A radiation tolerance study of the ALICE TPC Readout Control Unit 2

ALICE is a general-purpose detector that is designed to study the physics of quark-gluon plasma. The Time Projection Chamber (TPC) is one of the major detectors of ALICE. The TPC electronics consists of 4356 Front-end cards (FECs), which are controlled by 216 Readout Control Units (RCU). Each RCU connects to between 18 and 25 FECs using a multi-drop bus. In LHC Run1, the Readout Control Unit 1 (RCU1) performed even better than specification. However, in Run2 the energy of colliding beams is increased from 8 TeV to 14 TeV (maximum value) and higher luminosity, which leads to larger event size and higher radiation load on the electronics. As a solution, the Readout Control Unit 2 (RCU2) is designed to provide faster readout speed and improved radiation tolerance with respect to the RCU1. The RCU2 is conceptually similar to the RCU1 and it reuses the existing infrastructure and readout architecture of the TPC electronics. However, the multi-drop bus is split into four branches from the two branches and the bandwidth of the Detector Data Link (DDL) is increased from 1.60 Gbps to 3.125 Gbps. Correspondingly, the firmware is designed to utilize the improved parallelism. These actions ensure that the readout speed of the RCU2 can be improved by a factor of ~2 with respect to the RCU1. The flash-based Microsemi Smartfusion2 FPGA SOC is used as the main FPGA instead of the SRAM based Xilinx Virtex 2 Pro FPGA that was used on the RCU1. Because its configuration cells are immune to Single Event Effects, the radiation tolerance of the RCU2 was expected to be improved. The primary objective of this thesis has been to study the radiation tolerance of the RCU2. This is done through several irradiation tests, which are divided into two steps. To start with, the radiation sensitivity of the Smartfusion2 FPGA and all the hardware interface are characterized. Afterwards, a system-level irradiation test is performed. Actions have been taken against all the radiation related problems that were revealed during the irradiation tests. Running experience shows that radiation tolerance of the readout system based on RCU2 is about 10 times better as compared to the RCU1 for p-Pb collisions at similar energy level. The second objective of this thesis was to develop the firmware modules that realizes the readout algorithms. Development of the firmware has gone through three versions, the first prototype, the second prototype and the commissioning version, and important contributions were made to the first two versions. The integration and testing of the RCU2 is also an important task covered in this thesis. Functional tests were performed for the mass production, the irradiation tests at and the final installation at the TPC. Readout performance of the RCU2 has been characterized and the solutions aiming to further increase the readout speed have been proposed and verified.