Diamond Annual Review 2020/21

116 117 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Machine Operation and Development RichardWalker,Technical Director I n 2020/21, our 14 th year of operation has of course been unlike any other year due to the global pandemic, which has inevitably curtailed bothmachine operation and development to some extent. A total of 212 days (5,088 hours) had been scheduled for User Mode operation, including fve beamline start-up days. When the first UK lockdown was announced on 23 rd March 2020, we were in the first scheduled shutdown of the year. We resumed User Mode operation on 31 st March on schedule, however with a reduced number of operating hours – instead of the usual six day running (09:00 Wednesday to 09:00 Tuesday interspersed with Machine Development days), it was reduced to four days, settling eventually on a pattern with User Mode running from 09:00 Tuesday to 09:00 Saturday, with start-up/Machine Development on Mondays and we have been operating in that mode since then. This has resulted in the number of User Mode hours delivered this year being reduced to 3,445 hours. All operation was in standard multi-bunch mode (900 bunch train) with total current of 300 mA, there was no‘hybrid’or‘low-alpha’operation. The annual operating statistics are shown in Fig . 1. Despite the significantly increased number of switching off and on, the machine ran remarkably well, there were only 26 trips during User Mode throughout the year leading to our highest ever MTBF of 132 hrs. However, the uptime was poorer at 96.2%. This was primarily caused by three events, a failure in the cryo-plant, a loss of domestic water to the site whichmeant that staff had to leave and themachine be switched off and an electrical outage which resulted in a long recovery time, which together accounted for over half of the downtime. Radio frequency (RF) upgrades This year saw the first practical application of the Diamond synchrotron's new high-power solid-state amplifiers. With all of the original inductive output tube (IOT) RF amplifiers powering two superconducting cavities and two normal-conducting cavities in the storage ring, the first solid state amplifier is being used in the RF Test Facility, currently for high-power test and conditioning of the third normal-conducting cavity (Fig. 2). This solid- state amplifier can generate 80 kW without the high voltage required for IOT operation and uses the redundancy of multiple power transistor modules to provide a reliable high-power output. Once tests of the cavity are complete, the new cavity will be installed in the storage ring to provide further back-up of the superconducting cavities and cryogenic system. Two further solid-state amplifiers will be delivered in 2021, a 60 kW unit that will power the already installed second booster RF cavity, and a 120 kW amplifier that will power the 3 rd normal conducting cavity in the storage ring. Diamond-II update Significant progress has been made with the design of the Diamond-II machine upgrade in the last year during the Technical Design Report (TDR) phase, due to complete at the end of 2021. A revised lattice incorporating ‘anti-bends’ (actually quadrupole magnets which are offset horizontally to produce a bending field which is in the opposite direction of the principal bending magnets) has been adopted. A new solution for the vacuum system with a mixture of vessels with antechamber and conventional pumping and simple circular vessels with Non-Evaporable Getter (NEG) coating to provide the necessary pumping has also been adopted, both of which have been endorsed by the Diamond-II Machine Advisory Committee. Figure 3 shows the latest CAD model of one of the girder types that support the magnets and vacuum vessels, complete with vacuum pumps, ports for vacuum gauges etc. and photon extraction pipe. Design of all the major components is proceeding apace while various prototypes have been made and tested. Different materials for the girders have been tested to assess whether they offer improved performance compared to the standard use of machined steel (Fig. 4). The vacuum system for Diamond-II has to meet stringent requirements. It has to achieve the target pressure of 10 -9 mbar or lower in operation, it has to be able to handle high power densities due to synchrotron radiation from the bending magnets and the insertion devices and it has to present a smooth surface (low impedance) for the electron beam transport around the storage ring and to prevent heating by trapped radiofrequency (RF) energy. The dense magnetic lattice and small cross-section vacuum vessel (typically 20 mm diameter) mean that discrete pumps, as used on Diamond’s existing storage ring, are not adequate and a special low-outgassing and pumping NEG coating has to be applied to about half of the interior of the vacuum vessels to reach the specified pressure. For practical reasons, the storage ring vacuum system has to be manufactured as a series of smaller vessels, which are joined together with bolted flanges. There are more than a thousand of these joints around the storage ring; their design is critical as they have to provide a strong mechanical joint, a reliable ultra-high-vacuum (UHV) seal and near-perfect radiofrequency continuity with accurate alignment and a smooth internal surface throughout. In addition, the vacuum vessel has different cross-sections around the storage ring: in some places circular or elliptical and in other places a more complex shape such a ‘keyhole’. Furthermore, an all-metal construction is necessary to ensure high tolerance to radiation. The standard UHV Conflat® (CF) seal cannot be used as there is a 2 mm gap in the interior which would cause RF heating and potentially also beam instabilities. We have evaluated six different candidate designs of RF flange for Diamond-II, based on successful designs already in use at Diamond and at other accelerators. Figure 5 shows two examples, one (left) based on a standard UHV CF flange where the vacuum seal is made using a high purity copper gasket compressed between two circular knife edge flanges. The RF continuity is made by a separate aluminium gasket which closely conforms to the shape of the electron beam channel. The second (right) is again based on a standard CF flange but in this case, both the vacuum seal and the RF continuity are made simultaneously by a single copper gasket. The laboratory tests we have carried out on the flanges include vacuum leak tests and metrology after repeated making and breaking of the seal, bakeout to 200 °C and mechanical stressing as well as destructive testing where the test pieces are sectioned and examined under a microscope. Further tests are in progress and a full set of prototype vessels for an 8 m section of storage ring will be manufactured, assembled and tested before the final choices are confirmed. Figure 2: Diamond’s first 500 MHz solid-state amplifier (left) powering the third normal conducting cavity in the RF Test Facility (right). Figure 3: CAD model of one of the four girder types that are required for Diamond-II. Figure 4: Testing new girder materials for their vibration performance for Diamond-II. Figure 1: Mean Time Between Failures (MTBF) and Uptime by operating year. Figure 5: Some of the test pieces of RF compatible flanges for use in Diamond-II, with a keyhole (for transmitting undulator radiation (left) and a standard circular aperture (right).