In so-called cavity charges superfast jets with tip velocities in the range of 25 km/s were found. Such fast jets are strongly eroding by the perforation of the ambient air. The ablated jet material forms a tube around the jet and is optically protecting the core of the jet itself. With tests, where the jet is elongating in a vacuum, this problem does not exist or is drastically reduced. To get enough special resolution, the profile streak technique (PST) was used to obtain the diameter of the stretching jet as a function of the passing time or with respect Co its velocity gradient. With this special diagnostic technique the radius of these superfast jets was measured the first time. The used test method with typical results together with the applied evaluation technique will be described.