[Zurück]

E. Gruber, P Hischenhuber, S. Wampl, M. Simon, F. Aumayr:
"Controlling ion guiding through tapered glass - capillaries with temperature";
Poster: 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19), Frauenchiemsee/Germany; 18.09.2012; in: "Book of Abstracts, 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19)", (2012), S. 41.

1. INTRODUCTION
Guiding of highly charged ions (HCI) through tilted
insulating capillaries, both straight and tapered ones [1-5],
has developed from an area of basic research to a tool to
efficiently collimate and focus ion beams. Applications
range from nanoscale modifications of surfaces to
irradiation of single living cells. Consequently, parameters
are searched for to control and, possibly, optimize designer
ion beams. One such parameter is the electrical conductivity
of the insulating material [6]. Its strong temperature
dependence is the key to transmission control and can be
used to balance transmission instabilities arising from too
high incident ion fluxes which otherwise would lead to
Coulomb blocking of the capillary.
2. EXPERIMENTS
For our experiments we use a single tapered glass capillary
made of Borosilicate. The entrance diameter of the conical
shaped capillary is 0.86 mm, while the exit diameter is
82 􀀁m. After a 5 mm long straight section the conical part
of the capillary is about 5 cm long. The capillary is placed in
a specially designed copper cylinder surrounded by stainless
steel coaxial heaters and thermo shields. The temperature of
the copper parts is monitored by a K-Type thermocouple
and the heating power regulated by a PID controller. The
current experimental set-up allows for a controlled and
uniform temperature variation of the glass capillary between
room temperature and +90°C. Within such a moderate
variation of the temperature the conductivity changes by
three to four orders of magnitude [6].
A beam of Ar7+ ions with a kinetic energy of typically
4.5 keV is collimated to a divergence angle of less than 0.7°
and eventually hits a metallic entrance aperture directly in
front of the capillary. Transmitted ions are registered by a
position sensitive micro-channel-plate detector with wedgeand-
strip anode, located about 14 cm behind the sample.
Transmission rates are recorded for each capillary tilt angle
after steady-state conditions (i.e. a dynamical equilibrium)
are reached.
3. RESULTS
For our tapered capillary at room temperature we observe
"guiding" of the incident ions up to several degrees tilt angle
of the capillary with respect to the incoming ion beam. At
very small tilt angles (close to the straight direction),
however, we find a considerable suppression of the
transmission of ions. Such a "blocking" effect has been
reported previously by Nakayama et al. [7] and Kreller et al.
[8] for ions transmitted through tapered glass capillaries at
small angles, low beam energy and high beam intensity. It
can be attributed to repulsive Coulomb forces of a uniformly
charged ring-shaped region in the tapered part of the
capillary. In our experiments we demonstrate, that by
heating the tapered capillary (and thus increasing the
conductivity of the glass capillary) the excess charge can be
removed, and the blocking of the capillary can be
terminated.
4. CONCLUSION
The strong temperature dependence of the conductivity of
glass can be employed to successfully stabilize ion-guiding
against Coulomb blocking due to a high incident ion flux.