1. Electron Diagnostics with Coherent Radiation
LIFE LNF
February 20, 2009
Electron Diagnostics with Coherent Radiation
Enrica Chiadroni
LNF - INFN

2. 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..)

3. 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…

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

5. 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

6. 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)
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.

7. 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

8. 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?

9. 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 =
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).

10. 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

11. 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

12. 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 ~ 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:
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

13. 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

14. Bunch Length Measurement at FLASH with CDR
Form factor

15. Coherent Transition Radiation @
THz Code (B. Schmidt)

16. 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