The last edition of this letter ended with the optimistic remark that CMS after the end of LS1 would be perfectly prepared for the exciting times ahead of us. And indeed the final steps of LS1 went smoothly with CMS ready for the first beams. In April a cosmic test with magnetic field demonstrated the good progress of the commissioning of the detectors, the trigger and the DAQ. At this stage there were first indications that the Helium liquefaction system for the CMS solenoid had problems running for extended periods of time. As the system was dormant and kept at room temperature for more than one year during LS1 this was attributed to start-up problems and it was expected additional filter cleaning and regenerations could cure this problem.

Unfortunately this was not the case, but sustained magnet operation has been difficult throughout this year due to an apparent build up of contaminant in the filters, adsorbs, turbines and heat exchangers of the cold box. Simultaneously, mechanical damage to a cryogenic valve in the intermediate cryostat, situated in the experimental cavern, has restricted control flexibility in the helium liquefaction process. One contaminant repeatedly found in the accessible components (ie filters, adsorbers and the first two expansion turbines) is compressor oil (Breox B-35) and there is a growing body of evidence that the primary source of the erratic cold-box behavior is contamination with this oil, due to an oil-coalescer system maintenance error in late 2014, combined with the (known) lack of performance margin of the primary oil-removal system.

Besides very intensive, diagnostic measurements, which are complicated by the very nature of cryogenic installations, an invasive programme of filter, adsorber and turbine replacement has been undertaken as an interim measure, using the pre-scheduled technical stops of the LHC where possible. Together with revised procedures, based on empirical experience, this has allowed a reasonable duty cycle to be established. This pragmatic mode of operation has required more frequent maintenance stops to remove contaminants. As far as possible, these were synchronised with interruptions in LHC operation for physics and this far about 75% of the CMS 2015 pp collision data and almost all HI data have been taken at full magnetic field.

The Collaboration appreciates the priority being given to this issue by CERN management and particularly by CERN's Technology (TE) Department, which is responsible for the maintenance and operation of the CMS magnet external cryogenic system. Other CERN departments have also provided valuable assistance in skilled personnel and specialist equipment. A joint TE-CMS task force, meeting three times per week has steered the short- and long-term actions since May. As well as overseeing day- to-day actions in diagnosis and ad hoc optimization, the task force has conceived and launched a programme to ensure that the cryogenic system is fully revised and thoroughly cleaned of contaminants by the end of the year-end stop 2015-16. Orders have been placed for a replacement oil recovery system for the surface plant, as well as for specialized cleaning equipment, that will be used to circulate a solvent through the cold box (temporarily segmented into 4 separate circuits). Meanwhile the 300 m long high-pressure transfer line bringing helium from the surface to the underground cold box has been fully replaced. The fully developed plan for revising the cryogenic system defines the CMS critical path for the 2015-16 year-end technical stop (YETS) and aims to be completed within the time limits currently scheduled for first beam and first physics.

With the CERN management giving the cold box problems highest priority and great efforts of the technical groups together with CMS members it was possible to keep up the CMS magnet for a large fraction of the LHC running period.

Sadly, as it never rains but it pours, CMS recently developed a water leak in the first endcap disk on the +z-end. Due to slope of the LHC the water ran through the bottom of the vacuum tank of the solenoid and tripped the HV of muon chambers on the other end of the detector before the affected circuit could be closed. The repair will require a full opening of CMS during the coming YETS to repair the leak, dry out the inside of CMS and to check for possible collateral damage. Though currently it looks like only three muon chambers are affected, the exact scope of this repair can only be estimated when the detector is open.

Despite the complications faced in the operation of the detector the CMS collaboration also worked hard on the future of the detector. To keep the article short only three will be mentioned here.

In April the collaboration took the important decision to replace the current endcap calorimeters, which cannot be operated during HL-LHC, by a high granularity calorimeter (HGC) with tungsten as absorber and silicon detectors as active devices. This combined electromagnetic and hadronic calorimeter will allow investigating showers and their development with an unprecedented position resolution.

In June the Technical Proposal for the Phase II Upgrade of CMS was submitted. This document describes how CMS will be upgraded for the HL-LHC. It contains plans for replacing the silicon tracker and the endcap calorimeter and upgrades of the muon system, the trigger and the DAQ. In addition large parts of the infrastructure will have to be replaced or upgraded.

One of the major upgrade projects of CMS during still during LHC phase I is the installation of a new pixel detector with four layers and a reduced material budget. The installation is foreseen in the EYETS 2016/17 and the production is now in full swing to be ready in time for installation.

In summary, despite severe technical problems with its cold box CMS has demonstrated that the detector came out of LS1 very well prepared for 13TeV data taking. With some heroic efforts of CMS members and colleagues of the technical departments it was possible to record a large fraction of the 13 TeV pp data and most of the HI data, making this year a great success for physics analysis. The coming technical stop will once more require a full concentration of all forces for cleaning the cold box and in parallel open the detector and fix the recent water leak. For this CMS once more counts on the excellent collaboration with the technical groups as experienced several times in the past.

The recent LHC performance gives rise to great expectations for 2016 and CMS will do the utmost for being able taking data with a detector in perfect shape.