By regarding intense electron beams (∼340 keV, ∼2 kA, ∼10 ns) of a single pulse mode as a magnetic field source, we observed diffusion velocities of self‐magnetic fields of the beams through a 1.5 mm thick wall of a sintered superconducting Bi(2223) tube. Contrary to our prediction, fields of about 900 G out of the self‐magnetic fields diffused through the wall with an average diffusion velocity of 4×104m/s, almost independently of the operation temperatures for the range fromTc(∼103 K) to 80 K. Such a slow, viscous diffusion process accounts for effective functioning of superconducting tubes as lenses (Supertrons) for the short electron beams. ©1996 American Institute of Physics.