JET Neutral Beam Duct Optical Interlock
The JET Neutral Beam Injection (NBI) system is the most powerful neutral beam plasma heating system currently operating. Optical Interlocks were in- stalled on the beam lines in 2011 for the JET Enhancement Project 2 (EP2), when the heating power was increased from 23MW to 34MW. JET NBI has two beam lines. Each has eight positive ion injectors operating in deuterium at 80kV­125kV (accelerator voltage) and up to 65A (beam current). Heating power is delivered through two ducts where the central power density can be more than 100MW/m2. In order to deliver this safely, the beam line pressure should be below 2 ¥ 10­5 mbar otherwise the power load on the duct from the re-ionised fraction of the beam is excessive. The new Optical Interlock monitors the duct pressure by measuring the Balmer-a beam emission (656nm). This is proportional to the instantaneous beam flux and the duct pressure. Light is collected from a diagnostic window and focused into 1-mm diameter fibres. The Doppler shifted signal is selected using an angle-tuned interference filter. The light is measured by a photo-multiplier module with a logarithmic amplifier. The interlock activation time of 2ms is sufficient to protect the system from a fully re-ionised beam ­ a significant improvement on the previous interlock. The dynamic range is sufficient to see bremsstrahlung emission from JET plasma and not saturate during plasma disruptions. For high neutron flux operations the optical fibres within the biological shield can be annealed to 350°C. A self-test is possible by illuminating the diagnostic window with a test lamp and measuring the back scatter. We demonstrate an important technology for the protection of high power neutral heating beams and present the design and operational results.