Laser and electron beam processing of superconductors

Abstract

Scanning irradiations using high power density beams of CW-CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> lasers and electrons have been applied for the fabrication of superconducting Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al and Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Al,Ge) conductors. The materials to be irradiated were prepared in tape form and the scanning was carried out by moving the material in a stationary beam with the intension of future development to industrial applications. For the formation of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga, a tape composed of NbGa <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> compound layer on the Nb substrate was prepared by a continuous dipping process. The laser irradiation successfully produced a new layer containing A15 Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga by alloying the NbGa <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> layer and a portion of the underlying Nb substrate. The highest T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> attained by the irradiatioh is 15.9K (midpoint), and is increased to 18.2K by the following low temperature annealing at 700°C. Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al and Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Al,Ge) tape conductors were also fabricated by laser and electron beam scanning irradiations onto Nb-Al and Nb-Al-Ge composite tapes prepared by the conventional powder metallurgy method. The laser irradiation directly onto the Nb-Al composite produced a molten and then rapidly resolidified zone composed of α-Nb dendrite and A15 Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al. The T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> values are well over 16K, which can be increased by 1-2K by the subsequent annealing. A maximum J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> for reacted zone of 4.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was obtained at 23T and 4.2K. In order to minimize the loss of Al, the irradiation onto Nb-sheathed Nb-Al and Nb-Al-Ge composites was also carried out using both laser and electron beams. The J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> increases with increasing power density until the melting including Nb-sheath occurs, and then rapidly decreases at higher power densities due to the decrease of A15 volume fraction. The Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al and Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Al,Ge) tapes processed under proper conditions show excellent high field properties, the J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> exceeding well over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in magnetic fields up to 23T. Compared with the electron beam irradiation, the laser beam irradiation has a disadvantage that the absorption coefficient of energy into the material is sensitive to surface conditions neccesitating special care in handling the material surface. However, in the case of laser irradiation, operation in the pressurized atmosphere is possible, which is very effective to minimize the loss of volatile elements such as Al,Ga. In order to study the feasibility to large scale production, new continuous irradiation systems have been installed for both CW-CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser and electron, and the examinations to fabricate longer tapes of 10-100m are now in progress.