Some components in accelerator systems require very high currents. To provide these high currents in a conventional (i.e. non-superconducting) system, the duration of the current must be short in order to keep the required average power low and to make cooling of the system feasible.
Providing such a short current pulse is the task of pulsed-power systems. To achieve such a pulse, an energy store (e.g., a capacitor bank) is slowly charged. When the high current is required the energy storage device is discharged in a short time. The short period of time leads to a very high peak power.
Two applications for this technique are pulsed quadrupole lenses and the magnetic horn for focusing antiprotons.
Quadrupole magnets are used to focus an ion beam. However, one quadrupole focuses the beam only in one plane while it’s inevitable that the beam is defocused in the orthogonal plane. For this reason, two or three quadrupoles are always operated together. This allows focusing of the ion beam.
To achieve a high magnetic field strength, so-called air coils are used. This means that no ferromagnetic material is used to build the magnet ("iron-free"). This avoids saturation effects in the material. However, very high currents (in our case up to 400 kA) are required to produce the required magnetic fields. The four individual poles are formed by current conductors. In order to increase the field quality, the conductor geometry is adapted to form a „cos 2θ“ distribution.
In conventional systems, such currents lead to a melting of the conductors and to extreme energy costs. This problem can be circumvented by the use of superconductors or by pulsed operation of the magnet.
In our case, a pulsed quadrupole was developed and a first prototype has been built already. The circuit diagram of the first experiment is as follows:
The capacitor C1 is slowly charged to the operating voltage of 4.7 kV. This takes a few seconds. The stored energy is about 5 kJ. With a charging time of 10 seconds, this results in an average power of 500 W.
At a fixed time, the energy stored in the capacitor is rapidly supplied to the pulsed quadrupole. A simulation with LTSpice (Linear Technology, www.linear.com) shows the expected current and voltage profile:
It can be seen that the capacitor discharges in less than 200 μs. The average power during the current pulse is about 25 MW, which is 50000 times higher than the charging power.
The prototype is shown in the picture below.
In the upper part of the picture you see the quadrupole in its housing, while the power pulse unit (here a capacitor, the high voltage switch and a terminating resistor) is mounted under the magnet.
The associated high voltage power supply and the control electronics of the switch are not shown in the picture.
The electrical circuit shown here will soon be replaced by a more energy-efficient variant:
With the same voltage across the capacitor (and thus the same stored energy), this design will provide a current pulse that is more than twice as high as in the original design.
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