Fishbone Instability Research Articles

Nonideal fishbone instability

It is shown that there exists a nonideal collisionless fishbone mode in toroidal plasmas, which may lead to the formation of the magnetic islands and thermal crashes during fishbone activity. The mode arises because of the resonance interaction between the energetic trapped ions and the collisionless m=1 nonideal kink mode. The theory predictions are found to be consistent with experimental observations of fishbone oscillations accompanied by thermal crashes on the Axisymmetric Divertor Experiment Upgrade tokamak [Günter et al., Nucl. Fusion 39, 1535 (1999)].

Fishbone Instability Excited by Electrons in a Tokamak

Fishbone instability excited by electrons intokamak plasma is investigated theoretically and experimentally. Theanalysis of mode precession resonance indicates that the instability canbe excited by barely trapped suprathermal electrons. The fishboneinstability of electrons is more easily excited in the region of negativemagnetic shear than that of positive magnetic shear and it is firstobserved by using electron cyclotron resonance heating in an HL-1Mtokamak.

Low frequency fishbone mode induced by circulating particles in spherical tori

It is found that high β in low-aspect-ratio tori tends to stabilize the fishbone instability in a plasma with energetic circulating ions. The stabilization results from enhancement of the toroidal drift motion by large Shafranov shift, which makes it difficult to reconcile the condition of considerable energy exchange between the ions and the internal kink perturbation with the condition of the resonant wave–particle interaction.

Destabilization of internal kink modes at high frequency by energetic circulating ions.

A theoretical model is proposed to interpret the high-frequency fishbone instability observed in tangential neutral-beam-injection discharges in a tokamak. It is shown that, when the beam ion beta exceeds a critical value, energetic circulating ions can indeed destabilize the internal kink mode through circulation resonance at a high frequency comparable to the circulation frequency of the energetic ions. The critical beta value of the energetic ions, the real frequency, and the growth rate of the mode are in general agreement with the high-frequency fishbone instability observed in experiments.

Limitation of the operational regime due to MHD instabilities in conventional scenarios on ASDEX upgrade

In addition to the Greenwald density limit and the threshold conditions for H-mode access, neoclassical tearing modes (NTMs) pose the most severe constraint to tokamak operation in discharges with conventional (monotonic) current density profiles. We show, at hand of the example of ASDEX Upgrade the resulting constraints on density and useful heating power. At high densities, collisionality effects can lead to the appearance of a second stable regime for NTMs. A further limitation can arise from the appearance of fishbone instabilities which, depending on the orientation of the injected beams, can also lead to a clamping of the total plasma energy content. The resulting diagram is also useful as a guide for a strategy to expand the operating range by feedback stabilisation of the neoclassical tearing modes and for the orientation of the beamlines.

Internal kink instability during off-axis electron cyclotron current drive in the DIII-D tokamak

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile q(r) when q(min) approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number = m/n = 1/1. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.

The fishbone instability in ASDEX Upgrade

In ASDEX Upgrade the excitation of the fishbone instability has been experimentally investigated. The initial frequency of the fishbone oscillation scales with the toroidal precession frequency of the fast trapped ions. A new feature, the coupling of the fishbone mode with the edge localized mode (ELM) instability during high-β discharges, is found. Fishbones occur in ASDEX Upgrade only for βfast = βtor/(1+τE /τsd) above a certain threshold. It is thus possible to calculate their destabilization regime in terms of global plasma parameters, such as plasma current, plasma density, toroidal magnetic field and central electron temperature.

Wavelet analysis of fusion plasma transients

Analysis of transient signals in the diagnostic of fusion plasmas often requires the simultaneous consideration of their time and frequency information. The newly emerging technique of wavelet analysis contains both time and frequency domains. Therefore it can be a valuable tool for the analysis of transients. In this paper the basic method of wavelet analysis is described. As an example, wavelet analysis is applied to the well-known phenomena of mode locking and fishbone instability. The results quantify the current qualitative understanding of these events in terms of instantaneous frequencies and amplitudes and encourage applications of the method to other problems.

Fast detection of 14 MeV neutrons on the TFTR neutron collimator

Current mode operation of the NE451 ZnS scintillation detectors of the TFTR neutron collimator has enabled us to record the development of radial neutron emission profiles with much faster speed and higher accuracy than in the pulse counting mode. During high power deuterium–tritium (DT) operation, the intrinsic shot noise on the detector traces was so low that we could observe sawtooth instabilities and disruptions with good precision and, in addition, were able to identify precursor magnetohydrodynamic (MHD) activity and fishbone instabilities. These results demonstrate that in future tritium burning machines like ITER or TPX, the neutron collimator should be designed not only as a monitor of radial fusion power profiles but also as a wave detector for MHD activity.

Effects of trapped alpha particles on internal kink modes in tokamaks

The theory of linear interaction between internal kink modes and fusionproduced high-energy alpha particles in a toroidal plasma is extended by including finite-banana-width corrections to the alpha dynamics. An important implication of the theory for the stability properties of kink modes is that the finite-banana-width effects increase the value of the poloidal alpha beta required to attain the stable domain of operation as well as that required to excite the high-frequency fishbone instability. Furthermore, these effects lead to a reduction in the instability growth rate in both the low- and high-frequency unstable regimes.

Introduction to Confinement of Toroidal Plasma 6. Physics of Burning Plasma.

Physics associated with burning plasmas is reviewed.Behavior of alpha particles produced by fusion reaction and response of bulk plasmas to them are main subjects in this chapter. The thermalization and confinement of alpha particles are described.The alpha-particle heating enhances the fusion reaction, while the accumulation of helium ash reduces it through dilution of fuel ions.Stabilization of the sawtooth oscillation and destabilization of the fishbone instability and the TAE mode are examined.Finally, a prospect of future studies on burning plasma is presented.

The fishbone instability in the DIII-D tokamak

Although most DIII-D plasmas are stable to the fishbone instability, fishbones are sometimes observed when βp ≳ 1.5 and n̄e ⪅ 5 × 1013 cm−3. These bursts are usually of minor significance operationally; however, under one condition, over 50% of the beam power was lost. The angle of beam injection has little effect on the virulence of the instability, suggesting that the fishbone instability in DIII-D is the ion diamagnetic branch of the internal kink.

Tangential neutral-beam-driven instabilities in the Princeton beta experiment.

During tangential neutral-beam injection into the PBX tokamak, bursts of two types of instabilities are observed. One instability occurs in the frequency range 120-210 kHz and the other oscillates predominantly near the frequency of bulk plasma rotation (20-30 kHz). Both instabilities correlate with drops in neutron emission and bursts in charge-exchange neutral flux, indicating that beam ions are removed from the center of the plasma by the instabilities. The central losses are comparable to the losses induced by the fishbone instability during perpendicular injection.

Neutral flux measurements during neutral-beam injection on Doublet III (abstract)

Neutral particle flux parallel and perpendicular to the plasma current and at energies equal to and below the injection energy has been observed with the parallel and perpendicular charge exchange spectrometers on Doublet III. Initial neutral injection is marked by a rapid increase in the high-energy neutral efflux which usually relaxes over tens of milliseconds to a lower equilibrium value. As injected power is increased by the stepped addition of more neutral beams, the measured flux is not linearly additive, often increasing slightly or not at all. For very high values of injected power, in the 5–7-MW range, the neutral particle flux is often observed in bursts correlated with the rapid fall of giant sawteeth on soft x-ray signals. In many of these shots the fishbone instability is present and is observed with the perpendicular spectrometer, however the parallel flux does not correlate with fluctuations observed with magnetic probes unless the instability terminates rapidly, in which case both a soft x-ray drop and a burst of neutrals occurs. Some aspects of these measurements have been modeled by a particle transport code with particular emphasis on changes in the neutral density profile during high-power neutral injection.

Confinement of Fusion Reaction Products during the Fishbone Instability

The presence of fishbone events in the PDX tokamak is correlated with a reduction in the burnup of 0.8-MeV /sup 3/He ions. This reduction is probably caused by distortion of the /sup 3/He drift orbitals in the helical magnetic fields associated with the instability, resulting in the loss of about 70% of the confined /sup 3/He ions in a moderately strong fishbone. Such a nonresonant loss of particles with large orbits indicates that fishbone events may influence 3.5-MeV alphas in ignition experiments.

Study of High-Beta Magnetohydrodynamic Modes and Fast-Ion Losses in PDX

Strong magnetohydrodynamic activity has been observed in PDX neutral-beam-heated discharges. It occurs for βTq>~0.045 and is associated with a significant loss of fast ions and a drop in neutron emission. As much as 20%-40% of the beam heating power may be lost. The instability occurs in repetitive bursts of oscillations of ≤ 1 msec duration at 1-6-msec intervals. The magnetohydrodynamic activity has been dubbed the "fishbone instability" from its characteristic signature on the Mirnov coils.Received 29 September 1982DOI:https://doi.org/10.1103/PhysRevLett.50.891©1983 American Physical Society