SIS18 - Pulsed Magnets for Injection and Extraction of Ion Beam


The kicker consists of 9 fast, pulsed magnets. (Dimensions of each ferrite 23.5 cm * 12.5 cm * 25 cm (W * H * D)) In the "Windowframe" magnet, a current of up to 1600 A is driven along the red lines in the picture. This current produces a perpendicular magnetic field (green arrows), which bends the positive ion beam onto a circular path (orange). At the ESR, there is an identical system consisting of 3 modules.


Example Kicker magnet with flow of current
Draft: GSI Helmholtzzentrum für Schwerionenforschung GmbH

These magnets are installed in two vacuum tanks and are required for fast extraction of the beam for experiments with high beam intensity. The accelerator ring can be completely emptied within one circulation of the particles.

SIS18 Ring Accelerator Kicker magnet
Foto: GSI Helmholtzzentrum für Schwerionenforschung GmbH

Due to the acceleration with high frequency, there is no coasting beam in the beam pipe but packets -
so-called "bunches" - which can also be extracted individually by means of the kicker. High demands are therefore placed on the kickers:
The magnetic field must raise or fall within 100 ns, to use the gap between the bunches. The pulse height must be as flat as possible in order to keep the angular variation of the extracted beam packages small. In addition, the pulse length must be variable in order to extract the desired number of bunches at different beam energies. The maximum pulse length is 3 μs. Voltages of 80000 V are required for the generation of the current pulse of 1600 A. The pulse generation is effected by discharging a cable to an impedance-matched system.

Graphical representation of so-called "bunches"
Graphic: GSI Helmholtzzentrum für Schwerionenforschung GmbH


For the SIS18 kickers, the required high voltage is generated by means of a resonant charging method. A capacitor power supply charges a bank with 100 capacitors up to 260 V, which are then switched to a high-voltage transformer by a thyristor. Within a half wave, the 300 m long energy storage cable is charged up to 80000 V. The pulse generation is controlled by gas discharge switches (thyratrons). These switches can be ignited at the desired time and remain conductive until the current flow stops due to an empty energy storage. Therefore, in addition to the main thyratron, which connects the charged cable to the transmission lines, a dump thyratron is necessary to shorten the pulse. The functional scheme is described, inter alia, in the section 'Pulse generator from line circuits' on Wikipedia.

Functional layout kicker magnet
Graphic: GSI Helmholtzzentrum für Schwerionenforschung GmbH


The real construction of this circuit has a large space requirement. At the high voltage part the use of insulation oil is necessary. This is also used to cool the thyratrons. In addition to the diodes, a voltage divider for the high voltage measurement is also accommodated in the rectifier tank. The cable drums in the background, on which the energy storage cable is wound up, have a diameter of approx. 2.5 m.

Thyratron coupling
Draft: GSI Helmholtzzentrum für Schwerionen GmbH




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