1. Enrica Chiadroni University of Rome “Tor Vergata” and INFN-Roma2 Practice at the TTF VUV-FEL for Bunch Length Measurement @ SPARC REVIEW COMMITTEE University of Rome “Tor Vergata” October 4, 2004

2. New generation linear colliders and X-ray Free Electron Lasers require extremely low emittance (small transverse size) and high peak current (short bunch) electron beams The development of suitable beam diagnostics is necessary to produce and control such beams Good candidates are DR and SR due to their non-intercepting and parasitic nature (in use at DESY) and TR (to be used at SPARC) Very few, far from complete, experimental investigations of DR exist Scientific Motivation

3. Purposes Martin-Puplett interferometer construction Test of the Golay cell detector Measure the bunch length at the TTF VUV-FEL with CDR (Several measurements of the bunch length at TTF2 involve either different techniques, CSR, EOS,..., or different detectors) Know how transfer and hardware sharing on the the bunch length measurement at SPARC PhD Deadline

4. The Bunch Length Measurement @ SPARC  z = 1 mm Frequency domain technique by using CTR from a SiAl foil Test of the calibrated Golay cells

5. Coherent Radiation Theory njnj v rjrj z’ G  R Detector R>>  z By averaging with respect to the different positions of the particles in the bunch z

6. Incoherent Coherent The form factor F(  ) is given by the 3D Fourier transform of the normalized bunch distribution S(r)

7. Forward direction whereis the longitudinal distribution function of particles in the bunch F(  ) does not depend on the transverse bunch distribution Information about the longitudinal bunch distribution function S(r z )

8. Off-axis direction For large angles or large transverse beam sizes, the transverse bunch distribution will contribute to the form factor resulting in a longer apparent bunch length  Corrections are needed The transverse contribution can only be ignored in the limit: In our case,  z ~ 3 mm, r  (rms) ~ 0,3 mm tan  << 2  No corrections are needed

9. Transition and Diffraction Radiation Theory Due to the presence of optical inhomogeneities in the space, the field of the traveling charge induces currents on the screen surface which vary in time, thereby giving rise to the emission of radiation v z=0 z 11 22 Transition RadiationDiffraction Radiation TR is generated when a charged particle crosses the interface between two media of different permittivities DR is produced by a charged particle passing through an aperture in (or near the edge of) a conducting screen a

10. Transition radiation Diffraction radiation Maximum energy radiated at  0 =1/ . No energy is emitted at  =0 because of the radial polarization of the fields traveling with the electron bunch

11. Frequency Domain Technique : Autocorrelation measurement (1) Michelson Interferometer BS BS splits amplitudes Reflectivity coefficients depend on the BS material and thickness Part of radiation goes back to the source Incident radiation Moveable flat mirror detector Suitable BS for the FIR, such as Mylar, do not provide constant and equal reflectance and transmittance for all frequencies  Interference effects which spoil the efficiency

12. Frequency Domain Technique : Autocorrelation measurement (2) Martin-Puplett Interferometer P BS A BS splits polarizations Reflectivity coefficients depend on the diameter and spacing of wires P tot = P H +P V  Correlated fluctuations due to instabilities in the e-beam are eliminated Golay cell detectors Roof mirror Moveable roof mirror Incident radiation

13. Wire Grids and Roof Mirrors The component of E parallel to the plane of incidence is continuous on reflection, while the one perpendicular to it is inverted   x y Component of E parallel to the wires  total reflection Component of E orthogonal to the wires  total transmission

14. Data Analysis The split signals are recombined with variable delay and the radiation power is measured as a function of delay time  By Fourier transforming the autocorrelation function one obtains the CR power spectrum Main limitation of frequency-domain techniques is the strong suppression of the low frequency part of the spectrum due to: diffraction losses due to the interferometer apertures finite size of the radiator reduced acceptance and sensitivity of the detectors at long wavelengths It is therefore extremely important to have a reliable (relative) calibration of the detectors response vs. wavelength, extending to the longest possible wavelengths

15. The Golay Cell Flat response up to ≈1 mm Sensitivity: NEP ~ 10 -10 W/Hz 0.5 No cooling required Advantages Drawbacks Slow time response High sensitivity to abrupt temperature variations A study was made on existing detectors that led to the choice of An external band-pass amplifier to suppress the high frequency noise and to reduce the output impedance has been built

16. Study of suitable materials at mm-wavelengths First test of Golay cell detector Realization of the Martin-Puplett interferometer The interferometer will operate either in controlled atmosphere or in vacuum (few mbar) Detectors calibration Experimental Issues

17. Millimeter Wave Materials Common window materials are not transparent to FIR Plastic material (HDPE, TPX,…) are not good for UHV environment Flange window: quartz z-cut, diameter 60 mm, thickness 4,8 mm Detector window: diamond, diameter 6 mm, thickness 0,4 mm

18. First Test of the Golay Cell Detector @ TOSYLAB (TTF) by using SR Beam parameters: 30 bunches, 1 nC per bunch, ACC1 phase, -127 deg, blackbody 2 PE layers, thickness 12  m Chopper frequency 20 Hz 1 M  impedance, AC coupling beam 200 ms 50 ms

19. Experimental Layout @ TTF VUV FEL

20. Detector Calibration (1) The receiver is alternatively pointed towards a warm and a cold calibration target (e.g. ECCOSORB absorbing material, at room or liquid N 2 temperature) and the difference signal used as a measure of the (relative) cell sensitivity. The responsivity is obtained from the ratio of the output voltage to the power incident on the blackbodies and is measured as a function of wavelength over a wide band using a Martin-Puplett interferometer in conjunction with several band-pass filters Hot / Cold Method (Collaboration with the Univ. of Roma “La Sapienza”, Dott. M. De Petris)

21. Scalar Network Analyzer Local Oscillator mm-wave module waveguide Feedback Isolator waveguide... Channel B Channel A... Power meter Output power Oscilloscope Detector Chopper Local Oscillator to directly drive the mm-wave module Channel A: reference signal for normalization purposes Channel B: reflected power signal Source power 0 dBm Frequency bandwidth 75 GHz ÷ 110 GHz Detector Calibration (2) Coupling oscillator sources (Collaboration with the Univ. of Milano “Bicocca”, Dott. M. Gervasi)

22. Attenuator Chopper Power meter Golay cell Last waveguide mm-wave module Directional couplers Calibration Experimental Setup

23. Calibration Steps 1. Analyzer calibration Four source powers selected: 0 dBm, -10 dBm, -15 dBm, -20 dBm to get the most suitable output power Power meter output Reflected power depending on the optics

24. 2.Attenuator calibration Two local oscillators  VNA calibration to measure the return loss and the transmitted power from 1 to 2 Calibration Steps RF LO out Module Interface RF LO out Module Interface S1S1 S11S11 b1b1 S21S21 S12S12 S22S22 b2b2 S2S2 mm-wave test 1 2 Attenuator

25. Calibration Steps 4.Corrections due to the transition rectangular to circular aperture and to the horn antenna 3.Detector calibration The detector responsivity is strongly dependent on the aperture used where and

26. Calibration Steps A Gaussian profile either for the waveguide or the horn is assumed with L being the size of the waveguide

27. Preliminary Calibration Results In principle, it is possible exploring also the frequency bandwidth 26 GHz ÷ 40 GHz, but with a different setup  How do we accord the results? Even if a flat output power can be measured, it is not straightforward to have a flat power collected by the Golay cell  Improvement in the optical coupling is essential! The cell input power shows a strong cut off at low frequency (75/90 GHz), but an almost flat behavior for frequencies greater than 90 GHz The responsivity curves show still some disagreement at 80 – 95 – 105 GHz  To be figured out the reason!

28. To Do... Further detectors calibration with a different experimental setup in order to optimize the optical coupling between waveguides and the detector  Design and construction of a conical waveguide Cross check of results from first calibration by using the hot/cold method Bunch length measurement at TTF2 starting in November Data analysis Application of the method to bunch length measurements at SPARC