Measurement of LHCD edge power deposition through modulation techniques on Alcator C-Mod

• Main Authors: Parker, R. R. (Author), Chilenksi, M. A. (Author), Faust, Ian Charles (Contributor), Brunner, Daniel Frederic (Contributor), Labombard, Brian (Contributor), Baek, Seung Gyou (Contributor), Edlund, Eric Matthias (Contributor), Hubbard, Amanda E (Contributor), Hughes Jr, Jerry (Contributor), Terry, James L (Contributor), Shiraiwa, Shunichi (Contributor), Walk Jr, John R (Contributor), Wallace, Gregory Marriner (Contributor), Whyte, Dennis G (Contributor)
• Other Authors: Massachusetts Institute of Technology. Department of Mathematics (Contributor), Massachusetts Institute of Technology. Department of Nuclear Science and Engineering (Contributor), Massachusetts Institute of Technology. Department of Physics (Contributor), Massachusetts Institute of Technology. Plasma Science and Fusion Center (Contributor), Whyte, Dennis (Contributor)
• Format: Article
• Language: English
• Published: American Institute of Physics (AIP), 2017-05-08T12:04:57Z.
• Subjects: Article
• Online Access: Get fulltext

 The efficiency of LHCD on Alcator C-Mod drops exponentially with line average density. At reactor relevant densities (> 1 · 1020 [m[-3 superscript]]) no measurable current is driven. While a number of causes have been suggested, no specific mechanism has been shown to be responsible for the loss of current drive at high density. Fast modulation of the LH power was used to isolate and quantify the LHCD deposition within the plasma. Measurements from these plasmas provide unique evidence for determining a root cause. Modulation of LH power in steady plasmas exhibited no correlated change in the core temperature. A correlated, prompt response in the edge suggests that the loss in efficiency is related to a edge absorption mechanism. This follows previous results which found the generation of n||-independent SOL currents. Multiple Langmuir probe array measurements of the conducted heat conclude that the lost power is deposited near the last closed flux surface. The heat flux induced by LH waves onto the outer divertor is calculated. Changes in the neutral pressure, ionization and hard X-ray emission at high density highlight the importance of the active divertor in the loss of efficiency. Results of this study implicate a mechanism which may occur over multiple passes, leading to power absorption near the LCFS.