ATLAS Toroids Research Articles

Certification of a new generation of Resistive Plate Chambers for the Phase-1 BIS78 upgrade of the ATLAS Muon Spectrometer

In preparation for the coming years of LHC running at higher luminosity, two upgrade projects of the ATLAS Muon Spectrometer have been developed: the New Small Wheel project, improving the trigger in the end-cap regions, and the BIS78 project, dedicated to the transition region between the barrel and the endcaps (1< |η| <1.3). The BIS78 project will add 32 RPC triplets on the edges of the inner barrel sectors where the ATLAS toroid is present. These new generation of RPCs are characterized by thinner gas gaps (1 mm vs 2 mm of the legacy RPCs) as well as thinner resistive electrodes, together with a new high gain front-end electronics operating at lower voltage. Sixteen BIS78 stations have been already installed in the ATLAS Muon spectrometer between 2020 and 2021 and are being commissioned during this year. The status of the project, the status of the commissioning as well as the overall performances of the installed RPCs will be presented.

A Functional 3D-Printable Magnet Model of the ATLAS Toroid

The ATLAS detector is the largest particle detector at the LHC and one of the most complex machines ever built. It allows precise measurements of particles emanating from proton collisions. Due to its complexity, introducing the ATLAS detector in the high-school physics classroom can be challenging. Nonetheless, we show how to use 3D printing to provide a hands-on classroom activity by constructing a functional 3D model of the toroidal ATLAS magnet system. This model can be used to discover, visualize and explain the shape of a toroidal magnetic field and to start a discussion about the role of magnetic fields in particle detectors in general.

New Bridge Temperature Sensor for Superconducting Magnets and Other Cryogenic Applications

A few hundred temperature sensors are used to monitor the temperature behavior of the gigantic ATLAS toroid superconducting magnet system during cool down and normal operation. In order to guarantee good sensitivity of temperature measurements in the range from liquid helium to room temperature, two types of sensors are positioned at the same location: platinum resistance thermometers for the range 30–300 K and carbon composition resistors (Allen-Bradley) for the 4–30 K range. Both types are very well known for use in cryogenics and they have performed satisfactorily during 10 year of ATLAS operation. The sensors themselves are easily available and inexpensive and the main cost is for the many kilometers of cold and warm instrumentation cables, connectors, conditioners, and installation work. A reduction of the amount of measurement channels is an important issue and this motivated us to develop a new compact and robust sensor module covering the entire temperature range that would combine advantages of both platinum and carbon resistors. The solution is trivial, elegant, and simple. Two resistors with positive temperature derivative and two resistors with negative temperature derivative are connected in a full-bridge connection. We used two platinum and two carbon resistors. The output signal is a result of the subtraction of voltages across positive and negative temperature derivative resistors that makes this temperature bridge sensor very sensitive for the entire temperature range. Variable temperature characterization tests were performed in the helium gas environment in the CERN Cryogenic Laboratory. Our measurements have demonstrated that the bridge sensors have a full range sensitivity better than 0.1 mV/K at a supply current of 100 μ A. In the meantime, a few other superconducting detector magnets in experiments operating at CERN are equipped with these new, simple, and robust temperature sensors.

Fast Dump of the ATLAS Toroids

The toroidal magnet system of the ATLAS Detector at CERN consists of a Barrel Toroid (BT) and two End Cap Toroids (ECT-A and ECT-C). Each toroid is built up from eight racetrack coils wound with an aluminum stabilized NbTi conductor and indirectly cooled by forced flow liquid helium. The three toroids operate in series at 20.5 kA with a total stored energy of 1.5 GJ. In order to verify the reliability and effectiveness of the quench protection system, series of fast dump tests have been performed first of the single toroids and finally of the entire toroidal magnet system. In this paper a model to simulate the fast dump of the ATLAS toroids in single mode operation and in full system configuration is presented. The model is validated through comparison with measured data extracted from the ramp-and-quench runs. The calculated energy dissipation in the various coils is in very good agreement (within 1-2%) with the enthalpy changes estimated from the temperature measurements of the different parts of the cold masses. The results confirm the safe operation of such a gigantic and complicated magnet system which is now ready for continuous running of the ATLAS Detector.

Effects of Unequal Inter and Intra-Toroid Response Times on the Quench Performance of the System of Three ATLAS Toroids

The ATLAS Experiment at the LHC (CERN) has a toroidal magnet system composed of one barrel toroid and two end cap toroids powered in series at 20.5 kA with a total stored energy of 1.6 GJ. So far the three magnets have been tested independently. In preparation of the overall magnet system test, here we review the quench behavior under normal and faulty conditions including the information gained from the test of the three single magnets. In particular the consequences of variations in effective quench detection and quench heater activation times are studied. The effects on energy re-distribution, peak temperatures and internal voltage in the three toroids and between the 8 coils within a single toroid are investigated. Based on simulation studies partly verified by experiments, conclusions regarding the quench performance of the entire system are drawn.

Electro-Mechanical Behavior of the ATLAS Toroids in Fault Condition

The three toroids magnet in the ATLAS detector provides the magnetic field for the muon detection system. It consists of a barrel toroid and two end cap toroids, which are electrically and magnetically coupled. In order to guarantee a safe operation, the behavior of the toroid support structures have to be verified in fault conditions. The mechanical behavior of the ATLAS magnet system in the equilibrium operating condition has been calculated in the past. In this paper, results of 3D electro-magnetic calculations of unbalanced force distributions under fault conditions are presented and analyzed. In addition, the energy dissipation and acceptable operating ramp rate in the windings are discussed.

20.5 kA current leads for ATLAS Barrel Toroid superconducting magnets

Three pairs of 20.5 kA current leads for the ATLAS Toroid Magnets have been designed, manufactured and tested at Kurchatov Institute. The current leads have a high mechanical reliability and the vacuum tightness under 30 bars of internal pressure. The insulation between the current carrying parts and the mounting flange, the hydraulic connections and the temperature gauges withstand the overvoltage of at least 2 kV. The current leads are fully equipped with diagnostics needed for safety and control. The current leads were tested up to 24 kA. According to CERN's specification they were also tested in the absence of any cooling at very slow current discharge rate (5 A/s) from 20.5 kA to zero without any excessive overheating. Nowadays the current leads are successfully used at the ATLAS Magnet Test Facility at CERN.

Experimental characterization of resistive joints for use inside ATLAS toroids

The authors have investigated, both experimentally and theoretically, the thermo-electrical behavior of the ATLAS magnets resistive joints. These magnets exploit an Al-clad NbTi Rutherford superconducting cable, and the splices between different sections are performed by TIG-welding the Al matrices of the two cables to be connected. This technique is simple from a construction point of view, and we have shown that its performance is adequate for a safe operation of the magnets. The two main concerns during the design of these joints are the temperature rise due to Joule dissipation and the eddy currents induced under nonstationary conditions. We have devised a reliable model of these joints, that allows estimating their resistances and the induced eddy currents; later we have built and measured several sample joints to give experimental confirmation. The model requires, along with the joint geometry, the knowledge of the Rutherford-matrix interface resistance as well as the RRR of the aluminum matrix. In this paper we present the latest experimental data about the joint specific resistances, confirming the first results, and independent measurements of the interface resistance and Al RRR. All these quantities are characterized as a function of an applied magnetic field between 0 and 4 T.

Quench modeling of the ATLAS superconducting toroids

Details of the normal zone propagation and the temperature distribution in the coils of ATLAS toroids under quench are presented. A tailor-made mathematical model and corresponding computer code enable obtainment of computational results for the propagation process over the coils in transverse (turn-to-turn) and longitudinal directions. The slow electromagnetic diffusion into the pure aluminum stabilizer of the toroid's conductor, as well as the essentially transient heat transfer through inter-turn insulation, is appropriately included in the model. The effect of nonuniform distribution of the magnetic field and the thermal links to the coil casing on the temperature gradients within the coils is analyzed in full.

Stability and quench propagation velocities measurements on the 'racetrack' mock-up of ATLAS toroid coil

A mock-up of ATLAS toroid coil, which is one of the 3 detectors presently under construction for the future Large Hadron Collider at CERN, had been tested at Saclay. Various experiments have been led to check the validity of important technical options for the magnet. This paper focuses on thermal stability and quench propagation velocities measurements. For a 70/spl times/7 mm/sup 2/ aluminum-stabilized conductor carrying a 20 kA current, a 4-5 joule minimum quench energy and a 20 m/s longitudinal quench propagation velocity were found. We also studied the dependence of those features according to the operating current in the 5-20 kA range.

Quench propagation and protection analysis of the ATLAS Toroids

The ATLAS superconducting magnet system consists of the Barrel Toroid, two End Cap Toroids and the Central Solenoid. However, the Toroids of eight coils each are magnetically separate systems to the Central Solenoid. The Toroids are electrically connected in series and energized by a single power supply. The quench protection system is based on the use of relatively small external dump resistances in combination with quench-heaters activated after a quench event detection to initiate the internal dump of stored energy in all the coils. A rather strong quench-back effect due to eddy-currents in the coil casings at the transport current decay is beneficial for the quench protection efficiency in the event of heater failures. The quench behaviour of the ATLAS Toroids was computer simulated for normal operation of the quench protection system and its complete non-operation (failure) mode.

Thermal and electrical behavior of a resistive joint in the ATLAS toroids

ATLAS air core toroids exploit aluminum-clad NbTi/Cu superconducting cables. Several joints along the conductor are presently foreseen, e.g. between pancakes and between coils. The operating current (20.5 kA) is about 4 times larger than in the previous comparable magnets, and so the power dissipated in a similar joint is substantially higher, making the resistance value critical. In this work, the authors have developed some analytical models describing the temperature profile along the conductor given a localized or distributed heat source. By means of these models, they found the highest resistance permissible in order not to exceed a given local temperature rise. Later, they calculated, through a 2-D finite elements program, the specific resistance that can be expected when the joint is performed by TIG-welding the aluminum matrices. Such a value depends strongly on the aluminum-copper interface resistance and on the aluminum RRR in the welding region. They have shown that, with reasonable assumptions, specific resistances lower than 10/sup -9/ /spl Omega/m should be achieved. The results from the first measurements, confirming such a value, are reported and discussed.