ABSTRACT Following recent observations suggesting the presence of the geodesic acoustic mode (GAM... more ABSTRACT Following recent observations suggesting the presence of the geodesic acoustic mode (GAM) in ohmically heated discharges in the Mega Amp Spherical Tokamak (MAST) [J. R. Robinson et al., Plasma Phys. Controlled Fusion 54, 105007 (2012)], the behaviour of the GAM is studied numerically using the two fluid, global code CENTORI [P. J. Knight et al. Comput. Phys. Commun. 183, 2346 (2012)]. We examine mode localisation and effects of magnetic geometry, given by aspect ratio, elongation, and safety factor, on the observed frequency of the mode. An excellent agreement between simulations and experimental data is found for simulation plasma parameters matched to those of MAST. Increasing aspect ratio yields good agreement between the GAM frequency found in the simulations and an analytical result obtained for elongated large aspect ratio plasmas. (C) 2013 AIP Publishing LLC.
The acceleration of electrons in solar flares is simulated using an extended version of a full or... more The acceleration of electrons in solar flares is simulated using an extended version of a full orbit test-particle code previously used to compute losses of fusion alpha particles from tokamaks and to study flare proton acceleration. The magnetic and electric fields in the model are obtained from solutions of the linearized MHD equations for reconnecting modes at a magnetic X-point with zero equilibrium current. The magnetic field is assumed to have a small finite component perpendicular to the plane of the X-point. The version of the code used in the simulations is fully relativistic and includes Coulomb collisions. It is demonstrated that electrons cross the system boundary at distinct footpoints with the relative numbers and energy distribution at each footpoint sensitive to the longitudinal magnetic field component and collision time. These results are discussed in the context of recent flare observations showing asymmetric hard X-ray emission from the chromospheric footpoints of magnetic loops.
A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harm... more A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harmonics in the JET tokamak and in various space plasmas is proposed. It is suggested that, in each case, waves on the fast Alfven-ion Bernstein branch are excited by the magnetoacoustic cyclotron instability, which is driven by an energetic ion distribution with a local maximum at ul # 0, where vI is the velocity component perpendicular to the magnetic field.
Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium exp... more Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium experiments at sequential cyclotron harmonics of both fusion products and beam ions. The emission originates from the outer midplane plasma, where fusion products and ...
ABSTRACT Following recent observations suggesting the presence of the geodesic acoustic mode (GAM... more ABSTRACT Following recent observations suggesting the presence of the geodesic acoustic mode (GAM) in ohmically heated discharges in the Mega Amp Spherical Tokamak (MAST) [J. R. Robinson et al., Plasma Phys. Controlled Fusion 54, 105007 (2012)], the behaviour of the GAM is studied numerically using the two fluid, global code CENTORI [P. J. Knight et al. Comput. Phys. Commun. 183, 2346 (2012)]. We examine mode localisation and effects of magnetic geometry, given by aspect ratio, elongation, and safety factor, on the observed frequency of the mode. An excellent agreement between simulations and experimental data is found for simulation plasma parameters matched to those of MAST. Increasing aspect ratio yields good agreement between the GAM frequency found in the simulations and an analytical result obtained for elongated large aspect ratio plasmas. (C) 2013 AIP Publishing LLC.
The acceleration of electrons in solar flares is simulated using an extended version of a full or... more The acceleration of electrons in solar flares is simulated using an extended version of a full orbit test-particle code previously used to compute losses of fusion alpha particles from tokamaks and to study flare proton acceleration. The magnetic and electric fields in the model are obtained from solutions of the linearized MHD equations for reconnecting modes at a magnetic X-point with zero equilibrium current. The magnetic field is assumed to have a small finite component perpendicular to the plane of the X-point. The version of the code used in the simulations is fully relativistic and includes Coulomb collisions. It is demonstrated that electrons cross the system boundary at distinct footpoints with the relative numbers and energy distribution at each footpoint sensitive to the longitudinal magnetic field component and collision time. These results are discussed in the context of recent flare observations showing asymmetric hard X-ray emission from the chromospheric footpoints of magnetic loops.
A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harm... more A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harmonics in the JET tokamak and in various space plasmas is proposed. It is suggested that, in each case, waves on the fast Alfven-ion Bernstein branch are excited by the magnetoacoustic cyclotron instability, which is driven by an energetic ion distribution with a local maximum at ul # 0, where vI is the velocity component perpendicular to the magnetic field.
Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium exp... more Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium experiments at sequential cyclotron harmonics of both fusion products and beam ions. The emission originates from the outer midplane plasma, where fusion products and ...
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Papers by Ken Mcclements