Recent advances in MgB 2 conductors are leading to a new level of performance.Based on the use of proper powders, proper chemistry, and an architecture which incorporates internal Mg diffusion (IMD), a dense MgB 2 structure with not only a high critical current density J c , but also a high engineering critical current density, J e , can be obtained. In this paper, a series of these advanced (or second-generation, "2G") conductors has been prepared. Scanning electron microscopy and associated energy dispersive X-ray spectroscopy were applied to characterize the microstructures and compositions of the wires, and a dense MgB 2 layer structure was observed. The best layer J c for our sample is 1.07x10 5 A/cm 2 at 10 T, 4.2 K, and our best J e is seen to be 1.67x10 4 A/cm 2 at 10 T, 4.2 K.Optimization of the transport properties of these advanced wires is discussed in terms of Bpowder choice, area fraction, and the MgB 2 layer growth mechanism. PACS: 74.70.Ad; 74.25.Sv; 74.25.Qt; 74.62.Dh Keywords: MgB 2 , layer critical current density J c , engineering critical current density J e , internal magnesium diffusion (IMD)

Transport measurements of critical current density, J ct , in monocore powder-in-tube MgB 2 strands have been carried out at temperatures, T, of from 4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous-tube-forming-filling (CTFF) and internal-magnesium-diffusion (IMD).Strands with several powder compositions were measured, including binary (undoped) MgB 2 , 2% carbon doped MgB 2 , and 3% carbon doped MgB 2 . Magnetization loops (M-B) were also measured, and magnetic critical current density, J cm , values extracted from them. The transport, J ct (B) and magnetic, J cm (B), critical current densities were compared.Also studied was the influence of doping on the resistively measured irreversibility field, B irr and upper critical field B c2 . Critical current densities, J ct , and n-values were extracted from transport measurements and were found to be universally related (for all B and T) according to n∝ J ct m in which m = 0.52 ± 0.11. Likewise n was found to be related to B according to n ∝ B -p with a T-dependent p in the range of about 0.08~0.21. Further analysis of the field (B) and temperature (T) dependencies of n-value resulted in an expression that enabled n(B,T), for all B and T, to be estimated for a given strand based on the results of transport J ct (B) measurements made at one arbitrarily chosen temperature. Symbols B Magnetic field strength B irrResistively determined irreversibility field B irr,trans Apparent irreversibility field for magnetically induced current paths which must always include a portion transverse to the strand axis B c2Upper critical field B 0 (t)Temperature-dependent field-normalization quotient in the exponential form of J c (B,t) -see below B 00 Zero-temperature value of B 0 (t) CTFF Continuous tube forming/filling (process for strand fabrication) d 0 Inner diameter of the Nb chemical barrier (both CTFF and IMD strands) d i Inner diameter of the annular MgB 2 reaction layer (IMD strands) ∆M Full height of the magnetization loop normalized to the actual SC volume F p Bulk pinning force density F p,max Maximum value of F p vs B or B/B irr IMD Internal magnesium diffusion (process for strand fabrication) J c Abbreviation for the words "critical current density" J ct Transport-measured critical current density J cm Magnetically measured critical current density J cte Transport critical current density found by normalized I ct to the whole area inside the Nb chemical barrier J c2Critical current density transverse to the strand axis (magnetically induced) J c (B,t) Field-and temperature dependent J c J ct (B,t) Transport-measured value of field-and temperature dependent J c J c0 (t)Fitted zero-field temperature-dependent J c J ct0 (t)Fitted zero-field temperature-dependent J c based on transport measured field and temperature dependent J c equation J c00Fitted value of J c at zero field and zero temperature

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Conventional doping methods that directly add C or a C-bearing species to Mg+B powder have the disadvantage of adding C inhomogeneously, yielding either under-reacted regions or blocking phases. Pre-doped B powder provides a more homogeneous distribution of the C dopant in MgB 2 . Powders containing varying amounts of C were used to produce in-situ MgB 2 strands which showed high values of transport J c (10 4 A/cm 2 at 13.3T). Compared to SiC-added and malic acid-treated strands the pre-doped MgB 2 showed both higher values of B irr and transport J c , indicating that the pre-doping of B leads to more efficient C substitution into the B-sublattice.

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