Impact of beryllium surface layers on deuterium retention in tungsten

In ITER mixed Be/W layers are likely to form and their retention behavior is unknown. A series of ITER-grade tungsten samples have been exposed in PISCES-B to deuterium plasma at 300oC, with and without beryllium impurity seeding of the plasma. If the beryllium concentration is small, the majority of the incident beryllium is re-eroded from the sample surface and only a few nm thick layer of mixed Be/W remains. The W samples exposed without beryllium seeding showed blister formation. The inclusion of 0.2% beryllium impurity ion fraction within the plasma eliminated the surface blisters, however did not change the amount of retained deuterium in the samples. When the beryllium impurity concentration in the plasma is large (2.5%), a beryllium layer grows as the fluence increases. The deuterium retention also increases and exceeds the amount retained in either the pure tungsten, or the mixed Be-W layer on tungsten, case. JNM