Longitudinal uniformity of MgB₂ wires made by an internal magnesium diffusion process

Abstract

Single- and multi-core MgB2 wires have been fabricated by an internal Mg diffusion (IMD) process. Wires uniformity after drawing and/or rolling deformation has been studied by optical microscopy and x-ray micro-tomography. Several short samples taken lengthwise between both ends of the wire were annealed and their critical current measured. It was found that I c of the single-core barrier-free MgB2/Fe wire with high fill factor (32.5% of MgB2) vary by 100% towards the averaged I c. Multi-core wire having ∼15.0% of MgB2 and Cu outer sheath shows better uniformity with 25% deviation of I c over the 10 m length, which is still quite low and needs some improvements. It is expected, that improved in-length uniformity can be reached only for filamentary IMD wires with optimized deforming process and mechanically stronger outer sheath.

High-current-density Rutherford MgB2 cable sheathed by CuNi30 alloy

“…In comparison to the averaged critical current I c-av. (6 T) = 11.8 A, the range of I c deviation along the 100 m wire length is +14.1%and −10.3%, which is much better than for our previously published seven-filament Cu sheathed 10 m long IMD wire with 25% deviation of I c[12]. The improved uniformity of the present wire can be attributed to the mechanically stronger CuNi30 outer sheath in comparison to softer Cu.…”

“…AC loss in the cable samples with thickness of 0.85 mm and the rolled one of 0.78 mm were compared to see the possible effect of cable compaction on inter-strand coupling. and −10.3%, which is much better than for our previously published seven-filament Cu sheathed 10 m long IMD wire with 25% deviation of I c [12]. The improved uniformity of the present wire can be attributed to the mechanically stronger CuNi30 outer sheath in comparison to softer Cu.…”

“…Recently, 24-strand Rutherford cable was fabricated for an energy storage device using the filamentary wire provided by Sam Dong in which a deterioration of the current-carrying capacity by 25% was observed [9]. In contrast to PIT, MgB 2 wires made by internal magnesium diffusion (IMD) into B have much higher current densities [10,11], but the less uniform Mg-core/B-powder ratio affects the creation of MgB 2 and consequently leads to larger in-length variations of critical currents [12]. A Rutherford MgB 2 cable manufactured from IMD wires with reinforced Al + Al 2 O 3 material was successfully developed due to the considerably lower mass of Al in comparison to Cu [13].…”

Water ice-cooled MgB2 coil made by wind and react process

“…The mentioned differences in I c can be affected by the not ideally uniform temperature inside the heat-treated coil; see figure 2(c). This means that the in-length uniformity of the used MgB 2 wire with CuNi30 sheath is much better than that of the wire with Cu sheath made by a similar diffusion process having a variance of I c of 25% over a distance of 10 m [26]. The observed improvement in longitudinal uniformity of the wire in the present coil can be attributed predominantly to the applied CuNi sheath, which is mechanically stronger than copper.…”

“…The mentioned differences in I c can be affected by the not ideally uniform temperature inside the heat-treated coil; see figure2(c). This means that the in-length uniformity of the used MgB 2 wire with CuNi30 sheath is much better than that of the wire with Cu sheath made by a similar diffusion process having a variance of I c of 25% over a distance of 10 m[26]. The observed improvement in longitudinal uniformity of the wire in the present coil can be attributed predominantly to the applied CuNi sheath, which is mechanically stronger than copper.Figure8(b) shows the DC measured critical currents of the W&R coil in water ice compared with the I c of the short sample obtained by PC measurement below 36 K. Both I c (T) dependences have similar tendencies but different absolute values, which is caused by the applied measurement technique (DC or PC) as well as by different self-fields.…”