Method and apparatus for determining at least one characteristic of a superconductive film

Apparatus for measuring a characteristic of a film comprised of a material that is a superconductor below a critical temperature, comprising:

• means for maintaining the film at a temperature such that the film is in a superconducting state;
• means for inducing an alternating magnetic field at a localized region of a surface of the film, said inducing means including,
• a magnetic path confining means having a gap made therein, said magnetic path confining means having a drive winding coupled thereto;
• and
• current source means coupled to said drive winding for passing an alternating current therethrough for inducing an alternating magnetic field within the gap, said current source means being responsive to a control input signal for varying an amplitude of the alternating current so as to vary a magnitude of the alternating magnetic field;
• means, responsive to a current induced within the film by an entry of the magnetic field into the film, for determining a critical current density of the film within the localized region, said determining means including,
• a sense winding coupled to said magnetic path confining means;
• and
• means for detecting a magnitude of an electrical signal induced in said sense winding by said alternating magnetic field;
• said apparatus further comprising;
• means for translating the film relative to the determining means so that the critical current density of the film is determinable within a plurality of localized regions of the film.

A method of detecting a presence of a non-uniformity within a superconducting film, comprising the steps of:

• maintaining the film at a temperature such that the film is in a superconducting state;
• inducing an alternating magnetic field at a localized region of a surface of the film;
• responsive to a current induced within the film by an entry of the magnetic field into the film, determining a critical current density of the film within the localized region;
• determining, from the critical current density, a superconducting state permeability of the film;
• measuring the permeability of a magnetic circuit that includes the film, at a plurality of localized regions, by inducing the alternating magnetic field at a plurality of localized regions of the film, the alternating magnetic field being provided so as to induce a current density within the film that is less than the determined critical current density;
• and
• observing the measured permeability to detect a localized region wherein the measured critical current multiplied by the thickness of the film deviates from the determined superconducting state critical current, the deviation being due to a non-uniformity of the superconducting film within the localized region.

A method for measuring a critical current density (J.sub.c) at a selected region of a superconductive film, comprising the steps of:

• maintaining the film at a temperature so that the film is in a superconducting state;
• disposing a selected region of the film in proximity to an alternating magnetic field;
• detecting a magnitude of a current induced in the selected region of the film by the magnetic field;
• increasing the strength of the magnetic field so as to increase the magnitude of the induced current;
• and
• monitoring an amount of magnetic flux that is coupled into the selected region of the film to determine a magnitude of the current density at which the film switches from the superconducting state to a normal state, the determined magnitude of the current density being equal to J.sub.c.

A method for measuring a critical temperature (T.sub.c) at a selected region of a superconductive film, comprising the steps of:

• maintaining the film at a first temperature so that the film is in a superconducting state;
• disposing the selected region of the film in proximity to an alternating magnetic field;
• detecting a magnitude of a current induced in the selected region of the film by the alternating magnetic field;
• monitoring an amount of magnetic flux that is coupled into the selected region of the film to determine a magnitude of the current density at which the film switches from the superconducting state to a normal state, the determined magnitude of the current density being equal to J.sub.c ;
• providing the alternating magnetic field so as to induce into the selected region of the film a current that is less than J.sub.c ;
• and
• varying the temperature of the film from the first temperature to determine a temperature at which the selected region of the film switches from the superconducting state to a normal state, the determined temperature being equal to T.sub.c.

Apparatus for detecting a presence of a non-uniformity within a superconducting film, comprising:

• means for maintaining the film at a temperature so that the film is in a superconducting state;
• means for inducing an alternating magnetic field at a localized region of a surface of the film;
• measurement means, responsive to a current induced within the film by an entry of the magnetic field into the film, for determining a critical current density of the film within the localized region and for determining, from the critical current density, a superconducting state permeability of the film;
• said measurement means including means for measuring the permeability of a magnetic circuit that includes the film, at a plurality of localized regions of the film, by inducing the alternating magnetic field at a plurality of localized regions of the film, the alternating magnetic field being provided so as to induce a current density within the film that is less than the determined critical current density;
• and
• means, responsive to the measured permeability, for identifying a localized region wherein the measured critical current, when multiplied by the thickness of the film, deviates from the determined superconducting state critical current, the deviation being due to a non-uniformity of the superconducting film within the localized region.

Apparatus for measuring a critical temperature (T.sub.c) at a selected region of a superconductive film, comprising:

• means for maintaining the film at a first temperature so that the film is in a superconducting state;
• means for generating an alternating magnetic field at a selected region of a surface of the film;
• means for detecting a magnitude of a current induced in the selected region of the film by the magnetic field;
• means for monitoring an amount of magnetic flux that is coupled into the selected region of the film, said monitoring means being responsive to an output of said detecting means to determine a magnitude of a current density at which the film switches from a superconducting state to a normal state, the determined magnitude of the current density being equal to J.sub.c ;
• and
• means for controlling said generating means to generate the alternating magnetic field so as to induce a current into the selected region of the film that is less than J.sub.c, said controlling means further being operable for controlling said establishing means to vary the temperature of the film from the first temperature to determine a temperature at which the selected region of the film switches from the superconducting state to a normal state, the determined temperature being equal to T.sub.c.