M.J. Boland, Australian Synchrotron, Clayton, Victoria 3168, Australia T. Mitsuhashi, KEK, Ibaraki, Japan K.P. Wootton, The University of Melbourne, Victoria 3010, Australia
Abstract
A fast gated intensified CCD (ICCD) camera was used to observe the beam profile turn-by-turn in the visible light region.  Using the visible light from the optical diagnostic beamline on the storage ring at t he Australian Synchrotron an optical telescope was constructed to focus an image on the ICCD. The event driven timing system was then used to synchronise the camera with the injected beam. To over- come the problem of dynamic range between the amount of charge in an injected bunch and the stored beam, the beam was dumped by slowly phase flipping the RF by 180 de- grees between each one 1 Hz injection cycle. The injection process was verified to be stable enough so that measure- ments of the different turns could be captured on succes- sive injections and did not need to be captured in single shot.  The beam was seen to come in relatively cleanly in a tight beam but would then rapidly decohere due to the strong non-linear fields needed to run the storage ring at high chromaticity.  It would take thousands of turns for the beam to damp down again an  recohere into a tight beam spot again.  This measurement technique will be used to tune the storage ring injection process.
MOTIVATION
Observation of the beam profile can give extra informa- tion that is not available using other position diagnostics which are only sensitive to the centroid of the beam, such as BPMs, DCCT or striplines. Turn-by-turn beam profiles of the beam in the Australian synchrotron storage ring can be captured using the visible light Optical Diagnostic Beam- line which is equipped with an ICCD [1]. In May of 2012, the Australian Synchrotron changed its user beam mode from decay mode to top-up injection. Since the x-ray beamline shutter are open when the top- up beam is injected into the storage ring it has become more important to optimise the injection process. Two ar- eas that can be improved in the system are the injection kicker bump and the sextupole settings.  By observing the beam profile and motion during injection it is planned to improve the injection efficiency and the beam stability dur- ing injection. Turn-by-turn measurements of the transverse beam posi- tion can be made using electron BPMs [2], however these measure only the bunch centroid. Using a synchrotron light monitor, the transverse electron beam distribution can be measured.  Employing the ICCD camera, we are able to gate acquisition fast enough to measure single bunches on single turns. Hence, events on a highly reproducible cycle – like injection – can be accurately measured. Of particular interest is both closing the injection bump [3], as well as minimising the effect of sextupoles within the injection bump [4].
EXPERIMENTAL SETUP
The optical diagnostic beamline (ODB) of the Australian Synchrotron [5, 6] was used to image the visible light from the injected electron beam.  To accommodate this imaging apparatus, the lens in the optical chicane [5] was removed. Instead, the principal focusing optic was positioned on the optical table, as described elsewhere in these proceedings [7].  We form a real image of the electron beam photon source at the ICCD camera. The imaging system of the ICCD camera [1] can be trig- gered down to a shutter gate of 200 ps, sufficient to capture a single bunch of 23 ps which are spaced by 2 ns in the stor- age ring.  Acquisition using the ICCD was triggered using the programmable accelerator timing system [8] Event Re- nieclance of the converse of the area checans of the seem to bunch and the stored beam, the stored beam was dumped  g by slowly phase flipping the storage ring RF phase by 180 90 closk cycles of cardinos of colles of cardinos one of cardinos one of cardinos one of cardinos of cardinos one of cardinos Figure 1: Injected beam first 30 turns of the storage ring, imaged using ICCD camera.
1.794 ms Figure 2: ICCD images of the injected beam taken at intervals of 600 turns after injection. ISBN 978-3-95450-119-9 byright © 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY
RESULTS
The first few hundred turns of the beam were imaged turn-by-turn to establish the timing and performance of the system.  The injection process was determined to be sta- ble enough since the position and profile for a given turn was very similar from injection to injection.  The first 8 turns shown in Fig. 3 were then examined in more detail and compared with the model shown in Fig. 4. The po- sitions were in very good qualitative agreement and only turn 5 had a significantly different horizontal position com- pared with the measurement. Next the profile was exam- ined for the first 22 turns as shown in Fig. 1, where the dark field frame for turn −1 was subtracted from the sub- sequent frames to remove the background noise.  For the first 8 turns the beam seems to oscillate as expected about the central beam axis.  However, from turn 9 onwards the beam appears to decohere and filament into a large diffuse blob covering the whole of the injection coordinate space. As seen in Fig. 2 this diffuse beamprofile persists for sev- eral thousands of turns until the beam damps down into a smaller profile after about two damping times ( τ ≈ 4 ms).
ANALYSIS
The measured position of the injected beam for the first few turns in the storage ring are in excellent agreement with the model. Interestingly the beam appears to decohere af- ter about 8 turns in the ring which can be due to the strong sextupole fields that are present due to the high chromatic- ity used to damp current dependent instabilities. It is hoped that by tuning the injection kicker bump the motion in the beam during injection will be reduced. In addition the op- eration of a bunch-by-bunch transverse feedback system is planned so the chromaticity can reduced and maybe the fil- amentation of the beam will be reduced, thereby improving the injection efficiency.
CONCLUSIONS
A fast gated intensified CCD camera was used to observe the beam profile turn-by-turn in the visible light region. This measurement technique will be used to tune the stor- age ring injection process. The close agreement between measurements and tracking simulations illustrates the im- á portance of this technique in observing real electron beam motion on fast gated timescales. In the future this technique A will be used to fine tune the storage ring and the injection process. [1] Stanford Computer Optics 4Picos ICCD camera, http://www.
 Position in the first-turns in the storage ring performed by services.

D. Ziu.
 [1] Stanford Computer Optics 4Picos ICCD camera, http:// www.stanfordcomputeroptics.com [2] Y.-R.E. Tan, et al., Proceedings of PAC 2007, Albuquerque USA, FRPMN004, June (2007). [3] K. Fukami, et al., Nucl. Instrum. Meth. A, 694 , 1–5 (2012). [4] H. Tanaka, et al., Nucl. Instrum. Meth. A, 539, 547- (2005). [5] M.J. Boland, et al., Proceedings of EPAC 2006, Edinburgh UK, THPLS002, June (2006). [6] M.J. Boland, et al., Proceedings of APAC 2007, Indore, India WEPMA060, January (2007). [7] M.J. Boland, et al., These proccedings, TUPB72. [8] M.J. Spencer, et al., Proceedings of EPAC 2006, Edinburgh UK, TUPCH003, June (2006). [9] A. Terebilo, "Accelerator Toolbox for MATLAB", PUB-8732, May (2001).