Linear accelerator 3 (Linac3) has been up and running since the beginning of November, to make the most of Long Shutdown 2 (LS2): "Long shutdowns are ideal but rare opportunities to perform otherwise time-consuming tests or tests incompatible with the operation of the other machines," notes Detlef Küchler, ion source physicist. And after several years of research and development, mainly on the accelerator's source, the team at Linac3 indeed had some tests to run.
Built in 1994, Linac3 is the starting point for ions, mainly lead nuclei, collided by the LHC and used in fixed-target experiments at CERN. The source at the origin of the ion beams is of the ECR type (electron cyclotron resonance): oxygen gas and lead vapour (when lead is used) are injected into the source plasma chamber. A microwave is applied to create the plasma in which the lead and oxygen atoms are ionised; the plasma is confined by a magnetic field. At CERN, the same source is used for different types of ions.
A new shape for the micro-oven
To evaporate the lead, which is a metal, Linac3 uses a micro-oven: the solid sample of isotopically pure lead is placed in a small crucible, which measures a few centimetres and can contain up to 1.5 g of lead, and is heated by an electric current running through a filament that surrounds the crucible. After a certain time, generally two weeks, the crucible needs to be replaced by a newly filled one, although it usually still contains around half of the lead sample.
"We observed that a blockage of lead oxide, caused by the condensation of lead vapour, forms at the tip of the oven," explains Toke Kövener, fellow in the Linac3 team. "It seems that this blockage is the reason why the oven cannot evaporate the complete lead sample."
To prevent this phenomenon, during his PhD at CERN Kövener developed a new crucible shape that extends outside of the oven tip. "It showed promising results during our tests at the test stand: it was possible to operate the oven until the lead sample was fully consumed," Kövener adds. "I look forward to seeing how it operates inside the real source." The new oven will be installed for testing in Linac3 at the beginning of December.
A plasma chamber coated with aluminium
"Until now, the plasma chamber of the source was made entirely of stainless steel, but in January, we will test a new stainless steel chamber with a 20-micrometre thick internal coating of aluminium," explains Küchler. Indeed, some studies showed that an aluminium chamber allows for the production of a higher-intensity beam. A problem remains though: will the aluminium coating survive? "In 2005, we used a chamber made 100% of aluminium, but it turned out not to be resistant enough; the aluminium was quickly eroded by the plasma," Küchler adds. "I'm curious to see how the new chamber copes."
A remotely controlled extraction system
To produce the beam, an extraction system draws the lead ions from the plasma chamber. This extraction system consists of three electrodes. The gaps between these electrodes influence the intensity and emittance (the transverse dimensions) of the beam. Until now, these gaps were fixed and could only be changed when the source was vented and opened. "This year, we replaced the existing extraction system with a new system, where the main extraction gap can be controlled remotely. This allows for finer tuning of the extracted beam and may result in an increased beam intensity with a better emittance," explains Küchler.
In parallel to these tests, the produced beams are also used for the commissioning of the Linac3 radiofrequency (RF) cavities and of a new low-level RF system, used to accelerate the ions. The Low Energy Ion Ring (LEIR) will receive the first beam from Linac3 in June next year.
