Upgrade Plans for ATLAS Forward Calorimetry for the HL-LHC

The ATLAS detector was designed and built to study proton-proton (pp) collisions produced by the Large Hadron Collider (LHC) at centre-of-mass energies up to 14 TeV and instantaneous luminosities up to $10^{34}{\rm cm}^{-2}{\rm s}^{-1}$. At the higher instantaneous luminosity ($5\times 10^{34}{\rm cm}^{-2}{\rm s}^{-1}$) proposed for the High-Luminosity LHC (HL-LHC), some components of ATLAS will not operate properly, while others may not survive the dose that will be accumulated while collecting the proposed 3000 fb$^{-1}$ of pp collision data. For the ATLAS liquid argon (LAr) calorimeter, problems are anticipated in the forward region where the particle flux is particularly high. The existing Forward Calorimeter (FCal) was designed with very narrow LAr gaps (250-500 $\mu$m) in order to avoid problems due to ion build-up that would distort the electric field. At HL-LHC luminosities, these gaps are no longer sufficiently narrow. The resulting distortions of the electric field in the gaps would be exacerbated by significant voltage drops caused by the higher currents flowing across large protection resistors which are located within the cold volume of the endcap cryostats. There is also the possibility that the increased ionization load may result in heating sufficient to cause boiling of the liquid argon. Two distinct solutions to these problems are currently under study: one is the construction and installation of a new FCal with slight design modifications to address the problems referred to above. A second approach, which would not require the opening the cold volume of the cryostats, involves the installation of a small calorimeter in front of the FCal, resulting in a reduction of the particle flux to levels at which the existing device can still operate. This contribution will summarize the various forward calorimeter upgrade options, and discuss the procedure for adopting a solution. This process depends heavily on the results of radiation hardness of cold GaAs preamplifiers which are currently employed by one of the LAr sub-systems in the endcap cryostat; this is the topic of another contribution to these proceedings.