@article{7872,
  author = {B. Ayten and E. Westerhof and ASDEX Upgrade Team},
  title = {Non-linear effects in electron cyclotron current drive applied for the stabilization of neoclassical tearing modes},
  abstract = {Due to the smallness of the volumes associated with the flux surfaces around the O-point of a magnetic island, the electron cyclotron power density applied inside the island for the stabilization of neoclassical tearing modes (NTMs) can exceed the threshold for non-linear effects as derived previously by Harvey et al (1989 Phys. Rev. Lett. 62 [http://dx.doi.org/10.1103/PhysRevLett.62.426] 426 ). We study the non-linear electron cyclotron current drive (ECCD) efficiency through bounce-averaged, quasi-linear Fokker–Planck calculations in the magnetic geometry as created by the islands. The calculations are performed for the parameters of a typical NTM stabilization experiment on ASDEX Upgrade. A particular feature of these experiments is that the rays of the EC wave beam propagate tangential to the flux surfaces in the power deposition region. The calculations show significant non-linear effects on the ECCD efficiency, when the ECCD power is increased from its experimental value of 1 MW to a larger value of 4 MW. The nonlinear effects are largest in the case of locked islands or when the magnetic island rotation period is longer than the collisional time scale. The non-linear effects result in an overall reduction of the current drive efficiency for this case with absorption of the EC power on the low-field side of the electron cyclotron resonance layer. As a consequence of the non-linear effects, also the stabilizing effect of the ECCD on the island is reduced from linear expectations.
},
  year = {2014},
  journal = {Nuclear Fusion},
  volume = {54},
  pages = {073001},
  url = {http://stacks.iop.org/0029-5515/54/i=7/a=073001},
  doi = {10.1088/0029-5515/54/7/073001},
  language = {eng},
}