Fluxon pinning in niobium films

Abstract

Resistive losses induced by the presence of trapped magnetic flux in niobium superconducting films have been studied using 1.5 GHz microwaves. They are measured to span a very broad spectrum depending on the film-substrate interface and on the gas used in the sputtering discharge. An interpretation in terms of pinning by noble gas clusters is considered.

Superconducting niobium cavities, a case for the film technology

“…This has been described [6] as being the result of an anomalous behaviour of the upper critical field H c2 . Recent H c2 measurements [13] do not support this conjecture, the results being instead in conformity with what may be expected from a reasonable l-dependence of the upper critical field and leaving no room for any kind of exotic behaviour. The properties of R fl are best understood in terms of pinning, apparently dominated by "bubbles" of the noble gas atoms used in the sputtering discharge.…”

“…The sensitivity of the residual resistance to trapped magnetic flux [13], measured by R fl , is nearly two orders of magnitude larger for the bulk than it is for standard films. This has been described [6] as being the result of an anomalous behaviour of the upper critical field H c2 .…”

“…Such a belief has its roots in erroneous interpretations of early data, making no explicit distinction between the residual and BCS components of the surface resistance (the latter is dominant at LEP2) and accepting the conjecture [7][8][9] that most of the residual resistance of niobium films should be blamed on their small grain size, and therefore could not be reduced to negligibly small values. Now that an extensive set of data [10][11][12][13][14] is available which makes it possible to better analyse and understand the mechanisms contributing significant RF losses in niobium films, such a conjecture is no longer tenable and new light has been shed on the compared merits of the bulk and film technologies. The present paper aims at a brief and simple presentation of the arguments of relevance.…”

“…The data [10][11][12][13][14] reviewed in the present paper were obtained using single-cell cavities of a geometry similar to that of CEBAF and TESLA cavities, operated in the fundamental 1.5 GHz TM 010 mode. A detailed description of the experimental method, including the production of 1.5 Pm thick films coating the inner wall of copper cavities, the measurement of their surface resistance down to temperatures of 1.7 K, and the analysis of the data in terms of a limited number of variables are available in Reference [10] and are not repeated here.…”

“…To a good approximation its temperature dependence is given by R BCS v exp(-'/T) / T with ' being the energy gap. The fluxon induced resistance [13] …”