The present invention provides a method and apparatus for increasing the intensity of coherent population trapping resonances, used in atomic clocks and magnetometers, by pumping the atoms with light of alternating polarization. A water seat is configured to block the flow path. Thereby, the formation of a short circuit due to ion-beam irradiation may be hindered. After the suctioning off the vacuum in the padding is maintained for a certain time period, before renewed introduction of the active substance.