Beams and Beam Physics
23
FIGURE 1.17: Illustration of the first microtron. (From S. P. Kapitza,
The Microtron, Harwood Academic, London, 1978 [35]. Fig. 1.9, p. 14. With
permission: c
Taylor & Francis.)
follow the pattern
ω = ω 0 ,
ω 0
2
,
ω 0
3
,
ω 0
4
,
ω 0
5
, . . . .
(1.10)
This entails that the factor γ follows the sequence γ = γ 0 , 2γ 0 , 3γ 0 , 4γ 0 , . . . ,
which requires Δγ = 1 per turn. Since E = mc
2 = γm 0 c
2 , this means
ΔE = m 0 c
2 , and thus the necessary energy gain per turn must equal the rest
mass energy of the particle under consideration. For electrons, this means
ΔE = 511 keV and is thus possible; for protons, ΔE = 938 MeV and this is
not easily possible within the confines of a conventional magnet.
A very important further development of the concept of a microtron is
based on the fact that if the orbits of the particles are far enough separated
so that one can apply different magnetic fields for each orbit and can even
change the shape of the orbit away from circular, then by careful choice of the
orbit lengths, it is possible to maintain the synchronicity condition (1.10)
while maintaining the freedom to have any amount of acceleration that is
convenient. This is the basic idea behind CEBAF, the Continuous Electron
Beam Accelerating Facility, at Thomas Jefferson National Accelerator Facility
(TJNAF, Jefferson Lab, JLab), Newport News, Virginia, USA.