On the limiting mechanism of irradiation enhancement of Ic

Irradiation may significantly increase I_c in HTS. A systematic pattern occurs: R=I_c(afterirr.)/I_c(beforeirr.) increases at low defect density, d. It reaches a peak, and then it falls below 1 at high d. The pinning center mechanism, which causes R to increase, has been extensively studied. The falloff in R has not. It has been considered a secondary effect. Here, we will show that the fall-off plays an important role in determining the maximum I_c enhancement achievable. A phenomenological model to describe the R-vs.-d curve, over the entire d range, is proposed. The idea is that R is the product of two competing effects. (i) Irradiation damage acts as pinning centers, hence increases critical current density, J_c. (ii) Damage reduces the flow-area. Hence, it decreases the net critical current. Data on U/n processed Bi-2223 tapes are fitted to this model. The fitting indicates: (1) the reduction of the flow-area accounts for the majority of the R falloff; and (2) It is sufficient to describe J_c enhancement as linear with d, and it depending on field and temperature only through the ratio b=B/B_irr, where B_irr is the irreversible field before irradiation.