Automated GTO Measurement, Validation and Stack Selection System for the LHC Beam Dumping System

The LHC Beam Dumping System (LBDS) is a safety critical system ensuring extraction and safe disposal of LHC beams on dedicated graphite core dump blocks. It comprises 15 extraction and 10 dilution kicker generators per beam and generates up to 1 MA total current at 7 TeV beam energy. Both extraction and dilution generators employ similar stacks with 10 series connected GTO like thyristors as the main switches. To ensure safe operation of the GTO stack, the individual GTOs within the stack have voltage-sharing resistors and are continuously monitored with the goal to detect failure of an individual GTO. The monitoring system compares the voltage drop over the full stack to the voltage drop of the GTO in the lowest position within the stack and generates an alarm/interlock in case of abnormal behavior. The accuracy of the voltage surveillance system can be compromised by individual GTO leakage current variations if the latter is too high, compared to the current in the voltage sharing resistors. To minimize this risk, the Automated GTO Measurement and Stack Selection System (AMSSYS) was developed. It is based on simultaneous leakage current measurements of 5 GTOs at multiple (6) voltage levels within our dynamic range (100 V - 3 kV). AMSSYS selects automatically a set of 10 GTOs with best fitting parameters for one stack. These GTOs then all have a very similar behavior over the whole voltage range at the nominal operational temperature. This selection procedure minimizes forward blocking voltage (VAC) variations within the stack. The risk of a false detection of a GTO failure was significantly reduced, the sensitivity and reliability of the LBDS stack monitoring was improved. This paper outlines the development and design of AMSSYS and presents and compares different test programs for the GTO selection process.