A New Generation of In Situ ${\rm MgB}_{2}$ Wires With Improved $J_{\rm c}$ and $B_{\rm irr}$ Values Obtained by Cold Densification (CHPD)

By means of Cold High Pressure Densification (CHPD), the critical current density,
J
_c
, of binary and alloyed MgB
₂
wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB
₂
mass density of binary MgB
₂
wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C
₄
H
₆
O
₅
(malic acid) alloyed wires, densification also caused the enhancement of
B
_irr
, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic
J
_c
values were obtained for C
₄
H
₆
O
₅
added wires of 1 × 0.6 mm
²
cross section, the values of
J
_c
(4.2 K)=1 × 10
⁴
A/cm
²
for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of
B
_irr
^//
at 20 K was 11 T. The positive effects of cold densification on J
_c
and
B
_irr
on MgB
₂
was also observed on 150 mm long wires alloyed with C
₄
H
₆
O
₅
(malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding J
_c
(4.2 K) = 1 x 10
⁴
A/cm
²
at 13.1 T. Further improvements are expected after optimization.