STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

Using this process to verify that the reflector is operating correctly, virtually
no remnant coupling is observed through the back end of the recirculator. In fact,
the coupling introduced from the RF cavities (see Sections 2.2.3 and 3.3) prove to
be more of an operational concern [61].
6.3.2 Measured Effect on the Threshold Current
The threshold current was first established to be 1.8 mA and caused by the
2106 MHz mode without the reflector. The reflector was then activated and opera-
tion with 5.0 mA of average beam current was achieved with no indications of BBU.
Since direct observation was not possible, a BTF measurement of the 2106 MHz
mode was performed. The results of the measurement yielded a new threshold cur-
rent of (9.2 ± 0.4) mA; an increase by a factor of 5.1 (Fig. 6.8). Additional BTF
measurements were performed for the 2116 MHz mode (also located in cavity 7)
and the 2114 MHz mode (cavity 4). The concern was that by producing a reflection
in the betatron planes, these modes which were stabilized for decoupled optics may
become unstable. However, the results of the measurements indicate that the modes
remained stabilized and did not pose a threat for causing the instability.
6.4 Global Rotation
The reflector described in Section 6.3 interchanges horizontal and vertical phase
spaces, imaging each identically into the other. This will map a horizontal kick into
the vertical plane, and a vertical kick to the horizontal plane, and thus is effective
for suppressing BBU driven by modes with these specific polarizations. However,
it will equally map a diagonal kick back into a similar diagonal displacement, thus
failing to suppress BBU driven by an HOM of arbitrary polarization. Therefore
a more robust scheme for BBU suppression can be achieved by generating a 90 ◦
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