RD&D-Programme 2004 - SKB

When the welding chamber was built, the recommended maximum pressure was 0.5 millibar.
During testing it was found that the pressure had to be less than 0.2 millibar in order for the
welding process to be stable. If the pressure exceeds 0.25 millibar at maximum welding power,
extensive plasma formation occurs in the area around the weld pool. The plasma disturbs the
beam, resulting in poorer weld quality. An additional vacuum pump was installed. Along with
other measures, such as leak detection and sealing of the welding chamber, plus a minor rebuild,
this improved the vacuum level in the welding chamber to about 0.05 millibar.
The beam’s electron distribution has been measured regularly to determine the relationship
between electron distribution and weld quality and to assess the condition of the cathode,
see Figure 6-5.
Programme
Standard EBW machines are normally reliable. The equipment at the Canister Laboratory is
much more complicated than a standard machine. It is more like a prototype, and its reliability
has therefore been less than desirable. A number of measures are planned during the autumn of
2004 to improve reliability:
• Develop and improve the manufacture of cathodes. The goal of the work under way at TWI
is enable cathodes to be precision-made so that they generate uniform beams and welding
results at identical parameter settings. It must also be possible to mount the cathodes in the
welding machine without individual adjustment.
• Develop, test and rebuild the high-voltage cable’s connection to the gun to minimize the risk
of flashover.
• Rebuild the beam generation system so that the beam bore in the gun is simplified and the
risk of flashover is minimized. This work is expected to be finished during the autumn of
2005 so that the demonstration series of 20 lid welds is not delayed.
Figure 6-5. Electron density in beam focus. Green indicates low density and violet high density.
RD&D-Programme 2004 69