CERN Accelerating science

Tom summarized the issue with the SMP probe beam flag during the 2021 LHC pilot run. The probe beam flag was true even for nominal bunch intensity and allowed injection of a nominal beam into an empty LHC.

The SMP flags are generated from 3 SPS BCTs: the “low intensity” BCT4 and the “high intensity” BCT3/5. The BCT4 transmits data with a least significant bit (LSB) value of 1 x 108 charges. It is used for the probe beam flag. The BCT3/5 transmit data with a LSB value of
1 x 1010 charges. They are used for the setup and TED beam flags. A bug in FESA3 upgrade during LS2 meant that BCT4 was also encoding its data with a LSB=1 x 1010 charges. The interpretation by the SMP led to having the probe beam flag at true even for nominal bunches or higher intensities. This was quickly fixed but reminded of the weaknesses in the system.

Tom detailed the system architecture, as shown in Figure 1. The same FESA class is deployed for each of the 3 BCTS. BCT4 drives both A and B channels of the SMP without hardware redundancy. BCT3 and BCT4 are very old commercial systems.

Figure 1 SPS BCT / SMP architecture schematics.

A new 24-bit acquisition system has been installed on BCT5 during LS2 for the dI/dt. The ADC is located on surface. This will allow to change the SMP logic without hardware development. The noise level for high-intensity beams with the 24-bit acquisition is significantly lower than with the old 16-bit acquisition. However, the signal is affected by magnetic interference at flat top. It results in an measured intensity increase of 1 x 109 charges.

The proposal is to keep BCT3 and BCT4 as they are. BCT3 will provide the high intensity channel 3A and BCT4 the low intensity channel 4A. The BCT5 connection will be modified to send SMP directly from VFC-HD. The range 1 will feed the high intensity channel 3B and the range 3 channel will feed the low intensity channel 4B. This requires modifications on the SMP side (see next presentation of this meeting). BCT 5, thus, provides diverse redundancy.

Tom then presented the architecture for the LHC. There is still no redundancy for the 24-bit system, any issue would impact both systems. The plan for the YETS is to move the VFC to the operational crates. The new firmware also has the direct SMP transmission logic implemented. No change on the SMP side is required, the same headers will be used.

In conclusion, a direct transmission of the SMP frames has been implemented in the firmware of both the SPS and LHC 24-bit DC-BCT systems. In the SPS it is proposed to connect the BCT5 to both high and low intensity channels for improved redundancy. In the LHC the new VFC-HD will be installed during YETS to remove the third FEC and improve the redundancy. Also, the direct transmission from the 24-bit system to the LHC-SMP is possible.

Jan commented on the long history of the issue and thanked the team for improving the system. Jan asked which option should be chosen for the LHC. Tom replied that he proposes to use the direct connection with the VFC. Jan commented that then we should do that from the start. Daniel agreed.

Daniel asked why the same solution is not chosen for the B channel. Tom replied that the B channel is still on the 16-bit acquisition system.

Daniel added that the proposal should be written in a ECR.

Action: Draft ECR on the proposed DC-BCT and SMP changes for the SPS (T. Leven, R. Secondo).

                        The ECR for the LHC changes has since been prepared and approved. It is available in EDMS.