  An effective ion catcher is an important part of a radioactive beam facility
that is based on in-flight production. The catcher stops fast radioactive
products and emits them as singly charged slow ions. Current ion catchers are
based on stopping in He and H$_2$ gas. However, with increasing intensity of
the secondary beam the amount of ion-electron pairs created eventually prevents
the electromagnetic extraction of the radioactive ions from the gas cell. In
contrast, such limitations are not present in thermal ionizers used with the
ISOL production technique. Therefore, at least for alkaline and alkaline earth
elements, a thermal ionizer should then be preferred. An important use of the
TRI$\mu$P facility will be for precision measurements using atom traps. Atom
trapping is particularly possible for alkaline and alkaline earth isotopes. The
facility can produce up to 10$^9$ s$^{-1}$ of various Na isotopes with the
in-flight method. Therefore, we have built and tested a thermal ionizer. An
overview of the operation, design, construction, and commissioning of the
thermal ionizer for TRI$\mu$P will be presented along with first results for
$^{20}$Na and $^{21}$Na.
