Theory, Design and Tests on a Prototype Module of a Compact ...

12 2. LINAC AND SCL ACCELERATORS
at a differentiation of the structure with respect to the type <strong>and</strong> the
velocity of the particles.
Figure 2.4. Schematic view of the Wideröe drift-tube
linac. D are drift tubes, V is an alternating voltage
source, G are the gaps between the drift tubes <strong>and</strong> S
is the source of a continuos beam. B are used to group
the particles in bunches.
The Wideröe linac concepts was to apply a time-alternating voltage
to a sequence of drift tubes whose lengths increased with the particles
velocity, so the particles always arrive synchronously at each gap between
tubes. In figure 2.4 a schematic view is presented, D are drift
tubes connected to an alternating voltage source V that applies equal
<strong>and</strong> opposite voltages to sequential drift tubes, G are the gaps between
the drift tubes in which the electric force acts to accelerate the
particles, <strong>and</strong> S is the source of a continuous ion beam. For efficient
acceleration the particles must be grouped into bunches, shown by the
black dots, which are injected into the linac at the time when the polarity
of the drift tubes is correct for acceleration. The bunching can
be accomplished by using an RF gap B between the DC source <strong>and</strong>
the linac. The original Wideröe linac concept was applicable to heavy
ions <strong>and</strong> not suitable for acceleration to high energy of lighter protons
<strong>and</strong> electrons where the drift-tube lengths <strong>and</strong> the distances between
accelerating gaps would be too large, resulting in a small accelerating
rates, unless the frequency of accelerating field could be increased up
to 1 GHz. But in this range of frequencies the wavelengths are comparable
with the sizes of the drift tubes <strong>and</strong> propagation <strong>and</strong> radiation
effects should be included in the design.
Linear accelerators are quasi-periodic 4 structures where the single
elements, which could be resonant cavities, are coupled together either
electrically or magnetically.
2.1. Disc-loaded structure. It is often used for electrons. Starting
from the idea that the electric field in a uniform circular waveguide
cannot provide continuous acceleration of electrons, since the phase velocity
always exceed the velocity of light, one can think to lower the
4 The structure is quasi-periodic, since the longitudinal dimension of the single
element has to be adequate to the increased particle velocity during the travel.