LABORATORY EXPERIMENTS ON HYDROGEN AND IMPURITY BEHAVIORS IN LITHIUM-DEPOSITED ENVIRONMENT

Wall conditioning with deposition of thin lithium layers gives rise to low hydrogen recycling and reduction in oxygen and carbon impurities. To understand such effects, small-scale laboratory experiments are ca rried out mainly focusing on the chemical state of hydrogen involved i n the lithium layer. First, measurements of the methane yield from gra phite walls in a hydrogen glow discharge reveal that the lithium depos ition leads to similar to 25% reduction of chemical sputtering. Second , thermal desorption of hydrogen molecules from the lithium layer sugg ests existence of different states of hydrogen: one is weakly bound hy drogen which is released from the lithium layer at T similar to 200 de grees C, and the other is attributed to lithium hydride which decompos es for T > 400 degrees C. Finally, a new wall conditioning based on a Lithium borohydride (LiBH4) discharge is demonstrated, which enables c o-deposition of lithium and boron.