Series production and series tests of SIS100 dipoles for the FAIR ring accelerator are completed


Another decisive step has been taken on the way to completion of the large ring accelerator SIS100, the heart of the future accelerator center FAIR: The production of all 110 superconducting dipole magnets for the new heavy ion accelerator with a circumference of 1.1 kilometers has been completed, as have the corresponding cold tests at the final operating temperature of -269 degrees.

In the FAIR ring accelerator, various sophisticated magnets and entire magnet systems will ensure that the ion beam is precisely guided and focused. The superconducting dipole modules also belong to them. In total 110 dipole magnets were produced, 108 will be installed in the ring accelerator tunnel and two more are spare ones. The dipoles, that will mainly be needed for deflecting the particle beam, make up more than a quarter of all 415 fast ramped superconducting magnets utilized in the SIS100.

The successful production of these dipole modules and their testing represents the largest series of accelerator components ever manufactured by order of GSI. The completion is an important milestone on the way to installation in the tunnel, which is scheduled to begin in the second half of next year. Bilfinger Noell in Würzburg, one of the few European manufacturers of superconducting magnets, was contracted for series production.

The SIS100 dipole magnets are so-called superferric magnets, consisting of a superconducting coil and an iron yoke to guide the magnetic field. The particular feature of the magnets is the superconducting coil, in which a special superconducting cable is used. This nuclotron cable - originally developed for the ring accelerator Nuklotron at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia - is particularly suitable for generating rapidly ramped magnetic fields.

The cable consists of a copper-nickel tube. Around this tube strands of niobium-titanium, a common superconductor, are coiled. The original design was optimized with regard to the requirements of FAIR. It is cooled with liquid helium and operated at a temperature of 4.5 Kelvin (equivalent to 4.5 degrees Celsius above absolute zero at around -273 degrees). The design of the magnets allows to integrate vacuum chambers for the ion beam, whose wall temperature is also just above absolute zero. Thus, the chamber walls act like a super pump onto which the remaining particles of the beam vacuum keep attached. The extremely low remaining gas pressure made possible by is a mandatory precondition for the acceleration of heavy ion beams with highest intensities. Highest particle intensities are part of the specifications of the FAIR facility, which offers a wide variety of new experimental possibilities.

Each of the magnets, which weight about three tons and are three meter long, is subjected to a comprehensive test program: The quality control of the production as well as several tests under room temperature conditions are performed in Würzburg before shipment to Darmstadt. Among other things, the geometric precision of the inner aperture and the electrical properties of the coil were measured as part of the so-called FAT (Factory Acceptance Test). Bilfinger Noell succeeded in making the production so precise over the entire series that the deviations of the geometry of the field-determining pole shoes were always less than 50 micrometers from the nominal geometry.

After delivery to GSI, all 110 dipole modules were subjected to a SAT (Site Acceptance Test), which included performance tests at the final operating temperature of 4.5 K. To cool the magnets down to this temperature, GSI has built an elaborate, almost 700-square-meter test facility with cryogenic equipment for superconducting accelerator magnets (STF, Series Test Facility). It has four so-called feed boxes to connect the dipole modules for parallel testing in different phases. Using a specially procured high-power power supply unit, the modules could be supplied during the performance test with amperage up to 17 kiloamperes at rise rates of 28000 amperes per second.

The test program for all 110 dipole modules was carried out in years of cooperation by employees from various specialist areas and departments. In a final integration step, the thin-walled dipole chambers, produced by PINK Vakuumtechnik in Wertheim, are now being installed. (BP)