1. Spectral methods for measurement of longitudinal beam profile
Bunch shape and RF signal spectra
RF pick-up and signal spectrometer bunch length measurements
Self consistent calibration method

2. Measurement methods for short bunch length
Time Domain Methods
Frequency Domain Methods
streak camera
RF deflector
spectrometer
microwave spectroscopy
coherent radiation
conversion
into photons
bunch induced
RF field
time dependent
acceleration
conversion
into photons
conversion
into electrons
frequency
selective
element
deflection with time
dependent field
dispersive
bend
inverse Fourier transform +
assumptions on distribution
detection of transverse
electron distribution

3. Bunch Length Measurements with RF methods
For beam intensity and position measurement the bunch length affects the signal in proportion with bunch form factor
By sampling Fb at different frequencies information of bunch length and shape can be obtained !

4. Relation bunch spectrum / r.m.s. bunch length
Three bunch shapes with same qb and σb t
ib(t)
Fb(ω)
ω/2π

5. Best frequency choice for σb determination
Fb(ω)
σb- 20%
σb+ 20%
Best frequency for maximum sensitivity
ω/2π
ωopt/2π=0.159/σb

6. Range of microwave methods
ωopt/2π
[GHz]
Quasi optical systems
wave guide
systems
σopt [ps]

7. A very simple system rectangular waveguide with Fb(ω) diode power
Choose rectangular waveguide with ωcutoff≈ωopt . Connect waveguide to beampipe.
Detector will measure integrated spectrum integrated above ωcutoff
beampipe
rectangular
waveguide with
diode power
detector
oscilloscope L
Fb(ω)
ω/2π
ωcutoff
σb+ 20%
σb- 20%

8. Different configurations for coupling the electron bunch field
PhD thesis C.Martinez

9. Waveguide pick-up system design
DC block to protect RF detector against
beam induced
capton foil is better
but your vacuum group will kill you!
Courtesy L. Søby, CERN

10. Waveguide pick-up of CTF3 at CERN
Non intercepting bunch length monitor
RF in
Shorter bunches means more power
Courtesy L. Søby, CERN

11. A more sophisticated system CTF3(CERN) mm-wave spectrometer
Courtesy A. Dabrowski, CERN

12. Filter for mm-wave Reflection=low pass Transmission=high pass
PhD thesis C.Martinez

13. Example CTF3 mm-wave spectrometer
Example of one down mixing stage - RF-pickup
Example:
33 GHz beam harmonic (11th of 3 GHz)
ADC is 2 GS/s, typically use 4000 points, 2 micro second time window, delta t = 0.5 ns
Depending on the period of the bunch length variations along the pulse & parasitic noise optimize the choice of the second LO mixing stage
choose to down mix to a high frequency LO signal, choose 716 MHz
Beam acceleration
Beam harmonic #
Beam harmonic
Fixed first Mixing
Variable Mixing IF
IF (measured)
GHz 11
GHz
26.5 GHz
7.2 GHz
716 MHz
735 MHz
Example Schema K-band down mixing scheme
Courtesy A. Dabrowski, CERN

14. Example CTF3 mm-wave spectrometer
Courtesy A. Dabrowski, CERN

15. Self consistent calibration of spectral bunch length measurment
Measure RF signal S1 and S2 at two frequencies ω1 and ω2
Change machine setting to obtain a different (yet unknown) bunch-length
Measure again RF signal S1 and S2 at two frequencies ω1 and ω2
Compute response function R1 and R2 at ω1 and ω2 and bunch-lengths σA and σB for the two machine settings from knowledge of spectral shape
Store R1 and R2 for future measurements
Fb(ω)
ω/2π
Measurement A
Measurement B ω1 ω2

16. Spectrum of bunch trains
Envelope shape= Single bunch spectrum
ω/2π
Line spacing=1/bunch spacing

17. Thank you for your attention !
& Have fun measuring your beam parameters!
Re V
Im V