Speaker
Description
The boundary heat flux width ($λ_q$) is an important part of the power exhaust challenge in magnetic confinement fusion reactors. Understanding what sets $λ_q$ has largely been an empirical science [1], however physics understanding is progressing [3-6]. A database of $λ_q$ in H-mode indicated that the poloidal magnetic field ($B_p$) was the only significant parameter associated with the heat flux width: $λ_q~B_p^{-1.19}$ [1]. The maximum $B_p$ in the database was ~0.8 T, whereas ITER at 15 MA will be ~1.2 T.
C-Mod has been the only diverted tokamak capable of operating at reactor-relevant $B_p$, now with measurements up to 1.3 T. These new measurements in EDA H-mode clearly follow the inverse scaling of $λ_q$ with $B_p$ to values exceeding ITER-level. The heuristic drift (HD) model [4,5] has done a remarkable job of reproducing the trend and the magnitude of $λ_q$ in the database. The new high-field data from C-Mod are consistent with the HD model. Perhaps more importantly, the new data provide a benchmark for first principles models [6,7], one of which projects [6] to ~10 times larger $λ_q$ than the empirical $B_p$ scaling for ITER. In addition, we have assembled a database of $λ_q$ consisting of over 300 shots that span nearly the entire operating space of Alcator C-Mod (L-, H- and I-modes) under attached divertor conditions. As in earlier studies [8], $λ_q$ at fixed $B_p$ exhibit significant scatter that appears related to the core plasma confinement. We are presently exploring correlations of $λ_q$ with global and pedestal parameters; we will report on the latest results at this meeting. The database now includes a composite of measurements made by surface thermocouples and Langmuir probes. Improved spatial resolution and heat flux dynamic range over IR thermography allows for more accurate fits of $λ_q$ and resolving the role of transport into the private flux region. We find that the assumption of symmetric spreading of heat flux [1] is not appropriate under many conditions.
[1] T. Eich, et al., Nucl. Fusion 53 (2013) 093031. [2] R.J. Goldston, et al., Nucl. Fusion 52 (2012) 013009. [3] R.J. Goldston, J. Nucl. Mat. 463 (2015) 397-400. [4] C.S. Chang, et al., Nucl. Fusion 57 (2017) 116023. [5] B. Chen, et al., “Progress towards modeling…”, submitted to Phys. Plasmas. [6] B. LaBombard, et al., Phys. Plasmas 18 (2011) 056104.
| Country or International Organization | United States of America |
|---|---|
| Paper Number | EX/P6-9 |
