Publications

@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}

}

@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}

}