Hydrogen and helium trapping in tungsten under single and sequential irradiations

The trapping of hydrogen and helium in polycrystalline tungsten irradiated with 500 eV He⁺, H⁺ and D⁺ ions, individually or sequentially, has been measured by thermal desorption spectroscopy. Specimens irradiated with 500 eV He⁺ at 300 K show three He release peaks in the vicinity of ∼500, ∼1000, and ∼1200 K. The helium is thought to form He vacancy complexes or bubbles. Increasing the specimen temperature to 700 K does not significantly affect the trapping behavior of He. Sequential He⁺-D⁺ irradiation at 300 K results in the elimination of He release above 800 K. Instead, both D and He were released in the range 400-800 K. This is interpreted as interstitial D and He released from the near surface. Sequential He⁺-D⁺ irradiation at 700 K resulted in a reduced single He peak at ∼1000 K with very little release observed below 800 K; no D was trapped for irradiations at 700 K. Sequential D⁺-He⁺ irradiations at 300 K show that He trapping occurs in much the same manner as for the He⁺-only case while D retention is reduced at the near surface. Sequential D⁺-He⁺ irradiations at 700 K indicate that pre-irradiation with D⁺ has little or no effect on the subsequent trapping behavior of He.