High gloss future — Testing of first of series module for the UNILAC Alvarez upgrade


Highest quality and a mirror-like gloss: Inside the so-called Alvarez accelerator, a 55-meter-long part of GSI's linear accelerator UNILAC, the high-grade copper surface stands out. Due to the upgrading measures for operation with the new FAIR accelerator facility, which is currently under construction, the existing GSI facility is also undergoing many improvements. One of them is the replacement of the existing Alvarez with a new, improved accelerator structures of the Alvarez type. A first of series (FoS) module has now been completed and is undergoing testing.

The linear accelerator UNILAC (Universal Linear Accelerator) serves as the first accelerator stage to bring ions up to speed. The Alvarez section, located at the rear of the 120-meter-long UNILAC, brings them from 5% to 15% of the speed of light so they can be injected into the GSI ring accelerator, accelerated further and later transferred into the FAIR facility. Since the existing Alvarez, which is in operation for nearly 50 years, doesn’t meet FAIR's high requirements, the decision for its replacement was made.

The new components combine large dimensions in the meter range with high precision in the submillimeter range. Internal surfaces must be manufactured to the highest quality with roughness of just a few micrometers to apply the copper plating afterwards. For the GSI electroplating department, which specializes in large components, the copper plating itself is a huge challenge due to the necessary homogeneity. The special surface is necessary for the device to start its “glossy” future in the accelerator.

“Another specialty are the quadrupole magnets integrated into the structure's drift tubes, which provide beam focusing during acceleration. Manufacturing, installation and adjustment must be exactly right to guarantee the magnetic field quality,” explains accelerator physicist Dr. Lars Groening, who is head of the responsible department “UNILAC Post Stripper Upgrade”. “We have greatly improved the quadrupoles compared to the existing Alvarez: they focus more strongly and, in quasi-simultaneous operation with several ion species, ensure optimal focusing properties for each species through rapid switching. This is essential for FAIR.”

Many of GSI/FAIR’s technical departments are involved in this project. Following extensive planning, design and construction of the components took place. A FoS Alvarez component was delivered in 2019 and assembled on campus. Testing took place for specified properties such as dimensions, tolerances and surface quality of the inside, as well as low-power electromagnetic field characteristics. In the previous year 2020 the structure received its characteristic high gloss: it was successfully copper-plated at GSI's electroplating facility and is now ready for testing in high-power operation.

Once the FoS passes all tests, 25 sections will be manufactured in series production. They, too, must undergo a defined acceptance procedure and tests of the high-frequency electromagnetic fields. For this purpose, five sections with three-ton end caps at each side and the drift tubes will be assembled to one cavity, so that in total five cavities of the Alvarez type will be tested. Subsequent to the careful test campaign, the replacement of the existing Alvarez section with the five new Alvarez cavities can begin at the UNILAC tunnel, which is estimated to take about one and a half years. (CP)