Time trend observation of certain remarked bunches using a streak camera

A streak camera with two-dimensional sweep function enables the trend of the longitudinal beam profile in the ring. In the case of the multi-bunch measurement, the different bunch profile sit on same vertical position, thus, we could not distinguish the behavior of each bunch. We have been developed a trigger circuit to measure the bunch-by-bunch longitudinal beam profile using a sweep frequency, which is not the acceleration RF frequency divided by integer. The bunch profile of each bunch sit on different position in this measurement. We observed the increase of the synchrotron oscillation amplitude from the first bunch to the 20th bunch in the KEK-ATF Damping Ring by using this system. This paper describes the measurement system and the measurement results. INTRODUCTION The KEK-ATF is a test facility to develop the accelerator technologies for the future Linear Collider. [1][2] The damping ring (DR) produces extremely low emittance beam and supplies the low emittance beam to the ATF2 beam line to test the final focus system. The DR operates not only in single bunch mode, but also multi-bunch and multi-train mode. To understand the instability in the multi-bunch and the multi-train operation is one of important issue in the KEK-ATF. Some fundamental parameters of DR are listed in Table 1. The design fill pattern of the multi-bunch and multi-train operation in the DR is indicated in Fig. 1. The beam from the linac is not injected equally filled in the RF buckets. The linac accelerates one bunch train consist of up to 20 bunches in one pulse. We have two types of beam train, one has 1 to 10 bunches with the bunch spacing of 5.6ns and the other has 1 to 20 bunches with the bunch spacing of 2.8ns in single train. The bunch spacing can choose by controlling the laser timing of the photo-cathode RF gun. Each train has 154ns spacing, when the linac injects 3 trains to the DR. To diagnostic the longitudinal beam characteristics, a streak camera with synchro-scan and dual-axis sweep units, is used for measuring visible light of synchrotron radiation in the DR. We studied the damping time measurement [3] and the impedance measurement [4] using this streak camera for the single bunch operation. The theoretical work has been done for the case of multi-bunch operation [5]. From this work, some of the different longitudinal behavior from the first bunch to the following bunches are reported. So, it is important to observe the bunch-by-bunch longitudinal behavior of the instabilities in multi-bunch operation. We had used a steak camera with a sweep frequency of 1/8 of the acceleration RF frequency. Due to the slow horizontal sweep, the different bunches coming in later timing would be recorded in very close to the first one in the horizontal axis. Then different bunches would overlap each other in the horizontal direction. So, some other idea was necessary to observe the behavior of individual bunch. Table 1. Parameters of the DR Beam energy 1.3 GeV RF frequency 714MHz Harmonic number 330 Revolution period Synchrotron frequency 462ns 10.4kHz at Vc=300kV Figure 1: Beam filling pattern in DR. SINGLE BUNCH BEAM OBSERVATION In this section, some results of the streak camera measurement in single bunch operation in the DR is described. The important parameters of the streak camera, Hamamtsu C5680, are listed in Table 2. The dual-axis sweep function enables us the observation of the slow time-trend record of the longitudinal profile. Figure 2 shows results of observation for behavior of the beam after the injection, 2a: injection to 100s, 2b: 0.6msec after the injection to 0.7msec, 2c: 6msec after the injection to 6.1msec, and 2d: 60msec after the injection to 60.1msec, respectively. The vertical scale is 800ps in full range and the horizontal scale is 100s in full range. Because the turn-by-turn streak images are horizontally overlapped each other the change of the bunch length and the position were recorded continuously. Due to the large off-energy ___________________________________________ takashi.naito@kek.jp # Proceedings of IBIC2013, Oxford, UK WEPC34 Time Resolved Diagnostics and Synchronization ISBN 978-3-95450-127-4 761 C op yr ig ht c ○ 20 13 by JA C oW — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C -B Y3. 0) injection, and the large energy spread at the injection, the particles in the beam made the synchrotron oscillations with different phases. The amplitudes of the synchrotron oscillations are gradually damped and the phases are converged into the same phase. The measured dumping time of the bunch length is 24.5+/-4.5ms. This result is agrees well with the theoretical damping time 20.5ms. From the Fig. 2(d), no synchrotron oscillation is observed at the timing of 60ms after the injection. Table 2. Main Parameters of C5680 Streak Camera Vertical sweep (synchroscan) Streak time 280ps to 1/6fs Sweep frequency(fs) 75MHz to 165MHz