TLS Fast Orbit Feedback Upgrade

The Orbit Feedback System (OFB) at the Taiwan Light Source (TLS) was implemented two decades ago and has undergone multiple upgrades to enhance its performance. The loop bandwidth was constrained by the available hardware. The system is unable to eliminate perturbations from a fast source. In order to enhance the performance of orbit feedback, the system underwent an upgrade in 2008. The upgrade involved the replacement of the analog-type BPM electronics with digital BPM electronics, as well as the replacement of the corrector power supply with a high-performance switching-type power supply that has a wide bandwidth. Following the commissioning of the Taiwan Photon Source (TPS) in 2015, a decision was made to gradually migrate the control system of the Taiwan Light Source to the Experimental Physics and Industrial Control System (EPICS). This decision was made in order to facilitate resource sharing between the two facilities since EPICS had already been adopted by TPS. The Orbit Feedback System is a revitalized subsystem that now benefits from EPICS support. Additionally, the feedback computation unit has been upgraded to a Field Programmable Gate Array (FPGA), resulting in an increase in the calculating cycle from 2.5 kHz to 10 kHz. This report presents a concise overview of the integration of fast orbit feedback and the outcomes of the orbit stability enhancement following the upgrade.


Introduction
The Taiwan Light Source is a 3rd generation synchrotron light source situated at the National Synchrotron Radiation Research Center in Taiwan.The facility has been in operation since 1993, and its initial control system was a proprietary design that was self-defined.The documentation provided was incomplete and lacked a detailed manual.After a period of twenty years, numerous hardware components have become obsolete or are no longer in production.The majority of firmware and drivers lack support as well.The maintenance has been experiencing an increasing number of difficulties.As a consequence, upon the launch of a new project TPS, the open source software EPICS was adopted without any objections.Following the commissioning of TPS [1,2] and its opening for users, the control system of TLS has commenced a gradual migration to EPICS in order to mitigate the risk of obsolescence.
The previous orbit feedback system, along with the other TLS control systems, utilized a VME rack that was equipped with a CPU and IO card for interfacing purposes [3].Despite utilizing reflective memory to efficiently obtain orbit data without additional CPU usage, the implementation of singular value decomposition (SVD) for feedback correction [4][5] [6] still requires significant computational resources.Despite the potential for fast data streams up to 10 kHz, the outdated CPU was unable to effectively manage the speed, resulting in significant jitters and instability in the OFB system.Hence, in order to effectively process and keep up with the continuous stream of data, it is necessary to discard three out of every four samples during the process.The initial rate of OFB calculation was 2.5 kHz, which was obtained by dividing 10 by 4. As a consequence, the computation unit of OFB has been migrated to the Field-Programmable Gate Array (FPGA) instead of the conventional CPU in this upgrade.It is anticipated that the performance of OFB could be enhanced with an increase in the calculation cycle subsequent to the upgrade, in addition to the provision of EPICS support.The OFB infrastructure and performance of the OFB will be summarized in the reports.

Orbit feedback infrastructure
Figure 1 illustrates the infrastructure of the orbit feedback system, which is present in both the old and upgraded versions.The green blocks represent the previous orbit feedback with a computation rate of 2.5 kHz.The upgraded system comprises orange blocks and operates at a rate of 10 kHz.The newly established OFB has adopted I-Tech product, the gigabit data exchange (GDX) module, as FPAG solution for feedback computation [7].The GDX is equipped with Virtex 6 and has also been utilized for TPS FOFB, as reported in reference [8].It can perform 256*128 SVD computations in TPS and 64*64 SVD computations in TLS.Fig 2 displays the GDX photographs.As the time allotted for beamline users to utilize TLS is nearly scheduled, there are constraints on the maintenance and machine study time.In order to ensure uninterrupted normal operation during the migration to the new system, the summing hardware underwent revision to enable the addition of DC orbit correction, the old OFB, and the new OFB outputs.When one OFB is activated, the other should be deactivated.Once the new system underwent comprehensive testing and verification, all devices associated with the previous OFB were subsequently removed.The Intelligent Local Controller (ILC) responsible for DC orbit correction is scheduled for an upgrade later this year to support EPICS interface.

Performance comparison between the old and new upgraded OFB
The time series of R1BPM4 vertical position at 10 Hz is presented in Figure 3 below.It is evident that the stability of the position is significantly improved when the OFB is activated, as compared to when it is deactivated.In comparison to the previous OFB, the latest upgrade of the OFB system has also demonstrated a more efficient suppression of orbit vibration.
Figure 4 displays the time series of the overall vertical position of BPM at the top up mode for the purpose of comparing the previous and new OFB.The regular spikes occurring every minute are a result of the injection and can be disregarded.Figure 4 Based on the data presented in Figure 4, it is possible to calculate that the standard deviations of the vertical beam position are approximately 20~120 nm for applying new OFB; 75~340 nm for the old OFB.The variations in the orbit of the vertical beam have been reduced by one-third to one-fifth after OFB upgrade.The horizontal orbit's variation has been also reduced to 1/2~1/3 of its original value.5 displays the orbit spectrum obtained from a 10-second sampling period at a frequency of 10 kHz.The upper subplot pertains to the horizontal plane, while the lower subplot pertains to the vertical plane.The blue line is represented as FOFB off; the red line is as FOFB on.It can be observed that the stability of the orbit is significantly enhanced when the OFB is activated.Based on the spectral analysis, an approximate estimation suggests that the bandwidth of OFB is approximately 25 Hz for the horizontal plane and 50 Hz for the vertical plane.The bandwidth is restricted as a result of the presence of an aluminum vacuum chamber that has a thickness of 4 mm.Additionally, the integrated power spectrum density is illustrated in Figure 6.The upper subplot pertains to the horizontal plane, while the lower subplot pertains to the vertical plane.The blue line is depicted with FOFB off, while the red line is illustrated with FOFB on.

Conclusions
In order to prevent obsolescence and enhance orbit stability, a proposed plan for upgrading the TLS OFB has been put forth with the aim of migrating the supporting EPICS system.Following the upgrade, the rate of the calculation cycle has increased from 2.5 kHz to 10 kHz through the utilization of FPGA in place of VME CPU.The estimated bandwidth of OFB is approximately 25 Hz for the horizontal plane and 50 Hz for the vertical plane.The application of OFB significantly enhances the stability of the orbit.In comparison to its predecessor, the updated OFB demonstrates a greater capacity for mitigating orbit vibration.The newly established OFB has been conducting its operations since the year 2021.The performance and reliability of the system have thus far met expectations.

Figure 1 :
Figure 1: The orbit feedback system infrastructure.The green blocks represent the previous orbit feedback with a computation rate of 2.5 kHz.The orange blocks represent the upgrade orbit feedback with a computation rate of 10 kHz.

Figure 2 : 3 .
Figure 2: The gigabit data exchange (GDX) module for SVD computation of the TLS upgraded OFB.
(a) depicts the application of the previous OFB, while Figure 4(b) illustrates the use of the upgraded OFB.The results of the observation indicate that the position stability of the new OFB is superior to that of the previous version.

Figure 3 :Figure 4 :
Figure 3: The time series of R1BPM4 vertical position for new and old OFB on and off.

Figure 5 :
Figure 5: The orbit spectrum.The upper subplot is for horizontal; the lower one is for the vertical.The blue line is represented as OFB off; the red line is as OFB on

Figure 6 :
Figure 6: The orbit integrated power spectrum density.The upper subplot is for horizontal; the lower one is for the vertical.The blue line is represented as OFB off; the red line is as OFB on.

3. 3 .
Long-term Orbit Stability for 5 days when OFB On

Figure 7
Figure7illustrates the long-term orbit stability of TLS over a period of five days.The horizontal position is denoted by the red line, whereas the vertical position is represented by the green line.Two slow drifts have been identified as blue notices, attributed to the temporary instability of Linac and Booster caused by current decay.The study reveals that the long-term stability in both horizontal and vertical directions was maintained at a level of 5~10 microns for a period of five days, with the exception of the current decay.