[Zurück]

G. Wachter, K. Tökési, G. Betz, C. Lemell, J. Burgdörfer:
"A microscopic model for track formation by swift heavy ions";
in: "Book of Abstracts, 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19)", herausgegeben von: Max Planck Institut für Plasmaphysik; Book of Abstracts, 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19), 2012, S. 23.

1. INTRODUCTION
First experiments on guiding of highly charged ions (HCI)
through straight insulator nano-capillaries showed a
remarkable effect: after an initial charge up phase, the ion
beam could be steered by tilting the capillary axis while
remaining in the initial charge state indicating that the
transmitted ions never touched the inner walls [1].
Subsequent experiments confirmed this guiding effect also
for macroscopic glass capillaries, both straight [2,3] and
tapered ones [4]. The microscopic simulations revealed that
a self-organized charge up of the capillary walls due to
preceding HCI impacts leads to an electric guiding field,
which steers the incoming projectile ions along the capillary
axes [5]. Ion guiding ensues as soon as a dynamical
equilibrium of charge-up by the ion beam and charge
relaxation by bulk or surface conductivity is established.
The simulations showed that a stable transmission regime
required a delicate balance between incident ion flux and
charge relaxation via surface and bulk conduction,
conditions, which were obviously met in almost all cases
studied experimentally so far. In this contribution we show
that a key control parameter for guiding is the small residual
electric conductivity of the highly insulating capillary
material whose dependence of temperature 􀀂(T) is nearly
exponential.
2. EXPERIMENTS
We use a single straight macroscopic glass capillary (inner
diameter: 160 􀀁m; length: 11.4 mm) made of Borosilicate
(Duran) for which the guiding effect has been previously
established [2]. The current experimental set-up allows for a
controlled and uniform temperature variation of the glass
capillary between -30°C and +90°C [6]. Within such a
moderate variation of the temperature the conductivity
changes by almost five orders of magnitude. Beams of Ar7+
and Ar9+ ions with a kinetic energy of 4.5 keV are
collimated to a divergence angle of less than 0.5° and
eventually hit a metallic entrance aperture directly in front
of the capillary (120 􀀁m diameter). Transmitted ions are
registered by a position sensitive micro-channel-plate
detector with wedge-and-strip anode, located about 18 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
Experimental transmission curves are normalized with
respect to the transmission in forward direction (Fig.1).
Figure 1: Normalized transmission curves for 4.5 keV Ar9+ ions
guided through a glass capillary for different temperatures
ranging from 24°C to 88°C. The flux of the incident 4.5 keV
Ar9+ ions was kept constant at about 5000 ions entering the
capillary per second. The shaded area indicates the geometric
limit of transmission in the absence of guiding.
Our experiments [7] show that increasing the temperature of
a glass capillary and therefore its conductivity leads to a
reduction of guiding and, eventually, to a complete
disappearance of the guiding effect. This strong temperature
dependence can be employed to stabilize guiding against
Coulomb blocking due to a high incident ion flux [8].

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