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The in-vacuum undualtor (IVU) is one of the key technologies of the SPring-8 XFEL. This makes it possible to select a shorter magnetic period, and thus a lower electron energy. One concern on the use of IVUs is that the magnetic performance cannot be checked easily after assembly, because the magnetic array is placed inside the vacuum chamber and thus the conventional system is not available. To solve this, we have recently developed a new measurement system called "SAFALI", based on laser instrumentation and dynamic feedback of the magnetic sensor position. With the SAFALI system, we measured the magnetic field of the prototype IVU (18 mm period and 5 m length) for the SPring-8 XFEL and checked the reproducibility of the magnetic performance before and after vacuum chamber assembly. We also checked the variation of the magnetic performance due to temperature change and gradient in the measurement laboratory of SPring-8. In addition, we attached high-precision digital gauges to the magnetic array on 6 different longitudinal positions to observe correlation between the gap variation, temperature gradient, and undualtor phase error, which will be reported in this paper.
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