The ISIS linear accelerator (linac) consists of 4 radiofrequency (RF) accelerating tanks, accelerating hydrogen ions generated in the ion source to 37% of the speed of light before feeding them into the synchrotron for final acceleration. Tanks 1 and 4 were built at RAL in 1976, for ISIS’ predecessor, Nimrod. They are now showing their age, so a project is underway to replace tank 4 by 2018.
The vacuum vessel for Tank 4 is built in one piece, which means that access for maintenance to the drift tubes is extremely difficult. Additionally due to the X-rays generated by the fields in the tank and activation from beam lost during 30 years routine operation, maintenance is even harder.
A project has been under way since 2010 to design and build a replacement tank with a target installation date of 2018. This project aims to improve the reliability and maintenance issues with the tank and also to re-establish knowledge of tank design and construction at RAL.
A like-for-like 12 m replacement is being designed and built. The beam optic will remain the same, but the design has been re-engineered to ensure access is available to the drift tubes. The tank has been split into six 2 m sections with removable plates for access to the drift tubes and the quadrupoles they contain, making it much easier to maintain components. A 3D printed model of a 2 m section has been created and from this design a complete 2 m test section has been manufactured from mild steel and plated with 200 μm of copper.
The tank will sit on new support rails, which have been re-engineered to make them easier to install and align, and have been manufactured. Prototype drift tubes are being made at RAL. Each one can take weeks to manufacture. It is complicated to fit the drift tubes into the tank sections, as this must be done without damaging the copper plating. A bespoke sled and pulley system with special runners has been built for this task.
The complete tank, drift tubes and quadrupoles will require 58 kW of cooling water to keep their temperature stable.
The EHT Area (extra high tension – for extremely high voltage) used for the Cockroft-Walton electrostatic generator pre-injector for ISIS has been cleared out in preparation to become a test area for the new Tank 4. This has involved extending the area and installing new shielding. High power RF TH116 valves will be used to generate the accelerating power in the test tank, and this will be measured with a bead-pull system already being designed. Further R&D on the 2 m section and RF valves will continue in the test area once the 6 actual sections have been ordered.
The new linac tank sees design, construction and testing of such large accelerator components returning to RAL after almost 40 years. As Tank 1 is also expected to need replacing in the near future such work is vital. In addition there are extensive designs to replace the entire ISIS linac, with a view to increasing the energy at the same time.