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Evaluation of expected performance of Shintake beam size monitor for ATF2
| Content Provider | Semantic Scholar |
|---|---|
| Author | Yamaguchi, Yohei Komamiya, Sachio Orouku, Masahiro Suehara, Taikan Yamanaka, Takashi Kamiya, Yoshio Araki, Sakae Okugi, Toshiyuki Tauchi, Toshiaki Terunuma, Nobuhiro Urakawa, J. |
| Copyright Year | 2010 |
| Abstract | ATF2 is the final focus test facility for ILC to realize and demonstrate nanometer focusing. One of the goals of the ATF2 is a demonstration of a compact final focus system based on the local chromaticity correction. A designed beam size at the focal point is to be 37 nm in vertical. To achieve the goal, a beam size monitor capable of nanometer beam size measurement is inevitably needed. Shintake monitor satisfies the demands, and is installed at the virtual interaction point of the ATF2. Shintake monitor is a beam size monitor which uses laser interference fringe pattern to measure beam size. The beam test for the Shintake monitor was successful in measurement of signal modulation with the laser interference fringe pattern in November 2009. In April 2010, beam size of less than 1μm was achieved. We have studied the error sources, and evaluated the total error to be less than 10% for 1 minute measurement. This paper is about the evaluation of the Shintake monitor performance by analyzing beam tests data. Most systematic error sources are well understood, so that we can estimate accuracy of beam size measurement when the beam size reaches 37 nm. BEAM TESTS Beam tests for the Shintake monitor has performed at ATF2 virtual interaction point (IP). Figure 1 shows the Optical table of the Shintake monitor. Figure 1: Optical table of the Shintake monitor Electron beam comes from back of the optical table. Since upper limit of beam size measurement by the Shintake monitor is about 4 μm, the beam size was to be reduced to this size to find signal modulation. In November 2009 beam size reached the measurable range and we found signal modulation. In April 2010, the beam size of less than 1 μm was achieved. Figure 2 shows the beam size measurement when beam is 860 nm ± 40 nm (stat.) +0 -60 nm (sys.). The sources of this error are discussed in the following sections. Fringe Phase [rad] 14 16 18 20 22 24 26 28 30 C om pt on S ig na l [ ar b. u ni ts ] 0 10 20 30 40 50 Fringe Scan Figure 2: Beam size measurement STATISTICAL ERROR EVALUATION FROM BEAM TESTS Dominant sources of statistical error are: • Resolution of the gamma detector • Jitter of relative position between electron beam and the laser interference fringe pattern. Resolution of Gamma Detector Energy of the Compton signal due to the scattering of beam and laser light is significantly lower than background gamma energy. This makes the gamma detector difficult to measure signal energy. A gamma detector for the Shintake monitor is composed of multilayer scintillators. Owing to this structure the detector acquires information on shower development. Since the Compton signal and background are different in shower development due to the energy difference, they can be separated. Using this method in the analysis, the detector can identify the signal high resolution even in severe background conditions [1]. However, resolution of gamma detector is still one of the most dominant error sources due to high energy background. In the current condition, typical S/N is about 1, and the signal resolution is about 7% per bunch. Figure 3 shows resolution of the gamma detector. ___________________________________________ youhei@icepp.s.u-tokyo.ac.jp MOPE023 Proceedings of IPAC’10, Kyoto, Japan 1014 06 Beam Instrumentation and Feedback T03 Beam Diagnostics and Instrumentation Relative osition itter Between Electron Beam and aser nterference ringe Pattern Since phase of the fringe pattern determines the number of scattered photons by the beam electrons, the fringe phase jitter causes signal energy jitter. Beam position jitter causes signal energy jitter in the same way. In the beam size measurement, they are evaluated as relative position jitter between the beam and the fringe pattern, and cannot be treated separately. We evaluated the relative position jitter from beam tests. From our calculation, signal error from this effect is about 6%. More precisely, since signal amount and influence of relative position jitter is related to the interference fringe phase, signal error is also related to the phase. Figure 4 shows relation between signal error and the fringe phase. Fringe Phase [rad] 0 1 2 3 4 5 6 Si gn al E rr or [a rb . u ni ts ] 1 2 3 4 5 6 7 Fringe Phase vs. Signal Error Figure 4: Relation between signal error and the fringe phase measured by beam size measurement Black curve is fitting function expected by resolution of the gamma detector and relative position jitter influence. SYSTEMATIC ERROR EVALUATION FROM BEAM TESTS Several sources of systematic error reduce modulation depth. Their influence is written as ideal . meas CM M = , where Mmeas is a measured modulation depth, Mideal is an ideal modulation depth and C is modulation reduction factor. The measured modulation depth is shown as the product of all modulation reduction factors and the ideal modulation depth. Laser Polarization In principle, laser polarization never reduces contrast of the interference fringe. However P-polarized reflectance of the laser beam splitter is not exactly 50%, because the splitter is tuned for S-polarized light. So the existence of P-polarized light causes laser power imbalance and makes the contrast small. The contrast degradation reduces signal modulation. The modulation reduction factor from polarization is written as ( ) ideal down P up P down S up S . meas M P P P P P 2 M + = , where PS is a S-polarized laser power, PP is a Ppolarized laser power and P is the whole power of the laser. Superscripts show the light path. During the beam test in the December 2009, the modulation reduction factor was estimated to be 96.3%. After the beam test, we adjusted laser polarization with half wave plate, and now the factor is almost 100%. We calculate the modulation reduction factor from laser polarization to be less than 1%. Laser Alignment Accuracy If two lasers overlap only partially, the modulation becomes small [2]. The modulation reduction factor from misalignment is written as |
| Starting Page | 1014 |
| Ending Page | 1016 |
| Page Count | 3 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://accelconf.web.cern.ch/accelconf/IPAC10/papers/mope023.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
