Recent Progress in Quiescent H-mode (QH-mode) Research on DIII-D Tokamak
Event details
| Date | 09.09.2016 |
| Hour | 10:30 › 11:30 |
| Speaker | Xi Chen, General Atomics, San Diego, CA, USA |
| Location |
PPB 019
|
| Category | Conferences - Seminars |
In a recent discovery, the Quiescent H-mode (QH-mode) regime in DIII-D has been found to bifurcate into a new state at low torque in double-null shaped plasmas, characterized by increased pedestal height, width and thermal energy confinement. This regime has been sustained at net-zero NBI torque for 12 tau_E with excellent confinement (H98y2 ~ 1.46, beta_N~ 2). This provides an alternate path for achieving high performance ELM-stable operation at low torque, in addition to the low-torque QH-mode sustained with applied 3D fields. Measurements and simulation indicate that in the “wide-pedestal” state, the decreased edge (rho>0.9) ExB shear destabilizes broadband turbulence, which relaxes edge pressure gradients, improves peeling-ballooning stability and enables a wider and thus higher pedestal; in addition, increased outer-core (0.7<rho<0.9) ExB shear improves transport in that region and synergistically enhances the global energy confinement. In parallel, new experimental study and modeling of low, experimentally-relevant toroidal mode number (n≤5) Edge Harmonic Oscillation (EHO), which regulates the conventional QH edge, validate the proposed hypothesis of edge rotational shear in exciting the EHO. The observed minimum edge ExB shear required for EHO decreases linearly with pedestal electron collisionality, which is favorable for operating QH-mode in low collisionality and low rotation machines such as ITER. The ability to accurately simulate the EHO and maintain high performance QH-mode at low torque is an essential requirement for projecting QH-mode operation to ITER and future machines.
Practical information
- Informed public
- Free
Organizer
- Prof. P. Ricci
Contact
- Prof. P. Ricci