1. TESLA, 16/9/031 Smith-Purcell radiation and picosecond bunch diagnostics George Doucas and Wade Allison Sub-Dept. of Particle Physics, University of Oxford

2. TESLA, 16/9/032 Collaborators University of Oxford (J.H. Mulvey and M. Omori) Univ. of Essex (M.F. Kimmitt) Dartmouth College (J.E. Walsh +, J.H. Brownell and H.L. Andrews) ENEA, Frascati (G. Gallerano, A. Doria, E. Giovenale and G. Messina) Support from: Univ. of Oxford, British Council and Royal Society

3. TESLA, 16/9/033 Outline 1.Introduction 2.Early experiments at Oxford and recent results from Frascati. 3.The future (higher energy, shorter bunch, more theory at high g). 4.Summary of where we are now.

4. TESLA, 16/9/034 1. Introduction First observed in 1953 (Phys. Rev. 92, 1069, 1953) The term is now used to describe radiation produced from the interaction of a charged particle beam with a periodic structure, such as a grating. Is one aspect of the effect of the electromagnetic field of moving charge, such as transition and diffraction radiation, but with some distinct advantages…

5. TESLA, 16/9/035 1. Basic relationship q xoxo u Dispersion relation: l q nlnl Typically, in the far IR

6. TESLA, 16/9/036

7. 7 2. An elementary calculation A reasonably simple theory, capable of predicting behaviour under various experimental conditions is essential for any application. Not many papers with measured mW’s on the graphs!! Treatment based on assumption that a passing electron induces image charges on the surface of the grating. These are then ‘accelerated’ by the peaks and troughs of the periodic structure. (not the only approach!!) Accelerated charge produces radiation; objective is to find the angular distribution of the emitted intensity I.

8. TESLA, 16/9/038 2. An elementary calculation Final relationship, for the case of a single electron, at a height x o over a grating with period l and overall length Nl, is given by: or Term R 2 depends on the details of the grating profile; e is the ‘evanescent wavelength’, e ~  For high , good coupling is possible even at mm’s distance For a continuous beam of current I b, the emission is ‘spontaneous’ and the radiated power is given by changing 2p e 2 to 2p eI b.

9. TESLA, 16/9/039 3. Oxford results Phys. Rev. Lett., 69 (1992), 1761 First to observe incoherent SP radiation from an essentially continuous, low-density relativistic beam. Limited by range of emission angles accessible and electron beam position jitter. Nevertheless, reasonable agreement with predictions of surface current model of radiation process.

10. TESLA, 16/9/0310

11. TESLA, 16/9/0311 4. Frascati Phys. Rev. Sp. Topics-Accel. & Beams 5, 072802, (2002) Main motivation was to extend the range of emission angles accessible by light-collecting system. Confirm theoretical treatment by direct comparison of measured vs. calculated power. Improved experimental set-up and more reliable beam. Work supported by Royal Society.

12. TESLA, 16/9/0312 4. Frascati-experimental Microtron with discreet beam energies, starting at 1.8MeV, up to 5MeV, in steps of 0.8MeV. Most of the work at 1.8MeV ( g=4.52), some at g=10.3 Bunch length is approx. 15ps, bunch spacing 333ps. Bunch train duration is approx. 5 m s, with an average current of 200mA. Hence, each bunch has about 4.2x10 8 electrons. Normalized beam emittance is rather poor (~ 50  mm.mrad)

13. TESLA, 16/9/0313 Experimental Signal taken to detector through polished copper pipe (  3m long) Detector is InSb electron bolometer, liquid helium cooled. Note reference point for power calculation.

14. TESLA, 16/9/0314 Data ( g=10.3) E=4.75MeV, I=120mA, 400 mesh/inch filter in front of detector. Observed power levels orders of magnitude higher (tens of mW) than those expected from ‘incoherent’ theory. Spontaneous coherent enhancement of SP.

15. TESLA, 16/9/0315 Coherent enhancement For a bunch with N e electrons: there is possibility of coherent enhancement, if the ‘coherence integral’ S coh is not very small This is the ‘bunch form factor’, which depends on the distribution f (t) of the particles in the time domain.

16. TESLA, 16/9/0316 Coherent enhancement Begins to dominate as the wavelength of the radiation becomes comparable with the bunch length. Different assumed functions f (t) give very different angular distributions of coherent SP. Hence, coherent enhancement, not only increases the emitted power but it also provides a clear ‘signature’ of the time profile of the bunch, through a measurement of the angular ( i.e. wavelength) distribution of the radiation.

17. TESLA, 16/9/0317 Coherence & pulse shape Sample calculations, based on Frascati conditions (E=4.75MeV) Beam size was ~1x2mm and beam centroid about 2mm above grating. Assume pulse length of 16ps. Assume that 80% of particles are within this nominal length.

18. TESLA, 16/9/0318 Results-analysis Same data as before. Best fit for triangular shape, with 80% of particles inside 16ps. Shape is slightly asymmetric with respect to reference particle (t=0).

19. TESLA, 16/9/0319 Features 1.Simple experimental set-up. 2.Non-intercepting, valid for any charged particle beam, at almost all energies. 3.Ample radiated power. 4.Sensitivity to the bunch length and its harmonics can be optimized by matching it to the grating period. 5.Measurement of the spectrum of the radiation is facilitated by the natural dispersion of the grating.

20. TESLA, 16/9/0320 The future Interest in beam diagnostics for Linear Collider (LC-ABD bid to PPARC) Knowledge of the bunch longitudinal profile is important (beam-beam interaction)  needed by FONT. Need input from groups that measure beam size, position and backgrounds.

21. TESLA, 16/9/0321 Issues Do we understand  dependence?  new calculations in hand. Can we make precise predictions of coherent radiation for real bunches, gratings, beam pipe etc?  work in hand Can we measure the spectrum at high energies? Questions raised include… Background radiation  help from simulation groups Test facilities with known short bunches? Other periodic structures?  work in hand Detector selection, filters etc.  need to build up expertise. Radiation damage??

22. TESLA, 16/9/0322 a. FELIX Higher energy (45-50MeV), shorter bunch (1- 3ps) Simpler device, with no rotating mirrors but a series of collimated apertures, to detect simultaneously at a range of angles. IR detector array  preferably pyroelectric Direct comparison with Electro-Optic technique.

23. TESLA, 16/9/0323 ~ 300 mm

24. TESLA, 16/9/0324 Predictions for tests at FELIX If bunch were 3ps ‘triangular’, then.. Two different beam positions above grating, blue=1mm, red=5mm

25. TESLA, 16/9/0325 b. GeV region In parallel with these tests… New EM field calculations for a high g bunch, passing over a single wire (WA)

26. TESLA, 16/9/0326 x z A simple model Start with a fine wire along x-axis (radius 20μm) A relativistic bunch travels parallel to z, a distance b from the wire (in y)... then opposing currents I are induced in the wire... giving a radiated field like quadrupole radiation but compressed into flat disk-shaped lobes with θ x ~1/γ from the plane perpendicular to the wire I βc b

27. TESLA, 16/9/0327....expanding the calculation to an array of 10 such wires, 300μm pitch....then the two disks segment into azimuthal lobes around the wire axis eg at λ = 100μm (with exaggerated polar angle): Of course generally there is angular dispersion of the radiation by the grating according to wavelength.... As before the red arrow is the wire direction and the green arrow the beam.

28. TESLA, 16/9/0328 The dependence of the radiation reduction factor on λ for an rms bunch size of 30μm (0.1ps) λ 200 μm (m)

29. TESLA, 16/9/0329 Plot of radiated power against bunch size (in m) for red λ=100-250μm green λ=250-600μm zz

30. TESLA, 16/9/0330....and the good news: at high γ, the grating to beam separation can be up to ~γ  λ without serious loss of radiation flux. No problem!....and the bad news: for maximum flux the width of the grating should be ~γ  λ.... but it has got to be in the beam pipe! So this effect will be responsible for a substantial reduction in the flux from a grating. Calculations on these problems and other ideas continue... We have already learned a lot of things which upon reflection were simply understood We aim to predict the results of tests quantitatively, depending of course on whether we know the actual bunch length!

31. TESLA, 16/9/0331 Summary Coherent SP radiation can be used, in principle, to determine the Fourier transform of the longitudinal profile of finite-length bunches. Demonstrated (first time ?), using 14ps bunches from Frascati Microtron, at low energies (1.8 and 4.75MeV). Next runs are at FELIX, then… Final Focus Test Beam (FFTB) at SLAC (~ 1ps and 30 GeV). is one possibility. TESLA?