Magnetic flow pinning in Nb on helium bubbles

Field and temperature dependences of Fsub(p) pinning strength in niobium foils at different concentrations of rho pinning centres (PC) in the form of helium bubbles formed by He+ implantation with subsequent annealing are investigated. A particular attention is given to t reduced high temperature range. In this range specific features of Fsub(p) temperature dependence in average fields show the manifestation of additional to pinning on bubbles pinning mechanism. Possible causes of nonrealization of similarity law are discussed. Monotonic growth of Fsub(p) with rho is observed. Its features at high t and rho permit to suggest collective pinning manifestation on weak PC or small PC for account of quasiparticles scattering. The results obtained are compared with the theories of direct and statistical summing elementary pinning strength