Fishbone-like Instability Research Articles

Excitation of 2/1 fishbone-like modes by trapped energetic ions

A new branch of the m/n = 2/1 fishbone-like mode is found to be excitated by trapped energetic ions when the tearing mode is unstable. An energy-principle-based dispersion relation is derived and analyzed, which incorporates the wave-particle resonance and the resistive layer physics. Once the beta amplitude of trapped energetic ions exceeds a critical value, 2/1 fishbone-like instabilities would appear. The mode transition from tearing mode to 2/1 fishbone-like mode will be triggered with the increasing beta of trapped energetic ions. The total effects of the ideal magnetohydrodynamic potential energy and the adiabatic contribution of trapped energetic ions on the real frequency and growth rate of 2/1 fishbone-like modes are investigated; the effects of magnetic Reynolds number and magnetic shear are also studied. Both resistivity and magnetic shear play a stable role on 2/1 fishbone-like modes. An implication of the theory is conducive to understanding the 2/1 fishbone-like activities observed recently in HL-2A.

An overview of FTU results

Since the 2010 IAEA-FEC Conference, FTU has exploited improvements in cleaning procedures and in the density control system to complete a systematic exploration of access to high-density conditions in a wide range of plasma currents and magnetic fields. The line-averaged densities at the disruptive limit increased more than linearly with the toroidal field, while no dependence on plasma current was found; in fact, the maximum density of 4.3 × 1020 m−3 was reached at B = 8 T even at the minimum current of 0.5 MA, corresponding to twice the Greenwald limit. The lack of plasma current dependence was due to the increase in density peaking with the safety factor. Experiments with the 140 GHz electron cyclotron resonance heating (ECRH) system were focused on the sawtooth (ST) period control and on the commissioning of a new launcher with real-time steering capability that will act as the front-end actuator of a real-time system for ST period control and tearing mode stabilization. Various ECRH and electron cyclotron current-drive modulation schemes were used; with the fastest one, the ST period synchronized with an 8 ms modulation period. The observed period variations were simulated using the JETTO code with a critical shear model for the crash trigger. The new launcher was of the plug-in type, allowing quick insertion and connection to the transmission line. Both beam characteristics and steering speed were in line with design expectation. Experimental results on the connection between improved coupling of lower hybrid waves in high-density plasmas and reduced wave spectral broadening were interpreted by fully kinetic, non-linear model calculations. A dual-frequency, time-of-flight diagnostic for the measurement of density profiles was developed and successfully tested. Fishbone-like instabilities driven by energetic electrons were simulated by the hybrid MHD-gyrokinetic XHMGC code.

Observation of Electron Fishbone-Like Instabilities in EAST Heavy Impurity Ohmic Plasma

The transient burst of an internal kink mode is first observed in EAST heavy impurity ohmic plasma. The features of the electron fishbone-like mode are presented, and the fishbone-like instabilities are found to be driven by the trapped supra-thermal electrons. The processional frequency of the trapped supra-thermal electrons is calculated with different discharge parameters. The results indicate that the calculated processional frequency is consistent with the experimental observations. Furthermore, we also find that the frequency chirping of the long-lived mode is related to the evolution of the safety factor profile.

Off-axis fishbone-like instability and excitation of resistive wall modes in JT-60U and DIII-D

An energetic-particle (EP)-driven “off-axis-fishbone-like mode (OFM)” often triggers a resistive wall mode (RWM) in JT-60U and DIII-D devices, preventing long-duration high-βN discharges. In these experiments, the EPs are energetic ions (70–85 keV) injected by neutral beams to produce high-pressure plasmas. EP-driven bursting events reduce the EP density and the plasma rotation simultaneously. These changes are significant in high-βN low-rotation plasmas, where the RWM stability is predicted to be strongly influenced by the EP precession drift resonance and by the plasma rotation near the q=2 surface (kinetic effects). Analysis of these effects on stability with a self-consistent perturbation to the mode structure using the MARS-K code showed that the impact of EP losses and rotation drop is sufficient to destabilize the RWM in low-rotation plasmas, when the plasma rotation normalized by Alfvén frequency is only a few tenths of a percent near the q=2 surface. The OFM characteristics are very similar in JT-60U and DIII-D, including nonlinear mode evolution. The modes grow initially like a classical fishbone, and then the mode structure becomes strongly distorted. The dynamic response of the OFM to an applied n=1 external field indicates that the mode retains its external kink character. These comparative studies suggest that an energetic particle-driven “off-axis-fishbone-like mode” is a new EP-driven branch of the external kink mode in wall-stabilized plasmas, analogous to the relationship of the classical fishbone branch to the internal kink mode.

Dynamic polarization random walk model and fishbone-like instability for self-organized critical systems

We study the phenomenon of self-organized criticality (SOC) as a transport problem for electrically charged particles. A model for SOC based on the idea of a dynamic polarization response with random walks of the charge carriers gives critical exponents consistent with the results of numerical simulations of the traditional ‘sandpile’ SOC models, and stability properties, associated with the scaling of the control parameter versus distance to criticality. Relaxations of a supercritical system to SOC are stretched-exponential similar to the typically observed properties of non-Debye relaxation in disordered amorphous dielectrics. Overdriving the system near self-organized criticality is shown to have a destabilizing effect on the SOC state. This instability of the critical state constitutes a fascinating nonlinear system in which SOC and nonlocal properties can appear on an equal footing. The instability cycle is qualitatively similar to the internal kink (‘fishbone’) mode in a magnetically confined toroidal plasma where beams of energetic particles are injected at high power, and has serious implications for the functioning of complex systems. Theoretical analyses, presented here, are the basis for addressing the various patterns of self-organized critical behavior in connection with the strength of the driving. The results of this work also suggest a type of mixed behavior in which the typical multi-scale features due to SOC can coexist along with the global or coherent features as a consequence of the instability present. An example of this coexistence is speculated for the solar wind–magnetosphere interaction.

Fishbone-like instability in a looped-tube thermoacoustic engine

Quasi-periodic bursts of acoustic oscillations were observed during the start-up process in a looped-tube thermoacoustic engine. The acoustic oscillations have a constant frequency of 111 Hz, while the bursts have "quasi-periods" in the order of 14-25 s. The quasi-periodic bursts show a new mode of amplitude growth in this thermoacoustic engine. The envelope of the acoustic oscillations has a fishbone-like shape. The nature of the observed fishbone-like instabilities suggests a strong interaction between the acoustic and temperature field.

Lower hybrid wave produced supra-thermal electrons and fishbone-like instability in FTU

The fishbone-like internal kink instability driven by supra-thermal electrons generated by lower hybrid current drive is an important issue of burning plasma research. Indeed, the trapped particle averaged bounce characterizing the interaction of trapped alpha particles with low-frequency MHD modes in burning plasmas depends on energy, not on mass, hence the charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low-frequency MHD modes.Fishbone-like internal kink instabilities driven by electrons were observed during experiments on Frascati Tokamak Upgrade (FTU) and interpreted in terms of an oscillating ‘fixed point’ activity followed by one of ‘limit cycle’, produced by suprathermal electrons in the presence of a q-profile with qmin ≈ 1. As an interesting behaviour of the fast electron population produced by the LH power when the q-profile meets the condition qmin ≈ 1, the fast electron bremsstrahlung data of FTU show a marked redistribution in space across layers centred at r/a ≈ 0.20–0.33. This redistribution occurs during and in phase with the fishbone oscillation observed by the MHD diagnostic, with the same time scale and at the same radial position of the peak emission seen by x-ray tomography.

Observation of Fishbone-Like Instabilities Excited by Energetic Electrons on the HL-2A Tokamak

Strong burst of an internal kink mode is observed on the HL-2A tokamak. Features of the fishbone-like mode are presented. The fishbone-like instabilities can be driven during electron cyclotron resonance heating (ECRU) and can be excited on the high field side (HFS) by ECRH. It is found for the first time that the modes also present themselves on the low field side (LFS) during ECRH. Experiments show that the energetic electrons with energy of 35-70keV play a dominant role in the excitation mechanism, and the experimental results are also consistent with our calculation ones.

Overview of the FTU results

Steady internal transport barriers (ITBs) are obtained in FTU at ITER-relevant magnetic field and density (ne0 ≥ 1.3 × 1020 m−3) in almost full non-inductive discharges, sustained by lower hybrid (LH) and electron cyclotron (EC) RF waves sources. Similarly to ITER, only electrons are directly heated which in turn heat ions via collisions and no momentum is injected. Collisions do not affect the mechanisms of turbulence suppression and energy transport. At the highest densities the ion thermal conductivity remains ≤ the ohmic level, while the energy confinement time exceeds the ITER 97-L scaling by about 1.6 times. The ITB radius can be varied in the range 0.2 ≤ r/a ≤ 0.65 modifying the radial profile of the LH driven current, acting mainly on the safety factor q. A liquid lithium limiter (LLL) of innovative design, composed of a mesh of porous capillaries, has been tested successfully for the first time on a medium size tokamak. The LLL surface showed no damage up to the maximum thermal load of 5 MW m−2. With LLL cleaner plasmas are obtained and the particle recycling strongly drops; new interesting regimes of particle transport arise at high density, with highly peaked profiles. Significant progress in disruption mitigation by means of EC power has shown that they can be avoided when absorption occurs directly on the MHD islands driving the disruption. Feedback control/suppression of MHD tearing modes (TM, m = 2) with EC waves has been achieved relying on a real-time detection of the TM and of its radial location. Testing the collective Thomson scattering in ITER-relevant configuration has stressed that avoiding backscattered radiation to the source is very crucial. The theory of the evolution of fishbone-like instabilities driven by LH generated supra-thermal electrons in FTU is outlined, and its relation to the trapped α particles dynamics is stressed.

Observation of MHD induced fast ion losses on the CHS heliotron/torsatron

MHD induced beam ion losses were observed on the CHS heliotron/torsatron with an energy and pitch angle resolved scintillator detector. Beam ions escaping into the probe were characterized using orbit calculation codes. It was found that two types of ions, namely `passing boundary ions' and `trapped ions', were lost periodically during an m/n = 3/2 fishbone-like instability. There are thresholds in the mode amplitude for the ejection of fast ions. Above this threshold, losses are enhanced together with an increase of the mode amplitude. The excitation of the fishbone-like instability and the enhancement of MHD induced losses strongly depend on the degree of accumulation of beam ions in the plasma. Orbit calculations show that passing boundary ions, which have large pitch angle and vϕ close to zero (or close to the propagating velocity of the magneticfluctuation) near the q = 3/2 surface, are ejected by the instability.