Sticky situation for molecules in the MAX IV vacuum systems

Sticky situation for molecules in the MAX IV vacuum systems

The MAX IV 3 GeV storage ring vacuum system is the first of its kind, with an inner surface of the chambers coated with a non-evaporable getter material. A paper recently published in the Journal of Synchrotron Radiation summarises the results of this unique and successful design choice.

The MAX IV 3 GeV storage ring is a unique construction in many ways. It is the world’s first Multi Bend Achromat (MBA) ring and fourth-generation storage ring-based light source. The construction has a more compact design with more magnets bending and controlling the electron beam per unit length than has ever been used before. The unprecedented number of bending magnets results in a tiny beam of ultrabright and laser-like X-ray light that opens new science areas.

The magnets wrap around the vacuum tubes, which encapsulate the electron beam. The unique design with compact magnets requires a unique vacuum system as well. It is challenging to pump the gas molecules out of the resulting long, thin vacuum tubes – it would work a bit like trying to drink soda with a skinny straw. At the same time, circulating electrons are disturbed by any remaining molecules on their way. A new way of ensuring low pressure inside the storage ring vacuum system was needed. The novel solution for the vacuum system of the MAX IV 3 GeV storage ring originates from CERN, where it was developed for the construction of the Large Hadron Collider. Staff from CERN has collaborated with MAX IV to adapt and implement it for the 3 GeV storage ring.

The vacuum tubes’ inner surfaces are transformed into pumps by covering their inside with a thin layer of what is called Non-Evaporable Getter or NEG for short. The coating used at MAX IV is composed of titanium, zirconium and vanadium. The NEG coating, after activation, reduces the number of gas molecules inside the accelerator vacuum tubes and acts as a “sticky” surface that can capture gas molecules and permanently trap them so that they no longer disturb the beam.

“The ability to achieve a good vacuum was a worry that we all carried until the machines started at MAX IV. I’m proud and excited that we now have proof that this is a good solution for fourth-generation storage ring-based light sources,” says Eshraq Al Dmour, who heads one of the two engineering groups at MAX IV. “I was surprised by the fast conditioning of the 3 GeV ring vacuum system, as compared to conventional systems.”

During commissioning, the main challenge of the vacuum team was identifying and correcting spots that were heated up by the electron beam. The vacuum chambers around the ring have to be perfectly aligned. There is low tolerance for imperfections in their shape and finishing. The issues are now solved and pose no limitations to the accelerator performance.

Interruptions to light delivery are expensive, and the accelerator must be repaired on the fly, a bit like a Formula One car. Future developments are focused on minimising downtime.

“We have already done some developments to reduce the intervention time by using neon gas to vent the vacuum chambers. At the moment, the vacuum team is also looking towards preparing a spare vacuum section of the storage ring. It will be approximately 22 m long, which equals 5% of the 3 GeV storage ring circumference. The section will be ready to replace an existing cell in case of a serious incident” concludes Marek Grabski, leader of the vacuum team at MAX IV.

Marek Grabski and Eshraq Al-Dmour, Commissioning and operation status of the MAX IV 3 GeV storage ring vacuum system, J. Synch Rad. 28, 718 (2021), DOI: 10.1107/S1600577521002599