Fast and Precise Beam Energy Measurement at the International Linear Collider Michele Viti

04 November 2009Michele Viti2 Outline ILC overview Beam energy measurement An overview of my work and results –4-magnet chicane spectrometer Magnetic measurements Relative beam energy resolution –Laser Compton energy spectrometer Conclusions

04 November 2009Michele Viti3 ILC 30 km electrons/positrons linear accelerator Center-of-mass energy 500 GeV (upgradeable to 1 TeV) High luminosity (2*10^34 /cm^2*s) A machine for precise measurements

04 November 2009Michele Viti4 Precise measurements Well understood background, clean experimental environment Precise measurements. “Input” parameters well controlled, e.g. the center of mass energy at the interaction point (IP). Direct measurement of is very difficult

04 November 2009Michele Viti5 Precise measurements Solution: –Measurement the beam energy upstream ( ) and downstream of the IP for both beams plus a slow monitoring of. –Combine with the measurement of. Fast (bunch-to-bunch, good resolution), precise and non- destructive monitor for. Accuracy required for Similarly for the resolution. From now on as beam energy we refer to beam energy upstream the IP for electrons as well positrons.

04 November 2009Michele Viti6 Magnetic Chicane Energy Spectrometer At ILC baseline method for measurement is a 4-magnet chicane. d 12Offset d measured the by Beam Position Monitors, BPM, together with the B-field integrals of (1) and (2) give access to. Method well tested at LEP with an accuracy of. offset d magnets L BPM

04 November 2009Michele Viti7 Experiment T474/491 (SLAC) At End Station A (ESA) a 4-magnet energy spectrometer commissioned in 2006/2007 (experiment T474/491). Demonstrate the feasibility of the system (mainly BPMs and magnets).

04 November 2009Michele Viti8 End Station A ParameterILC-500SLAC ESA Repetition Rate 5 Hz10 Hz Energy 250 GeV28.5 GeV Bunch Charge 2.0 x Bunch Length 300  m  m Energy Spread 0.1%0.2% Bunches per train Beam Parameters at SLAC ESA and ILC Prototype components of the Beam Delivery System and Interaction Region. Characteristic: –Parasitic with PEP II operation –10 Hz train repetition and = 28.5 GeV –Bunch charge, bunch length, energy spread similar to ILC

04 November 2009Michele Viti9 Experiment T474/491 Institutes involved: SLAC, U.C. Berkeley, Notre Dame, Dubna, DESY, RHUL, UCL, Cambridge 2006, experiment T474: –April (2 weeks): Commission of cavity BPMS. –July (2 weeks): Commission of interferometer. 2007, experiment T491: –March (3 weeks): Commission and installation of magnets: first chicane data. –July (2 weeks): Additional new BPM in the centre of the chicane.

Magnetic measurements

04 November 2009Michele Viti11 Magnetic measurements B-field integral,, essential parameter for beam energy measurement. Need to be measured with an accuracy of 50 ppm to obtain

04 November 2009Michele Viti12 Magnetic measurements Nov 2006 – Feb 2007 measurements performed in the SLAC laboratories (DESY, Dubna, SLAC). Purpose of the measurements: –General understanding of the magnets Stability of the B-field and B-field integral. Monitoring of the residual B-field. B-field map. Temperature coefficient for B-field and B-field integral. –Development and tests of a procedure to monitor the B-field integral in ESA.

04 November 2009Michele Viti13 Magnetic measurements Important restriction: Monitor of the B-field integral: in ESA no device to measure directly this quantity. Solution: measure the B-field in one point and from that determine the integral. –Basic assumption When the field is changing in one point, it changes everywhere by the same amount. The field is assumed to be scaled B Z

04 November 2009Michele Viti14 Magnetic measurements Some results: B-field measured by NMR probe. In the lab: –Flip coil technique to measure B- field integral. –Calibration of the NMR –Comparison of the B-field integral calculated with the measurement. Error = mean + rms. Values close to the requirement. Not all the error sources visible in the figure (like calibration and alignment error for the flip coil).

04 November 2009Michele Viti15 Magnetic measurements The total error of the B-field integral using the one-point B-field measurement was Main contributions are alignment errors of the devices (flip coil). Several suggestions were proposed to improve the results.

Relative beam energy resolution

04 November 2009Michele Viti17 Relative Beam Energy Resolution 13At ESA, NMR probes in magnet 1 and 3 damaged. A complementary method to cross-check the absolute energy measurement was not implemented. Only relative energy measurements possible at ESA. offset d magnets L BPM

04 November 2009Michele Viti18 Relative Beam Energy Resolution dXbX0The offset d = Xb - X0 XbX0Xb measured by BPMs the X0 by extrapolation using BPMs upstream and downstream of the chicane. dd set to 5 mm, resolution required < 500 nm (in order to have ) Beam direction

04 November 2009Michele Viti19 Relative Beam Energy Resolution The BPMs measure the beam transverse position (X and Y) and angle (tilt) in the X-Z and Y-Z plane (X’ and Y’). X0X0 can be written as XbX0X0For zero current Xb=X0, the BPM measures directly X0. The coefficients (i=1,…,N and j=1,…,4) determined by a minimization procedure. Position, respectively, tilt of the monitor i upstream or downstream of the chicane

04 November 2009Michele Viti20 Relative Beam Energy Resolution Fundamental condition: the magnetic chicane must work symmetrically, i.e. the upstream path must be restored X0 downstream in order to use the BPMs downstream for X0 determination. Beam direction Ideal trajectory Wrong trajectory

04 November 2009Michele Viti21 Relative Beam Energy Resolution In ESA 4-magnet chicane not symmetric. For a given current the B-fields were different up to ~3%. X0BPMs downstream could not be used to determine X0. d Worse resolution for d.

04 November 2009Michele Viti22 Relative Beam Energy Resolution A resolution of 24 MeV was found for Relative resolution of dLargest contribution comes from the resolution on d (>2 microns).

Laser Compton Energy Spectrometer

04 November 2009Michele Viti24 Laser Compton Energy Spectrometer At LEP it was possible to have redundant beam energy measurements  cross check At ILC so far, complementary methods for upstream beam energy measurements not foreseen. We studied the feasibility of an upstream energy spectrometer based on Compton backscattering (CBS) events.

04 November 2009Michele Viti25 Laser Compton Energy Spectrometer Compton process with initial electron not at rest. Energy spectrum for electrons (photons) with sharp cut-off (Compton edge): Scattered particles strongly collimated in forward region.

04 November 2009Michele Viti26 Laser Compton Energy Spectrometer

04 November 2009Michele Viti27 Laser Compton Energy Spectrometer New approach Measure 3 positions of particles:, the center of gravity of the scattered photons, or, equivalently, one end point of the SR fan., position of beam, possible to measure with BPMs, position of the scattered electrons with minimum energy.

04 November 2009Michele Viti28 Laser Compton Energy Spectrometer Detailed GEANT4 Simulation: Beam parameters –Beam energies GeV (250 GeV default value). –Beam size in x (y) (2-5) microns. Geometrical parameters –Drift distance m. –B field 0.28 T, magnet length 3 m. Laser parameters –Smaller wavelength (e.g. green laser). –Pulsed laser with 3 MHz frequency. –Laser spot size microns. –Laser pulse energy must ensure 10^6 scatters e.g. 30 mJ for green laser. –Crossing angle ~8 mrad. Accuracy required for – < 1-2 microns – < 20 microns

04 November 2009Michele Viti29 Laser Compton Energy Spectrometer In practice Beam position measured with a cavity BPM (very well know and precise technique). Edge position –Diamond strip detector, –Quartz fiber detector, –Basic simulation shows that both are feasible. Photon detection, 2 possibilities –Center-of-gravity of backscattered photons, –One edge of the synchrotron radiation photons.

04 November 2009Michele Viti30 Laser Compton Energy Spectrometer In particular, Number of backscattered photons 4 orders of magnitude less than SR photons ~100 GeV, SR photons ~3 MeV. 1° option –thick absorber in front of the position detector –measure the profile of shower –signal from dominant –quartz fiber detector suitable.

04 November 2009Michele Viti31 Laser Compton Energy Spectrometer 2° option: –No absorber. –Measure one end point of the SR fan. –SR photons dominant. –Novel detector under development in DUBNA (Xenon gas detector). Main problem for both configurations: very high radiation dose ( GGy per year). Simulations demonstrate feasibility.

04 November 2009Michele Viti32 Conclusions I ILC represents the next generation of electron/positron collider, providing a unique environment for precise measurements. Beam energy essential information for precise measurements (e.g. top quark mass). Baseline method for upstream beam energy at ILC is BPM-based spectrometer. In the years 2006/2007 a prototype of such device was commissioned in the End Station A (experiment T474/491).

04 November 2009Michele Viti33 Conclusions II In the thesis an essential contribution was given In the experiment T474/491: –Monitor the B-field integral. An accuracy was found (ESA-SLAC note and PAC poster). –Determination of the resolution of the 4-magnet chicane. A value of was found (to be published…). A novel method based on Laser Compton scattering was studied and its feasibility demonstrated (NIM publication). –A proof-of-principle experiment is under study; proposal in preparation.