A betatron, especially in X-ray testing apparatus is provided, that includes a rotationally symmetrical inner yoke having two interspaced parts, an outer yoke connecting the two inner yoke parts, at least one main field coil, a toroidal betatron tube arranged between the opposing front sides of the inner yoke parts, and at least one contraction and expansion coil. In order to improve the transition from the turbomolecular zone to the molecular zone, the transition stage has a flow section that is continuously tapered in the tangential by limiting surfaces. The pre-treated water is supplied to a reverse osmosis module which separates the water into two streams by collecting fluids from both sides of pressurized membranes.