Biomedical Engineering Reference
In-Depth Information
[
16
]. Originally developed as a possible replacement for high current electron
accelerators for x-ray production it uses high voltage transmission lines and fast
switching devices to generate a pulsed electric field on he inside of a high gradient
insulating tube. A traveling wave is set up on the axis of the tube which pushes
the bunch of charged particles injected from an appropriate ion source forward.
Alternating insulators and conductors together with short pulse durations for the
applied voltage pulses have proven to provide electric field strength of 100 MV/m
and beyond. The design is based on the concept of a coreless induction accelerator
first introduced in 1970 [
17
].
The three critical technical components of the system are the high gradient
insulators (HGI) [
18
], dielectric materials with high bulk breakdown strength [
19
]
for the pulse forming lines, and very fast closing switches compatible with operation
at high voltage gradients [
20
].
Failure of an insulator in vacuum, i.e. surface flashover, occurs at a field
strength inversely proportional to the width of the applied electrical pulse. This
observation is the key motivation for the operational mode of the accelerator
proposed by Caporaso. A high gradient insulator (HGI) consists of alternating
layers of conductor and insulator, typically on a scale of less than 1 mm. The
HGI still exhibits the same inverse dependence of flashover field strength on pulse
width, but has an improved overall performance by a factor of 4 [
19
]. The reason
for this performance enhancement can be found by studying the trajectories of
electrons generated by field emission near the surface of the HGI. While in standard
monolithic insulators these electrons frequently bombard the surface, leading to an
amplification of the charge density, the periodic microstructure along the surface of
an HGI deflects the electrons away from the insulator surface, provided the ratio of
conductor to insulator thickness lies in a certain range [
21
].
While original coreless induction accelerators used liquid dielectrics it is highly
desirable to replace these with solid materials. Castable dielectric materials with
high bulk breakdown strength and variable permittivity have been developed for
some time and a particular solution is a blend of nanoparticles of BaSrTiO
2
and
various epoxy bases. Dielectric constants between about 3 and 45 have been
achieved by varying the concentration of the nanoparticles in the epoxy. A sample
transmission line of dimensions 4 cm
56 cm with embedded electrodes at 0.8
mm separation was constructed and charged repeatedly with 400 ns wide pulses at
increasing voltage amplitude. Failure occurred at 141 kV, representing a field stress
of 170 MV/m.
In order to generate the traveling pulse wave on the inside of the HGI tube in Fig.
30.2
“Blumlein” structures coupled to fast closing switches need to be employed.
These switches should exhibit fast rise times and low resistance in the “on” position.
One possible candidate under investigation is the use of wide band gap materials
illuminated below bandgap width by lasers. As the below bandgap light is able to
penetrate the material several centimeters one can place electrodes on opposite sides
of thin wavers to take advantage of the full bulk breakdown strength of the material
[
20
]. Initial tests performed by the group of G. Caporaso failed at a field strength
across the gap of only 30 MV/m, but it could be concluded that this was due to edge
