´Developments of 4GSR BPM Electronics
Si-Won Jang*, Garam Hahn, Jung Yun Huang, Changbum Kim, Dotae Kim, Gyujin Kim,Bokkyun Shin, DongCheol Shin, Donghyun Song, Beam Instrumentation Group, PAL, Pohang, South Korea
Woojin Song (POSTECH, Pohang)

Abstract
The emittance of the 4th-generation storage ring (4GSR) to be constructed in Cheongju-Ochang, Korea, is expected to be approximately 100 times
smaller than the existing 3rd-generation storage ring. With the decrease in emittance, more precise beam stabilization is required. To meet thisrequirement, the resolution of the beam position monitor (BPM) system also needs to be further improved. We have conducted research anddevelopment on the electronics of the BPM system for the 4GSR storage ring. In order to perform fast orbit feedback in the 4GSR storage ring, we need toacquire turn-by-turn beam position data, with a desired beam position resolution of 1 um. Additionally, prototypes of the bunch-by-bunch monitoring
system are being developed for the transverse feedback system and longitudinal feedback system. The internally developed electronics are intended tobe modified for future use as monitors for multi-bunch beam energy measurements at the end of the linear accelerator, by adjusting the logic
accordingly. In this presentation, we will describe more details of the current status of the development of the beam position monitor electronics for the4GSR in Korea.

Development strategy of 4GSR BPM electronics

The strategy for developing 4GSR BPM electronics is as follows: First, we aim to develop BBB(Bunchby Bunch) BPM electronics capable of providing the highest performance for bunch-by-bunch beam
position measurements. Subsequently, using this technology as a foundation, we will proceed todevelop TbT(Turn by Turn) BPM electronics specifically tailored for the 4GSR storage ring. By
integrating the front-end and back-end electronics, we will create Transverse & Longitudinal feedback
systems, referred to as TFS & LFS BPM electronics.
Following this, we plan to adapt the data processing logic of the BBB electronics into a single-passlogic. This adaptation will enable us to install these electronics along the LTB (Linac to Booster)
beamline in the linear accelerator backend. They will serve as multi-bunch beam energy measurementmonitors. The diagram below illustrates the development strategy for 4GSR BPM electronics.

4GSR SR BPM electronics & system configuration

For the 4GSR Storage Ring (SR), TbT (Turn by Turn) BPM electronics are currently in the process of
being developed based on the prototype of the BBB electronics. The timing system for the storage ringutilizes the Event Network to provide Trigger signals and the Machine Clock to the BPM electronics.
Additionally, SR BPM electronics employ SFP (Small Form-factor Pluggable) modules to provide X, Y,and Sum information at a speed of 375kHz for the Fast Orbit FeedBack (FOFB) controller, Fast Orbit
Interlock controller, and Fast DAQ server. However, for Slow Acquisition (SA) data required to calculate
the closed orbit of the storage ring, it is synchronized with a 10Hz Trigger and transmitted to the EPICS
system for beam position information.
Figures 4 depict the I/O port and data flow configuration for SR BPM electronics, while Figure 5illustrates the configuration of BPM cable connections for the storage ring.

Fig. 4 4GSR SR BPM electronics I/O and data flow configuration

Fig. 1 Development strategy of 4GSR BPM electronics

Development status of BBB BPM electronics

The BBB electronics are currently in the prototype development phase, and we are undergoing several
beam tests to ensure the system's stability. By configuring the desired storage ring parameters andturn counts, we have confirmed the capability of measuring the beam position for all bunches. Figures
2 and 3 below illustrate the circuit configuration of the developed prototype and provide the Bunch-byBunch beam position measurement results.

Fig. 5 The configuration of BPM cable connections for the storage ringThe feedback system for 4GSR is structured in two stages, with the first stage being the Slow Orbit
Feedback System (SOFB). This system calculates and corrects the closed orbit at a rate of 2Hz using a
total of 288 All BPMs and Correctors across the entire storage ring. The corrected closed orbit becomesthe new Reference Orbit (Ref. Orbit). Subsequently, the Fast Orbit Feedback (FOFB) system operates at
each cell, receiving TbT data from BPM electronics at 375kHz intervals and providing new set point
information to the Kicker MPS every 15.6kHz. Ultimately, the FOFB system aims to perform Fast Orbit
Feedback at a 10 KHz interval using Fast BPMs and Fast correctors to achieve a wide suppression
bandwidth. While there isn't a master feedback controller for the entire storage ring, the FOFB
controller exchanges information between adjacent cells at every turn. As a result, this system swiftly
corrects distorted beam orbits in each cell and calculates kick angles for orbit correction based on theoverall storage ring's orbit changes. Table 1 illustrates the target parameters of the 4GSR feedbacksystem.

Table 1. Specification goal of 4GSR Beam stability.

Fig. 2 Proto-type of 4GSR BBB BPM electronics

CONCLUSION

The Korean 4GSR project in Ochang is currently under construction with the goal of completion by2027. The beam position monitor electronics system for 4GSR is in the process of development, andinitial beam tests have been conducted based on the development of the 1st stage BBB BPM
electronics prototype. Based on this, BPM electronics for the storage ring and feedback systems forboth TFS (Transverse Feedback System) and LFS (Longitudinal Feedback System) are also underdevelopment. Additionally, preparations are underway for BBB energy measurement BPM electronics,
which will be used for multi-batch beam energy measurements at the end of the linear accelerator.
The aim is to complete the development of storage ring BPM electronics by the end of 2024.
Simultaneously, tests will be conducted at the Pohang Light Source-II (PLS-II) storage ring incollaboration with the ongoing development of the FOFB (Fast Orbit Feedback) controller.

REFERENCES[1] G.S. Jang et al,. “Low emittance lattice design for Korea-4GSR”, Nuclear Inst. and Methods inPhysics Research, A 1034 (2022) 166779[2] S.W. Jang, “Development of Button BPM Electronics for the Bunch by Bunch Feedback System of4GSR”, Proceedings of IPAC2022, Bangkok, Thailand, MOPOPT038.
Fig. 3 Beam test results of 4GSR BBB BPM electronics @ PLS-II