Disruptive Instability Research Articles

Disruptive instabilities in the TEXT-U tokamak

Disruptive instabilities in TEXT-U circular plasmas have been investigated. Hugill diagrams have been constructed for some single and consecutive disruptive shots, and the results indicated that the density limit boundary did not depend solely on plasma impurity content. Since the average radiated power intensity was typically only 30% of the input power, it is unlikely that major disruptions resulted from strong edge cooling and subsequent contraction of the plasma column. However, strong indications are found that a coupling between the m=1 and m=2 MHD modes could be the main triggering mechanism for the disruptions analysed, independently of where they occur in the Hugill operating space. After a major disruption has been triggered, the plasma current was observed to decay on a time-scale that varied considerably from one plasma discharge to another. This discrepancy was not found to depend, at least directly, on average quantities such as Zeff, plasma current, stored energy or plasma density. Finally, the installation of a set of locked mode detecting coils, for measuring the time derivative of the radial magnetic field associated mainly with the growth of the m=2 magnetic island, made it possible to observe that both minor and major disruptions were almost always preceded by a growth in Br. In particular, for some plasma discharges with long duration precursors, the characteristic fluctuations in the Mirnov coil signals and the soft X ray sawtooth oscillations are observed to disappear, which could suggest that a complete mode lock occurs before the disruption takes place. However, detailed investigation showed that the MHD modes were still rotating in the toroidal direction but with a very low frequency of about 50 Hz

Measurements of non-axisymmetric halo currents with and without ‘killer’ pellets during disruptions in the DIII-D tokamak

Non-axisymmetric halo currents are always observed during disruptive instabilities in DIII-D. These halo currents appear to have a helical structure which rotates toroidally in the electron current drift direction with frequencies ranging between 200 and 400 Hz prior to and during the initial plasma current quench phase of the disruption. Sometimes the halo current rotation locks at random toroidal phase angles during the plasma current quench. The total halo current rarely exceeds 30% of the pre-disruptive plasma current ( I po ) and peak-to-average toroidal peaking factors (TPF) are usually less than 3 during most disruptions. Neon ‘killer’ pellets have proven very effective in reducing both the total halo amplitude, often by as much as 50%, and the TPF from ∼ 3 to ∼ 1.2.

Radiative plasmas in TdeV

The main aspects of radiative plasma experiments carried out in the LH-heated TdeV tokamak are summarized. D 2, N 2 and Ne puffing, and divertor biasing, are assessed in terms of their ability to increase edge and divertor radiative losses and reduce neutralizer plate heat loads. Detailed patterns of radiation and plate power deposition are compared. Combined N 2 and D 2 puffing gives the best results. Biasing enhances divertor radiation but also causes an increase in power conducted and convected to one of the divertor legs. The improvements obtained are still insufficient due to disruptive instabilities that appear at high impurity puffing rates. The instability is triggered at the edge with N 2 and in the core with Ne.

Instabilities of a family of oblate stellar spheroids

We have examined the stability of a sequence of oblate elliptical galaxy models having the Stackel form suggested by Kuz'min & Kutuzov. We have employed the 2-integral DFs given by Dejonghe & de Zeeuw for which flattened non-rotating models are characterized by counter-streaming motion and are radially cool; we introduce net rotation in some models by changing the sign of the z-component of angular momentum for a fraction of the particles. We have found that all non-rotating and slowly rotating members of this sequence rounder than E7 are stable, and that even maximally rotating models rounder than E4 are stable. In the absence of strong rotation, the most disruptive instability, and the last to be stabilized by increasing thickness, is a lopsided (m=1) mode. This instability appears to be driven by counter-rotation in radially cool models. Its vigour is lessened as rotation is increased, but it remains strong even in models with net angular momentum 90% of that of a maximally rotating model before finally disappearing in maximally rotating models. Strongly rotating models are more unstable to bar-forming modes which afflict maximally rotating models with c/a >~ 0.5, but this mode is quickly stabilized by moderate fractions of counter-rotating particles. Bending instabilities appear not to be very important; they are detectable in the inner parts of the flatter models, but are less vigorous and more easily stabilized than the lop-sided or bar modes in every case. We briefly discuss the possible relevance of the lop-sided instability to the existence of many lop-sided disk galaxies.

Observation of disruptions in tokamak plasma under the influence of resonant helical magnetic fields

Disruptive instabilities were investigated in the small-tokamak TBR-1 during the application of resonant helical magnetic fields created by external helical windings. Indications were found that the main triggering mechanism of the disruption was the rapid increase of them=2/n=1 mode which, apparently after reaching a certain amplitude, interacts with other resistive modes: the internal 1/1 mode in the case of a major disruption, or with higherm components, as the 3/1 or 4/1, in the case of minor disruptions. After the coupling, the growth of the associated islands would create a chaotic field line distribution in the region between the corresponding rational magnetic surfaces which caused the gross particle transport and, finally, destroyed the confinement. In addition, investigations on higherZeff discharges in which a mixture of helium and hydrogen was used resulted in much more unstable plasmas but apparently did not alter the basic characteristics of the disruptions.

Size Optimization Studies for a Stellarator Reactor

Stellarators are steady state, have an absence of disruptive instabilities, have low recirculating power, and are natural divertors - all of which are intrinsic properties that make stellarators especially attractive as fusion reactors. The question is addressed of the minimum site requirement for a stellarator reactor, independent of the specific configuration chosen to optimize physics and technology aspects. A one-dimensional model is used to deduce by postulating specific plasma profiles the power balance between alpha-particle heating, radiation, and conductive losses in the plasma and to determine the minimum site compatible with the level of output power of the reactor and the operational limits due to plasma confinement, pressure, and density. Also considered is the influence on stellarator reactor size requirements of particle accumulation and of the presence of impurities in the plasma. Additionally, with regard to practical realization of the device, the limitations of wall power deposition and device aspect ratio are considered. Available stellarator reactor designs are reviewed based on these results. 17 refs., 14 figs.

Interstellar clouds in high-speed, supersonic flows: Two-dimensional simulations

We present a series of gasdynamical simulations of the interaction of a dense, cool interstellar cloud with a high-speed, supersonic wind that confines and accelerates the embedded cloud. Our goal is to attempt to determine if such clouds can survive various potentially disruptive instabilities, that occur at their peripheries, long enough to be accelerated to speeds which are comparable to the wind velocity. These simulations are performed using two-dimensional, Eulerian gas dynamics on both an axisymmetric (about the cloud axis) and 'slab' geometric grid. The spatial and temporal resolutions of the simulations are varied over a wide range to investigate the effects of small-scale instabilities on the overall acceleration of clouds and the development of large-scale, disruptive instabilities. Also, we study the effects of wind/cloud Mach number variations by changing the wind speed constant at about 12 km/s (which corresponds to a cloud temperature of 10,000 K). The current simulations track the evolution of clouds as they are accelerated to speeds approximately 4-5 times greater than their internal sound speeds. Furthermore, the models with the highest resolution were extended far beyond quasi-linear Rayleigh-Taylor growth times reaching 6-7 Rayleigh-Taylor growth times for the largest scale instabilities before being terminated because of the accumulation of errors at the rear grid boundary.

Modelling of transient electromagnetics in Tokamaks during off-normal conditions

During plasma disruption events in Tokamaks, a considerable amount of magnetic and thermal energy is associated to the transfer of plasma current into eddy and Halo currents. In this paper, a predictive numerical modelling is described concerning plasma-wall interactions during disruptive instabilities. Preliminary results are presented, giving an estimation of heat transfer interaction with electromagnetic phenomena. Eddy currents and heat deposition increase significantly with decreasing disruption time. An estimation of the order of magnitude of Halo currents and associated forces on plasma-facing conducting components is also presented.

The Thomson scattering systems of the ASDEX upgrade tokamak

The Thomson scattering system of the ASDEX upgrade (AUG) tokamak is described. One of the main objectives of AUG is to investigate plasma wall interaction in reactor relevant discharges with a magnetic divertor. The very successful Nd:YAG scattering system developed for its predecessor ASDEX, has been upgraded to give higher spatial and temporal resolution, reliability, and flexibility to different discharge conditions. The system consists of two independently operating devices, each using a cluster of six lasers: One measures the electron temperature and density along three possible vertical chords alternatively through the magnetic axis, or the inner or outer boundary layer; a second chord in the equatorial plane will always cover the magnetic center even in the case of considerable Shafranov shifts. An additional compact spectrometer has been designed for measurements with high radial resolution in the equatorial plane across the separatrix. A third system, using the laser beams for the vertical arrangement once again, has been designed for profile measurements in the energy deposition zone 2 cm above the outer divertor plate. Each laser is run at a repetition rate of 20 Hz and 1 J per pulse. A variety of synchronization modes are available, e.g., 20 Hz/6 J, 120 Hz/1 J etc., or repetitive bursts at 20 Hz. In this case the minimum delay between two pulses is presently limited to ≊30 μs by the existing data acquisition. This mode will be used for investigating fast phenomena such as sawteeth or disruptive instabilities. During the time intervals between the laser pulses the bremsstrahlung radiation (line integral) will be measured by the Thomson scattering detection system to calculate Zeff.

Density limit investigations on ASDEX

Density limit investigations on ASDEX have been performed under a variety of conditions: ohmically heated and neutral injection heated plasmas in H2, D2 and He have been studied in different divertor configurations, after various wall coating procedures, with gas puff and pellet fuelling, and in different confinement regimes with their characteristically different density profiles. A detailed description of the parametric dependence of the density limit, which in all cases is a disruptive limit, is given. This limit is shown to be a limit to the density at the plasma edge. Therefore, the highest densities corresponding to neRqa/Bt>30*1019 m-2.T-1 are obtained with centrally peaked ne profiles. Radiation from the main plasma at the density limit is always significantly below the total input power. The plasma disruption is due to an m=2 instability which for medium and high qa is preceded by one or more minor disruptions. In this range of qa, the disruptive instability is initiated by the occurrence of a Marfe on the high field side as a consequence of strong plasma cooling in this region. The duration of the Marfe increases with increasing distance between the plasma edge and the q=2 surface. After penetrating onto closed flux surfaces the Marfe leads to a current contraction and a subsequent destabilization of the m = 2 mode. In helium plasmas a strongly radiating, poloidally symmetric shell is observed before the density limit instead of a Marfe. An instantaneous destabilization of this mode is observed at low qa. Detailed measurements of plasma edge and divertor parameters close to the density limit indicate the development of a cold, dense divertor plasma before the disruption. Models describing the scrape-off layer and the divertor region predict an upper limit to the edge density at low divertor temperatures according to power balance considerations. Their relations to the experimental findings, especially the low field side cooling, ar

Application of laser-produced plasmas to a heliotron/torsatron device, SHATLET-M.

An l=2, m=12 modular-coil heliotron/torsatron device, SHATLET-M, has been filled with plasmas produced by the irradiation of an intense pulsed CO2 laser beam on a tiny freefalling deutrium pellet. Plasma parameters, namely, line-integrated density, spatially averaged beta value, their decay constants, and plasma currents are measured. The size of the deutrium pellets is in typical cases 0.2mm in diameter and 1mm in length. The pellets are irradiated by a CO2 laser pulse with a total energy of about 300J and a pulse width of 1μs. Within 10 μs, a toroidal plasma is rapidly formed that has a small ititial toroidal plasma current with a peak value of less than 150A. This initial toroidal current is rapidly damped in about 20μs leaving a net-currentfree toroidal plasma for another 200-400 μs. The diamagnetic loop measurement shows that the initial peak value of spatially averaged beta which may contain non-thermal components ranges from 3 to 6%, where the average densety is about 1×1013cm-3. The density increases in proportion to B1.8±0.2 (B is the magnetic field strength.), and the maximum averaged beta increases about in proportion to B. The decay time constants of the averaged beta values are less than about 100μs and no disruptive instability has been observed within these periods.

ERCH in low q plasma in T-10. The first plasma in T-15

The energy confinement time under microwave heating for various q values in T-10 linearly rises with the plasma density and with increase in the plasma column radius. At a given ECRH-power the energy increment in the plasma practically does not depend on the magnitude of the current, and the confinement time can be represented in the form: tau E= tau E.OH(POH/Ptot)alpha . The disruption study has shown that the development of an internal, minor or major disruption is accompanied by poloidal field perturbations, along with an ordinary MHD-activity at higher frequencies. An attempt to produce a device signalling the beginning of the disruptive instability development, in order to prevent it by external effects, has been made. The results of complex tests of the T-15 systems are given.

Resonant helical divertor experiments in ohmic and auxiliary heated JIPP T-IIU plasmas

A series of initial resonant helical divertor (RHD) experiments have been carried out in ohmically and auxiliary heated JIPP T-IIU plasmas. Disruptive and MHD instabilities make the interpretation of the RHD results difficult but an apparent increase in the energy confinement time is observed when the helical magnetic perturbation is applied. This may be due to the suppression of MHD activity or to a reduction in the edge convective heat losses. Magnetic island effects have been observed on the floating potential of a Langmuir probe array and energy scrape-off layer widths have been measured with and without helical perturbations during ICRF operation. Basic pump limiter data is presented including ion temperatures and C 4+ impurity profiles. Energy confinement times are reported in ohmically and NBI heated discharges.

Disruptive instabilities in the discharges of the TBR-1 small Tokamak

Minor and major disruptions as well as sawteeth oscillations (internal disruptions) were identified in the discharges of the small Tokamak TBR-1, and their main characteristics were investigated. The coupling of a growing m=2 resistive mode with an m=1 perturbation seems to be the basic process for the development of a major disruption, while the minor disruption could be associated with the growth of a stochastic region of the plasma between the q=2 and q=3 islands. Measured sawteeth periods were compared with those predicted by scaling laws and good agreement was found. The time necessary for the sawteeth crashes also agrees with the values expected from Kadomtsev's model. However, there are some sawteeth oscillations, corresponding to conditions of higher plasma Zeff, which showed longer crashes and could not be explained by this model.

Stability of narrow emission line clouds in active galactic nuclei

view Abstract Citations (29) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stability of Narrow Emission-Line Clouds in Active Galactic Nuclei Mathews, William G. ; Veilleux, Sylvain Abstract Clouds in the narrow-line region of active galaxies can be significantly eroded or destroyed by a variety of instabilities. Narrow-line clouds having column densities N <~ 10^22^ cm^-2^, at rest and in pressure equilibrium with an intercloud medium, are intrinsically unstable because the momentum transferred by continuum radiation is absorbed and raises the internal pressure above the ambient value. This results in a lateral flow of gas away from the sides of the clouds which can erode or ultimately destroy clouds before they reach desired velocities. Lateral flow instabilities can sharply limit the lifetime of clouds even though the radiation absorbed is otherwise dynamically insignificant. If these same clouds are pushed by a wind in the absence of radiation forces, they are quickly destroyed by Rayleigh-Taylor instabilities. Ironically, when radiation and wind forces act together, the clouds can be effectively stabilized against both lateral flow disruption and Rayleigh-Taylor instabilities, even when the dynamical pressure on the accelerating clouds greatly exceeds their internal thermal pressure. Stable clouds with columns N >~ 10^23^ can be accelerated to observed narrow-line velocities before the lateral flow instability becomes important. Very thick clouds having N >~ 10^24^ are not in hydrostatic equilibrium and may be too large to fit into narrow-line regions having modest or small covering factors. Clouds with column densities N ~ 10^23^ are the most likely candidates for the narrow-line region; they can be accelerated without appreciable disruption to observed velocities in just a few radial sound crossing times. Winds in the narrow-line intercloud medium provide the best means of rapid cloud acceleration. Narrow-line clouds falling radially inward under the influence of gravity are unable to reach appropriate velocities before the disruptive instabilities set in. Clouds in the narrow-line region, like their broad-line cousins, are more fragile and transient than was formerly thought. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/166996 Bibcode: 1989ApJ...336...93M Keywords: Active Galactic Nuclei; Emission Spectra; Line Spectra; Molecular Clouds; Taylor Instability; Gas Flow; Hydrodynamics; Interstellar Gas; Seyfert Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: SEYFERT; HYDRODYNAMICS; INSTABILITIES full text sources ADS |

Dynamic analysis of the TFTR bumper limiter

Of the many components and systems comprising a fusion reactor like the Tokamak Fusion Test Reactor (TFTR), few, if any, are subjected to more extreme and severe conditions than the protective armor for the interior vacuum vessel wall. The primary TFTR limiter, referred to as the Phase II bumper limiter, provides protection of the inner vacuum vessel wall from neutral beam shine through, normal plamsa loads, and major disruptive instabilities. The highly transient nature of forces and temperatures induced by these conditions produces significant dynamic responses in various parts of the bumper limiter structural system. This paper addresses the incorporation of a finite element dynamic analysis in the study of bumper limiter behavior when subjected to several electromagnetic load scenarios. These loads, due to a variety of realistic plasma conditions, were used in the interactive design and optimization of limiter components and support locations and flexibilities which enable the structure to perform within established criteria.

Dynamic analysis of the TFTR bumper limiter

Of the many components and systems comprising a fusion reactor like the Tokamak Fusion Test Reactor (TFTR), few, if any, are subjected to more extreme and severe conditions than the protective armor for the interior vacuum vessel wall. The primary TFTR limiter, referred to as the Phase II bumper limiter, provides protection of the inner vacuum vessel wall from neutral beam shine through, normal plamsa loads, and major disruptive instabilities. The highly transient nature of forces and temperatures induced by these conditions produces significant dynamic responses in various parts of the bumper limiter structural system. This paper addresses the incorporation of a finite element dynamic analysis in the study of bumper limiter behavior when subjected to several electromagnetic load scenarios. These loads, due to a variety of realistic plasma conditions, were used in the interactive design and optimization of limiter components and support locations and flexibilities which enable the structure to perform within established criteria.

The Pulsator tokamak

Pulsator was operated by IPP Garching from 1973 to 1979. It made two important contributions to international tokamak research. 1. In Pulsator, for the first time a tokamak plasma was influenced locally by an external, resonant, helical field. The accompanying formation of islands on magnetic surfaces, which affects transport and stability, provided the starting point for experimental investigation of the disruptive instability and its theoretical description. 2. By gas puffing during the discharge it was possible to attain a high-density regime which dramatically raised the then prevailing nτE values by almost two orders of magnitude. In this high-density regime an inward directed particle drift velocity and, as a consequence, under certain conditions an accumulation of impurities in the plasma core was found.

THE MEASUREMENTS OF SOFT X-RAY FLUCTUATIONS ON THE HT-6B TOKAMAK DEVICE

In this article the construction of measurement system on IIT-6B Tokamak which is highly sensitive to soft X-ray fluctuation is described and the primary results presented. When the average electron temperature Te is less than 100 eV, the wave shapes of typical soft X-ray fluctuations, the disruptive instabilities, the MHD disturbances, and some other interesting phenomena have been observed.

Economic ‘Stagflation’ and Public Support for the Political System

Declining trust in politicians and political institutions is one of the most dramatic and well-documented trends in American public opinion. Confidence in religious, educational and other institutions has also waned, but emphasis has focused on diminished political trust, both because it may summarize a wide range of diffuse grievances and because it might indicate an increased potential for disruptive action, political violence and instability. In the decade from 1968 to 1978, the level of political trust (measured by the conventional five-item CPS/NES index) was halved, the proportion of the public expressing moderate or high levels of trust falling from 64 to 33 per cent. The greatest decline in the index level (a drop of 14 points) occurred between 1972 and 1974.