Upgrade Plans for ATLAS Forward Calorimetry for the HL-LHC

Even though data-taking has just started with the LHC, plans are being developed to operate the machine and its detectors at up to 10 times the original design luminosity. This has a major impact on the Forward Calorimeter (FCal), which is exposed to some of the highest radiation rates in ATLAS. The FCal detector and its associated components were designed for operation at the maximum LHC luminosity of \(\text{10}^{\text{34}} \text{ cm}^{-2}\text{s}^{-1}\). However at the higher luminosities projected for the HL-LHC, operation of the FCal may be compromised. Beam heating in the FCal could lead to the formation of argon bubbles in the detector, the ionization rate will result in space charge effects that will reduce the signal and the current draw will result in a voltage drop across the HV current limiting resistors. Two possible solutions are being considered to maintain FCal operation at HL-LHC. One is a complete replacement of the FCal system. A replacement FCal would have a similar design to the current calorimeter except for additional cooling, lower value HV protection resistors and the use of smaller ionization gaps, as small as 100 microns in the first compartment. The second solution is the installation of a small warm calorimeter, named the Mini-FCal, to be placed in front of the FCal. This addition would reduce the ionization load in the first FCal compartment, which would keep a larger region of the FCal active and reduce the heat load to an acceptable level. The current concept for the Mini-FCal is a standard parallel plate calorimeter with copper absorbers and diamond sensors, which were chosen for their inherent radiation resistance. It is anticipated that neutrons will be the major cause of damage to the diamond sensors and the integrated flux of neutrons in the Mini-FCal after 3000 fb\superscript{-1} at the HL-LHC will be up to 2 x 10\superscript{17} neutrons/cm\superscript{2}. Recent irradiation tests carried out by members of the ATLAS LAr group show that these sensors can still operate after irradiation up to these levels although with a large reduction in signal.