Speaker
Description
See the full abstract here http://ocs.ciemat.es/EPS2019ABS/pdf/O2.109.pdf
Understanding the physics of the NTM onset and its suppression is a key problem in achieving controlled fusion. This requires better knowledge of the NTM threshold mechanism.
We solve the drift kinetic equation for the ion/electron response to the NTM magnetic perturbation, gi,e, assuming small magnetic island width, w<< 0, Vparallel is the parallel component of velocity. The particle distribution is then found to be attened across these drift islands rather than the magnetic island. This results in incomplete attening of the density/temperature pro le across the NTM island for w ~ rhotheta. As rho theta,e << rho theta,i, this effect is less signicant for electrons. To maintain plasma quasi-neutrality, an electrostatic potential is generated that slightly modifies the S island structure.
We identify a narrow collisional boundary layer in pitch angle around the trapped-passing boundary of width ~Sqrt(nu) , where nu is the collision frequency normalised to a characteristic drift frequency. In this region, collisions cannot be treated perturbatively and S no longer describes the streamlines. We provide a solution to the 2-D boundary layer problem, employing a momentum-conserving collision operator, allowing us to rigorously connect the trapped and passing regions. We show that the plasma response to the magnetic island is stabilising, providing a threshold island width for NTM instability of w<=wc = 2.67 rho theta,i [1,2] and 2.73 rho theta,i from full numerical and analytic solutions, respectively. This novel NTM threshold result appears to arise from the response of the electrons to the electrostatic potential required for quasi-neutrality.
References
[1] K. Imada et al., PRL 121, 175001 (2018)
[2] K. Imada et al., Nucl. Fusion 59 046016 (2019)
