Biomedical Engineering Reference
In-Depth Information
Fig. 30.4
A terawatt laser pulse is focused onto the front side of the target foil, where it generates
a blow-off plasma and subsequently accelerates electrons. The electrons penetrate the foil, ionize
hydrogen and other atoms at the back surface and set up a Debye sheath. The distribution of the
hot electron cloud causes a transversely inhomogeneous accelerating field
The problem of the very broad, exponential energy spectrum can be explained
by the inhomogeneous distribution of electrons in the sheath causing a transverse
inhomogeneous accelerating field. Ions originating from different locations on the
back surface of the target foil experience different accelerating fields. This problem
has been overcome by using micro-structured targets consisting of a thin high Z
metal foil and one or several small proton rich dots on the back surface [
34
].
If the transverse dimensions of such dots is smaller than the acceleration sheath
the produced protons are only subjected to the central part of the field and thus
experience a homogeneous potential. Following a proposal by Esirkepov et al.
[
35
] the group of H. Schwoerer [
34
] used an experimental set-up where the target
consisted of a 5
m of PMMA on the back
surface. The PMMA layer of the sample was microstructured, leaving PMMA dots
of (
m titanium foil coated with 0.5
m
2
. The laser was then aligned to hit the target foil directly opposite
of one of these dots. The laser used in this experiment was a 10 TW Ti:Sapphire
laser with 80 fs pulse duration, 10 Hz repetition rate, and a pulse energy on target of
600 mJ. The generated protons were energy analyzed and the spectrum exhibited a
narrow feature peaked around 1.2 MeV on top of a broad, exponential background.
This feature contained typically 10
8
protons per 24 msr, and had a full-width half-
maximum of 300 keV (or 25% of the mean energy).
20 20
)
