K. G. McClements

Solar flares as cascades of reconnecting magnetic loops

A model for the solar coronal magnetic field is proposed where multiple directed loops evolve in space and time. Loops injected at small scales are anchored by footpoints of opposite polarity moving randomly on a surface. Nearby footpoints of the same polarity aggregate, and loops can reconnect when they collide. This may trigger a cascade of further reconnection, representing a solar flare. Numerical simulations show that a power law distribution of flare energies emerges, associated with a scale-free network of loops, indicating self-organized criticality.

Ion cyclotron emission measurements during JET deuterium-tritium experiments

In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range v ≤ 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to tile last closed flux surface at major radius R approximately 4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs. In pure deuterium and mixed DT discharges ICE spectra are similar in form, but on changing from pure D to mixed D+T neutral beam injection at constant power, the intensity of the ICE rises in proportion to the increased neutron flux: this indicates that fusion alpha particles-and not beam ions-provide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfven-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles

THE EXCITATION OF OBLIQUELY PROPAGATING FAST ALFVEN WAVES AT FUSION ION CYCLOTRON HARMONICS

The theory of the magnetoacoustic cyclotron instability, which has been proposed as a mechanism for suprathermal ion cyclotron harmonic emission observed in large tokamaks, is generalized to include finite parallel wave number k∥. This extension introduces significant new physics: the obliquely propagating fast Alfven wave can undergo cyclotron resonant interactions with thermal and fusion ions, which affects the instability driving and damping mechanisms. The velocity–space distribution of the fusion ions is modeled by a drifting ring, which approximates the distribution calculated for the emitting region in tritium experiments on the Joint European Torus (JET) [Cottrell et al., Nucl. Fusion 33, 1365 (1993)]. Linear instability can occur simultaneously at the fusion ion cyclotron frequency and all its harmonics when the fusion ion concentration is extremely low, because the finite k∥ gives rise to a Doppler shift, which decouples cyclotron damping due to thermal ions from wave growth associated with fusi...

Collisionless shock and supernova remnant simulations on VULCAN

The VULCAN [C. N. Danson et al., Opt. Commun. 103, 392 (1993)] laser at the UK Central Laser Facility is being used for laboratory-based simulations of collisionless shocks. By ensuring that key dimensionless parameters in the experiments have values similar to those of supernova remnants (SNRs), the hydrodynamics and magnetic field of the experiment are scaled to those of a SNR. This makes it possible to investigate experimentally the physics of collisionless magnetized shocks in such objects. The experiments are providing data against which to test current theory. Collisionless shock formation and the interaction of two counterpropagating colliding plasmas permeated by a strong magnetic field are discussed.

Large-scale numerical simulations of ion beam instabilities in unmagnetized astrophysical plasmas

Collisionless quasiperpendicular shocks with magnetoacoustic Mach numbers exceeding a certain threshold are known to reflect a fraction of the upstream ion population. These reflected ions drive instabilities which, in a magnetized plasma, can give rise to electron acceleration. In the case of shocks associated with supernova remnants (SNRs), electrons energized in this way may provide a seed population for subsequent acceleration to highly relativistic energies. If the plasma is weakly magnetized, in the sense that the electron cyclotron frequency is much smaller than the electron plasma frequency ωp, a Buneman instability occurs at ωp. The nonlinear evolution of this instability is examined using particle-in-cell simulations, with initial parameters which are representative of SNR shocks. For simplicity, the magnetic field is taken to be strictly zero. It is shown that the instability saturates as a result of electrons being trapped by the wave potential. Subsequent evolution of the waves depends on the temperature of the background protons Ti and the size of the simulation box L. If Ti is comparable to the initial electron temperature Te, and L is equal to one Buneman wavelength λ0, the wave partially collapses into low frequency waves and backscattered waves at around ωp. If, on the other hand, Ti≫Te and L=λ0, two high frequency waves remain in the plasma. One of these waves, excited at a frequency slightly lower than ωp, may be a Bernstein–Greene–Kruskal mode. The other wave, excited at a frequency well above ωp, is driven by the relative streaming of trapped and untrapped electrons. In a simulation with L=4λ0, the Buneman wave collapses on a time scale consistent with the excitation of sideband instabilities. Highly energetic electrons were not observed in any of these simulations, suggesting that the Buneman instability can only produce strong electron acceleration in a magnetized plasma.Collisionless quasiperpendicular shocks with magnetoacoustic Mach numbers exceeding a certain threshold are known to reflect a fraction of the upstream ion population. These reflected ions drive instabilities which, in a magnetized plasma, can give rise to electron acceleration. In the case of shocks associated with supernova remnants (SNRs), electrons energized in this way may provide a seed population for subsequent acceleration to highly relativistic energies. If the plasma is weakly magnetized, in the sense that the electron cyclotron frequency is much smaller than the electron plasma frequency ωp, a Buneman instability occurs at ωp. The nonlinear evolution of this instability is examined using particle-in-cell simulations, with initial parameters which are representative of SNR shocks. For simplicity, the magnetic field is taken to be strictly zero. It is shown that the instability saturates as a result of electrons being trapped by the wave potential. Subsequent evolution of the waves depends on the...

The detection of wave activity in the solar corona using UV line spectra

The SUMER and CDS instruments on the Solar and Heliopheric Observatory spacecraft (SOHO), due to be launched in 1995, may enable us to identify the dominant mechanism responsible for solar coronal heating. In this paper we examine, in particular, the possibility that Alfvén or acoustic waves, propagating through the corona and heating the ambient plasma, could be detected through the measurement of ultra-violet line widths. The contribution of wave broadening to the total line width depends on the orientation of the magnetic field with respect to the line of sight. CDS may be used to identify the magnetic field geometry in a particular region. The spatial resolution provided by SUMER, superior to that of previous instruments, should then make it possible to discriminate between different broadening mechanisms. In the case of lines produced by heavy ions in the low corona, we find that the line width produced by an Alfvén wave flux sufficiently high to heat the active corona corresponds to a Doppler temperature of up to twenty times the kinetic temperature.

Physics of energetic particle-driven instabilities in the START spherical tokamak

The recent use of neutral beam injection (NBI) in the UKAEA small tight aspect ratio tokamak (START) has provided the first opportunity to study experimentally the physics of energetic ions in spherical tokamak (ST) plasmas. In such devices the ratio of major radius to minor radius R0/a is of order unity. Several distinct classes of NBI-driven instability have been observed at frequencies up to 1 MHz during START discharges. These observations are described, and possible interpretations are given. Equilibrium data, corresponding to times of beam-driven wave activity, are used to compute continuous shear Alfven spectra: toroidicity and high plasma beta give rise to wide spectral gaps, extending up to frequencies of several times the Alfven gap frequency. In each of these gaps Alfvenic instabilities could, in principle, be driven by energetic ions. Chirping modes observed at high beta in this frequency range have bandwidths comparable to or greater than the gap widths. Instability drive in START is provided by beam ion pressure gradients (as in conventional tokamaks), and also by positive gradients in beam ion velocity distributions, which arise from velocity-dependent charge exchange losses. It is shown that fishbone-like bursts observed at a few tens of kHz can be attributed to internal kink mode excitation by passing beam ions, while narrow-band emission at several hundred kHz may be due to excitation of fast Alfven (magnetosonic) eigenmodes. In the light of our understanding of energetic particle-driven instabilities in START, the possible existence of such instabilities in larger STs is discussed.

Neutral beam stabilization of sawtooth oscillations in JET

Recent experiments in the Joint European Torus (JET) have provided evidence of sawtooth stabilization by fast ions arising from deuterium neutral beam injection (NBI). A possible theoretical basis for the interpretation of the observed sawtooth period behaviour is investigated and predictions are compared with experimental results, using a sawtooth period model developed to predict the sawtooth period in the International Thermonuclear Experimental Reactor (ITER). Unlike the case of ion cyclotron resonance heating, a detailed comparison between theory and experiment for NBI has not yet been made. In the model employed in this paper, a beam ion contribution to the internal kink potential energy has been incorporated, using a simple analytical expression valid in the limit of isotropic fast particles. This analytical expression has been found to compare well with detailed calculations performed with a hybrid kinetic/MHD code NOVA-K, using fast particle distribution functions computed with a plasma analysis code TRANSP. The beam ion contribution has been implemented in a transport code PRETOR and a few representative JET discharges have been analysed and modelled. The beam ion term is found to be sufficiently stabilizing to produce simulated sawtooth periods in agreement with the experimental results. Sawtooth periods computed without taking this term into account are much shorter than the measured periods. The model indicates that sawteeth are triggered in these JET discharges by excitation of the internal kink in the semi-collisional ion-kinetic regime: this was found by previous authors to be the sawtooth trigger most likely to be relevant to ITER. The role of beam ions in determining the sawtooth period in JET is thus found to be similar to the predicted role of α-particles in ITER.

Experiment on collisionless plasma interaction with applications to supernova remnant physics

Results from a scaled, collision-free, laser-plasma experiment designed to address aspects of collisionless plasma interaction in a high-plasma β supernova remnant (SNR) are discussed. Ideal magneto-hydrodynamic scaling indicates that the experimental plasma matches the SNR plasma at 500 ps. Experimental data show that the magnetic field can alter the plasma density profile when two similar plasmas interact in a colliding geometry. These results are not explained by magnetic-field pressure; they do, however, suggest magnetic field penetration that localizes the plasma particles to the Larmor radius, which appears smaller than the size of the experiment and the particle mean-free paths and may thus increase the effective collisionality of the interacting plasma system.

Collective electric field effects on the confinement of fast ions in tokamaks

The injection of neutral particle beams counter to the plasma current direction in the Mega-Ampere Spherical Tokamak (MAST) [A. Sykes, R. J. Akers, L. C. Appel et al., Nucl. Fusion, 41, 1423 (2001)] leads to substantial losses of energetic beam ions and also rapid toroidal rotation. The electrodynamic consequences of energetic ion loss on tokamak plasmas are explored in light of results from the MAST counterinjection experiments and test particle calculations of the current density due to escaping ions. Previous authors have noted that there are two possible consequences of such a current: either a compensating bulk plasma return current is set up, or the plasma behaves as an insulator, with the energetic ion current balanced by a displacement current rather than a conduction current. Radial electric fields and hence toroidal flows occur in both cases, but higher fields are predicted in the insulating case. Such fields are important because they can confine both fast ions and bulk plasma (via the suppress...