Improved pinning by multiple in-line damage

Columnar pinning centres provide the largest pinning potential, Upin ,b utnot the greatest Jc or pinnable field, Bpin .C haracteristics of ion-generated columnar defects which limit Jc and Bpin are discussed, including reduction of the percolation path, and the need for a larger number of columns of damage, for pinning, than are usually estimated. It is concluded that columnar pinning centres limit Bpin to less than 4 T, and also severely reduce Jc .T hegoal of maximizing Upin ,v ia columnar centres, appears to have obscured a more rewarding approach and resulted in neglect of a large regime of ion interactions. Evidence is reviewed that multiple in-line damage (MILD), described herein, can provide orders of magnitude higher Jc and Bpin ,d espite providing lower Upin .T he MILDpinning centre morphology is discussed, and it is estimated that for present-day large grain high Tc superconductors, a Jc value of ∼10 6 Ac m −2 is obtainable at 77 K, even when crystal plane alignment and weak links are not improved. In addition, the pinned field is increased by over an order of magnitude. An experiment is proposed to confirm these calculations, directly compare MILD pinning to continuous columnar pinning, and determine the optimum MILD structure. Applications of MILD pinning are discussed.