High-Voltage Power Supply for Accelerator Beam Kicker Magnets

 

The kicker magnet itself is typically a ferrite-loaded or air-core structure with low inductance to facilitate fast field rise and fall times. The high-voltage power supply, often referred to as a kicker pulser, charges a pulse-forming network (PFN) or a set of capacitors to a high voltage, typically ranging from tens of kilovolts to over 100 kV for large rings. This stored energy is then discharged into the magnet via a fast high-voltage switch.

 

The choice of switching element is critical. For the fastest rise times and highest repetition rates, hydrogen thyratrons are historically used, capable of switching tens of kilovolts in nanoseconds. However, solid-state switches, such as stacks of IGBTs or MOSFETs, are increasingly common due to their long life, high reliability, and precise control. These switches must be configured in series to withstand the full voltage and in parallel to handle the high peak currents, which can reach thousands of amperes.

 

The shape of the current pulse through the magnet, and hence the magnetic field pulse, must be extremely flat-topped to provide a uniform deflection across the entire extracted bunch. Any ripple or droop on the flat-top will cause a variation in the kick strength, leading to beam loss or poor injection efficiency. Achieving this flatness requires careful design of the PFN or the use of active pulse-shaping techniques. A PFN is a network of inductors and capacitors designed to produce a square pulse when discharged into a matched load. The impedance of the PFN must be precisely matched to the characteristic impedance of the magnet and its connecting cables to minimize reflections that would distort the pulse.

 

For applications requiring more complex pulse shapes, such as a fast rise followed by a slow decay or a series of multiple pulses, active pulsers are used. These systems employ multiple independently controlled switch stages. By timing the turn-on and turn-off of these stages, the voltage applied to the magnet can be modulated in real-time, creating a custom waveform. This is often used in bunch-by-bunch extraction schemes where the kicker must be turned on and off within the gap between circulating bunches.

 

Timing and synchronization are paramount. The kicker pulse must be synchronized with the arrival of the beam bunch at the magnet location with sub-nanosecond precision. This requires a master timing system that distributes a stable clock and triggers to all pulsers. The delay between the trigger and the start of the magnetic field must be stable and known. Any jitter in this delay will cause the bunch to see a slightly different kick from pulse to pulse, leading to transverse oscillations and beam instability.

 

Furthermore, the high-voltage pulser must operate in a radiation-hard environment, particularly in the extraction region of a high-energy accelerator. All components, especially semiconductors, are subject to displacement damage and single-event effects from the stray radiation field. Special shielding and radiation-tolerant component selection are required.

 

The power supply for a kicker magnet is therefore a highly specialized piece of equipment, representing a convergence of pulsed power engineering, high-speed switching, and precision timing. Its performance directly determines the efficiency and quality of beam transfer between accelerators and the ability to perform time-sensitive experiments. Without these fast, stable, and reliable high-voltage pulsers, the complex choreography of particle beams in modern accelerator complexes would be impossible.