Improvement of the CERN SPS electrostatic septa ion traps
At CERN, the SPS synchrotron is equipped with a slow extraction channel towards the fixed target beam lines in the North Area This channel includes five consecutive electrostatic septa, where the field free region and the active high field region are separated by an array of tungsten-rhenium wires. The field-free region provides for the circulating beam, while the high field region is used to deflect the extracted beam. Since the residual gas can be ionized by the orbiting beam, low energy ions could cross the wire array and enter the high field region and cause high voltage breakdown when accelerated onto the cathode. To prevent low energy ions from entering this high electric field region, a vertical field is applied to the orbiting beam using so-called `ion traps' for active protection. The vertical field is created by electrodes placed inside the region containing the circulating beam. Due to electromagnetic coupling onto the ion trap electrodes observed with the high frequency LHC beam (25 ns spaced bunches), the efficiency of the ion traps is greatly reduced. This leads to increased vacuum activity (electron cloud related) as well as high spark rates both in the main field and between the ion trap electrodes and their grounded support. In view of the SPS performance increase required for HL-LHC, this paper highlights the upgrades and improvements required to obtain a stable ion trap field and significantly reduce the number of breakdown events observed with the LHC beam in the accelerator.
A New Spark Detection System for the Electrostatic Septa of the SPS North (Experimental) Area
Electrostatic septa (ZS) are used in the extraction of the particle beams from the CERN SPS to the North Area experimental zone. These septa employ high electric fields, generated from a 300 kV power supply, and are particularly prone to internal sparking around the cathode structure. This sparking degrades the electric field quality, consequently affecting the extracted beam, vacuum and equipment performance. To mitigate these effects, a Spark Detection System (SDS) has been realised, which is based on an industrial SIEMENS S7-400 programmable logic controller and deported Boolean processor modules interfaced through a PROFINET fieldbus. The SDS interlock logic uses a moving average spark rate count to determine if the ZS performance is acceptable. Below a certain spark rate it is probable that the ZS septa tank vacuum can recover, thus avoiding transition into a state where rapid degradation would occur. Above this level an interlock is raised and the high voltage is switched off. Additionally, all spark signals acquired by the SDS are sent to a front-end computer to allow further analysis such as calculation of spark rates and production of statistical data.
OASIS : A new system to acquire and display the analog signals for LHC
GENERAL ARCHITECTURE To cope with the user requirements [1] for the Large Hadron Collider (LHC) era, a new system has to be developed for the acquisition and display of analog signals in the accelerator domain. OASIS, the Open Analog Signals Information System, extends the capabilities of the existing ‘new Analogue observation system’ (nAos) in many fronts: new acquisition hardware is supported to provide higher analog bandwidth to the users, a new distributed triggering scheme has been designed for greater flexibility and the software has been completely redesigned with openness in mind to allow easy upgrading and maintainability. This paper describes the architecture chosen to satisfy the new user requirements. The solution involves three tiers and makes use of the J2EE platform to communicate between the GUI and the middle tier. The front-end tier runs on Linux PCs in CompactPCI format, to support fast digitizer modules. A first prototype system, used for the extraction tests from the Super Proton Synchrotron (SPS) to the LHC, is also presented. We chose for the OASIS system a three-tier