Abstract Details
Abstracts
Author: Michael D Campanell
Requested Type: Pre-Selected Invited
Submitted: 2019-02-22 02:51:21
Co-authors: G.R. Johnson, M.V. Umansky
Contact Info:
Lawrence Livermore National Laboratory
7000 East Ave
Livermore, CA 94550
USA
Abstract Text:
Conventional theoretical models of tokamak scrape-off layer plasmas set boundary conditions assuming the surfaces are below the plasma potential. Recently, our studies found that when emission is sufficiently strong, the plasma-surface interaction switches to an inverse sheath regime where the surface is above the plasma potential and ions are confined [1]. Inverse regimes may have several important consequences in magnetic fusion. Inverse sheath theory explains why emissive probes often float above plasma potential in experiments [2]. Simulation studies [3] suggest that inverse sheaths cause formation of ion-ion plasmas in negative ion sources, an important issue for the design of ITER neutral beams. In a new study, we showed that if an inverse sheath forms at a divertor plate, it will cool the target plasma to a sub-eV temperature, even if the upstream plasma is tens of eV or hotter [4]. This extreme cooling effect was not predicted by other studies of thermionic emission in divertors [5] because the sheath was assumed to be space-charge limited (SCL). Under SCL theory, the target plasma can remain arbitrarily hot relative to the thermionic temperature. Our work shows that an inverse sheath is more stable and it constrains the nearby plasma to the tenths-of-eV thermionic temperature. It follows that use of thermionic divertor plates with inverse sheaths may offer an alternative method of inducing radiative detachment, avoiding the usual need to inject impurities that can contaminate the core plasma.
[1] M. D. Campanell and M. V. Umansky, PoP 24, 057101 (2017) and PRL 116, 085003 (2016)
[2] B. F. Kraus and Y. Raitses, PoP 25, 030701 (2018)
[3] Z. Zhang et al., PSST 27, 06LT01 (2018)
[4] M. D. Campanell and G. R. Johnson, PRL 122, 015003 (2019)
[5] M. Komm et al., PPCF 59, 094002 (2017)
* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.
Comments:
