1. Optical Diagnostics of High- Brightness Electron Beams Victor A. Verzilov Synchrotrone Trieste ICFA AABD Workshop, Chia Laguna, Sardenia

2. Introduction “ID” of a high-brightness beam  high charge per bunch (1 nC and more)  small transverse and longitudinal beam dimensions  extremely small normalized emittances  high peak current  space-charge effects in the beam dynamics Two missions of beam diagnostics  Provide instruments for study of the physics  Assist in delivering high quality beams for applications Every machine is as good as its diagnostics

3. Introduction (continue) Vertical and horizontal emittances Transverse beam profile Beam trajectory Energy and energy spread Bunch length Longitudinal bunch shape Charge per bunch Current (peak and average) Bunch-to-bunch jitter Some of the parameters are measured by traditional methods, others require specific techniques and instrumentations For high-brightness beams control of following parameters is essential

4. Specific requirements Take into account space charge forces Resolution from several millimeters to few tens of micrometers in both longitudinal and transverse plane Large dynamic range both in terms of beam intensity and measuring interval Non-invasive Single-shot Real time Jitter-free and synchronized Usual (stability, reliability,etc)

5. Optical diagnostics and others Optical diagnostics are based on analysis of photons generated by a beam in related processes or make use of other optical methods (lasers, etc.) This talk reports the current status of optical diagnostics of high- brightness beams Reasons  significant progress  make an essential part of available tools  impossible to cover everything Other techniques  wire scanners  zero phasing  transverse rf deflection cavity  high-order BPM

6. Outline Transverse and longitudinal profile measurements give the largest amount of information about beam parameters Transverse plane  Spatial resolution is a key issue  Survival problem for intercepting monitors  Non-invasive methods  Emittance measurement issues Longitudinal plane  Coherent radiation is a primary tool Direct spectral measurements Fourier transform CDR vs CTR  Electro-optical sampling

7. Transverse plane OTR vs inorganic scintillators at a glance OTR instantaneous emission linearity (no saturation effects) high resolution surface effect: thickness doesn’t matter small perturbation to the beam (small thickness) small radiation background (small thickness) can be used in a wide range of  relatively low photon yield (limitation in pepper-pot measurements) Scintillators ( YAG:Ce, YAP:Ce, oth.) high sensitivity no grain structure time response ~ 100ns conformance to HV radiation resistance bulk effect

8. TR spatial resolution FWHM resolution is 2-3 times of the classical PSF scales as ~  tails problem; mask can help high-resolution is experimentally confirmed [CEBAF(4 GeV) SLAC (30 GeV)] OTR resolution is determined by the angular acceptance

9. Scintillator resolution A.Murokh et al. BNL-ATF Recent experiment at BNL expressed concerns about micrometer-level resolution. Strong discrepancy in the beam size compared to OTR and wire scans was observed. Confirmed at ANL 220 MeV @ 0.8 nC 30-40% discrepancy Q=0.5nC

10. Instantaneous heating. TR case N.Golubeva, V.Balandin TTF Temperature limits Si  Melting - 1683 °  Thermal stress – 1200° Al  Melting - 933 °  Thermal stress – 140-400° Si: 1GeV @ 300um. For Al values ten times smaller

11. Heating by a bunch train 20um 50um 9MHz 1MHz N.Golubeva, V.Balandin TTF Two cooling processes contribute to the temperature balance  Radiation cooling ~ temperature to the power of 4  Heat conduction depends on the thermal conductivity and temperature gradient Si@9MHZ Si @ 20 um 1nC

12. 90° Thompson scattering W.P.Leemans et al. LBNL Noninvasive Both transverse and longitudinal profiles Synchronization Powerful laser Limited applicability e-beam: 50 MeV@1.5nC laser: 50mJ@0.8  m; 50-200fs photons:30keV@10 5 ph/bunch 66  m FWHM

13. Diffraction radiation Diffraction radiation is emitted when a particle passes in the proximity of optical discontinuities (apertures ) DR characteristics depend on the ratio of the aperture size to the parameter  DR intensity ~ e -a/   and is strongly suppressed at wavelengths <a/ 

14. TR vs DR from a slit Transition radiation Diffraction radiation

15. Effect of the beam size Angular distribution depends on the relative particle position with respect to the aperture and can be used to measure the beam size Strong limitation is a low intensity in visible and near infra-red Energy and angular spread, detector bandwidth are interfering factors Still has to be proven experimentally A.Cianchi PhD Thesis

16. Emittance measurement. Multislit vs quadscan S.G.Anderson et all PRSTAB 5,014201(2002) Measure of spaces-charge dominance Pepper-pot (multislit) Quadscan 3 screens Widely used techniques drift High-brightness beam at “low energy” Space-charge forces LLNL 5MeV@50-300pC

17. Longitudinal plane Small longitudinal bunches are crucial for many applications Bunch lengths are on a sub-ps time scale Conventional methods often do not work Several new techniques have been developed Coherent radiation has become a primary tool to measure the bunch length and its shape in the longitudinal plane It is very powerful tool with nearly unlimited potential towards ever shorter bunches

18. Radiation from a bunch All particles in a bunch are assumed identical. No angular and energy spread.

19. Radiation zoo Any kind of radiation can be coherent and potentially valuable for beam diagnostics  Transition radiation  Diffraction radiation  Synchrotron radiation  Undulator radiation  Smith-Parcell radiation  Cherenkov radiation Nevertheless, TR is mostly common  Simple  Flat spectrum

20. Bunch form-factor and coherence wavelength is much shorter than bunch dimensions radiation is fully incoherent particles emit independently total intensity is proportional to N wavelength is of the order of bunch dimensions radiation is partially coherent some particles emit in phase increase in total intensity wavelength is much longer than bunch dimensions radiation is fully coherent all particles emit in phase total intensity is proportional to N 2 F=0 0 <F< 1 F=1

21. Form-factor and bunch shape For the normalized longitudinal distribution of particles in the bunch  (z) By inverse Fourier transform Symmetric bunch Transverse coherence comes first. Unless the beam is microbunched.

22. Bunch shape and form-factor Bunch shapes with the same rms bunch lengths Although, in principle, the bunch shape can be retrieved from a measurement, be care, this could be ambiguously. The bunch size, however, is recovered reliably. Form-factors

23. Kramers-Kronig analysis If F(  ) is determined over the entire frequency interval, the Kramers-Kronig relation can be used to find the phase. Both real and imaginary part of the form-factor amplitude are to be known to recover the asymmetry of the bunch shape. By inverse Fourier transform Real part is the observable R.Lai and A.J.Sievers NIM A397

24. Kramers-Kronig analysis.Experiment Spectral intensity has to be defined over a significant spectral range. Errors are produced when asymptotic limit are attached to the data to complete the spectral range. Front-tail uncertainty. Analytical properties of the bunch shape function have to be taken into account. Confirmed by recent SASE results! TESLA TDR

25. Polychromator Single-shot capable Narrow bandwidth Discreteness T.Watanabe et al. NIM A480(2002)315 Tokio University 1.6ps900fs Results are consistent with streak camera and interferometer measurements

26. Hilbert -Transform spectrometer M.Getz et al., EPAC98 TTF Josephson junction Wide bandwidth More R&D is necessary T= 4-78K f= 100-1000GHz

27. Coupled to a frequency domain. Fourier spectroscopy Measurement in the time domain is a measurement of the autocorrelation of the radiation pulse. Precise Established Time consuming

28. Low-frequency cut-off All experimental data suffer to a different extent from the low frequency cut-off. There is a number of reasons which cause the cut-off: detector band, EM waves transmittance, target size etc. Data analysis usually consists in assuming a certain bunch shape and varying the size parameter for the best fit to undisturbed data.

29. Analysis in the time domain (TR case) A.Murokh,J.B.Rosenzweig et al Filter function Model bunch shape Coherent spectrum Autocorrelation curve

30. TR. Finite-size screen r screen  The effect comes into play when the screen size is comparable or smaller than  The TR spectrum from a finite size target is a complex function of the beam energy, target extensions, frequency and angle of emission. r=20 mm  d=0.05 rad 1mm2mm

31. Coherent diffraction radiation Bunch length was measured for slit widths 0 to 10 mm. Effect of the target finite size was proved. M.Castellano et al. PRE 63, 056501 TTF

32. Coherent diffraction radiation.Result DR and TR results are consistent in a wide range of slit widths. CDR can be successfully used for bunch length measurements. Very promising for ultra- high power beams, because non-invasive. M.Castellano et al. PRE 63, 056501 TTF 225MeV @ 1nC

33. Electro-optic sampling (EOS) Noninvasive Fast response ~40 THz Linearity&dynamic range Jitter dependent Modulation of the polarization of light traveling through a crystal is proportional to the applied electric field Collective Coulomb field at R is nearly transverse

34. EOS Single-shot option Single shot On-line Nearly jitter-free Make use of a long pulse with a linear frequency chirp Bunch time profile is linearly encoded onto the wavelength spectrum

35. EOS Single-shot option.First prove Resolution ≈  Chirp  Pulse width  ~300fs ~70 fs achievable ( ) 1.72 ps I.Wilke et al., PRL, v.88, is.2,2002 FELIX e-beam: 46MeV@200pC 0.5x4x4mm 3 ZnTe crystal laser: 30 fs@800nm,chirp up to 20ps

36. Conclusions Beam diagnostics has significantly advanced to meet specific requirements of high-brightness beams Wide choice of available techniques from which one can select Lack of suitable (simple and reliable) non-invasive methods for measurements in the transverse plane (near-future projects) In the longitudinal plane CDR is likely OK Difficulties with measurements at μm and sub-μm level in the transverse plane