated that the algorithm can be used to analyze single-mode MHD instabilities in toroidal plasmas.

Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ∼ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

ELM driven divertor target currents on TCV

There is currently both considerable interest in the physics of ELM transport in the scrape-off layer (SOL) and concern over the impact of the ELM power and particle loads on the divertor targets of future fusion reactors. This paper describes some experimental observations of the reaction of target floating potentials and currents during ELMs in TCV, relying principally on fast measurements of these parameters using tile embedded Langmuir probe arrays. Clear evidence is presented for rapid modifications in the local target floating potentials occurring long before the characteristic rise of hydrogenic excitation emission due to local recycling provoked by the arrival at the target of the ELM ion flux. This precursor activity appears to be synchronous with the growth of MHD modes in the main chamber. Simple conditional averaging is used to derive a ‘coherent ELM’ and thus generate the radial distribution of the current to probes held at the target potential. At some locations, the coherent ELM can also be used to estimate the time evolution of the local target electron temperature, density and power flux, even though these quantities are not directly measured. The time delays between the reactions of currents, floating potentials and derived temperature are consistent with the expections of recently published kinetic simulations of ELM energy transport down a one-dimensional SOL plasma. The strong potential variations observed during the ELM are the result of current flows at the targets. These currents are generally of opposite sign at inner and outer divertors and are thus consistent, at least in part, with a thermoelectric origin in which the driven current is produced by an in/out divertor temperature asymmetry near the strike point of nearly a factor 2, probably due to the generation of divertor asymmetries by the ELM heat pulse. Such asymmetries are commonly observed in low to medium density L-modes for the particular TCV magnetic equilibrium studied in this paper. The total current balance during the ELM is satisfied only to within a factor 2, so that, whilst some of the driven current flows parallel to field lines in the SOL, there is an apparent additional negative current to the inner divertor during the ELM whose origin remains unexplained. It might, however, be due, in part, to increased plasma–wall interaction in the main chamber during the ELM event.

Amplification of MHD waves in swirling astrophysical flows

Recently it was found that helical magnetized Hows efficiently amplify Altven waves (Rogava et al. 2003). This robust and manifold nonmodal effect was found to involve regimes of transient algebraic growth (for purely ejectional flows), and exponential instabilities of both usual and parametric nature. However the study was made in the incompressible limit and an important question remained open - whether this amplification is inherent to swirling MHD flows per se and what is the degree of its dependence on the incompressibility condition. In this paper, in order to clear up this important question, we consider full compressible spectrum of MHD modes: Alfven waves (AW), slow magnetosonic waves (SMW) and fast magnetosonic waves (FMW). We find that helical flows inseparably blend these waves with each other and make them unstable, creating the efficient energy transfer from the mean flow to the waves. The possible role of these instabilities for the onset of the MHD turbulence, self-heating of the flow and the overall dynamics of astrophysical flows are discussed.

Upgrade of the RFX fast protection system in view of the new operating scenarios and machine modifications

This paper deals with the upgrade and the revision of the RFX fast protection system, which follow the recent modifications to the power supply circuits and the installation of a new copper shell and of 192 saddle coils for MHD mode control. The experimental evidences and the new requirements due to the machine and circuit modifications, which called for the revision of the system, are described, together with its new architecture.