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suppress BBU.
As mentioned in Section 6.4.2, a true 90 ◦ rotation was not achieved in the
FEL Upgrade. For the sake of the simulations, a fictitious optics was generated to
simulate the effect of a rotator such that a 90 ◦ rotation is achieved from cavity 7
back to itself. As the mode polarizations deviate from 0 ◦ and 90 ◦ , a rotation remains
more effective at suppressing BBU than the reflector.
Discussion
Several important conclusions can be drawn from the aforementioned simula-
tions. The first is that varying the angle at which the two orthogonal polarizations
deviate from 0 ◦ and 90 ◦ did not improve the effectiveness of the suppression tech-
niques. In fact, in the special case of the local reflector, as the deviation increased
the threshold current became rapidly smaller, from approximately 25 mA to 3 mA
when the rotation angle was 45 ◦ . Although such a decline does not occur with the
rotator, neither does changing the polarization angle bring any significant advantage.
The primary objective of this simulation study, however, was to bring attention
to the fact that HOM orientations can play an important role in the choice of
suppression techniques, which until recently, has been overlooked [89]. One of the
salient conclusions is that a global rotation is clearly a more robust suppression
technique for arbitrarily polarized higher-order modes. This can be understood
by recalling that the threshold current is inversely proportional to M ∗ , defined in
Eq. (4.19). For a local reflection, M14 = M32 which will increase M ∗ . A rotator on
the other hand, with M14 = −M32 will, in general, tend to decrease the value of M ∗
and thereby increase the threshold current.
It is important to keep in mind that these simulations were performed for the
Jefferson Laboratory FEL Upgrade Driver, which is comprised of 3 cryomodules
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with a small number of dangerous HOMs which are well separated in frequency. As