Week 19: Getting connected

Added March 5th, 2010 in Shutdown Weekly

How many wires does it take to monitor a tokamak?
As you can imagine, tens of thousands of signals are gathered from the JET machine and used to monitor it. Some are used during the short periods of plasma operation to gain experimental results from each JET pulse. Some of these signals are connected to alarms to warn the technicians of important plant failures. Others are quietly and continuously recorded to help the engineers to diagnose faults retrospectively.

A multiple-redundancy is built in for all the delicate instrumentation components. When one sensor fails there is usually another one that can be used to make the same measurement, helping us to keep the machine running reliably.

Many of the in-vessel sensors are embedded under tiles in the divertor region. These include thermocouples (to measure tile temperatures) Langmuir probes to measure charge deposition) and various magnetic sensor coils (to measure plasma properties).

As part of the current shutdown, a full survey of the in-vessel wiring is being undertaken. During the last few weeks the tiles have been removed from the divertor, exposing sockets for the electrical connections to the sensors. David Croft and Simon Cramp have worked closely with the remote handling team. Special test boxes have been installed to allow more than 500 wires to be checked for electrical continuity and isolation. This will also help to diagnose that the fault is in a sensor which might be replaced before re-installation.

Once the condition of the wiring is established, a strategic decision can be made to optimise the use of the wiring that is in good condition.

Background information
“Shutdown” in JET terms means to close the experiment down periodically to maintain and upgrade the fusion device. After this period experiments are going to be performed leading to better understanding of how to build and operate JET′s successor ITER. The current shutdown started on 26 October 2009. It is the fifth to be carried out remotely since commissioning JET in 1983. During the shutdown 86,000 components will be changed inside the JET torus.