Upgrade Plans for ATLAS Forward Calorimetry for the HL-LHC

The upgrade of the LHC Collider foresees increased instantaneous luminosity 3-7 times the original design value of 10$^{34}$ cm$^{-2}$ s$^{-1}$. The increased particle flux at this high luminosity phase of the LHC (HL-LHC) will have an impact on many sub-systems of the ATLAS detector. In particular, in the LAr forward calorimeter (FCal), which was designed for operation at LHC luminosities, the associated increase in the ionization load at HL-LHC luminosities poses a number of problems that can degrade its performance, related to beam heating, space charge effects in the LAr gaps and HV drop due to increased current draws over the HV current-limiting resistors. One solution to these problems, which would require the opening of both ATLAS endcap cryostats, is the construction and installation of a new FCal, with cooling loops, narrower LAr gaps, and lower value protection resistors. The signal performance of the current FCal and of a possible narrow-gap FCal has been measured in a dedicated test-beam campaign at the high-intensity proton beam facility in Protvino, Russia. These results compare well to simulated signal behavior in both the nominal and critical particle density regimes. A second proposed upgrade solution, which does not involve opening the cryostats, is the installation of a small Mini-FCal calorimeter in front of the current FCal, resulting in a reduction of the particle flux to levels at which the existing device can operate normally. Several copper- sampling designs are proposed with different technologies in the sensitive volume of the Mini- FCal: diamond sensors, high-pressure xenon gas, and liquid argon. The talk will review the design of the ATLAS forward calorimeter and discuss the effects that are expected to degrade the performance at HL-LHC luminosities as well as the different upgrade scenarios proposed.