2021
8.
Neto, E. Lascas; Graves, J. P.; Raghunathan, M.; Sommariva, C.; Pfefferlé, D.; Contributors, J. E. T.
Heavy Impurity Transport in Tokamaks Subject to Plasma Rotation, NTV and the Influence of Saturated Ideal MHD Perturbations Journal Article
In: Plasma Physics and Controlled Fusion, vol. 64, no. 1, pp. 014002, 2021, ISSN: 0741-3335.
Links | BibTeX | Tags: impurities, internal kink, neoclassical transport, plasma flow, VENUS-LEVIS
@article{netoHeavyImpurityTransport2021,
title = {Heavy Impurity Transport in Tokamaks Subject to Plasma Rotation, NTV and the Influence of Saturated Ideal MHD Perturbations},
author = {E. Lascas Neto and J. P. Graves and M. Raghunathan and C. Sommariva and D. Pfefferl\'{e} and J. E. T. Contributors},
doi = {10.1088/1361-6587/ac3964},
issn = {0741-3335},
year = {2021},
date = {2021-11-01},
urldate = {2021-11-01},
journal = {Plasma Physics and Controlled Fusion},
volume = {64},
number = {1},
pages = {014002},
publisher = {IOP Publishing},
keywords = {impurities, internal kink, neoclassical transport, plasma flow, VENUS-LEVIS},
pubstate = {published},
tppubtype = {article}
}
2016
7.
Cooper, W A; Brunetti, D; Duval, B P; Faustin, J M; Graves, J P; Kleiner, A; Patten, H; Pfefferlé, D; Porte, L; Raghunathan, M; Reimerdes, H; Sauter, O; Tran, T M
Saturated ideal kink/peeling formations described as three-dimensional magnetohydrodynamic tokamak equilibrium states Journal Article
In: Physics of Plasmas, vol. 23, no. 4, pp. 040701, 2016.
Abstract | Links | BibTeX | Tags: internal kink, MHD equilibrium
@article{cooper-2016,
title = {Saturated ideal kink/peeling formations described as three-dimensional magnetohydrodynamic tokamak equilibrium states},
author = {W A Cooper and D Brunetti and B P Duval and J M Faustin and J P Graves and A Kleiner and H Patten and D Pfefferl\'{e} and L Porte and M Raghunathan and H Reimerdes and O Sauter and T M Tran},
doi = {10.1063/1.4945743},
year = {2016},
date = {2016-04-06},
journal = {Physics of Plasmas},
volume = {23},
number = {4},
pages = {040701},
abstract = {Free boundary magnetohydrodynamic equilibrium states with spontaneous three dimensional deformations of the plasma-vacuum interface are computed for the first time. The structures obtained have the appearance of saturated ideal external kink/peeling modes. High edge pressure gradients yield toroidal mode number n = 1 corrugations for a high edge bootstrap current and larger n distortions when this current is small. Deformations in the plasma boundary region induce a nonaxisymmetric Pfirsch-Schl\"{u}ter current driving a field-aligned current ribbon consistent with reported experimental observations. A variation in the 3D equilibrium confirms that the n = 1 mode is a kink/peeling structure. We surmise that our calculated equilibrium structures constitute a viable model for the edge harmonic oscillations and outer modes associated with a quiescent H-mode operation in shaped tokamak plasmas.},
keywords = {internal kink, MHD equilibrium},
pubstate = {published},
tppubtype = {article}
}
Free boundary magnetohydrodynamic equilibrium states with spontaneous three dimensional deformations of the plasma-vacuum interface are computed for the first time. The structures obtained have the appearance of saturated ideal external kink/peeling modes. High edge pressure gradients yield toroidal mode number n = 1 corrugations for a high edge bootstrap current and larger n distortions when this current is small. Deformations in the plasma boundary region induce a nonaxisymmetric Pfirsch-Schlüter current driving a field-aligned current ribbon consistent with reported experimental observations. A variation in the 3D equilibrium confirms that the n = 1 mode is a kink/peeling structure. We surmise that our calculated equilibrium structures constitute a viable model for the edge harmonic oscillations and outer modes associated with a quiescent H-mode operation in shaped tokamak plasmas.
2015
6.
Cooper, W A; Brunetti, D; Faustin, J M; Graves, J P; Pfefferlé, D; Raghunathan, M; Sauter, O; Tran, T M; Chapman, I T; Ham, C J; Aiba, N; team, The MAST; contributors, JET
Free boundary equilibrium in 3D tokamaks with toroidal rotation Journal Article
In: Nuclear Fusion, vol. 55, no. 6, pp. 063032, 2015.
Abstract | Links | BibTeX | Tags: internal kink, MHD equilibrium, plasma flow
@article{cooper-2015,
title = {Free boundary equilibrium in 3D tokamaks with toroidal rotation},
author = {W A Cooper and D Brunetti and J M Faustin and J P Graves and D Pfefferl\'{e} and M Raghunathan and O Sauter and T M Tran and I T Chapman and C J Ham and N Aiba and The MAST team and JET contributors},
url = {http://stacks.iop.org/0029-5515/55/i=6/a=063032},
doi = {10.1088/0029-5515/55/6/063032},
year = {2015},
date = {2015-05-22},
journal = {Nuclear Fusion},
volume = {55},
number = {6},
pages = {063032},
abstract = {The three-dimensional VMEC equilibrium solver has been adapted to numerically investigate the approximate toroidal rotation model we have derived. We concentrate our applications on the simulation of JET snakes and MAST long-lived modes under free boundary conditions. Helical core solutions are triggered when 〈β〉 exceeds a threshold value, typically 2.7% in JET-like plasmas. A large plasma current and edge bootstrap current can drive helical core formations at arbitrarily small 〈β〉 in which the ideal saturated internal kink coexists with an ideal saturated external kink structure of opposite phase. The centrifugal force linked with the rotation has the effect of displacing the plasma column away from the major axis, but does not alter significantly the magnitude of the edge corrugation of the plasma. Error field correction coil currents in JET-like configurations increase the outer midplane distortions by 2 cm. The edge bootstrap current enhances the edge modulation of the plasma driven by the core snake deformations in MAST.},
keywords = {internal kink, MHD equilibrium, plasma flow},
pubstate = {published},
tppubtype = {article}
}
The three-dimensional VMEC equilibrium solver has been adapted to numerically investigate the approximate toroidal rotation model we have derived. We concentrate our applications on the simulation of JET snakes and MAST long-lived modes under free boundary conditions. Helical core solutions are triggered when 〈β〉 exceeds a threshold value, typically 2.7% in JET-like plasmas. A large plasma current and edge bootstrap current can drive helical core formations at arbitrarily small 〈β〉 in which the ideal saturated internal kink coexists with an ideal saturated external kink structure of opposite phase. The centrifugal force linked with the rotation has the effect of displacing the plasma column away from the major axis, but does not alter significantly the magnitude of the edge corrugation of the plasma. Error field correction coil currents in JET-like configurations increase the outer midplane distortions by 2 cm. The edge bootstrap current enhances the edge modulation of the plasma driven by the core snake deformations in MAST.
5.
Pfefferlé, D
Energetic ion dynamics and confinement in 3D saturated MHD configurations PhD Thesis
Swiss Institute of Technology Lausanne (EPFL), 2015.
Abstract | Links | BibTeX | Tags: drift-kinetic, fast particles, guiding-centre, Hamiltonian, internal kink, magnetic ripple, MHD equilibrium, neoclassical transport, neutral beam injection, stellarator, VENUS-LEVIS
@phdthesis{pfefferle-thesis,
title = {Energetic ion dynamics and confinement in 3D saturated MHD configurations},
author = {D Pfefferl\'{e}},
url = {https://infoscience.epfl.ch/record/207958},
doi = {10.5075/epfl-thesis-6561},
year = {2015},
date = {2015-05-04},
publisher = {EPFL},
school = {Swiss Institute of Technology Lausanne (EPFL)},
abstract = {In the following theoretical and numerically oriented work, a number of findings have been assembled. The newly devised VENUS-LEVIS code, designed to accurately solve the motion of energetic particles in the presence of 3D magnetic fields, relies on a non-canonical general coordinate Lagrangian formulation of the guiding-centre and full-orbit equations of motion. VENUS-LEVIS can switch between guiding-centre and full-orbit equations with minimal discrepancy at first order in Larmor radius by verifying the perpendicular variation of magnetic vector field, not only including gradients and curvature terms but also parallel currents and the shearing of field-lines. By virtue of a Fourier representation of the fields in poloidal and toroidal coordinates and a cubic spline in the radial variable, the order of the Runge-Kutta integrating scheme is preserved and convergence of Hamiltonian properties is obtained. This interpolation scheme is crucial to compute orbits over slowing-down times, as well as to mitigate the singularity of the magnetic axis in toroidal flux coordinate systems. Three-dimensional saturated MHD states are associated with many tokamak phenomena including snakes and LLMs in spherical or more conventional tokamaks, and are inherent to stellarator devices. The VMEC equilibrium code conveniently reproduces such 3D magnetic configurations. Slowing-down simulations of energetic ions from NBI predict off-axis deposition of particles during LLM MHD activity in hybrid-like plasmas of the MAST. Co-passing particles helically align in the opposite side of the plasma deformation, whereas counter-passing and trapped particles are less affected by the presence of a helical core. Qualitative agreement is found against experimental measurements of the neutron emission. Two opposing approaches to include RMPs in fast ion simulations are compared, one where the vacuum field caused by the RMP current coils is added to the axisymmetric MHD equilibrium, the other where the MHD equilibrium includes the plasma response within the 3D deformation of its flux-surfaces. The first model admits large regions of stochastic field-lines that penetrate the plasma without alteration. The second assumes nested flux-surfaces with a single magnetic axis, embedding the RMPs in a 3D saturated ideal MHD state but excluding stochastic field-lines within the last closed flux-surface. Simulations of fast ion populations from NBI are applied to MAST n=3 RMP coil configuration with 4 different activation patterns. At low beam energies, particle losses are dominated by parallel transport due to the stochasticity of the field-lines, whereas at higher energies, losses are accredited to the 3D structure of the perturbed plasma as well as drift resonances.},
keywords = {drift-kinetic, fast particles, guiding-centre, Hamiltonian, internal kink, magnetic ripple, MHD equilibrium, neoclassical transport, neutral beam injection, stellarator, VENUS-LEVIS},
pubstate = {published},
tppubtype = {phdthesis}
}
In the following theoretical and numerically oriented work, a number of findings have been assembled. The newly devised VENUS-LEVIS code, designed to accurately solve the motion of energetic particles in the presence of 3D magnetic fields, relies on a non-canonical general coordinate Lagrangian formulation of the guiding-centre and full-orbit equations of motion. VENUS-LEVIS can switch between guiding-centre and full-orbit equations with minimal discrepancy at first order in Larmor radius by verifying the perpendicular variation of magnetic vector field, not only including gradients and curvature terms but also parallel currents and the shearing of field-lines. By virtue of a Fourier representation of the fields in poloidal and toroidal coordinates and a cubic spline in the radial variable, the order of the Runge-Kutta integrating scheme is preserved and convergence of Hamiltonian properties is obtained. This interpolation scheme is crucial to compute orbits over slowing-down times, as well as to mitigate the singularity of the magnetic axis in toroidal flux coordinate systems. Three-dimensional saturated MHD states are associated with many tokamak phenomena including snakes and LLMs in spherical or more conventional tokamaks, and are inherent to stellarator devices. The VMEC equilibrium code conveniently reproduces such 3D magnetic configurations. Slowing-down simulations of energetic ions from NBI predict off-axis deposition of particles during LLM MHD activity in hybrid-like plasmas of the MAST. Co-passing particles helically align in the opposite side of the plasma deformation, whereas counter-passing and trapped particles are less affected by the presence of a helical core. Qualitative agreement is found against experimental measurements of the neutron emission. Two opposing approaches to include RMPs in fast ion simulations are compared, one where the vacuum field caused by the RMP current coils is added to the axisymmetric MHD equilibrium, the other where the MHD equilibrium includes the plasma response within the 3D deformation of its flux-surfaces. The first model admits large regions of stochastic field-lines that penetrate the plasma without alteration. The second assumes nested flux-surfaces with a single magnetic axis, embedding the RMPs in a 3D saturated ideal MHD state but excluding stochastic field-lines within the last closed flux-surface. Simulations of fast ion populations from NBI are applied to MAST n=3 RMP coil configuration with 4 different activation patterns. At low beam energies, particle losses are dominated by parallel transport due to the stochasticity of the field-lines, whereas at higher energies, losses are accredited to the 3D structure of the perturbed plasma as well as drift resonances.
2014
4.
Cooper, W A; Hirshman, S P; Chapman, I T; Brunetti, D; Faustin, J M; Graves, J P; Pfefferlé, D; Raghunathan, M; Sauter, O; Tran, T M; Aiba, N
An approximate single fluid 3-dimensional magnetohydrodynamic equilibrium model with toroidal flow Journal Article
In: Plasma Physics and Controlled Fusion, vol. 56, no. 9, pp. 094004, 2014.
Abstract | Links | BibTeX | Tags: internal kink, MHD equilibrium, plasma flow
@article{cooper-2014,
title = {An approximate single fluid 3-dimensional magnetohydrodynamic equilibrium model with toroidal flow},
author = {W A Cooper and S P Hirshman and I T Chapman and D Brunetti and J M Faustin and J P Graves and D Pfefferl\'{e} and M Raghunathan and O Sauter and T M Tran and N Aiba},
url = {http://stacks.iop.org/0741-3335/56/i=9/a=094004},
doi = {10.1088/0741-3335/56/9/094004},
year = {2014},
date = {2014-08-13},
journal = {Plasma Physics and Controlled Fusion},
volume = {56},
number = {9},
pages = {094004},
abstract = {An approximate model for a single fluid three-dimensional (3D) magnetohydrodynamic (MHD) equilibrium with pure isothermal toroidal flow with imposed nested magnetic flux surfaces is proposed. It recovers the rigorous toroidal rotation equilibrium description in the axisymmetric limit. The approximation is valid under conditions of nearly rigid or vanishing toroidal rotation in regions with significant 3D deformation of the equilibrium flux surfaces. Bifurcated helical core equilibrium simulations of long-lived modes in the MAST device demonstrate that the magnetic structure is only weakly affected by the flow but that the 3D pressure distortion is important. The pressure is displaced away from the major axis and therefore is not as noticeably helically deformed as the toroidal magnetic flux under the subsonic flow conditions measured in the experiment. The model invoked fails to predict any significant screening by toroidal plasma rotation of resonant magnetic perturbations in MAST free boundary computations.},
keywords = {internal kink, MHD equilibrium, plasma flow},
pubstate = {published},
tppubtype = {article}
}
An approximate model for a single fluid three-dimensional (3D) magnetohydrodynamic (MHD) equilibrium with pure isothermal toroidal flow with imposed nested magnetic flux surfaces is proposed. It recovers the rigorous toroidal rotation equilibrium description in the axisymmetric limit. The approximation is valid under conditions of nearly rigid or vanishing toroidal rotation in regions with significant 3D deformation of the equilibrium flux surfaces. Bifurcated helical core equilibrium simulations of long-lived modes in the MAST device demonstrate that the magnetic structure is only weakly affected by the flow but that the 3D pressure distortion is important. The pressure is displaced away from the major axis and therefore is not as noticeably helically deformed as the toroidal magnetic flux under the subsonic flow conditions measured in the experiment. The model invoked fails to predict any significant screening by toroidal plasma rotation of resonant magnetic perturbations in MAST free boundary computations.
3.
Pfefferlé, D; Graves, J P; Cooper, W A; Misev, C; Chapman, I T; Turnyanskiy, M; Sangaroon, S
NBI fast ion confinement in the helical core of MAST hybrid-like plasmas Journal Article
In: Nuclear Fusion, vol. 54, no. 6, pp. 064020, 2014.
Abstract | Links | BibTeX | Tags: confinement, fast particles, guiding-centre, internal kink, MHD equilibrium, neoclassical transport, neutral beam injection, VENUS-LEVIS
@article{pfefferle-nbi,
title = {NBI fast ion confinement in the helical core of MAST hybrid-like plasmas},
author = {D Pfefferl\'{e} and J P Graves and W A Cooper and C Misev and I T Chapman and M Turnyanskiy and S Sangaroon},
url = {https://iopscience.iop.org/article/10.1088/0029-5515/54/6/064020},
doi = {10.1088/0029-5515/54/6/064020},
year = {2014},
date = {2014-05-23},
journal = {Nuclear Fusion},
volume = {54},
number = {6},
pages = {064020},
abstract = {Energetic ions are found to be transported strongly from the core of MAST hybrid-like plasmas during long-lived mode (LLM) magnetohydrodynamic activity. The resulting impact on the neutral beam ion deposition and concurrent current drive is modelled using the guiding-centre approximation in the internal kinked magnetic topology. General coordinate guiding-centre equations are extended for this purpose. It is found that the kinked core spirals around the position of strongest ionization, which remains geometrically centred, so that a large fraction of the population is deposited in the high shear external region where the plasma is almost axisymmetric. Those particles ionized in the low shear region exhibit exotic drift motion due to the strongly non-axisymmetric equilibrium, periodically passing near the magnetic axis and then reflected by the boundary of the kinked equilibrium, which in this respect acts as a confining pinch. Broad agreement is found against experimental measurement of fast ion particle confinement degradation as the MAST LLM amplitude varies.},
keywords = {confinement, fast particles, guiding-centre, internal kink, MHD equilibrium, neoclassical transport, neutral beam injection, VENUS-LEVIS},
pubstate = {published},
tppubtype = {article}
}
Energetic ions are found to be transported strongly from the core of MAST hybrid-like plasmas during long-lived mode (LLM) magnetohydrodynamic activity. The resulting impact on the neutral beam ion deposition and concurrent current drive is modelled using the guiding-centre approximation in the internal kinked magnetic topology. General coordinate guiding-centre equations are extended for this purpose. It is found that the kinked core spirals around the position of strongest ionization, which remains geometrically centred, so that a large fraction of the population is deposited in the high shear external region where the plasma is almost axisymmetric. Those particles ionized in the low shear region exhibit exotic drift motion due to the strongly non-axisymmetric equilibrium, periodically passing near the magnetic axis and then reflected by the boundary of the kinked equilibrium, which in this respect acts as a confining pinch. Broad agreement is found against experimental measurement of fast ion particle confinement degradation as the MAST LLM amplitude varies.
2013
2.
Cooper, W A; Chapman, I T; Schmitz, O; Turnbull, A D; Tobias, B J; Lazarus, E A; Turco, F; Lanctot, M J; Evans, T E; Graves, J P; Brunetti, D; Pfefferlé, D; Reimerdes, H; Sauter, O; Halpern, F D; Tran, T M; Coda, S; Duval, B P; B, Labit; Pochelon, A; Turnyanskiy, M R; Lao, L; Luce, T C; Buttery, R; Ferron, J R; Hollmann, E M; Petty, C C; van Zeeland, M; Fenstermacher, M E; Hanson, J M; Lütjens, H
Bifurcated helical core equilibrium states in tokamaks Journal Article
In: Nuclear Fusion, vol. 53, no. 7, pp. 073021, 2013.
Abstract | Links | BibTeX | Tags: helical core, internal kink, MHD equilibrium
@article{cooper-2013,
title = {Bifurcated helical core equilibrium states in tokamaks},
author = {W A Cooper and I T Chapman and O Schmitz and A D Turnbull and B J Tobias and E A Lazarus and F Turco and M J Lanctot and T E Evans and J P Graves and D Brunetti and D Pfefferl\'{e} and H Reimerdes and O Sauter and F D Halpern and T M Tran and S Coda and B P Duval and Labit B and A Pochelon and M R Turnyanskiy and L Lao and T C Luce and R Buttery and J R Ferron and E M Hollmann and C C Petty and M van Zeeland and M E Fenstermacher and J M Hanson and H L\"{u}tjens},
url = {http://stacks.iop.org/0029-5515/53/i=7/a=073021},
doi = {10.1088/0029-5515/53/7/073021},
year = {2013},
date = {2013-06-11},
journal = {Nuclear Fusion},
volume = {53},
number = {7},
pages = {073021},
abstract = {Tokamaks with weak to moderate reversed central shear in which the minimum inverse rotational transform (safety factor) qmin is in the neighbourhood of unity can trigger bifurcated magnetohydrodynamic equilibrium states, one of which is similar to a saturated ideal internal kink mode. Peaked prescribed pressure profiles reproduce the 'snake' structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state. Snake equilibrium structures are computed in simulations of the tokamak \`{a} configuration variable (TCV), DIII-D and mega amp spherical torus (MAST) tokamaks. The internal helical deformations only weakly modulate the plasma\textendashvacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.},
keywords = {helical core, internal kink, MHD equilibrium},
pubstate = {published},
tppubtype = {article}
}
Tokamaks with weak to moderate reversed central shear in which the minimum inverse rotational transform (safety factor) qmin is in the neighbourhood of unity can trigger bifurcated magnetohydrodynamic equilibrium states, one of which is similar to a saturated ideal internal kink mode. Peaked prescribed pressure profiles reproduce the 'snake' structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state. Snake equilibrium structures are computed in simulations of the tokamak à configuration variable (TCV), DIII-D and mega amp spherical torus (MAST) tokamaks. The internal helical deformations only weakly modulate the plasma–vacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.
2012
1.
Pfefferlé, D; Graves, J P; Cooper, W A
Exploitation of a general-coordinate guiding centre code for the redistribution of fast ions in deformed hybrid tokamak equilibria Proceedings Article
In: Journal of Physics: Conference Series, pp. 012020, 2012.
Abstract | Links | BibTeX | Tags: fast particles, helical core, internal kink, MHD equilibrium, VENUS-LEVIS
@inproceedings{pfefferle-first,
title = {Exploitation of a general-coordinate guiding centre code for the redistribution of fast ions in deformed hybrid tokamak equilibria},
author = {D Pfefferl\'{e} and J P Graves and W A Cooper},
url = {http://stacks.iop.org/1742-6596/401/i=1/a=012020},
doi = {10.1088/1742-6596/401/1/012020},
year = {2012},
date = {2012-12-03},
booktitle = {Journal of Physics: Conference Series},
journal = {Journal of Physics: Conference Series},
volume = {401},
number = {1},
pages = {012020},
abstract = {Self-consistent fast ion distributions are usually obtained using a code that solves the guiding-centre equations, with an appropriate fast ion source (e.g. NBI pinis) and sink (e.g. collision operators). Straight field-line coordinate systems, such as Boozer coordinates, are ordinarily convenient due to the simple separation of longitudinal and cross-field motion, and the simple expression of magnetic differential operators. However, these coordinates are found to be near-singular at the boundary of the internal helical region associated with an n = m = 1 infernal mode. These important configurations are associated with many tokamak phenomena, including snakes and long-lived modes [1] in spherical or more conventional devices. Such internal helical states occur when there is a radially extended region where the safety factor is close to unity. Recent calculations predict the possibility of helical equilibria in ITER hybrid scenarios [2]. The ANIMEC code [3] conveniently produces an equilibrium helical state despite choosing for example an axisymmetric fixed boundary. The corresponding magnetic field in these coordinates can now be fed to the newly devised Particle-In-Cell (PIC) code VENUS-LEVIS, which has been upgraded with phase-space Lagrangian guiding-centre orbit equations [4], embodying full 3D anisotropic electromagnetic fields and a formulation that is independent of coordinate choice, despite retaining intrinsic Hamiltonian properties. The simulations are applied to MAST experiments where the presence of a long-lived mode can effect confinement of neutral beam ions, potentially affecting NBI heating and current drive [1]. Neighbouring equilibria from ANIMEC, one helical in the core and the other axisymmetic, permits a precise means of identifying the effect of 3D geometry on the simulated confinement properties of MAST's neutral beam fast ion population. In agreement with the compared experimental data from MAST neutron camera, a significant fraction of particles are pushed out of the helical core region affecting both the measured radial neutron distribution and the heating and current drive properties of the neutral beam population.},
keywords = {fast particles, helical core, internal kink, MHD equilibrium, VENUS-LEVIS},
pubstate = {published},
tppubtype = {inproceedings}
}
Self-consistent fast ion distributions are usually obtained using a code that solves the guiding-centre equations, with an appropriate fast ion source (e.g. NBI pinis) and sink (e.g. collision operators). Straight field-line coordinate systems, such as Boozer coordinates, are ordinarily convenient due to the simple separation of longitudinal and cross-field motion, and the simple expression of magnetic differential operators. However, these coordinates are found to be near-singular at the boundary of the internal helical region associated with an n = m = 1 infernal mode. These important configurations are associated with many tokamak phenomena, including snakes and long-lived modes [1] in spherical or more conventional devices. Such internal helical states occur when there is a radially extended region where the safety factor is close to unity. Recent calculations predict the possibility of helical equilibria in ITER hybrid scenarios [2]. The ANIMEC code [3] conveniently produces an equilibrium helical state despite choosing for example an axisymmetric fixed boundary. The corresponding magnetic field in these coordinates can now be fed to the newly devised Particle-In-Cell (PIC) code VENUS-LEVIS, which has been upgraded with phase-space Lagrangian guiding-centre orbit equations [4], embodying full 3D anisotropic electromagnetic fields and a formulation that is independent of coordinate choice, despite retaining intrinsic Hamiltonian properties. The simulations are applied to MAST experiments where the presence of a long-lived mode can effect confinement of neutral beam ions, potentially affecting NBI heating and current drive [1]. Neighbouring equilibria from ANIMEC, one helical in the core and the other axisymmetic, permits a precise means of identifying the effect of 3D geometry on the simulated confinement properties of MAST's neutral beam fast ion population. In agreement with the compared experimental data from MAST neutron camera, a significant fraction of particles are pushed out of the helical core region affecting both the measured radial neutron distribution and the heating and current drive properties of the neutral beam population.