Radiation Tests on the Complete System of the Instrumentation of the LHC Cryogenics at the CERN Neutrinos to Gran Sasso (CNGS) Test Facility

Abstract

There are more than 6000 electronic cards for the instrumentation of the LHC cryogenics, housed in crates and distributed around the 27 km tunnel. Cards and crates will be exposed to a complex radiation field during the 10 years of LHC operation. Rad-tol COTS and rad-hard ASIC have been selected and individually qualified during the design phase of the cards. The test setup and the acquired data presented in this paper target the qualitative assessment of the compliance with the LHC radiation environment of an assembled system. It is carried out at the CNGS test facility which provides exposure to LHClike radiation field. I. THE CRYOGENIC INSTRUMENTATION ELECTRONICS The cryogenic instrumentation electronics are placed all around the LHC tunnel and in protected areas. Concerning the tunnel electronics, radiation was a main constraint since the beginning of the design phase. Space or military technologies were incompatible with the budget of the project and instead, Components Off The Shelf (COTS) were selected, qualified for operation under radiation and finally used [1, 2]. Adversely, the protected areas electronics have not been designed radiation-tolerant, as the radiation levels in the protected areas were quite underestimated. Many of the components of the protected areas electronics are the same as the tunnel ones; nevertheless, there are several components for which no information exists for their performance under radiation. The aim of the tests at the CNGS facility is to validate the complete systems (rather than individual components) in both cases: tunnel and protected areas electronics. The cryogenic instrumentation electronics (in the cases of tunnel and protected areas as well) are divided into conditioners, measuring temperature, pressure, liquid helium level and digital status, and into actuator channels, providing AC and DC power to the areas where helium needs to be heated-up. Figure 1 shows the architecture of the system, in the case of conditioner channels. A conditioner card holds two independent channels. Each channel has a front end ASIC taking measurements on a sensor. The resulting waveform is sent for digitization to the ADC. A 16 bit word is then sent to the FPGA for the first stage of processing and the formatting of the data provided to the communication card. Up to 15 channels may be interfaced with the same communication card, which implements the WorldFIP protocol and places the data on the Fieldbus. Figure 1: System architecture The system offers very high accuracy, due to its auto calibrating features [1]: For each measurement on a sensor, there is a measurement on a high precision reference resistance which permits the correction of the gain drifts. The polarity of the input of the amplifier is inverted so as to correct its offset. Finally, the excitation current is applied in both directions in order to compensate for the thermocouple effects, as well as any dc offsets of the wiring. II. RADIATION TOLERANCE STRATEGY The radiation, in the case of the LHC tunnel electronics, was faced in two main ways: an elaborate components selection and a set of mitigation techniques [1]. A. Components Selection Customized development of a radiation hard front end ASIC and of a linear voltage regulator for power supplies and references. Use of anti-fuse FPGAs. Selection of a Fieldbus agent (implementing the WorldFIP protocol) that uses signal transformers instead of optical insulators. Qualification for operation under radiation of all the components in dedicated facilities [2,3].