Matteo Solfaroli Camillocci

Impact of the Voltage Transients After a Fast Power Abort on the Quench Detection System in the LHC Main Dipole Chain

A Fast Power Abort in the LHC superconducting main dipole circuit consists in the switch-off of the power converter and the opening of the two energy-extraction switches. Each energy-extraction unit is composed of redundant electro-mechanical breakers, which are opened to force the current through an extraction resistor. When a switch is opened arcing occurs in the switch and a voltage of up to 1 kV builds up across the extraction resistor with a typical ramp rate of about 80 kV/s. The subsequent voltage transient propagates through the chain of 154 dipoles and superposes on the voltage waves caused by the switch-off of the power converter. The resulting effect caused intermittent triggering of the quench protection systems along with heater firings in the magnets when the transient occurred during a ramp of the current. A delay between power converter switch-off and opening of the energy-extraction switches was introduced to prevent this effect. Furthermore, the output filters of the power converters were modified in order to damp faster the voltage waves generated after the power-converter switch-off and to lower their amplitude. Finally, snubber capacitors were added in parallel to the extraction switches to help the commutation process by reducing the arcing effect and thus smoothing the voltage transient. A set of dedicated tests has been performed in order to understand the voltage transients and to assess the impact of the circuit modifications on the quench detection system. The results have been compared to the simulations of an electrical model of the LHC main dipole circuit developed with the Cadence suite (PSpice based).

Beam systems without failures - what can be done?

The beam dumps triggered by interlocks not related to the magnet powering are discussed. This concerns the systems like the RF, the transverse feedbacks, beam instrumentation, beam dumping system, collimators and control systems. An analysis of the reasons of these dumps is presented together with a possible strategy to mitigate the effect of these failures. BEAM DUMP ANALYSIS This study was meant to analyze the major causes of beam dumps not related to the powering system, in order to identify solutions and possible bottlenecks in view of the operation beyond 5 TeV. All beam dumps of 2010 and 2011 have been analyzed, excluding those related to powering system and those happened during MD periods as not representative of the sample needed for the analysis. They have finally been sorted by energy on: • Dumps at 450 GeV • Dumps during the ramp • Dumps at 3.5 TeV Injection energy In order to avoid compromising the results of the analysis with a large number of not fully significant dumps at injection energy, only the fills with intensity of both beams higher than 3E10 p have been considered. As the beam intensity in 2010 was sensibly smaller than what used in 2011, this choice resulted in a very weak statistics. Indeed, only 2 events were found; one due to losses in injection region and the other one to a BLM card electronic failure. More interesting is the statistics for the 2011 when 38 events took place, as shown in Figure 1. Figure 1: beam dumps at injection energy In Table 1 the details of the beam dumps at 450 GeV are shown. It can be noticed that most of the events are due to beam instabilities (leading to losses and beam excursion) rather than system failures. Table 1: Events at injection energy in 2011 Beam dump cause Number of events (2011)