Speaker
Mr
Devon Battaglia
(USA)
Description
The L-H power threshold (P_LH) on the National Spherical Torus Experiment decreases with larger X-point radius (R_x) and the amount of lithium evaporated on the divertor surfaces. The edge T_e (where T_e ~ T_i in the L-mode edge) at the L-H transition decreases 30 – 40% with larger R_x, but is fairly independent of the edge density, neutral fueling rate and lithium conditioning. These observations are consistent with the X-transport theory, which predicts that the edge radial electric field (E_r) just inside the plasma separatrix must become more negative as T_i or R_x are increased in order to counteract non-ambipolar neoclassical ion orbit loss in a diverted plasma. Consequently, the E x B shearing rate, which is predicted to be favorable for triggering and sustaining H-mode, increases with both T_i and R_x. Self-consistent E_r calculations using the XGC0 code provide insight into the dependence of the shearing rate on the magnetic geometry and edge T_i. For example, the shearing rate remains constant as R_x is reduced from 0.64m to 0.47m only if the edge T_i profile is increased by 25%. Increasing the neutral recycling rate does not significantly alter the T_i profile needed to maintain the same shearing rate, but does require more core heating to maintain the critical edge temperature. This is consistent with the experimental observations that P_LH varies with R_x and divertor recycling, while T_e at the L-H transition only depends on R_x. This agreement between theory and experiment provides a valuable tool for interpreting the hidden variables in the empirical P_LH scaling relationships and for optimizing the heating requirements for ITER and other advanced tokamaks. XGC0 simulations are used to examine other known P_LH dependences, including the ion grad-B drift direction, X-point height, ion species and plasma current.
Supported by US DOE contracts DE-AC02-09CH11466 and DE-AC05-00OR22725.
Country or International Organization of Primary Author
USA
Primary author
Mr
Devon Battaglia
(USA)
Co-authors
Dr
Ahmed Diallo
(PPPL)
Dr
Benoit LeBlanc
(Princeton Plasma Physics Laboratory)
Dr
Choong-Seock Chang
(Princeton Plasma Physics Laboratory and KAIST)
Dr
Jonathan Menard
(Princeton Plasma Physics Laboratory)
Dr
Rajesh Maingi
(Oak Ridge National Laboratory)
Dr
Ron Bell
(Princeton Plasma Physics Laboratory)
Dr
Seung-Hoe Ku
(Princeton Plasma Physics Laboratory)
Dr
Stanley Kaye
(Princeton Plasma Physics Laboratory, Princeton University, Princeton NJ, 08543 USA)
Dr
Stefan Gerhardt
(Princeton Plasma Physics Laboratory)
