# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 9 Micromegas for Precise Timing  
A MIP forms multiple ionization clusters when passing through the drift region of a Micromegas detector. The number of clusters and their locations are distributed according to the average ionization length of the particle in the given gaseous medium. The location of the clusters may vary around 100 um for a MIP affecting the distance of the last cluster to the mesh. The gas type and the electric field limit the drift velocity of the electrons. The spread of the electrons during the primary ionization process and their diffusion in gas while drifting toward the micromesh result in a time jitter of several nanoseconds for the electrons when reaching the mesh. As a result, a Micromegas can not achieve a sub-nanosecond time resolution for vertex separation of MIPs as demanded at future HEP experiments [114]. PICOSEC-Micromegas is an R&D project that was initiated in order to overcome this limitation in time resolution by introducing a new detection concept for Micromegas detectors [115]. Figure 26 illustrates this concept. A Cherenkov radiator [116] and a photocathode are placed in front of the gaseous volume. The passage of a charged particle through the Cherenkov radiator produces UV photons, which are then absorbed in the photocathode, and primary electrons are created on the bottom surface of the photocathode. These electrons are subsequently preamplified and then amplified in the two high-field stages, thereby, inducing a signal that is measured between the anode and the mesh. Figure 25: The Forfire Micromegas prototype. An extra mesh allows the decoupling of the photocathode from the detector, the use of reflective mode, and an extra amplification stage. Electrons emitted from the photocathode surface experience the same electric field along the same distance to the mesh. The gaseous volume is needed only to amplify the electrons and to induce a readable signal on the anode. A Micromegas-like detector with a grounded mesh between two electric fields with parallel field lines is chosen. A negative voltage is applied to the cathode and a positive voltage to the anode to provide a unidirectional electric field in both regions. With the grounded field, the voltage will only drop until the mesh when a spark occurs. The other region is inversely polarized and stays unaffected by the spark. Additionally, fields can be independently tuned with a grounded mesh, and field scans are easily doable. In the PICOSEC-Micromegas, the length of the drift gap is reduced to the same order as the amplification gap and is operated with a similar electric field. In this field configuration, a first preamplification of the electrons happens in the drift gap, improving the time resolution as it reduces the drift time of the primary electrons and, mainly, the spread in the time of arrival of the first electron into the Micromegas amplification gap [117]. A single-anode prototype was used to demonstrate the proof-of-principle by reaching a time resolution as low as \((24.0\pm 0.3)\) ps in a particle beam with 150 GeV muons. This result is reached with a drift field of 23.75 kV/cm and an amplification field of 21.48 kV/cm. In this unusual configuration, where the drift field is comparable to the amplification field, preamplification occurs in the short drift region of 200 \(\upmu\)m. This detector was operated with a 18 nm CsI photocathode providing \((10.40\pm 0.40)\) photoelectrons per muon and with the gas mixture used in the COMPASS experiment consisting of 80% neon, 10% ethane, and 10% CF4[118]. An even higher increase of the drift field, with constant voltages, is achieved with reduced drift gaps down to 120 \(\upmu\)m. With this configuration, a single photoelectron time resolution of \(44\pm 1\) ps is reached in the laser with a drift field of 44 kV/cm and an amplification field of 21 kV/cm [119].