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A rapid heating and quenching (RHQ) operation along a long-length of jellyroll Nb/Al precursor over 300 m has been established. The Jc varies sensitively with the joule-heating current (IRHQ) that determines the temperature from which a precursor wire is quenched, excluding the "plateau" region where the bcc supersaturated-solid solution (Nb(Al)ss) only forms. The RHQ operation using such a plateau condition can suppress a scatter of Jc from point to tail of a long-length of wire. The standard deviation of Jc (4.2 K) at 21 T along a long-length of wire was about 5%, which was less than the variation of Jc with IRHQ in the plateau region (±7%). The mechanical deformation process needed when incorporating the Cu stabilizer also plays an important role in ensuring uniform heat treatment of the coil that is wound with a long-length of wire and, hence, large heat capacity. Although a temperature ramp-up rate might differ from place to place in the coil, the deformation processing makes such a ramp-up rate insensitive to these critical parameters. The coils using 300.5 m and 254 m lengths of Cu-clad RHQT JR Nb3Al conductor generated additional 2.3 T and 3.2 T in bores of 90.2 mmφ and 64.6 mmφ, respectively, indicating that a long-length of RHQT JR Nb3Al conductor can be trustworthy for practical coil applications.