The doctoral program has two application deadlines during the year and is open to students who are currently pursuing a doctoral program or who are in the process of planning a doctoral program within a technical subject. In total, there are 50 places for doctoral students at CERN each year and if you are admitted to the program, you join a group at CERN where you normally work for 24 months.
A separate NTNU-CERN Doctoral Degree Program has recently been set up as part of the collaboration between CERN and NTNU. These positions are first triggered at NTNU, and if one is awarded the position, the application for CERN's doctoral degree program is a formality.
For more information, see CERN's official doctoral program page.