apparatus for dissociating cleaning gas such as nf 3 for use in semiconductor manufacturing includes a housing enclosing a microwave resonator to which microwave energy is applied , a plasma tube within the housing and within which cleaning gas flows and is dissociated by the microwave energy . the plasma tube has two ends into and out of which the cleaning gas flows . a first and a second structural assembly thermally protects and seals each end of the plasma tube against atmospheric leaks . each structural assembly has a metal collar and a sealing o - ring fitting tightly around a respective end of the plasma tube . each metal collar includes a thin layer of elastomeric material of high thermal conductivity for conducting heat through the collar away from the end of the plasma tube thereby protecting the o - ring from heat damage . this permits the apparatus to operate more efficiently . a fan forces ambient air over the apparatus .