Bunch compression and the emittance growth due to CSR

1
Bunch compression and the emittance growth due
to CSR

• Institute for Molecular Science, UVSOR
• Miho SHIMADA
• High Energy Accelerator Research Organization,
  KEK
• Kentaro HARADA

2
Outline

• Beam dynamics studies for the 5 GeV ERL
• Bunch compression down to lt100fs
• Optimization of R56, sextupole magnet, RF phase
  is important.
• Preserving emittance with and without bunch
  compression
• Beam envelope optimization is efficient to
  suppress emittance growth.
• Towards user experiment at the test ERL
• Short bunch for THz radiation (CSR)

3
Preservation of low emittance
4
Layout from merger section to insertion devices
section

• Test Facility Main Parameter
• Injection energy 5 MeV
• Full energy 165MeV
• Injection bunch length 1 psec2 psec
• Bunch length after compression 0.1 psec
• Initial projected emittance 100 nmrad

Generation of 0.1-ps electron bunch with low
emittance gives us useful information for the
5GeV ERL
5
Optics of the achromatic and isochronous arc
section(No bunch compression)
Twiss parameter bx, by and hx
R560
Injectormerger
Insertion device
6
Mechanism of emittance growth due to CSR wake
(2) Energy change depending on the longitudinal
position
(1) CSR emission from the bunch tail catches up
with the bunch head
Bunch head
Bunch tail
(3) Displacement of bunch slices

• CSR wake induces the projected emittance growth

Y. S. Derbenev et al , TESLA-FEL
7
Minimization of the emittance growthdue to the
CSR wake
qPhase
qCSR
qPhaseqCSR
(x, px) at the arc exit
(x, px) at the arc exit
Large emittance growth
Minimized emittance growth
The emittance growth is minimized when qPhase
coincides with qCSR (direction of CSR kick).
R. Hajima, Nuclear instruments and Methods in
Physics Research A 528 (2004) 335-339
a, b, g Twiss parameter, f bending angle
8
Evolution of the projected emittance(without
bunch compression)
Transverse Phase Space at the end of arc section
Initial 100nm rad

• Charge, 77pC/bunch
• Emittance growth can be suppressed below 140 nm
  rad.
• The r.m.s bunch length is maintained within the
  accuracy of a few (results not shown here)

9
Bunch compression at the arc section

• Rough estimation of fRF corresponding to R56

• Rs 0.9 is introduced to compressed up to 0.1 ps
  from 1 ps.
• R56 of ARC section was varied from -0.1 to -0.35
  m.
• RF phase shift, fRF, was determined by following
  R56.

10
Bunch length after bunch compression I
Longitudinal phase space
0.77 pC
0.5ps
77 pC

• CSR wake expands the bunch length.
• It is difficult to produce a 0.1ps short bunch
  with bunch charge of 77 pC
• Small R56 is better for bunch compresstion
  (except R56 -0.1)

0.5ps
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Bunch length after bunch compression II
Longitudinal phase space
0.77 pC
0.5ps
7.7 pC

• Electron particles shift to the center of the
  bunch by CSR effect.
• As a result, the bunch length is slightly shorten.

0.5ps
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Projected emittance growth after bunch compression
7.7 pC
CSR kick
77 pC

• Emittance growth with 77 pC is a few mm-rad.
• It is difficult to keep the initial emittance
  even with 7.7pC
• Emittance growth with 0.77 pC at large R56 is
  less than a few

13
Transverse phase space after the arc section
(7.7 pC, 0.1ps)
Optimized
Not Optimized
CSR kick
CSR kick
ex 151.2 nm rad
ex 191.8 nm rad

• Optimization of the direction of the phase
  ellipse is effective for compensation of the
  projected emittance growth.
• At the optimized optics --- qCSR qPhase 1.178

14
Bunch compression
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For what the test ERL can be used?

• User experiments with Terahertz coherent
  radiation
• SR of terahertz region, from 1THz to 10 THz, is
  enhanced by coherent radiation, when the rms
  electron bunch length is shorter than 1ps.
• Transverse beam size should be less than the
  wavelength to keep the coherent enhancement --
  (un-normalized ex lt 100 mm rad)
• User experiments with laser Compton Xrays
• It is a challenging task to produce the Xray with
  sub-pico seconds.

So we need the electron bunch with the bunch
length less than sub-pico second, and bunch
charge as large as possible.
16
For what the test ERL can be used?
THz radiation from short bunch
THz region
100 fs
1 ps
High intensity THz radiation is expected when 0.1
ps bunch is produced
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Introducing the sextupole magnets

• 2 families of sextupoles were introduced in the
  arc section.
• Optimize numerically the optics to produce the
  0.1 ps short bunch with various electron charge
  by using elegant.
• Emittance growth is not critical issue for THz
  radiaion.

18
Comparison between with and without sextupole
magnets
charge 77pC, 1ps (rms), 0.1mm-mrad
R56 -0.1m
with CSR, without sextupole
with CSR, with sextupole
T566 lt 0 helps the bunch compression With CSR
390 fs (rms), 6.9mm-mrad
59 fs (rms), 5.6mm-mrad
distortion by CSR
1ps
100fs
optimized by R. Hajima
19
Bunch compression of a 154pC bunch
charge 154pC, 1ps (rms), 0.1mm-mrad
re-optimized Acc phase, sext
with CSR, with sextupole
with CSR, with sextupole
T566 -1.23m
235 fs (rms), 30.7mm-mrad
110 fs (rms), 13.5mm-mrad
Acc phase 109.596 ? 108.466
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Higher charge per bunch
charge 308pC, 1ps (rms), 0.1mm-mrad
charge 616pC, 1ps (rms), 0.1mm-mrad
with CSR, with sextupole
with CSR, with sextupole
224 fs (rms), 32.4mm-mrad
188 fs (rms), 92.6mm-mrad
acc phase 106.754
acc phase 108.287
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Summary

• No bunch compression
• The projected emittance growth due to CSR is not
  significant with the bunch length of 1 ps, up to
  77pC
• Bunch Compression
• 77pC(100mA)
• It is challenging task to compress the bunch
  length down to 0.1 ps.
• 7.7pC(10mA)
• The projected emittance growth can be suppressed
  by the optimization of the optics in the arc
• 0.77pC(1mA)
• The projected emittance growth is less than a few
  

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simulation by elegant (25MeV)
R56 -0.1m, acc phase (module-1 / 2) 90 /
126.7 initial 1ps (rms), 0.1mm-mrad
100pC
sext-on T566 -0.037m
sext-off T566 0.38m
355 fs, 9.4 mm-mrad
640fs, 13 mm-mrad
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Bunch length after bunch compression I
Longitudinal phase space
0.77 pC
8x10-5 (m)
77 pC

• CSR wake expands the bunch length.
• It is difficult to produce a 0.1ps short bunch
  with charge of 77 pC

3.5x10-4 (m)
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Bunch length after bunch compression II
Longitudinal phase space
0.77 pC
8x10-5 (m)
7.7 pC

• Electron particles shift to the center of the
  bunch by CSR effect.
• As a result, the bunch length is slightly shorten.

8x10-5 (m)
25
Projected emittance growth after bunch compression
7.7 pC
CSR kick
77 pC

• Emittance growth with 77 pC is very huge.
• It is difficult to keep the low emittance even
  with 7.7pC
• Emittance growth at 0.77 pC is less than a few

26
Transverse phase space after the arc section
Optimized
Not Optimized
CSR kick
CSR kick
ex 355.8 nm rad
ex 832.6 nm rad
(7.7 pC, 0.1pC)

• Optimization of the direction of the phase space
  ellipse is effective for compensation of the
  projected emittance growth.

27
Bunch compression with 0.77 pC
R56 -0.1 m
R56 -0.3 m
8x10-5 (m)
4x10-4 (m)