Electronic regulation of the SPS extraction quadrupole current pulse shape for improved stability of the extracted beam

In order to minimise the event pile-up and therefore optimise the detection efficiency, Chorus and Nomad experiments ask for a long and rectangular spill profile. At present the fast-slow extraction is generated by driving the beam into a quadrupolar-octopolar resonance by exciting a quadrupole magnet with a semi-trapezoidal current [1]. The trapezoidal pulse shape is obtained by discharging a capacitor into the magnet coils. After a few milliseconds of undamped discharge a fixed resistor is switched into the circuit. The attenuation is then higher and the sine wave continues with a lower gradient. The two gradients can be adjusted by varying the initial capacitor voltage and the time at which the resistor is switched into the circuit. A further degree of freedom in determining the spill shape has been added by allowing the possibility of changing the second slope value independently of the initial conditions. This task is achieved by means of a variable current source added in parallel to the fixed resistor. This current source consists of a circuit based on IGBT (Insulated Gate Bipolar Transistor) switches controlled in their linear characteristics. With this configuration, an adjustment of the second slope from damped to undamped discharge is possible within IGBT safe operating area limits. Tests have been performed during operation on both SPS fast/slow extractions. In all the cases the combination of a careful adjustment of the machine parameters (mainly QH and xH), of the kicker settings and of the current provided by the current source result ed in an improvement of the detection efficiency for the neutrino experiments.