Batchelor, D.

Chang, C.

Hertle, E.

Huttel, E.

Judin, V.

Müller, A.S.

Nasse, M.J.

Schuh, M.

Steinmann, J.L.

Müller, A. S.

Nasse, M. J.

Steinmann, J. L.

KITopen

  DESIGN OF A COMPACT SETUP TO MEASURE BEAM ENERGY BY DETECTION OF COMPTON BACKSCATTERED PHOTONS AT ANKA* C. Chang, D. Batchelor, E. Hertle, E. Huttel, V. Judin, A.-S. Müller, M. J. Nasse, M. Schuh, J. L. Steinmann, Karlsruhe Institute of Technology, Karlsruhe, Germany Abstract One of the most important parameters of accelerators is their beam energy. So far, the method of resonant depolarization was used to accurately determine the energy at 2.5 GeV of the ANKA electron storage ring, which, however, becomes cumbersome for lower energies. A good alternative is the detection of Compton backscattered photons, generated by laser light scattered off the relativistic electron beam. To achieve compactness and integration into the storage ring, the setup of transverse scattering is proposed instead of conventional head-on collision. The feasibility has been studied by comparison between simulations of Compton backscattered photons by AT and CAIN 2.35 and actual measurement of background radiation with an HPGe (High Purity Germanium) spectrometer. The layout of the setup is also included in the paper. INTRODUCTION Compton backscattering (CBS), sometimes also referred to as laser-Compton scattering or inverse Compton scattering, describes the process of (laser) photons (energy EL) scattering off of relativistic electrons (energy Ee). The scattered photons with energy Es follow the kinematics illustrated in Eq. 1 and Fig. 1, where φ is the collision angle between the incoming laser and the electrons and θ is the scattering angle between the scattered photons and the initial electrons. The electron velocity divided by the speed of light is denoted by β:  LsL eE (1- cos )E1 cos E / E [1-cos( - )]    . (1)  Figure 1: Scheme of CBS. For θ=0, the energy of the scattered photons reaches its maximum and forms a sharp cut-off edge in the energy spectrum. For typical CBS measurements at storage rings we have Ee>>mc2>> EL (mc2 is the electron rest energy) and φ>0. This leads to an approximation for the cut-off energy Emax as shown in Eq. 2. The electron beam energy Ee can then be determined from the known values of mc2, EL, φ, and the measured Emax using  2emax 2 4e2LEEm cE4E sin2. (2) MOTIVATION The ANKA storage ring [1] operates from 0.5 GeV (injection energy) to 2.5 GeV (normal user operation). Several times a year ANKA offers special user operation at 1.3 and 1.6 GeV, e.g., to generate coherent synchrotron radiation in the THz regime using a low-αc optics [2]. Previously, precise energy calibration at 2.5 GeV was successfully achieved by resonant spin depolarization [3]. For lower energies, however, this technique requires very long measurement times. Here CBS is more suitable as it does not require a polarized electron beam. So far, several facilities have reported energy measurements based on CBS using a head-on collision geometry (φ=π) with relative accuracies reaching 10-4 to a few 10-5 [4-9]. Compared to the traditional CBS method, we are currently realizing for the first time a transverse configuration (φ=π/2). This setup has several advantages: It is very compact and can therefore also be used at rings with restricted space. Furthermore, the transverse setup reduces Emax by a factor of two, which makes measurements and especially detector calibration considerably easier because available calibration sources have limited upper energies. The transverse configuration can in principle also be converted easily into a versatile laser wire diagnostics tool. SETUP AT ANKA Figure 2 shows the transverse CBS setup for energy measurements currently under construction at ANKA. The interaction point is located at one long straight section. The gamma photons generated by CBS propagate in a narrow cone along the direction of the electron beam. The photons with the maximum (cut-off) energy Emax are concentrated on the propagation axis. We therefore plan to use a collimator in front of the HPGe spectrometer to collect these photons and reduce the background level. We chose a laser emitting in the mid-infrared range (CW monochromatic CO2 laser with EL=0.117 eV) to ensure  ___________________________________________  *Work supported by the European Union under contract PITN-GA-2011-289191   THPME112 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83494Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation  that Emax is within the detectable range of commercially available HPGe spectrometers (up to ~10 MeV). The laser can be tightly focused to match the vertical size of the electron beam and therefore maximize the signal rate.  Figure 2: Energy measurement setup by detection of Compton backscattered photons at ANKA. MEASUREMENT ACCURACY The relative width of the cut-off edge ΔEmax/Emax can be derived from Eq. 2 [10]:   e Lmax maxmax e maxLE R(EE EE E )R( t2E E n /) a 2      . (3) Here “  ” refers to the square root of the quadratic sum of the individual terms, which are (values are for ANKA): ΔEe/Ee: energy spread of the electron beam (~10-4–10-3);  ΔEL/EL: relative stability of the laser photon energy and radiation line width (<10-5 for the used laser system);  ΔR(Emax)/R(Emax): energy resolution of the HPGe detector at Emax (~10-3); Several sources for Δφ: (1) orbit drift during measurement <10-5 rad; (2) horizontal angle of electron beam ~ a few 10-5 rad or less; (3) horizontal angle of laser <10-5 rad. Therefore, ΔEe/Ee and ΔR(Emax)/R(Emax) are the dominant contributions that widen the cut-off edge to ~10-3. Furthermore, to determine the average value of Emax, an erfc-like function [4,7,8,10]can be fit to the edge curve. The statistic relative uncertainty of determining Emax depends on the photon density at the cut-off edge dNγ/dEs (Emax) and can be estimated as [10]   E max max maxmaxmax maxstatist csiE / EdNEE (E )E2d  . (4) In our case ΔEmax/Emax is around 10-3. The maximum energy Emax would be around 0.2 MeV, 1.5 MeV, 2.3 MeV and 5.6 MeV for an electron beam energy of 0.5 GeV, 1.3 GeV, 1.6 GeV and 2.5 GeV, respectively. For example, to reduce statistic uncertainty to below 10-4 for a 1.3 GeV beam, dNγ/dEs(Emax) must be higher than 100 counts/keV. If it reaches ~1000 counts/keV, the statistic uncertainty can be further reduced to a few 10-5. The systematic uncertainty of determining Emax is limited by the accuracy of the energy calibration of the HPGe detector, which can reach a few 10-5 [8]. This is potentially the limit of the traditional head-on collision setup, if enough spectral data has been recorded to reduce the statistic uncertainty. Once we get the average value of Emax and its relative uncertainty, we can calculate the electron beam energy using Eq. 2, and its relative uncertainty can be calculated as [10]  LEEe E maxe max LE 2E 2E 2 tan / 2   . (5) Here σEL/EL is the relative uncertainty of the average laser photon energy, which is in our case much smaller than 10-5. The angular deviation σφ comes from (1) obit drift during measurement (<10-5 rad), (2) measurement error of the electron orbit due to the limited beam position monitor accuracy (on the order of 10-5 -10-4 rad), and (3) misalignment of the laser (estimated as ~10-4 rad). Thus, the total uncertainty can be up to a few 10-4. For traditional head-on collision setups this term can be neglected (second order dependence ~σφ2/4), since tan(φ/2)=1 for φ=π/2 and approaches infinity for φ=π. For the transverse setup, however, this term needs to be considered as it has an impact on energy measurement accuracy. The aim of this project is to achieve an energy measurement of the electron beam with a relative uncertainty of a few10-4. SIGNAL-TO-NOISE RATIO  Besides the determination of the collision angle, another challenge of the transverse CBS method is the much lower interaction time in contrast to the head-on collision scheme. Therefore a feasibility study has been carried out comparing a simulation of CBS photons with an actual background measurement for the low-αc mode at 1.3 GeV. The background was measured at the long straight section of the IMAGE beamline, see Fig. 3. The HPGe detector was a Canberra GX3018, with an energy resolution of 1.80 keV (FWHM) at 1.33 MeV and an active volume of 139 cm3 (diameter 58 mm, length 52.5 mm). The full energy peak efficiency for ~1.5 MeV photons is estimated to be at least several percent. The results are showed in Fig. 4.  If we focus 10 W of laser power to 100 µm rms to   Figure 3: Background measurement at IMAGE beamline. Proceedings of IPAC2014, Dresden, Germany THPME11206 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-132-83495 Copyright©2014CC-BY-3.0andbytherespectiveauthors  overlap the vertical size of the electron beam in low-αc mode at the interaction point (96 µm rms as simulated by AT) and use a typical 40 mA electron beam current, the spectrum of CBS photons reaching the detector during 20 minutes can be simulated with CAIN 2.35 (Fig. 5). If we assume 5% full energy peak efficiency, the photon density at the edge is found to be ~4200/keV and ~2800/keV, for 16 mm2 and 4 mm2 collimators, respectively. Both are enough to reduce the statistic relative uncertainty of determining Emax to a few10-5.  Since both the CBS photon and the background radiation level (mainly from gas bremsstrahlung, see Table 1) are proportional to electron beam current and detection time, a signal-to-noise ratio of around 2.5 can be estimated.   Figure 5: Simulated CBS photon spectrum around the energy edge for a 40 mA electron beam in low-αc mode at 1.3 GeV, for a 10 W laser and 20 minutes acquisition time. The collimator has the same location as the beamline slits in the background measurement. SUMMARY At ANKA, energy measurements by detection of CBS photons are especially useful for energies lower than 2.5 GeV, for example in the low-αc mode. Compared to conventional head-on collision methods previously used Table 1: Average Photon Count Rate (photons/mA/s) Slit size/collimator area 16 mm2 4 mm2 Background (measured) 0.779 0.478 Signal (simulated, ~5% full energy peak efficiency) 1.98 1.32 at several facilities, a transverse scheme is adopted at ANKA for its high usability. Despite its comparatively low laser-electron interaction time, background measurements and simulations with typical parameters for the low-αc mode have indicated that we can expect a signal to noise ratio exceeding 2.5. Furthermore, the photon density at the spectrum edge is enough to reduce the statistic relative uncertainty of determining Emax down to a few10-5. To achieve accurate energy measurements with a transverse setup, a high wavelength stability of the laser, an accurate determination of Emax, and finally a good knowledge of the collision angle are required. For transverse geometries, the collision angle accuracy is most likely the limiting parameter, whereas for head-on collision schemes the absolute energy calibration of the HPGe detector is the most challenging factor finally.  ACKNOWLEDGMENT Thanks to R. Klein at MLS, Berlin for the inspired discussion. Thanks to C. Wilhelm and S. Kaminski at KSM/KIT for lending us the HPGe detector and a lot of valuable suggestions and instructions. Also thanks to Prof. Hübers from DLR, Berlin, for making the laser available to us. Thanks to E. Bründermann from RUB, Bochum for all the supports. Thanks to S. Bauer, et al. for supporting our measurements at the ANKA IMAGE beamline. Thanks to N.J. Smale for instructive discussions and operating the machine. Thanks to M. Suepfle at the IR beamline for testing our components. Thanks to the entire THz group and all colleagues at ANKA, in particular: M. Hagelstein, A. Voelker and T. Fischboeck. REFERENCES [1] D. Einfeld et al., WPPH094, PAC2001. [2] A.-S. Müller et al., Beam Dynamics Newsletter 57, ICFA, p. 154, 2012. [3] A.-S. Müller et al., THPKF022, EPAC2004. [4] R. Klein et al., Nucl. Instrum. Methods Phys. Res., Sect. A 384, 293 (1997). [5] R. Klein et al., Nucl. Instrum. Methods Phys. Res., Sect. A 486, 545 (2002). [6] R. Klein et al., Phys. Rev. ST Accel. Beams 11, 110701 (2008). [7] V.E. Blinov et al., Nucl. Instrum. Methods Phys. Res., Sect. A 598, 23 (2009). [8] C. Sun et al., Phys. Rev. ST Accel. Beams 12, 062801 (2009). [9] J.Y. Zhang et al., Nuclear Physics B Proceedings Supplement 00, 1-11 (2011). [10] M.N. Achasov et al.,  the beam energy calibration   system for the BEPC-II collider,  arXiv: 0804.0159v1   (2008) , http://arxiv.org/pdf/0804.0159v1.pdf  Figure 4: Results of a background measurement acquired for 2000 s in low-αc mode at 1.3 GeV: (a) using 16 mm2 slits with 1.92-1.20 mA electron beam current; (b) using 4 mm2 slits with 1.1-0.82 mA electron beam current. The red squares mark the cut-off edge area of CBS photons. THPME112 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83496Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation

Design of a compact setup to measure beam energy by detection of Compton backscattered photons at ANKA

https://publikationen.bibliothek.kit.edu/170099394/6992461

Design of a compact setup to measure beam energy by detection of Compton backscattered photons at ANKA

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