Studies and optimization of Pohang Light Source-II superconducting radio frequency system at stable top-up operation with beam current of 400 mA

Abstract

After three years of upgrading work, the Pohang Light Source-II (PLS-II) is now successfully operating. The final quantitative goal of PLS-II is a top-up user-service operation with beam current of 400 mA to be completed by the end of 2014. During the beam store test up to 400 mA in the storage ring (SR), it was observed that the vacuum pressure around the radio frequency (RF) window of the superconducting cavity rapidly increases over the interlock level limiting the availability of the maximum beam current storing. Although available beam current is enhanced by setting a higher RF accelerating voltage, it is better to keep the RF accelerating voltage as low as possible in the long time top-up operation. We investigated the cause of the window vacuum pressure increment by studying the changes in the electric field distribution at the superconducting cavity and waveguide according to the beam current. In our simulation, an equivalent physical modeling was developed using a finite-difference time-domain code. The simulation revealed that the electric field amplitude at the RF window is exponentially increased as the beam current increases, thus this high electric field amplitude causes a RF breakdown at the RF window, which comes with the rapid increase of window vacuum pressure. The RF accelerating voltage of PLS-II RF system was set to 4.95 MV, which was estimated using the maximum available beam current that works as a function of RF voltage, and the top-up operation test with the beam current of 400 mA was successfully carried out.

Reduction of RF accelerating voltage of Pohang Light Source-II superconducting RF cavity for stable top-up mode operation

“…earlier than the other SCs as studied in our previous work [9]. V loss/turn is 1242.2 keV because the energy losses by radiation in the bending magnets and IDs are estimated to be 1042.2 keV and 200 keV in the PLS-II storage ring, respectively.…”

“…When the value of V acc is greater than 3.4 MV, the lower the value of V acc , the more effective it is for reducing the bunch charge density. However, the problem is that the V acc of the storage ring cannot be freely reduced because its minimum value is constrained to the operating beam current as we have reported in our previous study [9].…”

“…This section reports on how the RF accelerating voltage is reduced by lowering the Q ext value for stable 400 mA operation. By modifying the equation (3) of our previous work [9], the relation between the maximum available beam current (I b,max ) and V acc of the storage ring can be expressed as follows:…”

Troubleshooting of the beam dump problem caused by helium vessel pressure fluctuation during the cryogenic adsorber regeneration process of a superconducting RF system for synchrotron user facilities

“…After implementing each solution in the second half of 2018, there was no beam dump associated with the cryogenic adsorber regeneration process, as shown in figure 2. With other efforts for stable operation of the SRF system [8,9], the total number of SRF faults decreased from 18 times in 2018 to 8 times in 2019 and 5 times in 2020. Therefore, the MTBF reached 31.5 days in 2020.…”