Electron Diagnostics with Coherent Radiation LIFE Meeting @ LNF February 20, 2009 Electron Diagnostics with Coherent Radiation Enrica Chiadroni LNF - INFN

Outline CR theory Sources TR, SR, ER, DR, THz  Comb structure Characterization of the source Power, # of photons, energy, etc. Frequency Cut-off Detection systems (Golay cells, pyrodetectors, bolometers, etc..)

Scientific Motivation CR can be used as a powerful longitudinal diagnostics for high-brightness electron beams a powerful source in the THz frequency range We focus on sources that use ultra-short high-brightness electron bunches to create short, broadband THz radiation pulses with high peak power Examples of sources are CTR, CDR, CSR, CER, etc…

The Coherent Radiation Detector v nj R >> sz rj q G z z’ where

The form factor is typically different from zero for wavelengths equal or longer than the bunch length. Measuring the coherent spectrum it is possible to reconstruct the bunch length and even its longitudinal structure. By inverse Fourier transforming

Possible Scenarios SPARC Development Possible Applications Production of an electron bunch with a longitudinal density modulation directly from the source  M. Boscolo et al., NIM A 577 (2007) 409 - 416 Possible Applications 1. Source for dedicated experiments 2. Diagnostic tool for the bunch itself Production of coherent radiation at the frequency of the comb itself: at this frequency, the intensity emitted by the comb is the same emitted by a single micro-pulse in which all the electrons of the comb are compressed. Generation of two narrow micro-pulses perfectly synchronized each other but with a variable time delay, to perform pump-and-probe measurements.

The Form Factor of a Comb Longitudinal Structure (1) 300 mm 3 mm Form factor of a single sub-pulse s = 50 mm Form factor of the comb structure 3 mm Spectrum of a square pulse

The Form Factor of a Comb Longitudinal Structure (2) Which kind of coherent spectrum one can expect from two micro-bunches of s = 300 mm with variable spacing?

Source Characterization Parameters for the single bunch optimization (M. Boscolo et al., A Possible THz Radiation Source with a Train of Short Pulses in the SPARC High-Brightness Photoinjector, Proceedings of EPAC Conference 2008, Genova, Italy) Q = 300 pC  Ne = 1.865 109 Ebeam = 84 MeV sz = 55 mm The total spectral energy radiated by the bunch is where d2U/dwdW is the G-F distribution ( but corrections due to the finite size target are needed).

Experimental Considerations The form factor of any kind of electron distribution can be retrieved by means of Fourier transform spectroscopy  Frequency domain technique The Source Any radiation process which does not change the electron distribution is a good candidate  Transition Radiation, Diffraction Radiation, Synchrotron Radiation, Edge Radiation, Spontaneous Undulator Radiation (but not SASE), Comb Structure

Frequency Domain Technique Martin-Puplett Interferometer Roof mirror BS splits polarizations Reflectivity coefficients depend on the diameter and spacing of wires Incident radiation P BS Moveable roof mirror A Correlated fluctuations due to instabilities in the e-beam are eliminated Detectors MAIN LIMITATIONS low and high frequencies suppression  Reliable calibration of detectors  Transfer function of the system ii) Phase information missing  Kramers-Kronig technique

Golay Cell and Pyroelectric Detectors Based on LiTaO3 crystal 100 mm Advantages Gold layer Flat response up to 3 mm Diamond window: 6 mm Sensitivity: NEP ~ 10-10 W/Hz0.5 No cooling required Chromium layer Characteristics Operating spectral range: 0.1 – 3 THz (No-flat response!!) Active element: 2 mm x 3 mm Sensitivity: NEP@20 Hz ~ 10-8 W/Hz0.5 No cooling required Fast time response (~ hundreds of ms) A Golay cell detector is a thermo-acoustic detector consisting of a small cell filled with a gas, typically Xenon because of its low thermal conductivity, and a sensitive heat absorbing film with low thermal capacity which ensures a flat response to different frequencies. The frequency dependence is given, in principle, only by the properties of the window material used. Pyroelectric detectors, being thermal detectors, produce a signal in response to a change in their temperature. Below a temperature Tc, known as Curie point, ferroelectric materials (Triglycine sulfate, Lithium, Tantalate) exhibit a large spontaneous electrical polarization. If the temperature is altered by an incident radiation, the polarization changes and, if electrodes are placed on opposite faces of a thin dielectric, forming a capacitor (Fig. 3.13), the change in polarization can be observed as an electrical signal. Drawbacks Slow time response (~ ms) Response optimized for sinusoidal signal High sensitivity to abrupt temperature variations

System Calibration DETECTOR CALIBRATION  Collaboration with the Univ. of Roma “La Sapienza”, M. De Petris HOT-COLD METHOD for four selected wavelengths: 850 mm, 1.1 mm, 1.4 mm, 2.1 mm Z-cut QUARTZ VACUUM WINDOW CALIBRATION “La Sapienza”, P. Calvani, S. Lupi, D. Nicoletti

Bunch Length Measurement at FLASH with CDR Form factor

Coherent Transition Radiation @ THz Code (B. Schmidt)

Large angles are required to extract long wavelength SR from bending magnets, because the ”natural” opening angle in this case increases up to several tens milliradians in the far-infrared range. However, the situation changes dramatically if a straight section is introduced between two bends, like in Fig. 1(a). Long-wavelength radiation emitted by relativistic electrons in this setup is called Edge Radiation (ER), and presents a significantly smaller opening angle than standard SR frombends