How high a field has been and can be achieved in superconducting bulk niobium cavities: the role of RRR

Abstract This Brief Note investigates the relationship between the residual resistivity ratio (RRR) and the maximum achievable surface magnetic field in superconducting bulk niobium (Nb) cavities. Data spanning the 1980s to the 2020s, with RRR values ranging from 30 to 500, are compared with theoretical performance limits, including the lower critical field ( B c 1 ), the superheating field ( B sh ), and the thermal runaway field ( B run ). The analysis reveals that modern Nb cavities are approaching B run and the metastability region above B c 1 across the entire RRR spectrum, but they remain well below the fundamental limit set by B sh . Achieving B sh requires not only a defect-free surface but also ensuring that B run exceeds B sh . The latter condition, often overlooked, is not always met at 2 K.