Muon spectroscopy experiments can be carried out with either no applied magnetic field, or with a field applied either parallel or perpendicular to the muon's spin. In the current MuSR instrument, the muon spins are antiparallel to the beam direction. Any applied perpendicular magnetic fields then cause the muons to travel in a curved path. Super MuSR will enable the application of much larger fields, which would cause the muon beam to curve so much that it would miss the sample.
To get around this issue, the new instrument will rotate the muon spins in flight so the applied perpendicular magnetic field can be along the beam direction. Although new to ISIS, this configuration has been used at the Paul Scherrer Institut (PSI) in Switzerland and TRIUMF in Canada for over 30 years.
Super MuSR will have two spin rotators, each rotating the muon spins by 34 degrees each, resulting in 68 degree rotation. They will create a magnetic field to change the spin direction and balance the force this generates with an electric field to ensure the beam doesn't curve.
The electric field will be supplied by two large electrodes held at plus and minus 192 kV. These need to be polished to an extremely high level and kept clean to avoid sparks coming from imperfections on the surface.
These electrodes have just been delivered to RAL, with the Metrology team in Technology inspecting them before they will be sent to the Vacuum Processing Group within ASTeC in Daresbury for cleaning.
The Metrology team visually checked the surfaces for scratches, dents and marks before using their large Co-ordinate Measuring Machine (CMM) to measure the flatness of the electrode face and the profiles of the rounded edges. The CMM detects any small imperfections from manufacturing that would affect the quality of the instrument's performance, with an accuracy of a few 1000ths of a mm.
In Daresbury, the electrodes will be ultrasonically cleaned, using sound waves to remove all dirt, grease and oil-based residue from manufacturing that simple cleaning cannot take off. Ultrasonic cleaning has been recommended by the team at PSI, who use it to clean their electrodes and enable them to achieve high voltages while minimising failures. The ultrasonic bath at Daresbury is one of the few places this cleaning could be done, as it requires a suitably sized bath and the expertise to handle this type of component.
Once the ultrasonic cleaning is complete, the electrodes will return to ISIS to be included in the pre-build of the Super MuSR instrument ahead of its installation in 2027.
