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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
-Four conductors leading the current
pulse
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 Difficulties:
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 -Small Spot size
generate a cos(2θ)-distribution. For shielding of the electromagnetic pulse and
-High field intensities ~ 3,6T
protection of beamline components close to the quadrupole a laminated barrier of
-Lorentz-Forces on the conductors
panel sheet has to be built.
-Epoxy resin for mechanical stability
-Laminated shielding of electromagnetic
pulse to protect beamline components
-High currents of around 400kA
-Handling the field aberration (offsets, crank…)
-stable construction
-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
0.048 – 0.068 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
Current (red) through lens and voltage (black) occurring at the semiconducor
stack (LTSpice)
solution:
Conductors built of many thin
drilled and isolated lead strands
Circuit structure:
Pulse duration:
- short pulse duration for a short stress of Lorentz forces on the
conductors
DÜRR MF-Kabel
- 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
Variation of the shielding‘s wall thickness for the example of
Vacoflux at 400 kA