Before the commissioning of SPring-8, the in-vacuum hybrid undulator d
eveloped at SPring-8 had been brought to the ESRF for the first beam t
est in the summer of 1996. The purpose of this test was to investigate
the influence of the in-vacuum undulator on the beam and check its va
cuum system. However, heating by the resistive wall impedance turned o
ut to be a critical issue for the in-vacuum undulators.