Advanced magnet powering
Advanced Magnet Powering for high stored energy detector magnets. These comprise developments, such as a Free Wheel System (FWS), a Persistent Current Switch (PCS), compact, high-performance Quench Protection Dump Units (QPDU) that will improve the stability and efficiency of the operation of such magnet systems while minimizing energy consumption in an effort to be more economical and ecologically sound.
Snubber for supressing arcing in superconducting detector magnet circuits
Since the start of the EP R&D effort in 2020, in the topic of Advanced Powering emphasis was on the development of a snubber for the ATLAS Toroid Circuit which powers the superconducting ATLAS Toroid Magnet. The ATLAS Toroid Circuit suffers from a recurring problem where with each slow dump the contact resistance and thus the operating temperature of the breakers increases. This is problematic as a too-high operating temperature necessitates maintenance during a run, which interferes with the data-taking of ATLAS. This problem is expected to be caused by arcing, where the stored magnetic energy due to parasitic inductances in the system (i.e. long busbar lengths) is discharge as an arc in the breaker contacts. This arc damages the contacts thus increasing their resistance, and moreover results in a voltage spike in the system.
To suppress this arcing, we are investigating the possibility of installing a snubber. A snubber comprises diodes, capacitors, and a resistor parallel to the capacitors. At the opening of the breaker, the capacitors temporarily conduct the current, thus smoothly redistributing the current to the run-down unit without having a voltage spike and an arc between the contacts. The capacitors absorb the energy stored in the parasitic inductances and this energy is subsequently slowly discharged over the parallel resistor.
Following circuit studies, a snubber demonstrator was built (Fig. 1). This snubber features a diode, two capacitors, and a parallel resistor. The snubber demonstrator was installed into a circuit (Fig. 2) where a large amount of parasitic was deliberately introduced for the purpose of testing the effectiveness by which the snubber prevents arcing over the breaker. The snubber demonstrator was designed such that the current and voltage per capacitor matches that in the Toroid Circuit under pessimistic assumptions. Similar to the ATLAS Toroid Circuit, the demonstrator circuit also features a superconducting magnet with a relatively large amount of stored energy, which is to be discharged over the copper coil.
Fig. 1. Snubber demonstrator, featuring diodes, capacitors, a breaker, a parallel resistor, and a large copper coil for the purpose of introducing parasitic inductance.
Fig. 2. Snubber demonstrator circuit.
In a series of tests (Figs. 3, 4, and 5) in the Cryolab at CERN (thanks to support from TE-CRG) it is was shown that the snubber absorbs over 99% of the parasitic stored magnetic energy, thus allowing a safe discharge of the superconducting magnet without arcing in the breaker and without a voltage spike. The behaviour of the snubber demonstrator is thus fully consistent with expectations and gives confidence in the overall progress towards the full-size snubber.
Fig. 3. Testing in the CERN Cryolab.
Fig. 4. At the opening of the breaker, the capacitors in the snubber demonstrator are charged over a period of 30 ms. As a result, the parasitic inductance is absorbed by the capacitors, and the current is smoothly redistributed from the power converter to the run-down-unit, i.e. the copper coil. This result is consistent with expectations and shows the excellent effectiveness of the snubber demonstrator.
Fig. 5. Following the charging of the snubber demonstrator with parasitic stored magnetic energy at the moment of breaker opening, the energy is slowly and safely discharged over the parallel resistor.
As a next step, a design of the full-size snubber is prepared (Fig. 6), and in parallel the capacitors are undergoing performance and endurance testing.
Fig. 6. A design of the full-size snubber is presently under development.