An ion laser with a gas discharge vessel, for example a helium-selenium ion laser, utilizing cataphoretic vapor transport provides monochromatic exit radiation in a parallel beam of high intensity. The discharge takes place in a tube of high silica glass surrounded at some spacing by an envelope tube that is subject to cooling and is therefore gas-tight for helium. The envelope tube has a configuration or partial partition that provides condensation chambers for the vapor. Advantages are increased power especially for compact and short configurations, with high reliability, safety, and service life.