Beryllium in JET: a report on the operational experience

The evaporation of beryllium inside the JET (Joint European Torus) has changed considerably the behavior of the machine and the plasma performance. Recovery from disruption was fast and density control was easily achieved. The operational space with respect to Z_eff-values, dilution, density, and temperature could be considerably extended. The dilution was reduced and consequently the fusion parameter was doubled to 5×10²⁰ m^-3 keV. The equivalent Q_DT-value, albeit nonstationary, approaches 0.5. Beryllium tiles were installed on the belt limiter, requiring for the first time an intervention into the vessel with full biological protection. The control of the beryllium environment was successful and installation times did not require more manhours than previously for in-vessel work. Experimental results from operation with the beryllium limiter are not yet available. Preliminary findings indicate the possibility of obtaining very low Z_eff-values (<1.3-1.8) for additionally heated discharges. Energies of up to 180 MJ were deposited on the limiter, twice the amount which can be taken by the graphite limiter before the occurrence of the carbon bloom. At the edges of the beryllium limiter tiles, hot spots develop (as was also observed for the graphite)