Neoclassical tearing modes, both rotating and locked, grow because, once a small seed island size is exceeded, high parallel transport eliminates density and temperature gradients and hence the bootstrap current, near the island O-point. For positive shear, this current deficit promotes for further island growth The saturated island size depends on the bootstrap current density j_bs in the vicinity of the rational surface and constitutes a key b-limiting process for long pulse tokamaks. The time scale associated with a 0.1m island will be ~ 10 sec in ITER. Localized Electron Cyclotron Current Drive (ECCD), appropriately modulated in time and space to be deposited near the island O-points, can replace the bootstrap current deficit and hence stabilize the island. This work shows that a 50%"on"-50%"off" modulation scheme will stabilize neoclassical tearing modes when 0.68 j_cd > j_bs, where j_cd is the driven current density near the mode rational surface , calculated as if the power were continuously on. Unmodulated ECCD can affect the saturated island width. With a proper choice of poloidal and toroidal launch angles, ray-tracing calculations of electron cyclotron current drive indicate the driven current can be localized to within Da/a = 0.10 of the q=2 surface with a current drive efficiency of g= 0.15. This leads to j_cd = 0.15 MA/m², compared to j_bs = 0.10 MA/m² representative of ITER. Specifications for an ECCD current drive capability for ITER will be presented. For locked modes, ECCD sources will be required at four toroidal locations spaced by roughly 90 degrees.

* ITER San Diego, 11025 North Torrey Pines Road, La Jolla, CA, USA.

  ITER Garching, Max-Planck-Institut fur Plasmaphysik, D-85748, Garching bei Munchen, GERMANY