Biomedical Engineering Reference
In-Depth Information
Not only is the combination of a low energy cyclotron and a linac comparable
in size and carries only 1 quarter of the weight of a super conducting cyclotron
[
12
] for carbon ions of 700 tons, but the transverse emittance of the linac beam is
about 5 times smaller than the one of standard accelerators. This makes it possible
to use smaller and lighter magnets for the beam delivery system. A high-frequency
proton linac rotating around the patient has been patented by TERA [
13
]. A 6 GHz
radiofrequency has been chosen for TULIP (TUrning LInac for Proton therapy) to
reduce the length of the linac sections. The beam is deflected after the cyclotron
by a first bending magnet, accelerated to an intermediate energy using part of the
SLC structure, bent to the horizontal direction again, accelerated to the full energy,
and finally bent to a vertical down direction. Such a rotating linac can provide a
proton beam cycling at hundreds of Hertz, ideal for treatment schemes like distal
edge tracking [
14
].
30.3
Novel Technology Approaches
Modern particle accelerators have become known to grow ever bigger with each
increase of energy range being explored. Synchrotrons increase in radius to allow
higher and higher energies to be confined on a closed orbit with existing magnet
technology. The currently largest circular accelerator, the LHC at CERN, is
occupying an underground tunnel of 27 km circumference. This has led to the
understanding that the design of the next generation accelerators will be based on
linear geometries. But even these machines are getting larger and larger, and can
easily occupy 100's of meters or more. The main reason for this is that all accelerator
schemes currently used rely on accelerating particles in a vacuum waveguide with a
relatively modest upper limit in break down voltage of a few tens of MeV per meter.
To achieve drastic progress in reducing the size of future accelerators an entirely
new approach is needed, and medical applications may possibly benefit from these
developments.
During the last decade new technologies for the production and linear accelera-
tion of heavy charged particles have been emerging and are frequently discussed,
amongst other applications, in the context of hadron therapy. The direct drive
accelerator (DDA) [
15
], the dielectric wall accelerator (DWA), and the acceleration
using laser produced plasmas are the most important examples and the two latter
ones will be discussed here.
30.3.1
Dielectric Wall Accelerator
The dielectric wall accelerator (DWA) system, employing a variety of advanced
concepts to achieve extreme high electric field gradients, is being developed at
the Lawrence Livermore National Laboratory by the group of G. J. Caporaso
