Reminder of WG2 goals The primary goals of WG2 are to assess MGPD performance, define common test standards and solve or get around related physics issues. Its role is also to understand better the phenomena at play in gas detectors. Major progress has been made in this direction in close relation with other working groups. Over the last 5 years, a wide variety of characterisation results have been reported in WG2. Benchmark performance as well as sensitivity to various kind of particles have been measured with different MPGD technologies. Beside compiling results, WG2 is also the natural place to discuss and agree upon test standards which enable comparison and eventually selection of best suited detectors for a given application. At the beginning of the collaboration, sparking, ageing and charging-up were identified as the main physics issues limiting the performance and a wider spread of MPGDs. Dedicated studies were carried out to reveal the underlying mechanisms and eventually find solutions. In this section, the WG2 activity and achievements are summarised, connections to other working groups are underlined. Common characterisation Several RD51 groups, by producing and sharing results, have shed a new light on basic phenomena in gases which are discussed below. Sometimes at the interface between working groups, they contributed to the definition of new design rules of MPGDs and to the development and validation of software tools. The process of signal amplification by electron avalanche impacts a lot on the performance of the detector (timing, gas gain, stability, signal to noise ratio, ageing etc...). Avalanche development could be studied in several ways, for instance, by measuring the relation between gas gain and applied voltage. Using measurements performed in a large variety of geometry and experimental conditions, mechanisms such as UV photon feedback and Penning effect could be quantitatively studied, providing important inputs to Monte Carlo programs. In GEM-like geometries, the electrodes are not facing the avalanche region and photon feedback is naturally suppressed. This advantage allows operation in pure gases and thus imaging of cherenkov radiation (RICH) and search of Dark Matter (DM) by conversion of cherenkov/scintillation light on photo-sensitive coatings. In addition, DM experiments require a large target mass and thus the use of liquid argon or xenon. Operation in cryogenic conditions is still very challenging : the necessary large increase in electric field makes the detector stability very sensitive to the electrode surface quality. For this reason, double phase configurations with extraction of primary electrons from the liquid to the gas phase are investigated. In 2011, this concept has been successfully applied to a liquid argon TPC prototype read out by a LEM (Large THGEM) and strips (Fig. 1 right). Electron multiplication and primary ionisation are statistical processes which set a limit to the energy and spatial resolution. Related fluctuations should be minimised and are being measured with improved precision by means of GridPix detectors (a combination of a Micromegas grid on top of a CMOS pixel chip acting as anode). Thanks to an unprecedented granularity and a high sensitivity to single electrons, GridPix was used to measure some moments of the gas gain distribution (as well as the Fano factor) and to discriminate between various models of avalanche development. In order to optimise pixel detectors for high tracking accuracy, the detailed path of electrons must be predicted. But at the vicinity of the MPGD holes, field gradients and diffusion make this task difficult. In cascaded structures, the field configuration is even more complex and calculating the path probabilities of electrons remains even more challenging. Ion drift is also of interest in single photon detectors to minimise backflow to the photo-cathode. For the time being, these probabilities are inferred from current measurements under continuous irradiation of the detectors. They are necessary inputs to validate field solver programs and microscopic tracking algorithms. From the measurements compiled in WG2 it appears that in most MPGD geometries, close to 100% of the primary electrons can be transferred from the drift to amplification region and are multiplied by avalanche. Ion backflow can be reduced easily to the percent level (compulsory for an LC-TPC). Lower fractions are possible in Micromegas of smaller pitch and in GEM/THGEM stacks too, probably because of the misalignment between foils. GEM-THGEM foils can easily be coated with photo-sensitive materials (e.g. CsI). Assembled in a stack, they provide enough gas gain to detect single photo-electrons. These gaseous photo-multipliers (GPM) reach similar efficiencies as traditional PM tubes and have no sensitivity to magnetic fields. But to preserve the photo-cathode from ion impact, innovative and complex designs have recently been proposed (COBRA, flower-THGEM) and are being characterised. Another challenge is to efficiently extract and detect single photo-electrons. The hole pitch of THGEMs being relatively large, photoelectrons liberated between holes can be lost due to the particular shape of the field in this region (Fig. 1 left). This effect is now understood and can be minimized with an adequate setting of the fields. Gas discharges Gas discharges induce dead time and are potentially harmful to the detector elements and electronics. As a central topic in WG2, several facts on discharges could be compiled and made available to the RD51 community. Because the spark probability is a function of the total charge released in the gas, operation at low gas gain is obviously preferred but not always possible. The ionising background is also relevant as well as the gas mixture (more precisely the atomic number of the atoms). Electrode geometry and surface quality are other parameters that can be optimised for low spark rate. It terms of potential damage to the detector, the electrical energy stored in the mesh or foils rises quadratically with the applied voltage. Sparks are thus less energetic in gas mixtures with strong Penning effects which have lower workable voltages. An important milestone achieved in WG2 is the development of efficient protections against sparks. For applications where rate capability is not mandatory, external diode networks are fine (T2K-TPC). In systems with a very large number of channels (LC-DHCAL), the number of passive components becomes prohibitive but integration of smaller diodes inside the ASICs is investigated. The integration of resistive elements to readout electrodes can be used to limit the spark current and the resulting voltage drop. It is an appealing solution for tracking in high rate environments (SLHC-MAMMA) or when the spread of avalanche signals onto readout electrodes is desirable (LC-TPC). Several configurations of conductors, resistors and insulators were tested by the MAMMA groups. The final configuration uses resistive strips capacitively coupled to readout strips through a separating insulating layer. The prototype behaviour under irradiation is remarkable and shows virtually no voltage (nor efficiency) drop even at high gains. The LC-TPC Micromegas configuration is more conventional, the pad plane being coated with an insulating and a resistive layer. Avalanche charge is evacuated laterally, inducing signals on several adjacent strips. An excellent resolution both in the rφ plane and drift direction is achieved as well as full protection against discharges. In GridPix detectors, the insulating layer is absent to fully exploit the high granularity of the pixel readout. High resistivity materials deposited by wafer post-processing technology have been tried. Protection against discharges has been demonstrated in several gas mixtures. In multi-stage structures, the gain is distributed over several foils and the actual gain of each foil is low (a few tens). Also, diffusion between foils reduces the charge density and thus the spark rate. In applications with constrains on material budget or thickness, one foil is used. Sparking then matters and resistive GEMs have been developed. Charging-up and ageing Insulating materials are always present in MPGDs. Resistive elements can also be incorporated to the design. Depending on the exact configuration, the avalanche charge may not be readily evacuated to ground, causing time dependent variations of field and thus gain. MPGDs such as GEM or micro-Bulk are prone to charging-up but, by a careful choice of the geometry, this effect can be attenuated or even suppressed. With resistive elements, the time constants can in principle be calculated from the conductivity, permittivity and geometry. Resistive MPGDs with high rate capability have been built and successfully tested in the RD51 test beam facility (Bulk Micromegas LC-TPC module). Radiation tolerance is an attractive feature of MPGDs which makes them competitive with solid-state sensors for tracking in hard radiation environment like the one at SLHC. Two main ageing studies were reported in WG2. The first study was performed by the NIKHEF Detector R&D group who exposed a thin GridPix chamber to an intense strontium source, accumulating a total charge equivalent to more than 10 years of SLHC. With a drift gap of 1 mm, this very fast detector (called GOSSIP) can measure track vectors. It is a candidate for the upgrade of the inner layers of the vertex detector at future SLHC. In view of the upgrade of the endcap muon spectrometer of ATLAS at SLHC, the MAMMA collaboration conducted irradiation tests of resistive strip Bulk Micromegas. Several tests were actually done using various particles composing the LHC background (neutrons, gamma, X-rays and charged particles). In total, the charge equivalent to 5 to 10 years of SLHC was accumulated during the tests. By constantly monitoring the current of the mesh, no degradation of performance was observed. Figure 1 : High resolution scan of a THGEM surface with single photo-electrons revealing the 800 μm pitch hole pattern and small less efficient zones between holes (top). Two-dimensional view and strip signals of a muon traversing the liquid argon Large GEM TPC (right).