Over 140 digitiser cards are being used in the machine protection systems of the Large Hadron Collider (LHC) at CERN. The cards are used to check that the particle beams have been accurately deflected. Incredible precision is required for this as two high-energy particle beams travel at close to the speed of light in opposite directions in the 27km LHC accelerator ring.
They are then made to collide at four locations around the ring for runs of several hours. The particles are so tiny that the task of making them collide is akin to firing two needles 10 kilometers apart with such precision that they meet halfway.
The energy stored in the two beams is so high that, in the case of loss of control of the beams, the LHC machine could suffer serious damage. To safely dispose of the beams in the case of an emergency, or at the end of a collision run, the LHC Beam Dumping System (LBDS) must extract the full beams from LHC in one revolution and transport them to an absorber block located about 700m away from the LHC rings. After every beam dump, post-operation check systems will make sure that all elements within LBDS performed correctly and that the beams were cleanly extracted.
A key part is checking that the fast-pulsed magnets, called kicker magnets, have accurately deflected the particle beams from the LHC rings to the extraction lines. Digitiser cards are used to capture the current pulse waveforms to check that they had the correct shape and were correctly synchronised with the beam. To give an idea, the LBDS extraction magnet current pulses themselves are about 20kA with rise times of less than 3 µs and a synchronisation error below 20ns. At LBDS alone, for every beam dump event, nearly 500 analogue signals are acquired using digitiser cards and more than 150 synchronisation signals are captured using digital I/O cards.
“We have developed our own waveform acquisition and analysis framework called IPOC (Internal Post Operation Check),” explained Nicolas Magnin, Software Team Leader for the Accelerator Beam Transfer group at CERN. “It is programmed using C++ on Linux and it includes a hardware abstraction layer that enables us to interface with many types of digitizer cards. We use a variety of digitiser cards from Spectrum Instrumentation to cover a range of kicker magnets used for all the CERN accelerators that have different requirements in terms of bandwidth and dynamics. This variety of cards enables us to cover a bandwidth range from 10MS/s to 500MS/s and a resolution from 8 to 16 bits depending on the application.”
LBDS HV generators in the LHC tunnel. Each blue computer contains several digitisers to capture internal current of HV generators for every beam dump
A very high level of precision is naturally required for the measurements of these kicker pulse events. For example, the most demanding systems require a pulse-to-pulse reproducibility error for the delay of lower than 10ns, and for the amplitude of lower than 0.5% within a dynamic range of 16. To achieve this, the precision of the acquisition has to be even greater by an order of magnitude. As the kicker current signal dynamic is not fixed, the various input ranges of the digitizer are used to optimize the signal-to-noise ratio of the acquired waveform. Almost all waveforms are saved in a logging database for later offline analysis. Also, all waveform analysis results (for instance: delay, length, rise time, fall time, flat top amplitude, etc.) are stored in the logging database to extract trends over time to check system stability, temperature dependency, etc.