EGAS41 - Swansea University

41 st EGAS CP 194 Gdańsk 2009
Low energy-spread ion bunches from a laser-cooled gas
M.P. Reijnders ∗ , N. Debernardi, G. Taban, S.B. van der Geer, P.H.A. Mutsaers,
E.J.D. Vredenbregt, O.J. Luiten
Department of Applied Physics, Eindhoven University of Technology,
P.O Box 513, 5600 MB Eindhoven, The Netherlands
∗ Corresponding author: M.P.Reijnders@tue.nl
Pulsed and continuous ion beams are used in applications such as focused ion beams. The
smallest achievable spot size in focused ion beam technology is limited by the monochromaticity
of the ion source. Here we present energy spread measurements on a new concept,
the ultra-cold ion source, which is based on near-threshold ionization of laser cooled atoms
[1]. The low energy spread is important for focused ion beam technology because it enables
milling and ion-beam-induced deposition at sub-nm length scales with many ionic
species, both light and heavy.
In the experiment, Rubidium atoms are captured in a magneto optical trap (MOT)
inside an accelerator structure where they are ionized by a pulsed laser in a DC electric
field. Time-of-flight measurements show two orders of magnitude lower energy spread than
in the current industry standard reached with Gallium liquid-metal ion sources. Bunches
with energy of only 5 eV can be produced with a root-mean-square energy spread as low
as 0.02 eV. We show that the slowly moving, low-energy-spread ion bunches are ideal
for studying intricate space charge effects in pulsed beams, i.e., the transition from space
charge dominated dynamics to ballistic motion. [2]
10
1
LMI source
U [eV]
0.1
0.01
12.6 fC
2.8 fC
0.41 fC
0.11 fC
0.026 fC
1 10 100 1000
U[eV]
Figure 1: Measured energy spread σU as function of the beam energy U for various bunch
charges (scatter plots), together with particle tracking simulations (dotted and solid curves).
References
[1] S.B. van der Geer et al., J. Appl. Phys. 102, 094312 (2007)
[2] M.P. Reijnders et al., Phys. Rev. Lett. 102, 034802 (2009)
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