1. March 2006 The International Linear Collider at the IP – from feedback hardware to electromagnetic backgrounds FONT collaboration P Burrows, G Christian, C Clarke, C Swinson, H Dabiri Khah, G White, S Molloy Tony Hartin

2. March 2006 Outline Hardware Hardware Ground motion and the need for stablisation Ground motion and the need for stablisation IP feedback and the FONT project IP feedback and the FONT project FONT results and BPM sensitivity FONT results and BPM sensitivitySimulation Modification of GEANT for low energy transport Modification of GEANT for low energy transport 20 mrad crossing angle with different sets of accelerator parameters 20 mrad crossing angle with different sets of accelerator parameters Origin of BPM spray Origin of BPM spray Variation with Solenoid and DiD magnetic field Variation with Solenoid and DiD magnetic fieldTheory The background generators: GUINEA-PIG and CAIN The background generators: GUINEA-PIG and CAIN New, non-linear sources of e+e- backgrounds at the IP New, non-linear sources of e+e- backgrounds at the IP

3. March 2006 Luminosity Loss at IP due to ground motion Relative offsets in final Quads due to fast ground motion leads to beam offsets of several y (2.7 nm for ILC 500 GeV). Correct using beam-based feedback system near IP or by active mechanical stabilization of Quads or both.

4. March 2006 Intra-train Beam-based Feedback Intra-train beam feedback is last line of defence against relative beam misalignment Key components: Beam position monitor (BPM) Signal processor Fast driver amplifier E.M. kicker Fast FB circuit TESLA TDR: principal IR beam-misalignment correction

5. March 2006 (FONT1 +) FONT2: beamline configuration Dipole and kickers BPMs

6. March 2006 FONT2 results: feedback BPM (Jan 04) Feedback on Beam flattener on Beam starting positions Delay loop on beam startbeam end 123 4 Latency 54ns, correction factor 14/1

7. March 2006 BPM noise from backgrounds Noise form factor sinc(π.f.T) Secondary emission down to 100 eV needs to be considered Geant3 minimum transport is 10 keV! Copper strips with 1 mm gap to wall Noise from secondary charges crossing strip-wall gap ~1pm error for each charge absorbed or emitted. 1e6 hits per b.c. would be a problem!

8. March 2006 Geant model of 20mrad design Background primaries are traced through from the IP Optimal position for the BPM can be established BPM hits vary with parameter sets, geometry and magnetic fields. BPM 93443

9. March 2006 Geant3/4 mod for Low Energy Transport Geant3 X-section parametrization wrong below 10kev cut Recode Geant3 to parametrize real data from NEA site

10. March 2006 Geant3 total BPM hits and emitted charges LowE mods reveal significant increase in BPM hits at 100eV cut. Generally Factor of 5 increase in BPM compared to default Geant cut of 1 MeV, and factor of 2 increase against Geant default minimum cut Can define Geant areas (ROIs) around BPMS which are tuned for Low Energy particles Worst case scenario (scheme14 in the 20mrad case) ~ 10 5 hits per strip per bc

11. March 2006 Background Primaries – ILC parameter Sets High Lumi sets produce the greatest numbers of background pairs Guinea-Pig and Cain include L-L, B-H and B-W processes Beam field is treated linearly i.e. equivalent photon method scaled by the number density of beam field photons BUT.. The beam fields are intense – 10% of Schwinger critical field What about non-linear sources of background pairs? High LumiScheme14 Low PScheme13 Large YScheme12 Low QScheme11 NominalScheme10 USSCScheme9 1 TeV TESLAScheme 8 High LumiScheme7 Low PScheme6 Large YScheme5 Low QScheme4 NominalScheme3 USSCScheme2 500 GeV TESLA Scheme 1

12. March 2006 Where does BPM spray originate from? (20mrad) X,Y view at z=3.12m E+ and E- have different trajectories spray originates from annulus around extraction line B

13. March 2006 Relation of E distribution of BPM spray- producing pairs to Solenoid field Increase in leads to energy peak shifting With stronger magnetic field, (a)higher energy pairs, and (a)higher energy pairs, and (b) pairs with lower transverse momentum will produce spray on the mask hole edge (b) pairs with lower transverse momentum will produce spray on the mask hole edge Energy(MeV) B S

14. March 2006 The impact of a Detector Integrated Dipole(20mrad) (Seryi & Parker, Phys Rev ST Accel Beam 8, 041001 (2005) Introduced to offset Y displacement at the IP due to solenoid field Steers more primaries into Lumi Cal (K. Busser). Extra BPM spray sources 30% more spray hits delivered to the BPM

15. March 2006 Is there another non-linear source of pairs at the IP? Known: multiphoton pair production rate described by Sokolov-Ternov and onset governed by beam parameter Y=E/E c ~0.3. Scheme1 has Y=0.054, Scheme14 has Y=0.376 2 nd order process rather than 1 st order Rules for onset are different Calculation is complicated, but simplified when the photons are co-linear ee nk k b ee nk kk bb 21 Unknown: Multiphoton Breit-Wheeler Unknown: Multiphoton Breit-Wheeler 2 b k 1 b k e-e+

16. March 2006 Resonances in multiphoton B-W Multiphoton Breit-Wheeler Resonances Multiphoton Bremstrahlung (non-resonant) Ordinary Breit-Wheeler )1()( 2 22 m nk q Pairs created in intense e-m field have a quasi-level structure and resonant transitions can occur (Zeldovich, 1967) 2 nd order IFQED x-section can exceed normal x-sections by orders of magnitude (Oleinik, JETP 25(4) 697, 1967) 2 nd order Breit-Wheeler process in CIRCULARLY POLARISED field shows the same feature hepwww.ph.qmul.ac.uk/~hartin/thesis

17. March 2006 Experimental evidence for the IFQED processes 1 st order: One photon pair production Experiment E144 SLAC. 46 GeV beam with Nd:glass laser peak intensity 0.5x10 18 Wcm -2. Up to 4 photons contributed to each event Meyerhofer et al (1996) other non- linear phenomena such as electron mass shift observed 2 b k 1 b k e-e+ 1b k e-e+ 2 nd order: Substantial theoretical studies but no experimental efforts yet! BUT potentially more detectable because of resonances

18. March 2006 The field of the relativistic charge beams With low disruption, approximate to a constant crossed e-m field perpendicular to direction of propagation SIMPLIFICATION: Beamsstrahlung photons k 1 and k 2 emitted forward. Assume they are collinear COMPLICATION: Symmetry of the field seen by the synchrotron photons

19. March 2006 Including the external field in IFQED calculations Operator Method: quantum interaction of electron and external field photons but electron trajectory is considered classical. Due to Baier et al (JETP 28(4) p.807, 1969) Full quantum treatment: Horrendously complex but potentially doable with Vermaserens FORM Semi-classical method: Dirac equation is solved exactly for interaction with a classical plane- wave e-m field. Most common method. Used originally by Narozhnyii, Nikishov and Ritus in the mid 1960s

20. March 2006 IFQED – Dirac Equation Solution Exponential dependency on external field 4-potential Fourier Expansion in contributions of n external field photons Different external field polarisations lead to different form factor functions Circular polarisation Bessel functions n Jn(Q) Linear polarisation Generalised Bessel–type functions Constant crossed field-Azimuthally symmetric Airy functions n Ai(n Q) Constant crossed field –Nonazimuthally symmetric New AiJ functions n AiJn(Q)

21. March 2006 Calculation of Resonance widths The Electron Self Energy must be included in the Multiphoton Breit-Wheeler process This is a 2 nd order IFQED process in its own right. Renormalization/Regularization reduces to that of the non-external field case The Electron Self Energy in external CIRCULARLY POLARISED e-m field originally due to Becker & Mitter 1975 for low field intensity parameter =(ea/m) 2. Has been recalculated for general ESE in external CONSTANT CROSSED field is due to Ritus, 1972 Optical theorem: the imaginary part of the ESE is the same form as the Sokolov-Ternov equations

22. March 2006 Where do the resonances occur? Beamsstrahlung photon E S >> 0.511 MeV Beam photon E B < 0.511 MeV Processes which give/take energy to the field allowed and mass shell can be reached for physical values For collinear beamstrahlung photons, resonance condition is r (external field photons) ~ E S /E B Resonance PeakResonance Width

23. March 2006 Notes on the cross-section calculation Full trace contains ~ 100,000 terms Full trace contains ~ 100,000 terms Dramatically simplified by Special centre of mass-like reference frame Assume beamsstrahlung photons and beam field photons are collinear Only insert Imaginary part of self energy to get resonance width And the PRELIMINARY results…..?

24. March 2006 Results: Results: Stimulated Breit-Wheeler (Non- Resonant) Compare Stimulated Breit- Wheeler process with ordinary Breit-Wheeler process Examine the resonant and non- resonant contributions to the cross-section separately Nonresonant Stimulated Breit- Wheeler cross-section only a few percent of the ordinary Breit- Wheeler cross-section Can be neglected as a source of extra pairs

25. March 2006 Results: Results: Stimulated Breit-Wheeler (Resonant) Differential cross-section can exceed the ordinary Breit-Wheeler process Stimulated Breit-Wheeler Cross- section up to 2 orders of magnitude greater than ordinary breit-wheeler Transverse production of pairs seems favoured PROVISO – calculation for special reference frame. Need to generalise the case!

26. March 2006 Summary and Future Work FONT project continues with digital version and experiments planned at ATF2 (KEK) GEANT tweaking Modified Geant 3 for low energy transport Modified Geant 3 for low energy transport Ran pair files through Geant 3 simulation of BDS Ran pair files through Geant 3 simulation of BDS BPM hits are within a factor of 10 of problem levels. More detailed studies are needed. Simulation support for planned ESA tests LowE modification's reveal an increase in hits by a factor of 2 and were important to take into account 2 nd order, nonlinear interactions of beamsstrahlung photons with the beam fields should be taken into account because the cross- sections are potentially resonant and can exceed 1 st order and linear ordinary cross-sections – established by substantial theoretical work by several groups Preliminary calculations of the Stimulated Breit-Wheeler process (simplified case) suggests that this will be an issue at the ILC

27. March 2006 3 GeV electrons Focussed Tabletop TeraWatt Laser low spec laser Australian Synchrotron Experimental detection of Stimulated Breit- Wheeler resonances