Lithium-based surfaces controlling fusion plasma behavior at the plasma-material interface

Abstract

The plasma-material interface and its impact on the performance of magnetically confined thermonuclear fusion plasmas are considered to be one of the key scientific gaps in the realization of nuclear fusion power. At this interface, high particle and heat flux from the fusion plasma can limit the material's lifetime and reliability and therefore hinder operation of the fusion device. Lithium-based surfaces are now being used in major magnetic confinement fusion devices and have observed profound effects on plasma performance including enhanced confinement, suppression and control of edge localized modes (ELM), lower hydrogen recycling and impurity suppression. The critical spatial scale length of deuterium and helium particle interactions in lithium ranges between 5-100 nm depending on the incident particle energies at the edge and magnetic configuration. Lithium-based surfaces also range from liquid state to solid lithium coatings on a variety of substrates (e.g., graphite, stainless steel, refractory metal W/Mo/etc., or porous metal structures). Temperature-dependent effects from lithium-based surfaces as plasma facing components (PFC) include magnetohydrodynamic (MHD) instability issues related to liquid lithium, surface impurity, and deuterium retention issues, and anomalous physical sputtering increase at temperatures above lithium's melting point. The paper discusses the viability of lithiumbased surfaces in future burning-plasma environments such as those found in ITER and DEMO-like fusion reactor devices. V C 2012 American Institute of Physics. [http://dx.

Effect of lithium in the DIII-D SOL and plasma-facing surfaces

“…Ex-Situ analysis of samples exposed to lithium, using DiMES, indicated that lithium once deposited, can diffuse or intercalate to depths up to 1000 nm. This is consistent with observations from other work [10,13]. In addition, 25 graphite samples exposed during the entire 2013 campaign showed that lithium deposited in the tiles was very non-uniform, consistent with observations by MDS.…”

“…We note that the direction of SOL flow discussed above is consistent with previous DIII-D 13 C experiments [12].…”

“…This was confirmed using Parks' pellet ablation model [11] where ionization occurred outside the separatrix for typical DIII-D pulses. Hence, the dominant flow might be expected from the injector directly to the vicinity of the strike points, as has been reported for 13 C trace impurity injection [12].…”

“…The observed distribution of LiI radiation discussed in Section 2 persists for many discharges after lithium injection; thus lithium is retained in the graphite tiles that make up the DIII-D PFCs. Although Li retention has been observed in other tokamaks [13] the absence of Li before this work and a precise measurement of injected lithium allowed a careful assessment of lithium retention and also it's presence in ensuing tokamak discharges.…”

Deuterium Uptake in Magnetic-Fusion Devices with Lithium-Conditioned Carbon Walls

“…This dramatic increase in surface oxygen content during deuterium bombardment is surprising, and leads to unexpected consequences for the deuterium chemistry. In particular we found that when lithiated carbon is bombarded with low-energy deuterium ions ( 50-500 eV), significant chemical shifts in the oxygen XPS photoelectron O1s spectrum are observed experimentally [14,17,[21][22][23]. The chemical changes were observed as stronger shifts in the O1s spectra when compared to those found in Li1s and C1s XPS spectra for lithiated carbon (and also compared to laboratory XPS experiments with lithium- only foils [21]), challenging the notion that the uptake of deuterium occurred by hydride bonding with lithium atoms.…”