1. Iron-free high current pulsed air-core coil quadrupoles
C.Tenholt 1, P.Spiller 2, U. Blell 2, O. Kester 1,2
1 J. W. Goethe-Universität Frankfurt, Germany; 2 GSI Darmstadt, Germany
Motivation
Quadrupole lenses are built to focus ions or to transfer them between synchrotrons. For some applications an extremely high field gradient is needed, which cannot be achieved with warm iron magnets since they are limited by saturation of the iron yoke. One alternative are superconducting magnets. However, their disadvantage is the necessity of a complex cryogenic system. As in the field of strong final focusing only certain bunches are chosen we are able to work with pulsed applications, which can provide a similar magnetic gradient as superconducting magnets. The current needed to reach strong magnetic field gradients for typical energies and sizes of SIS18 (18Tm accelerator at GSI) beams is about 400kA. The electrical pulse duration needs to be only few 10µsec long. This can be achieved by discharging a capacitor bank through an array of semiconductors. The current pulse is applied to four conductor arrangements forming a quardupole, which may be connected in series. Each conductor is built of many separate lead strands, which are drilled and isolated against each other. In this way, the impact of skin effect is compensated considered over the length of the lens. As the amount of current is regulated by the cross section of the sum of wires, its shape should generate a cos(2θ)-distribution. For shielding of the electromagnetic pulse and protection of beamline components close to the quadrupole a laminated barrier of panel sheet has to be built.
Four conductors leading the current
pulse
-Epoxy resin for mechanical stability
Laminated shielding of electromagnetic
pulse to protect beamline components
Difficulties: High currents of around 400kA
Small Spot size Handling the field aberration (offsets, crank…)
High field intensities ~ 3,6T stable construction
Lorentz-Forces on the conductors Magnetic shielding
Distribution of magnetic flux density simulated with CST at 400kA
Regulation of current amount by the cross section of the conductor:
Essential for quadrupoles  current distribution of cos(2θ) along the blue line
Magnetic field simulation in CST* at a current peak of 400 kA (on the right)
*CST EM STUDIO release Version 2012
Comparison of a pulsed lens's magnetic field with an ideal magnetic field of a quadrupole lens, from the lens's centre (left) going to the conductors (right at – m)
Oscillating circuit consisting of capacity bank the semiconductor switch and lens (L1-486nH)
Forces on the coils`conductor:
Caused by the Lorentz-Force there are huge impacts on the conductors of the lense during the time of the current pulse:
Approximated values
Force outwards (length: 600 mm) : ~200 kN
Problem due to short pulse:
Skin-Effect
solution:
Conductors built of many thin drilled and isolated lead strands
Current (red) through lens and voltage (black) occurring at the semiconducor stack (LTSpice)
Circuit structure:
Pulse duration:
short pulse duration for a short stress of Lorentz forces on the conductors
Low voltage for low energy consumption
High capacitance for a high current amplitude
- Adjustment of the capacitance to the lenses‘ inductivity and additional optimizing of the recovery diodes‘ resistor
DÜRR MF-Kabel
Variation of the shielding‘s wall thickness for the example of Vacoflux at 400 kA