Vol 44, No 7 (2018)

The use of lithium as a material of the tokamak in-vessel plasma-facing components made it necessary to develop appropriate diagnostic instruments. For the T-10 and T-11M tokamaks, devices have been developed that allow one to investigate the processes of lithium transport in the tokamak scrape-off layer, the dynamics of lithium deposition at different temperatures of the collecting surface in real time by using a piezoelectric quartz detector, adsorption and desorption of the plasma-forming gas by lithium, and the influence of the electric field on the process of lithium collection. The plasma parameters are monitored using Langmuir probes. The developed devices can be used to extract lithium deposited on the tokamak vessel wall without breaking vacuum conditions. For these purposes, a gateway and a vacuum input without bellows have been designed on the basis of an innovative liquid-metal coupling.

The lithium emitter−collector scheme has been successfully used on limiter tokamak T-11M for a long time and is therefore one of the most mature concepts of liquid lithium-assisted power and particle exhaust. In the present paper, a possible application of the emitter−collector scheme to the divertor tokamak T-15 is analyzed with the 2D transport code SOLPS4.3. Modeling indicates that placing the lithium limiter at the outer midplane of the vessel results in the widest possible spreading of lithium over the scrape-off layer, whereas lithium deposition in this case is localized primarily at the upper outer target plate. The power exhaust capability of the lithium emitter−collector scheme is also studied. It is often presumed on the basis of a simple 0D analysis that the noncoronality can bring the lithium radiation capability in line with the other low-Z impurities typically involved in the divertor power exhaust (such as carbon and nitrogen). However, detailed 2D modeling shows that in spite of the significant increase of the Li cooling rate due to the noncoronal effects, the contribution of lithium radiation to divertor power balance remains marginal, unless the lithium inventory in the edge becomes close to the deuterium one.

The edge plasma parameters were measured by means of a Mach probe in a lithium experiment on the T-11M tokamak. The angular and radial distributions of the ion saturation current, along with the radial distribution of the electron temperature, were obtained in different modes of tokamak operation. The radial distributions of the electron temperature and ion saturation current in the main operating mode (L-mode) revealed a peak in the scrape-off-layer of the vertical limiter (lithium emitter), which can indicate the formation of a magnetic island in this region. The measured plasma flow velocity along the magnetic field was found to be close to one-half of the ion sound velocity for Li⁺ ions.

Molybdenum (Mo), tungsten (W), and stainless steel (SS) are widely used as important structure materials and first wall materials in fusion devices, while liquid lithium (Li) limiter/divertor can provide an attractive option for withstanding high heat load and solving life-time problem of first wall. Studying the compatibility of these materials exposed to liquid Li is significant for the application in Mo, W, and SS in fusion reactors. The corrosion behaviors of Mo, W, and 304SS exposed to static liquid Li at 600 K up to 1320 h under high vacuum with pressure 10⁻⁴ Pa were investigated. After exposure to liquid Li, it was found that the weight loss of Mo, W, and 304SS increases with corrosion time, but the total amount is moderate. 304SS specimens produce a non-uniform corrosion behavior because of Cr, Ni, and carbon (C) elements selectivity depletion and formation of carbides compound near surface. Mo and W surface microstructures are unchanged. 304SS surface hardness increases with corrosion products because these particles include C element, which increases by 49 HV after exposed to liquid Li for 1320 h, while Mo and W surface hardness are unchanged by the reason of their excellent corrosion resistance.

One of the primary conditions necessary for the success of magnetic fusion reactors is the ability to mitigate damage to the first wall during ELMs and plasma disruptions. A potential solution involves the use of flowing liquid metals such as lithium as a first wall, but ensuring its stability under the extreme environments in the reactor would be imperative. The conditions leading to instabilities on the free surface of flowing liquid lithium (LL) layers on a substrate and in a porous material are investigated using both analytical methods and computational modeling, with consideration for the effects of LL velocity, LL layer thickness, substrate porosity, LL permeability, and hydrogen (H) plasma velocity. Linear stability analysis is used to predict the critical velocity and wavelength-dependence of wave growth, as well as the onset of instability. The modeling of LL flows is performed on a flat substrate and in a porous material for various LL thicknesses, LL and H plasma velocities to analyze the conditions leading to droplet formation and ejection.