sub- ps bunch length measurements a t the jlab fel with coherent transition radiation (ctr)
DESCRIPTION
Sub- ps bunch length measurements a t the JLab FEL with coherent transition radiation (CTR) a nd “Martin- Puplett ” interferometer. Pavel Evtushenko, JLab. The key principles of the diagnostic Operation of the Matrin-Puplett interferometer An examples of data - PowerPoint PPT PresentationTRANSCRIPT
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Sub-ps bunch length measurementsat the JLab FEL
with coherent transition radiation (CTR) and “Martin-Puplett” interferometer
Pavel Evtushenko, JLab
The key principles of the diagnostic
Operation of the Matrin-Puplett interferometer
An examples of data
Low frequency cut-off
Data Evaluation in frequency domain
Pulsed beam vs. CW beam measurements
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ps and sub-ps bunch length measurements using CTR
1. Transition radiation is produced when the electron bunch passes a boundary of two media.
2. Respond time is zero. Shape of the radiation pulse is a
“copy” of the electron bunch shape.
3. When the wave length of the radiation becomes more than the bunch length the radiation becomes COHERENT. ( L )
4. Power is proportional to:incoherent radiation N coherent radiation N2
5. Measurements of the radiation spectrum give information about the bunch length.
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Coherent radiation power spectrum
€
˜ E b(ω ) ∝ ˜ E S k(ω )e iωτ k
k
∑
€
Pb(ω ) = ˜ E b(ω ) ˜ E b*(ω ) = ˜ E S k(ω )e iωτ k ˜ E S
*j(ω )e−iωτ j
j
∑k
∑
€
Pb(ω ) = Ps(ω ) N e + e iω (τ k −τ j )
j ≠k
∑k
∑ ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
€
PS (ω ) = ˜ E S (ω ) ˜ E S*(ω )
€
e iωτ k
k
∑ ≡ N e fb(t)e iωtdt∫ = ˜ f b(ω )
€
Pb(ω ) = PS (ω ) N e − N e( N e +1) ˜ f b(ω )2
[ ]
€
˜ f b(ω )2
€
Eb(r r ,t) = ES k(
r r ,t −τk )
k
∑ - Coulomb field of electron bunch
- Fourier transform of the Coulomb field
- Power spectrum of the bunch
- Power spectrum of a single electron
- Bunch Form Factor (BFF)
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The Martin-Puplett interferometer operation
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The Martin-Puplett interferometer
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The Martin-Puplett interferometer
)tt(g)t(gE2RT)t(E 0||out
dt)t(g)t(g)t(gRTE)(U 2
||2
o
)t(gE)t(E 0in
the autocorrelation function ismeasured with the help of the MPI
The Wiener-Khintchine theorem says: “the Fourier transform of the autocorrelation functionis the power spectrum”.
longitudinal field profileat the MPI entrance
longitudinal field profileat the MPI exit
detectors measureintensity IE2
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The MPI scan
16 17 18 19 20 210.05
0.10
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Am
plitu
de o
f the
Gol
ay c
ell s
igna
l, V
mirror position, mm
Golay #1 Golay #2
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Interferogram - the normalized difference
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Inte
nsity
, a. u
.
mirror position, mm
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The use of two detectors: raw data vs. interferogram
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Bunch length reconstruction
2
t 2t
t
e2c
Qtn
2teCP~
2t4
0 eCe1)(f fit
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
diffraction on the Golay cell input window
4
0e1
2
0e1
Gol
ay c
ell a
pertu
re tr
ansm
issi
on
Frequency, THz
the Gaussian shapeof the bunch is assumed
its power spectrum isalso Gaussian
low frequency cut-offdiffraction on the Golaycell input window
two filter functionswere considered:
20e11F filter
40e12F filter
The fit function is used
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Jlab FEL CTR interferogram
step size – 5 um, 128 steps
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Jlab FEL CTR spectrum and the fit function
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JLab FEL (layout and longitudinal matching)Requirements on phase space:• Long bunch in linac• high peak current (short bunch) at FEL
– bunch length compression at wiggler• “small” energy spread at dump
– energy compress while energy recovering– “short” RF wavelength/long bunch get slope and curvature right
E
f
E
f
E
f
E
f
modified Martin-Puplettinterferometer (step scan)is used with CTR;only tune (pulsed) beam
Michelson interferometer(rapid scan)is used with CSR;CW beam
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Interferometers
We use two different interferometers;essentially both are a modificationof the Michelson interferometer.
The two interferometers differ inimplementation; Beam splitter Polarizer Detector Focusing element Mirror position measurements
Modified Marin-Puplett interferometer:(step scan device) beam splitter & polarizer (wire grids) detector (Golay cell) focusing (Plano-convex lens) mirror position is set by step motorUsed with CTR
Michelson interferometer:(rapid scan device 2 sec/scan) beam splitter (silicon) detector (pyroelectric) focusing (parabolic mirrors) mirror position is measured by another built-in interferometerUsed with CSR
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Pulsed beam measurements; RMS 148 fs
The mod. Martin-Puplett interferometer (the Happek device) measurements
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CW beam measurements 0.31 mA; RMS 147 fs
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CW beam measurements 0.62 mA; RMS 151 fs
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CW beam measurements 1.25 mA; 163 fs
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CW beam measurements 2.51 mA; 145 fs
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Pulsed beam measurements vs. CW beam
The bunch length does not change with beam current.