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

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

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

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

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

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)

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.

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

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