PRAMANA — journal of

c Indian Academy of Sciences 

physics

Vol. 69, No. 6 December 2007 pp. 1209–1214

Test-beam programs for devices to measure luminosity and energy and optimize the luminosity W LOHMANN DESY, Platanenallee 6, D-15738 Zeuthen, Germany E-mail: [email protected] Abstract. An overview is given on the test-beam programs to perform detector and system studies for luminosity and energy measurements and beam diagnostics for luminosity optimization. Keywords. Test-beam; detector R&D. PACS Nos 29.27; 41.75; 42.60

1. Introduction The ILC will be a collider for precision measurements. The luminosity will be measured using Bhabha scattering as a gauge process. A highly performing and mechanically precise calorimeter in the very forward direction will be used. In addition, to tune the beams to highest luminosity within a bunch train a fast feedback system based on highly precise beam position monitors is planned. Additional information is obtained from a calorimeter measuring e+ e− pairs produced by beamstrahlung photons, the BeamCal. The determination of particle masses, e.g. for the Higgs boson or top-quark, requires a control of the beam energy on the 10−4 level. A beam momentum spectrometer up and downstream of the IP and a synchrotron radiation device downstream of the IP are planned to match this requirement. For these particular critical devices studies with test-beams are vital to prove the principle. An overview on the test-beam activities is given.

2. Very forward calorimeters Two compact electromagnetic calorimeters are planned in the very forward region of the ILC detector. The innermost one, the BeamCal, is hit by low energy e+ e− pairs stemming from beamstrahlung, as shown in figure 1a. The simulation is done using the GuineaPig program [1]. The sum of these depositions is about 10 TeV per bunch crossing for nominal accelerator parameters and a centre-of-mass energy of 500 GeV. Integrated over one year this corresponds to an accumulated dose of

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Figure 1. The distribution of energy deposited per bunch crossing on the front face of BeamCal by e+ e− pairs originating from beamstrahlung photons. (left); A diamond sensor of 10 × 10 mm2 size and 300 μm thickness assembled for the test-beam (right).

10 MGy. Sensors used in the calorimeter must withstand these radiation doses. Due to the strong dependence of the depositions on the beam-parameters, a good linearity over a range of 104 also is required. A material thought to be sufficiently radiation hard is CVD diamond. Samples from different manufacturers, as shown in figure 1b, are prepared for a test in a high intensity electron beam. Monte Carlo simulations have shown that the average energy of electrons and positrons in the shower inside the calorimeter is about 10 MeV. Hence we will use a 10 MeV electron beam, as delivered by the DALINAC accelerator at the TU Darmstadt, for a quantitative study of the sensor performance as a function of the absorbed dose. The accelerator is tunable for currents between 1 nA and 1 μA, allowing to collect doses in the range of MGy in about a week. An alternative option is the use of a Microtron, delivering electrons of similar energy and intensity, available in JINR, Dubna. Studies of the linearity were done using the fast extraction mode of the CERN PS, delivering up to 106 particles of several GeV energy within 10 ns. The results are promising. However, due to the limited precision of the flux measurement, a second proof will be necessary. To measure precisely the position and energy of high energy electrons, both calorimeters must be very compact. Prototypes, equipped with ultra-thin silicon or diamond sensor planes, are foreseen for test-beam studies in a few years. Part of these measurements can be done at DESY, where an electron beam of a few GeV is available. Within the EUDET framework a pixel telescope will be installed, allowing a scan over the sensor pad structure. Finally, high energy electrons will be needed to study the performance.

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Figure 2. Schematic view of the beam momentum spectrometer planned at the ‘end station A’. Four dipole magnets 10D45 form the magnetic chicane. Beam position monitors, BPM 1 to BPM 6 are placed up- and downstream and in the center of the chicane (left). The beam energy spectrum mapped to the synchrotron radiation stripe detector (right).

3. Beam energy measurements and the end station A program At SLAC’s end station A a program to test components for the beam delivery system has started. Of particular interest here are the devices to measure the average beam momentum and the energy spectrum of the spend beam. The beam energy is 28.5 GeV, bunch charge and length are 2 × 1010 and 300 μm, very similar to the ILC. Usually one bunch is accelerated, a two-bunch operation with 20–400 ns bunch spacing is possible. The average beam momentum will be determined by the deflection in a magnetic field. A magnet chicane consisting of four dipole magnets, as shown in figure 2a, will be used. The position of the beam is precisely determined by beam position monitors up- and downstream of the chicane and in its center. The displacement of the beam in the center of the chicane with respect to the up- and downstream measurements is used to determine the beam momentum. Initially, SLAC LINAC BPMs will be used. In a later stage these BPMs will be replaced by new ones designed at UC London or developed at SLAC. The magnets of the chicane was commissioned and installed in 2007. The synchrotron radiation from the last bending magnet will be used as an alternative analyzer of the beam energy and the energy spectrum. As can be seen from figure 2b, the intensity distribution of the synchrotron radiation vs. the spatial coordinate is a mapping of the beam energy spectrum. A prototype of this synchrotron stripe detector is installed and will be tested. Two data taking periods of about two weeks are planned at ESA in the years 2006 and 2007. In addition to the beam energy measurements the ESA program contains collimator wake-field studies for different collimator geometries, investigation of electromagnetic interference effects (EMI) for detector electronics near the beam-pipe, bunch length measurements and BPM tests using a spray beam to simulate the

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Figure 3. Schematic view of the laser-wire scanner. A laser beam crosses the beam axis and samples several bunches. The analysis of the Compton photons in the gamma detector allows the determination of the transverse beam profile (left). Part of the laser beam injection into the beam-pipe of PETRA (right).

beamstrahlung pair background [2]. A mock-up of the beam instrumentation between IP and the first magnets will be built for these tests. The primary LINAC beam will be available up to 2008. Test-beams in the LCLS era are subject of discussions. A beam up to 50 GeV and secondary electron, pion and proton beams between 1 and 25 GeV seem feasible. Investments in the ESA safety system will also be necessary after 2008.

4. Beam diagnostics with a laser-wire In the operation of the linear collider we would like to collect maximum luminosity under stable conditions. Laser wires are devices to monitor the transverse particle beam size and the transverse beam emittance, allowing to optimize the accelerator and beam delivery system settings [3]. The expected beam sizes are in the range between 500 nm and 10 μm, where conventional wire scanners are at the limit of their resolution. The principle of operation is shown in figure 3a. A facility to determine the transverse profile of bunches with a laser-wire is operated at PETRA. Part of the installations can be seen in figure 3b. The PETRA ring was upgraded in 2007. Studies with the calorimeter will be continued up to 2008 at ATF in KEK. Emittance measurements are planned at ATF2 after 2008. In addition, the system at PETRA will be upgraded to continue the program after 2008 at PETRA III.

5. Program at the ATF facility A wide research program was launched for beam-line instrumentation at the ATF in KEK and is now organized internationally. ATF, as sketched in figure 4, delivers an electron beam of 1.3 GeV and a bunch size in the μm range. Three bunches 1212

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Figure 4. Scheme of the ATF facility. In the extraction line the laser-wire device and the FONT facility are installed. A high quality beam will be delivered in the ATF2 project.

with 150 ns or two bunches with 300 ns between bunches can be delivered as a train. Two sets of nano-BPMs are installed and a resolution of 20 nm was measured. High resolution nano-BPMs might also be essential for the beam momentum spectrometer. The information from nano-BPMs will be used for a fast feed-forward and feedback to correct the beam direction. At ILC such a system will be necessary to align the two beams for maximum luminosity. The FONT4 [4] project aims a fast feed-back with a total latency of less than 150 ns. For ILC-like bunches, delivered by ATF, the stabilization of the third bunch on a μm level is anticipated. First components are installed. The ATF2 project will extend the extraction beam-line of ATF to a final focus beam-line for the ILC [5]. The goal is to achieve a beam size of 35 nm and nanometer stability simultaneously. In addition, advanced beam instrumentation like a precise beam size monitor (shintake) and cavity BPMs will be developed and tested and the above-mentioned FONT and laser-wire projects will be continued.

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W Lohmann References [1] D Schulte, Study of electromagnetic and hadronic background in the interaction region of the TESLAS collider, Ph.D. Thesis, University of Hamburg, 1996, TESLA-97-08 [2] T Markiewicz, ILC beam tests in end station A, These proceedings [3] LBBD Collaboration: homepage at: http://www.pp.rhul.ac.uk/ lbbd/ [4] FONT: Feedback on nanosecond timescale, project homepage at: http://hepwww.ph.qmul. ac.uk/ white/FONT/ [5] ATF homepage at: http://atf.kek.jp/; ATF2 proposal KEK Report 2005-2, CERN-AB2005-035, DESY 05-148, ILC-Asia-2005-22, JAI-2005-002, SLAC-R-771, UT-ICEPP05-02, August 2005; ATF2 proposal Vol. 2; KEK Report 2005-9, CERN-AB-2006004, DESY 06-001, ILC-Asia-2005-26, JAI-2006-001, SLAC-R-796, UT-ICEPP-05-04, February 2006

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