Spectral methods for measurement of longitudinal beam profile

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

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

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 !

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

Best frequency choice for σb determination Fb(ω) σb+ 20% σb- 20% Best frequency for maximum sensitivity ωopt/2π=0. 159/σb ω/2π

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

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

Different configurations for coupling the electron bunch field Ph. D thesis C. Martinez

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

Waveguide pick-up of CTF 3 at CERN Non intercepting bunch length monitor RF in Shorter bunches means more power Courtesy L. Søby, CERN

A more sophisticated system CTF 3(CERN) mm-wave spectrometer Courtesy A. Dabrowski, CERN

Filter for mm-wave Reflection=low pass Ph. D thesis C. Martinez Transmission=high pass

Example CTF 3 mm-wave spectrometer Example of one down mixing stage - RF-pickup Example: 1. 2. 3. 4. 33 GHz beam harmonic (11 th 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) 2. 99855 GHz 11 32. 984 GHz 26. 5 GHz 7. 2 GHz 716 MHz 735 MHz Example Schema Kband down mixing scheme Courtesy A. Dabrowski, CERN

Example CTF 3 mm-wave spectrometer Courtesy A. Dabrowski, CERN

Self consistent calibration of spectral bunch length measurment 1. 2. 3. 4. Measure RF signal S 1 and S 2 at two frequencies ω1 and ω2 Change machine setting to obtain a different (yet unknown) bunch-length Measure again RF signal S 1 and S 2 at two frequencies ω1 and ω2 Compute response function R 1 and R 2 at ω1 and ω2 and bunch-lengths σA and σB for the two machine settings from knowledge of spectral shape 5. Store R 1 and R 2 for future measurements F (ω) b s ea M t en t. A rem en em ur asu Me B ω1 ω2 ω/2π

Spectrum of bunch trains Fb(ω) En ve lop es ha pe =S ing le b un ch sp ec tru m ω/2π Line spacing=1/bunch spacing

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