Quench protection heater studies of the 3rd 1-m model magnet for the KEK-LHC low-β quadrupoles

In superconducting magnets with large energies, quench protection heaters (QPHs) are necessary to prevent localized quenches. If the full energy of a magnet is dissipated into a small volume, the magnet may suffer irreparable damage. The QPHs are used to heat the surface of the coil to increase the size of the normal zone so the heat is dissipated over a larger volume. As a result, the maximum temperature after a quench will be reduced. The KEK low-β quadrupole 1-m model magnets for the Large Hadron Collider (LHC) have four QPHs on the outer surface of the coil. A capacitive power supply with 7.2 mF/channel and a maximum charge of 400 V is used to fire the heaters, yielding a maximum energy of 576 J/channel. The QPH set-up used at CERN has an initial current of 54 A and a time-constant of 118 ms. Because the power supply used for the KEK 1-m model is not identical to the power supplies used at CERN for the 6-m production magnets, a similar set-up is achieved by connecting two power supply channels in parallel and adding an external resistor to each circuit giving an initial current of 52.6 A with a time-constant of 109 ms. Several aspects of the performance of the QPHs for the third 1-m model magnet (LHCIRQ03) have been studied: full energy dump (compared with 2nd model, LHCIRQ02), full charge on QPHs at 215 T/m, determine the minimum magnet current at which the QPHs can initiate a quench, and spot heater tests (both with and without the QPHs)