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Cloud of Li vapors increases Li coating lifetimes on TZM Mo

In support of the NSTX-U (the spherical tokamak NSTX of Princeton University, upgraded) development programme at Princeton's Plasma Physics Laboratory (PPPL), a series of experiments were conducted at Magnum-PSI to test how a lithium (Li) coating would hold under conditions comparable to the NSTX-U divertor conditions.

The first experiments showed that the Li-coating can shield the divertor region longer than previously expected. The vaporized lithium produced a cloud of vaporized Lithium that serves as a shield to mitigate the plasma fluxes on the wall. A high percentage of the eroded wall material had been confined to the surface of the sample target, which is great - as it keeps impurities from drifting into other areas of the machine.

Figure: plasma beam in Magnum-PSI [Source: DIFFER]

During discharges on Li-coated graphite, the high confinement at the target decreased after five seconds. In contrast, Li coating lifetimes on TZM Mo (Titanium-Zirconium-Molybdenum alloy) are strongly extended - lasting more than 30 seconds - due to re-deposition fractions R(Li) > 0.85.

The experiments at Magnum-PSI are used to further develop a re-deposition model to determine lifetime of Li coatings in NSTX-U.