First technical plasma following 2007 Shutdown

At the end of November 2007, following a very successful Shutdown period, JET entered a Restart phase. During this period, the JET machine and its many systems, are commissioned in readiness for the next Experimental Campaigns which are planned to start in April 2008. Due to the complexity of the facility the progress of a Restart cannot be entirely predicted and frequently the team has to react flexibly to unforeseen situations. It is therefore satisfying to report that JET obtained its first technical plasma on the 5th of February - an important milestone in the Restart programme. Additionally, in the same week, pulses with plasma currents up to 2 Mega-Ampere and up to 20 seconds duration were achieved.

From the beginning of the Restart period the JET vessel is pumped by various pumping systems. Typical vacuum pressures between pulses are usually below 10E-6 mbar. During the early part of the Restart, leaks are sometimes experienced and must be subsequently localised, often requiring a bit of detective work. Leaks on Tokamaks can appear in a variety of locations such as welds, feedthroughs and vacuum windows.

After a certain quality of leak tightness is established the plasma-facing vessel surfaces are conditioned. The objective of this process is to release any gas particles which are on the inner wall surfaces. If this process is skipped too many particles would be released during the experiments leading to high levels of impurities in the plasmas. Vessel conditioning techniques are vessel baking up to 320°C and glow discharge cleaning.

All of JET's systems have to be commissioned during a Restart. For instance the power supplies feeding the toroidal and poloidal field coils have to be gradually commissioned up to their full availability. Commissioning the Neutral Beam Injection (NBI) Systems requires an extensive programme. After the NBI power supplies and several safety systems are commissioned the beams are fired for short periods onto targets, which are brought into the beam. During this phase the power of the beams is gradually increased and the alignment of the beams is checked and optimised. A comparable programme has to be performed for the various Radio Frequency Heating Systems.

During the 2007 Shutdown several diagnostic systems were dismantled for refurbishment or simply to gain access to other equipment. These were reinstalled at the end of the Shutdown or at the beginning of the Restart. Furthermore, a number of diagnostics have to be realigned and recalibrated in order to optimise the quality of the data measured during experiments.

Due to the complexity of the JET machine it is a very challenging undertaking to Restart JET within a tight timescale. Work during nights and at weekends is a common means to accelerate this process. Restarts at JET are managed in a very strict manner and all people involved are very focused on achieving one objective: To bring JET back to its full capability as quickly as possible so that the Experimental Campaigns can resume.

11th February, 2008