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The Theory of the Kink Mode during the Vertical Disruption Events in Tokamaks

Description: This paper explains the locked m/n = 1/1 kink mode during the vertical disruption event when the plasma has an electrical contact with the plasma facing conducting surfaces. It is shown that the kink perturbation can be in equilibrium state even with a stable safety factor q > 1, if the halo currents, excited by the kink mode, can flow through the conducting structure. This suggests a new explanation of the so-called sideway forces on the tokamak in-vessel components during the disruption event. __________________________________________________
Date: January 14, 2008
Creator: Zakharov, Leonid E.
Partner: UNT Libraries Government Documents Department

Stabilization of Sawtooth Oscillations by the Circulating Energetic Ions

Description: The influence of the well-circulating energetic ions on the ideal kink instability and semi-collisional tearing mode are studied. It is found that the precession of these ions can be a key factor that affects the instability: it can lead to the stabilization of the mentioned instabilities, the effect being weakly dependent on the direction of the injection. The developed theory is consistent with the experimental observations of the stabilization of sawtooth oscillations during the negative-ion-based neutral beam injection in JT-60U.
Date: May 26, 2004
Creator: Kolesnichenko, Ya.I.; Marchenko, V.S. & R.B.White
Partner: UNT Libraries Government Documents Department

Experimental Verification of the Kruskal-Shafranov Stability Limit in Line-Tied Partial Toroidal Plasmas

Description: The stability properties of partial toroidal flux ropes are studied in detail in the laboratory, motivated by ubiquitous arched magnetic structures found on the solar surface. The flux ropes studied here are magnetized arc discharges formed between two electrodes in the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys. Plasmas, 4, 1936 (1997)]. The three dimensional evolution of these flux ropes is monitored by a fast visible light framing camera, while their magnetic structure is measured by a variety of internal magnetic probes. The flux ropes are consistently observed to undergo large-scale oscillations as a result of an external kink instability. Using detailed scans of the plasma current, the guide field strength, and the length of the flux rope, we show that the threshold for kink stability is governed by the Kruskal-Shafranov limit for a flux rope that is held fixed at both ends (i.e., qa = 1).
Date: July 19, 2011
Creator: Oz, E.; Myers, C. E.; Yamada, M.; Ji, H.; Kulsrud, R. M. & Xie, J.
Partner: UNT Libraries Government Documents Department

Equilibrium and Stability of Partial Toroidal Plasma Discharges

Description: The equilibrium and stability of partial toroidal flux ropes are studied in detail in the laboratory, motivated by ubiquitous loop structures on the solar surface. The flux ropes studied here are magnetized arc discharges formed in the Magnetic Reconnection Experiment (MRX). It is found that these loops robustly maintain their equilibrium on time scales much longer than the Alfven time over a wide range of plasma current, guide eld strength, and angle between electrodes, even in the absence of a strapping fi eld. Additionally, the external kink stability of these flux ropes is found to be governed by the Kruskal-Shafranov limit for a flux rope with line-tied boundary conditions at both ends (q > 1).
Date: January 4, 2011
Creator: E. Oz, C. E. Myers, M. Yamada, H. Ji, R. Kulsrud, and J. Xie
Partner: UNT Libraries Government Documents Department

Numerical Calculations Demonstrating Complete Stabilization of the Ideal Magnetohydrodynamic Resistive Wall Mode by Longitudinal Flow

Description: The cylindrical ideal magnetohydrodynamic (MHD) stability problem, including ow and a resistive wall, is cast in the standard mathematical form, ωA⋅x = B⋅x, without discretizing the vacuum regions surrounding the plasma. This is accomplished by means of a finite element expansion for the plasma perturbations, by coupling the plasma surface perturbations to the resistive wall using a Green's function approach, and by expanding the unknown vector, x, to include the perturbed current in the resistive wall as an additional degree of freedom. The ideal MHD resistive wall mode (RWM) can be stabilized when the plasma has a uniform equilibrium ow such that the RWM frequency resonates with the plasma's Doppler-shifted sound continuum modes. The resonance induces a singularity in the parallel component of the plasma perturbations, which must be adequately resolved. Complete stabilization within the ideal MHD model (i.e. without parallel damping being added) is achieved as the grid spacing in the region of the resonance is extrapolated to 0 step size
Date: May 20, 2009
Creator: S. Smith, S.C. Jardin, J.P. Freidberg, L. Guazzotto
Partner: UNT Libraries Government Documents Department

Kink instabilities and the gas-embedded Z-pinch

Description: A comparison of a previous gas-embedded Z-pinch experiment (by E. Smars) is made using a nonlinear theory of kink instability (by W. Manheimer, et al.). Good agreement is found for some experimental cases while for others the theory predicts too fast a growth of the kink. To account for this discrepancy the evolution of topologically different secondary currents is postulated. (auth)
Date: April 30, 1975
Creator: Hartman, C.W.
Partner: UNT Libraries Government Documents Department

Current Driven Rotating Kink Mode in a Plasma Column with Non-Line-Tied Free End

Description: First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. Instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition.
Date: March 28, 2006
Creator: Furno, I; Intrator, T P; Ryutov, D D; Abbate, S; Madziwa-Nussinov, T; Light, A et al.
Partner: UNT Libraries Government Documents Department

Effects of Boundary Conditions and Flow on the Kink Instability in a Cylindrical Plasma Column

Description: An experimental investigation of the kink instability is presented in a linear plasma column where one end is line-tied to the plasma source, and the other end is not line-tied and therefore free to slide over the surface of the end-plate. This latter boundary condition is a result of plasma sheath resistance that insulates, at least partially, the plasma from the end-plate. The helical m = 1 kink mode is observed to grow when the plasma current exceeds a threshold and, close to the criticality, is characterized by an axial mode structure with maximum displacement at the free axial boundary. Azimuthal rotation of the mode is observed such that the helically kinked column always screws into the free axial boundary. The kink mode structure, rotation frequency and instability threshold are accurately reproduced by a recent kink theory [D. D. Ryutov, et al., Phys. Plasmas 13, 032105 (2006)], which includes axial plasma flow and one end of the plasma column that is free to move due to a perfect non-line-tying boundary condition which is experimentally verified. A brief review of the kink theory and its predictions for the boundary conditions relevant in the present experiments are presented.
Date: February 6, 2007
Creator: Furno, I; Intrator, T P; Lapenta, G; Dorf, L & Ryutov, D D
Partner: UNT Libraries Government Documents Department

A new paradigm for 3D collisionless magnetic reconnection.

Description: A new paradigm is emerging for 3D magnetic reconnection where the interaction of reconnection processes with current aligned instabilities plays an important role. According to the new paradigm, the initial equilibrium is rendered unstable by current aligned instabilities (lower-hybrid drift instability first, drift-kink instability later) and the non-uniform development of kinking modes leads to a compression of magnetic field lines in certain locations and a rarefaction in others. The areas where the flow is compressional are subjected to a driven reconnection process on the time scale of the driving mechanism (the kink mode). In the present paper we illustrate this series of event with a selection of simulation results.
Date: January 1, 2002
Creator: Lapenta, G. M. (Giovanni M.)
Partner: UNT Libraries Government Documents Department

Magnetohydodynamics stability of compact stellarators

Description: Recent stability results of external kink modes and vertical modes in compact stellarators are presented. The vertical mode is found to be stabilized by externally generated poloidal flux. A simple stability criterion is derived in the limit of large aspect ratio and constant current density. For a wall at infinite distance from the plasma, the amount of external flux needed for stabilization is given by Fi = (k2 {minus} k)=(k2 + 1), where k is the axisymmetric elongation and Fi is the fraction of the external rotational transform. A systematic parameter study shows that the external kink mode in QAS can be stabilized at high beta ({approximately} 5%) without a conducting wall by magnetic shear via 3D shaping. It is found that external kinks are driven by both parallel current and pressure gradient. The pressure contributes significantly to the overall drive through the curvature term and the Pfirsch-Schluter current.
Date: January 3, 2000
Creator: Fu, G.Y.; Ku, L.P.; Cooper, W.A. & Hirshman, S.H.
Partner: UNT Libraries Government Documents Department

Vertical and Kink Mode Stability Calculations for Current Carrying Quasiaxial Stellarators

Description: Vertical and kink mode stability are essential for strongly shaped tokamaks, to preclude disruptive plasma termination. Similarly, the design of stellarators with significant current, such as quasiaxial stellarators (QAS) must be carefully examined to ensure adequate vertical and kink stability. The CAS3D [1] and TERPSICHORE [2] MHD stability code packages are being used to evaluate the stability of QAS configurations. CAS3D has previously been applied to predict stability of the W7-X stellarator, under construction at IPP, Greifswald, Germany. As part of a multifaceted effort to develop an interesting compact quasiaxial stellarator configuration for a modest sized experiment at PPPL, these two code packages are being used for three dimensional calculations of internal and external MHD stability, including extensive benchmarking of MHD stability for axisymmetric and nonaxisymmetric cases. Optimized configurations have been identified for the proposed NCSX experiment which are stable to ballooning, kink and the periodicity-preserving modes and have acceptable confinement. Neoclassical particle transport is being studied and optimized for good confinement [7]. The three dimensional global ideal MHD stability code packages CAS3D and TERPSICHORE are found in agreement in numerous benchmarking studies and both predict global stability for the NCSX design point configuration. CAS3D has extended TERPSICHORE calculations for NCSX showing stability of the kink and periodicity-preserving modes for this stellarator even without a conducting wall. There is still a need for comprehensive comparisons of detailed measurements of MHD instabilities to results of global stability calculations for both rippled tokamak and stellarator experiments.
Date: July 1, 1999
Creator: Kessel, C.; Nuehrenberg, C.; Fu, G-Y.; Redi, M.H.; Cooper, W.A. & al, et
Partner: UNT Libraries Government Documents Department

Comment on "Wall Forces Produced During ITER Disruptions" by H. R. Strauss, R. Paccagnella, and J. Breslau (PHYSICS OF PLASMAS 17, 082505 (2010)

Description: The paper by H.R. Strauss presents numerical simulations, which pretend to describe the disruption instability in ITER device. The simulations were performed with numerical code M3D, described in Ref.[7] of the paper.
Date: October 20, 2010
Creator: Zakharov, Leonid E.
Partner: UNT Libraries Government Documents Department

Edge Plasma Boundary Layer Generated By Kink Modes in Tokamaks

Description: This paper describes the structure of the electric current generated by external kink modes at the plasma edge using the ideally conducting plasma model. It is found that the edge current layer is created by both wall touching and free boundary kink modes. Near marginal stability, the total edge current has a universal expression as a result of partial compensation of the δ-functional surface current by the bulk current at the edge. The resolution of an apparent paradox with the pressure balance across the plasma boundary in the presence of the surface currents is provided.
Date: November 22, 2010
Creator: Zakharov, L.E.
Partner: UNT Libraries Government Documents Department

Luminosity Loss due to Beam Distortion and the Beam-Beam Instability

Description: In a linear collider, sources of emittance dilution such as transverse wakefields or dispersive errors will couple the vertical phase space to the longitudinal position within the beam (the so-called ''banana effect''). When the Intersection Point (IP) disruption parameter is large, these beam distortions will be amplified by a single bunch kink instability which will lead to luminosity loss. We study this phenomena both analytically using linear theory and via numerical simulation. In particular, we examine the dependence of the luminosity loss on the wavelength of the beam distortions and the disruption parameter. This analysis may prove useful when optimizing the vertical disruption parameter for luminosity operation with given beam distortions.
Date: June 30, 2005
Creator: Wu, Juhao; Raubenheimer, T.O.; Chao, A.W.; Seryi, A.; /SLAC; Sramek, C.K. et al.
Partner: UNT Libraries Government Documents Department

Phenomenological Theory of the Kink Instability in a Slender Plasma Column

Description: When one deals with a plasma column whose radius a is much smaller than its length L, one can think of it as of a thin filament whose kink instability can be adequately described simply by a 2D displacement vector, {xi}{sub x} = {xi}{sub s}(z,t); {xi}{sub y} = {xi}{sub y}(z,t). Details of the internal structure of the column such as the current, density, and axial flow velocity distribution would be lumped into some phenomenological parameters. This approach is particularly efficient in the problems with non-ideal (sheath) boundary conditions (BC) at the end electrodes, with the finite plasma resistivity, and with a substantial axial flow. With the sheath BC imposed at one of the end-plates, we find instability in the domain well below the classical Kruskal-Shafranov limit. The presence of an axial flow causes the onset of rotation of the kink and strong axial ''skewness'' of the eigenfunction, with the perturbation amplitude increasing in the flow direction. We consider the limitations of the phenomenological approach and find that they are related to the steepness with which the plasma resistivity increases at the plasma boundary with vacuum.
Date: November 18, 2005
Creator: Ryutov, D D; Furno, I; Intrator, T P; Abbate, S & Madziwa-Nussinov, T
Partner: UNT Libraries Government Documents Department

Luminosity Optimization With Offset, Crossing Angle, and Distortion

Description: In a linear collider, sources of beam jitter due to kicker noise, quadrupole vibration and long-range transverse wakefields will lead to beam offsets and tilts at the Intersection Point (IP). In addition, sources of emittance dilution such as short-range transverse wakefields or dispersive errors will lead to internal beam distortions. When the IP disruption parameter is large, these beam imperfections will be amplified by a single bunch kink instability which will lead to luminosity loss. In this paper, we study the luminosity loss and then the optimization required to partially cancel the luminosity loss both analytically and with direct simulation.
Date: June 15, 2005
Creator: Wu, Juhao & Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Mode Structure of Disruption Precursors in TFTR Enhanced Reversed Shear Discharges

Description: The mode structure of the disruption precursors in the TFTR enhanced reversed shear parameters has been studied by using T(subscript e) fluctuation profiles and q profiles obtained from Electron Cyclotron Emission (ECE) and Motional Stark Effect (MSE) measurements. The observed profiles of the radial displacement associated with the MHD modes were consistent with the displacement profiles expected from the ideal MHD external kink mode. The observed mode frequencies differ from the plasma toroidal rotation frequency measured with CVI charge-exchange recombination light. The independence of the mode frequency from the plasma rotation frequency supports the ideal MHD hypothesis. Possible causes of the frequency difference are discussed.
Date: January 1, 1998
Creator: Fredrickson, E.; Taylor, G.; Manickam, J.; Okabayashi, M.; Batha, S. & al, et
Partner: UNT Libraries Government Documents Department

Observation and control of resistive wall modes

Description: Two approaches to achieving long-time scale stabilization of the ideal kink mode with a real, finite conductivity wall are considered: plasma rotation and active feedback control, DIII-D experiments have demonstrated stabilization of the resistive wall mode (RWM) by sustaining beta greater than the no-wall limit for up to 200 ms, much longer than the wall penetration time of a few ms. These plasmas are typically terminated by an m = 3, n = 1 mode as the plasma rotation slows below a few kHz. Recent temperature profile data shows an ideal MHD mode structure, as expected for the resistive wall mode at beta above the no-wall limit. The critical rotation rate for stabilization is in qualitative agreement with recent theories for dissipative stabilization in the absence of magnetic islands. However, drag by small-amplitude RWMs or damping of stable RWMs may contribute to an observed slowing of rotation at high beta, rendering rotational stabilization more difficult. An initial open-loop active control experiment, using non-axisymmetric external coils and a new array of saddle loop detectors, has yielded encouraging results, delaying the onset of the RWM.
Date: December 1998
Creator: Strait, E. J.; Garofalo, A. M. & Austin, M. E.
Partner: UNT Libraries Government Documents Department

Stabilization of the resistive wall mode using a fake rotating shell

Description: Tokamak plasma performance can, in theory, be greatly improved if the so called resistive wall mode is stabilized. This can be achieved by spinning the plasma rapidly, but such a scheme is not reactor relevant. A more promising approach is to apply external feedback in order to make a resistive shell placed around the plasma act like a perfect conductor. A scheme is outlined by which a network of feedback controlled conductors surrounding the plasma can be made to act like a rotating shell. This fake rotating shell combined with a stationary conventional shell (e.g. the vacuum vessel) can completely stabilize the resistive wall mode. The gain, bandwidth, current, and power requirements of the feedback amplifiers are extremely modest. A previously proposed stabilization scheme (the intelligent shell) is also investigated, and is compared with the fake rotating shell concept. The main disadvantage of the former scheme is that it requires a high gain.
Date: November 1, 1995
Creator: Fitzpatrick, R. & Jensen, T.H.
Partner: UNT Libraries Government Documents Department