1. Viliam Senaj MKD meeting 23/05/2017
MKD A/B2 XPOC problems
Viliam Senaj
MKD meeting 23/05/2017

2. XPOC2 problem after machine stop 17/18 May 2017
Frequent XPOC2 interlock on T_del, T_rise, R_2 … (almost every 1st low energy pulse after 6.5 TeV)
Observed problems are consequence of slower compensation part reaction resulting in later T_ref100%; slightly reduced I_ref100% (~ -3A for first low energy pulse) and consequently shorter T_del, T_rise

3. XPOC2 problem after machine stop 17/18 May 2017
Most likely due to higher losses in compensation circuit we observed lower I_max2 and I_end (~ 5A lower) influencing R_max2 and R_end

4. No visible effect at 6.5 TeV
Non of these problems were observed on 6.5 TeV leading to conclusion that the reason is not related to eventual contact degradation

5. Compensation capacitor current sharing differences
Main difference was observed in current sharing behaviour between two compensation capacitors

6. 6.5 TeV pulse – compensation capacitors current sharing
6.5 TeV prior to 450 GeV – compensation capacitors negative current undershoot - at instant of freewheel path conduction start (opposite polarity on GTO)
Compensation capacitors have ferromagnetic housing (steel); negative current leads to remnant magnetisation that needs to be re-magnetised during following (low energy) pulse (hysteresis effect and associated losses)

7. 6.5 TeV before machine stop and after adding magnets
Before machine stop – capacitor in branch B saw some repolarisation after the negative current and needed less repolarisation during the next low energy pulse – reason for faster rising current and higher current in br.B
Adding magnets did not influence the current “tail” at 6.5 TeV but lead to lower remnant magnetisation with the same effect – faster take-over of compensation current; in principle adding magnet on the other capacitor should further “speed-up” compensation current take-over.

8. Compensation capacitor current sharing
Partial recovery of compensation capacitors sharing behaviour after adding magnets:
15/05- before machine stop
22/05 after machine stop
23/05 – after adding 10 small “white board” magnet with correct polarity on br. B capacitor housing

9. Situation before and after adding magnets to compensation capacitor of branch B
With magnets
After machine stop