ISAC LEBT
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ISAC LEBT Richard Baartman
Published online: 10 October 2013 © Springer Science+Business Media Dordrecht 2013
Abstract The ISAC electrostatic Low Energy Beam Transport (LEBT) system is described. Keywords Charged particle optics · electrostatic
1 Introduction In the isotope separation on-line (ISOL) technique, radioactive ions are created at rest and accelerated with a static potential to an energy of only a few tens of keV. At this energy, the most efficient way to transport the particles to the experimenter is with electrostatic bending and focusing elements. The reason for this can be understood from the Lorentz force F = q(E + v × B) (E is the electric field and B the magnetic; v is the velocity). For v c, and typically attainable fields, the first term is much larger than the second. Moreover, up to the sparking limit, electric fields (∼10 kV/cm) are far more economical than magnetic fields (1 T). Lastly, electrostatic fields have advantages when it is required to transport ions of widely different mass and charge. Given only electric fields, all particles that have been accelerated from rest by the same potential will follow the same trajectory irrespective of mass or charge. One is free to choose any length scale for the optics that transports and focuses the radioactive ion beam particles: widely separated quadrupoles with large apertures can in principle transport as well, with same acceptance, as small apertures and many quads, and the overall cost is little different. However, the more focusing there is
ISAC and ARIEL: The TRIUMF Radioactive Beam Facilities and the Scientific Program. R. Baartman (B) TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3, Canada e-mail: [email protected]
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per unit length, the less effected is the beam by perturbations such as stray magnetic fields and misalignments. This favours many small quadrupoles over few large ones. 2 Standard modules In ISAC, we chose a length scale of 1 metre as a good compromise between economics and sensitivity. By this is meant that a focusing unit, consisting of two quadrupoles, has a length of 1 metre. The Twiss β-function is thus also roughly 1 metre, and beam radius for a typical emittance of π = 10 π mm-mrad is 3 mm. ISAC quadrupoles have length of 50 mm and aperture of 50 mm, with generally 25 mm dia. aperture ground planes at entrance and exit. This requires the electrode voltages to be ±2 kV for a 60 keV particle energy, and yields acceptance in the 100 to 200 π μm range. Bends are also electrostatic, and their electrodes are spherical. This gives equal focusing in the bend and non-bend planes. Bend radius is 25.4 cm in all cases. There are 10 identical 90◦ bend sections, consisting of two 45◦ benders sandwiching a triplet tuned to make the section achromatic. The periodic transport sections have cells consisting of two quads; each cell has length 1.0 m, but the quads, rather than equally separated, are arranged as doublets (see Fig. 1). This allows that a 45◦ spherical bender switch can be inserted into any long dr
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