STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
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old current is on the order of several Amperes. As a consequence of this method,<br />
the fundamental frequency is also shifted and the cavity must be set to zero accel-<br />
erating gradient, essentially making it a drift space. Because these measurements<br />
were performed in 2004, the effect was measured specifically for the 2114.156 MHz<br />
mode as it was causing BBU at the time (see Section 5.3). While the mechani-<br />
cal tuner is capable of shifting the fundamental mode frequency by ±200 kHz, the<br />
2114.156 MHz mode shifted less than 100 kHz. Ultimately, because methods such<br />
as point-to-point focusing proved to be so effective, the effects of these frequency<br />
shifts on the threshold current were never investigated experimentally.<br />
7.1 Cavity-based Feedback<br />
Cavity-based feedback aims to directly lower the QL of a dangerous HOM. Two<br />
feedback mechanisms were experimentally tested and are discussed in the following<br />
two sections.<br />
7.1.1 Active Q-damping Circuit<br />
The idea of the damping circuit is as follows: couple voltage from one of the<br />
HOM ports and using a narrowband filter select the HOM frequency of interest,<br />
shift the signal by 180 ◦ in phase, amplify the signal and then return it to the cavity<br />
through the same HOM port. A schematic of the setup is given in Fig. 7.1. By<br />
adjusting the gain and phase shift, the quality factor of the HOM could be reduced<br />
by a factor of 20. However, for these optimally tuned values of the gain and phase<br />
shift, the feedback loop became very sensitive to external disturbances, such as<br />
motion in the cables and the effects of the amplifier warming up. To avoid issues<br />
with stability, the feedback circuit was set to reduce the QL of the 2106 MHz mode<br />
163<br />
by a factor of 4.8 (see Fig. 7.2). The threshold current was determined to be 2.3 mA