Abstract
In the CTF3 (CLIC test facility 3) run of November 2007, a vertical beam instability has been found in the combiner ring during operation. After a careful analysis, the source of the instability has been identified in the vertical deflecting modes trapped in the rf deflectors and excited by the beam passage. A dedicated tracking code that includes the induced transverse wakefield and the multibunch multipassage effects has been written and the results of the beam dynamics analysis are presented in the paper. The mechanism of the instability was similar to the beam breakup in a linear accelerator or in an energy recovery linac. The results of the code allowed identifying the main key parameters driving such instability and allowed finding the main knobs to mitigate it. To completely suppress such beam instability, two new rf deflectors have been designed, constructed, and installed in the ring. In the new structures the frequency separation between the vertical and horizontal deflecting modes has been increased, changing the position of the rods inside the cells, and special antennas have been inserted to absorb the power released by the beam to the modes. The deflectors have been made in aluminum to reduce the costs and delivery time and have been successfully tested and installed in the ring. The design, the realization procedures, and the rf test results are illustrated.
Highlights
CTF3 [1] is the third test facility of the compact linear collider (CLIC) project [2,3]
In particular the fact that the measured Á frequency of the oscillation was about 48 MHz shifted with respect to frf, were the first strong hint that the instability was due to the vertical deflecting modes (VDMs) trapped in the rf deflector (RFD) and excited by the beam
Vertical trapped modes in the RFDs of the CTF3 combiner ring (CR) have been identified to be the cause of the vertical beam instability found in the November 2007 run
Summary
CTF3 [1] is the third test facility of the compact linear collider (CLIC) project [2,3]. The phenomenology of such instability is described in detail in [7] and can be summarized as follows: (i) the profile of the vertical oscillation as a function of the bunch positions did not change shot by shot, in other words the amplitude and phase of the oscillations were remarkably stable, every pulse, showing almost exactly the same trace on the scope; (ii) the measured Á frequency of the oscillation with respect to frf was $48 MHz; (iii) the instability was stronger increasing the train length or the bunch charges; (iv) changing the temperature of the deflectors by þ8C did not change the scenario; (v) the instability occurred both in the case of a single and of recombined train; and (vi) a better steering inside the deflectors seemed to yield a weaker instability, even if no systematic study was done All these characteristics, in particular the fact that the measured Á frequency of the oscillation was about 48 MHz shifted with respect to frf, were the first strong hint that the instability was due to the vertical deflecting modes (VDMs) trapped in the RFDs and excited by the beam. A brief description of the machine operation with the new devices is reported in the fourth section
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