Experiments with Lithium Gettering in T-10 Tokamak

The two experimental campaign with lithium gettering of the T-10 chamber were carried out. The experiments were aimed to prove the lithium abilities to obtain high performance discharges with lithium cover of the wall and limiters with reduced recycling of the working gas and the impurities level in ohmic and heated with the high ECR power tokamak discharges. The second goal of the T-10 experiments were to extend the operational space of discharge parameters in T-10 limiter configuration with graphite limiters in conditions of the clean plasma with low recycling and to test compatibility of the Li-gettering with the proper work of the primary vacuum windows of the T-10 ECRH system. The lithium element was manufactured on the base of the kapillar porous system. The lithium element was placed in the same port as the graphite rail and circular limiters. It was introduced to the vacuum chamber by means of the special transport vacuum system. The inner heater was able to heat it up to the 550 C. Typically, about one gram of Li was evaporated each time at the temperature 450 C during 20 minutes. Such technique was sufficient to reduce density decay time from about 1.5 s to 0.075 s. The significant decrease of the recycling was sustained during 10 discharges. The SXR emission decreased in a factor of 10. The central radiative losses and the total radiative losses decreased in a factors of 4 and 2 respectively. This corresponds to the decrease of the high Z impurities in a factor of 4, oxygen in 7, while the main carbon impurity decrease up to 2 times and Zeff from 1.8 to 1.15. Thus the experiment showed significant decrease of the impurities and recycling. At the same time the influence of the lithium gettering on the windows transmission and stable work of the high power ECRH system were not observed. The problem of plasma-wall interaction is now one of the main obstacle on the construction of the power tokamak-reactor. Several approaches to this problem are currently under investigation and testing on tokamaks. The lithium protection of the wall, proposed in [1] is now one of the promising solution of the problem. Several tokamaks investigate this approach using lithium either as the wall getter, or the limiter material [2-5], since the successful TFTR experiment [6]. Although all these experiments showed not only compatibility of lithium with the tokamak discharges, but even significantly improved the discharge performance, a lot of unresolved questions are still exist. So the main goal of T-10 experiments with lithium gettering is the investigation of lithium behavior and discharge characteristics in OH and high power ECR heated discharges. Apart from that these experiments had a practical goal to reduce the recycling and impurities level and provide clean plasma with density control in T-10 tokamak with the graphite limiters and typically high deuterium recycling. Experiments should also prove the compatibility of lithium with the reliable work of T-10 high power ECR system with respect to the possible damage of the gyrotrons vacuum windows. There were also questions about the techniques of the conditioning of the installation with lithium after long opening to atmosphere. Taken this into account, only the moderate Li gettering was used during the first campaign in 2006 [8]. This was done by positioning of the Li head away from the chamber in the port. As the