Alpha particle-driven toroidal rotation in burning plasmas

The mechanism of a torque intrinsically produced by alpha particles and the subsequent possibility to create significant toroidal rotation and shear are numerically investigated. In steady-state DEMO plasmas, regardless of the magnetic configuration, the orbit-following Monte Carlo code OFMC predicts that co-directed collisional torque and a counter-directed j-vector x B-vector torque always emerge due to the gradient of the source profile of alpha particles and both of them virtually cancel each other out, as analytically predicted earlier. The magnitude of each torque is enhanced in the reversed shear configuration compared with the normal shear configuration, provided that the source gradient is finite and similar in both cases. The resultant rotation velocity estimated by the TASK/TX transport code is far below the threshold to stabilize resistive wall modes (RWMs) through intrinsic alpha-driven torque alone. It is estimated that a neutral beam injection at a moderate power level may be capable of producing toroidal rotation sufficient to stabilize RWMs.