Machine Operation and Development

Figure 1: Mean Time Between Failures (MTBF) and Uptime by operating year.

The annual operating statistics are shown in Figure 1. The Mean Time Between Failures (MTBF) for the year was 79.9 hours, a little disappointing when compared with the previous two years but nevertheless still exceeding the target minimum of 72 hours. The reduction in MTBF this year is mainly attributed to an elevated number of trips due to power outages, BPM (Beam Position Monitor) electronics faults and front-end vacuum interlocks. The latter two are problems that have been known for some time, and for which solutions have been devised and are in the process of being implemented. Despite these issues the overall uptime (beam delivered as a percentage of scheduled hours) remained high at 98.2%.

Figure 2: The cavities are installed in straights 16 and 18, on either side of straight 17 which
hosts the two superconducting cavities.

In response to the risk of extended downtime in the event of further conducting cavities, of the Higher-Order Mode (HOM) damped cavity design currently in use at ALBA and BESSY. The two cavities were delivered in February and March 2017 and after testing and conditioning in the RF Test Facility were installed in the ring in August and November 2017. The cavities are installed in straights 16 and 18, on either side of straight 17 which hosts the two superconducting cavities, and in each case upstream of insertion devices (Fig. 2).

The first normal conducting cavity has been successfully conditioned in the ring and was operated for the first time in user mode during the two day lowalpha period in March 2018 without a trip, with an accelerating voltage of 400 kV. The additional voltage provided by the cavity allowed the voltage on the superconducting cavities to be reduced, which gave a significant improvement in the trip rate compared to previous low-alpha periods. The second cavity will be brought into operation in the summer of 2018 following delivery and installation of its transmission line and circulator.

A digital low-level RF system (for stabilisation of cavity RF voltage and phase) is being developed in collaboration with ALBA. The first system is in operation on the first normal conducting cavity and has already demonstrated superior stability compared to the analogue system currently in use on the superconducting cavities. Further systems are in construction and will be deployed on the second normal conducting cavity and later also on the two superconducting cavities.

Figure 3: Schematic showing the operation of the longitudinal multi-bunch feedback (LMBF)
system (a) and a view of the kicker cavity installed in the storage ring (b).

A Longitudinal Multi-Bunch Feedback (LMBF) system has recently been implemented, in order to combat potential longitudinal instabilities of the electron bunches due to higher order modes in the normal conducting cavities, as well as to provide valuable additional diagnostic features.

The LMBF system provides monitoring and correction of the precise relative timing of the individual bunches in the storage ring. All bunches should ideally arrive at a fixed phase relative to the driving field in the accelerating cavities. However, there was a risk when installing the normal conducting cavities that interactions with the beam could lead to ‘coupled bunch instabilities’ which could limit the beam current. Having detected the start of an instability in the beam the LMBF is capable of suppressing it by acting on the beam with a suitable correction signal.

Conceptually, the system is similar to the Transverse Multi Bunch Feedback (TMBF) already in use since 2008 (Fig. 3a). A pickup registers the arrival time of each bunch, which is then processed in a digital feedback controller. This feedback controller is continuously calculating 500 million control values per second, which it sends through an amplifier to a kicker to influence each bunch. The whole feedback process takes only a few microseconds and relies on the predictability of the motion of a bunch over such short periods.

Figure 4: The 'missing sextupole' scheme consists of removing a sextupole magnet from the standard double-bend achromat (DBA) cell, and in its place installing a mini-wiggler magnet as a
source for the new DIAD beamline.

To implement the LMBF a new kicker cavity needed to be designed, manufactured and installed. The fabricated structure was installed in April 2017 in straight 22 just upstream of the insertion device and is only recognisable by the eight feedthroughs connecting power from the amplifier to the complex structure on the inside (Fig. 3b).

The feedback processor is based on commercial MicroTCA hardware, while FPGA and EPICS components were developed in-house. During this project the opportunity was taken to update the TMBF to the new hardware, which now allows synchronised measurements of the horizontal, transverse and longitudinal position of each individual bunch, a formidable new diagnostic tool.

Figure 5: Replacement girder under construction; the arrow indicates the position of the
'missing sextupole' and the new narrow gap vacuum vessel where the DIAD mini-wiggler will
be installed.

The Dual Imaging and Diffraction (DIAD) beamline (K11) is the last new beamline to be funded and is currently under construction. Originally envisaged to be installed on a 'Superbend' (a bending magnet with higher magnetic field than the standard bending magnets in the ring) it was later proposed to be installed on an insertion device, using a second Double-Double Bend Achromat (DDBA) cell (as described in last year’s Annual Review). Calculations however indicated that a second DDBA cell posed a significant risk to the operation of the ring, and so another scheme was investigated, the 'missing sextupole' scheme (Fig. 4). This involves removing a sextupole magnet from one of the achromats and using the space created to install a short 10-pole wiggler magnet as the source for the DIAD beamline. Having confirmed its feasibility in simulations, this mode of operation has now been thoroughly tested during machine development periods by switching off the relevant magnet and retuning the machine. Meanwhile the new vacuum vessels have been designed and constructed, and the modified girders are being prepared (Fig. 5). These are due to be installed in the June 2018 shutdown.