Protective device for a vacuum seal

An extreme ultraviolet (EUV) light source comprising:

• a chamber defining an interior in which a target material is provided;
• at least one sealing apparatus in fluid communication with the interior and configured to seal a first fitting and a second fitting to thereby define an internal environment for target material, the sealing apparatus comprising:
• a vacuum sealing device chemically reactive to the target material in the internal environment, the vacuum sealing device fixed between the first and second fittings;
• and
• a protective device chemically non-reactive to the target material, the protective device positioned between the internal environment and the vacuum sealing device, wherein the protective device defines a test pathway between the internal environment and the vacuum sealing device when at a test temperature at which the target material is non-migrating, the test pathway including one or more through pathways within the protective device.