Electron
Electron Bunch Compression and Coherent
Effects
Effects at the SDL
Henrik
Graves*,,
G. Lawrence
Lawrence Carr*,
Carr , Adnan
Adnan Doyuran*,
Doyuran , William S. Graves
Henrik Loos*,
Loos , G.
Eric
D.
Johnson*,
Samuel
Krinsky*,
James
Rose*,
Brian
Sheehy*,
V.
Eric D. Johnson ,
Krinsky ,
Rose ,
Sheehy , Timur V.
Shaftan*,
Yu*
Shaftan , John
John Skaritka*
Skaritka and Li-Hua
Li–Hua Yu
* National
Upton, NY 11973,
11973,USA
USA
National Synchrotron
Synchrotron Light Source, Brookhaven National Laboratory, Upton,
Abstract.
Abstract. The
The DUVFEL
DUVFEL accelerator
accelerator in
in the
the Source
Source Development
Development Lab of NSLS/BNL generates a
high
highbrightness
brightnesselectron
electronbeam
beam from
from aa laser
laser driven electron
electron source and a magnetic bunch compressor.
This
This beam
beam isis used
used for
for different
different PEL
FEL experiments
experiments in
in SASE
SASE and
and future
future HGHG configurations. The
compression
compression of
of the
the electron
electron beam
beam to high peak current while preserving the transverse properties
isis of
of great
great importance
importance to
to the
the performance
performance goals of these FELs. In this paper we report on the
experimental
experimental methods
methods to
to characterize
characterize the
the longitudinal
longitudinal properties
properties of the electron beam and the
measured
measured results
results for
for various
various settings
settings of
of the
the DUVFEL
DUVFEL accelerator.
accelerator. The observed effects on the
electronbeam
beam spectra
spectra and
and time
time profiles
profiles during compression
compression are most likely due to coherent effects
electron
whiletheir
their exact
exact origin
origin is
is still
still subject of ongoing investigation.
while
INTRODUCTION
INTRODUCTION
The Deep
Deep Ultra
Ultra Violet
Violet Free
Free Electron
Electron Laser
Laser Facility
Facility (DUVFEL)
(DUVFEL) [1]
The
[1] in
in the
the Source
Source DeDevelopment Lab
Lab (SDL)
(SDL) at
at NSLS/BNL
NSLS/BNL is
is designed
designed to
to produce
produce fully
velopment
fully coherent
coherent radiation
radiation
in the
the deep
deep ultraviolet
ultraviolet by
by using
using the
the high-gain
high–gain harmonic
harmonic generation
generation process
process [2].
The
in
[2]. The
facility consists
consists of
of aa photoinjector
photoinjector [3]
facility
[3] with
with drive
drive laser,
laser, aa magnetic
magnetic bunch
bunch compressor,
compressor,
and the
the 10
10 m
m long
long NISUS
NISUS undulator.
undulator. The
The layout
layout is
is shown
in Fig.
Fig. 1.
and
shown in
1. The
The facility
facility has
has
50 m
50m
Bend
Bend
Dump
Dump
NISUS 10m
10m
NISUS
undulator
undulator
Time domain
domain
Time
diagnostics
diagnostics
Coherent IR
IR
Coherent
diagnostics
diagi^stics
Bend
Bend
Linac tanks
tanks
Linac
210 MeV
MeV
210
75
MeV
75 MeV
1.6
1.6 cell
cell gun
gun
with
with copper
copper
cathode
cathode \
55 MeV
MeV/
Bunch compressor
compressor
Bunch
Dump
Dump
30
30 mJ,
ml, 100
100 fs
fs
Ti:Sapphire laser
Ti:Sapphire
laser J
FIGURE 1.
1. Schematic
Schematic layout
layout of
of the
the DUVFEL
DUVFEL accelerator
accelerator in
in the
the Source
FIGURE
Source Development
Development Lab.
Lab.
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
849
TABLE 1. Parameters of the DUVFEL Photoinjector.
Laser
Tripled Ti:Sapphire multipass amplifier
Pulse energy
Pulse length, rms
< 400 juJ
0.3 - 3 ps
Gun
BNL Gun IV, Cu cathode
Number of cells
Voltage gradient
1.6
<110
MV/m
Linac
SLAC type
Frequency
Final energy
Bunch charge
Normalized emittance at 200 pC
2856 MHz
200 MeV
< 1 nC
2.5 jum
recently demonstrated SASE gain of more than 3 x 105 at 400 nm [4]. Following the
successful SASE experiments, preparations are underway to perform direct optical seeding at 266 nm, and to add an additional undulator for High Gain Harmonic Generation
(HGHG) at 200 nm.
In addition to its primary use as electron beam source for the EEL program the
accelerator serves as a research tool for high brightness electron beam generation. A
summary of the main accelerator parameters is given in Table 1. The drive laser for the
photocathode is an amplified and frequency tripled 100 fs TiiSapphire laser. The electron
beam is generated in a BNL type IV gun with a copper photocathode. Four SLAC type
acceleration structures provide beam energy for up to 200 MeV. A four dipole chicane
is used as a magnetic bunch compressor to achieve peak currents of several hundred
amperes.
There are both time domain and frequency domain diagnostics to measure the electron
beam longitudinal profile near the end of the linac. Far infrared (FIR) coherent radiation
can be extracted from the accelerator and coupled into a microwave spectrometer.
The time profile of the electron beam may be measured using the linac RF and a
magnetic spectrometer. Of particular interest in the electron beam studies with the
DUVFEL accelerator is the electron bunch compression in the magnet chicane, since this
determines the available peak current for PEL experiments and the longitudinal shape
of the bunch. Coherent radiation emitted by short electron bunches can modulate their
energy spectrum, which may be converted into a time modulation in the magnetic bunch
compressor. The next section introduces the diagnostic methods used for the longitudinal
studies presented thereafter.
EXPERIMENTAL METHODS
Measurement techniques both for the properties of the accelerated electron beam as well
as for the initial conditions when the beam is generated in the photocathode have to be
850
Delay
Delay
100 fsfs IR
IR
lOOfsIR
100
250
250fs
fs blue
blue
250
fs
blue
psUV
UV
55 ps
5psUV
Photo
Photo
Photo
diode
diode
diode
BBO
crystal
FIGURE2.
2. Setup
Setup for
for the
the pulse
pulse shape
shape measurement
measurement of
of the
the
photocathode
UV
laser.
The
FIGURE
measurement
the photocathode
photocathode UV
UV laser.
laser.The
Thedifference
difference
FIGURE
2.
Setup
for
the
pulse
shape
difference
frequencyof
ofthe
the266
266nm
nmUV
UV and
and the
the 800
800 nm
nm IR
is generated
generated
in
BBO
crystal
and
detected
with
frequency
of
the
266
nm
UV
and
frequency
the
800
nm
IR light
light is
generatedin
inaaaBBO
BBOcrystal
crystaland
anddetected
detectedwith
withaa a
photodiodewhile
whilescanning
scanningthe
thedelay.
delay.
photodiode
while
scanning
the
photodiode
employedto
tobe
beable
ableto
to compare
compare experimental
experimental results
results with
with
We
employed
to
be
able
simulations.
describe
the
employed
with simulations.
simulations.We
Wedescribe
describethe
the
instrumentationand
and measurement
measurement results
results in
in the
the
subsections
below,
beginning
instrumentation
and
with
the
instrumentation
the subsections
subsections below,
below,beginning
beginningwith
withthe
the
UVlaser
lasertime
timeprofile
profileused
used to
to produce
produce the
the electron
electron beam.
beam.
UV
laser
time
profile
UV
electron
beam.
UV laser
laser diagnostic
diagnostic
UV
UV
laser
diagnostic
Thebroad
broadbandwidth
bandwidthof
of the
the Ti:Sapphire
Ti:Sapphire seed
seed laser
laser
of
up
to
The
broad
bandwidth
of
the
TiiSapphire
10
nm
supports
structures
The
seed
laser of
of up
up to
to 10
10nm
nmsupports
supportsstructures
structures
in
the
generated
UV
light
which
can
be
as
short
as
a
few
100
fs.
Therefore,
pulse
in
the
generated
UV
light
which
can
be
as
short
as
a
few
100
fs.
Therefore,
in the generated UV light which can be as short as a few 100 fs. Therefore,pulse
pulse
shape
measurements
with
a
ps
streak
camera
are
not
sufficient
to
resolve
these
features.
shape
measurements
with
a
ps
streak
camera
are
not
sufficient
to
resolve
these
features.
shape measurements with a ps streak camera are not sufficient to resolve these features.
Furthermore,the
the frequency
frequency multiplication
multiplication from
from IR
IR
to
UV
in
nonlinear
crystals
Furthermore,
the
frequency
multiplication
seems
Furthermore,
from
IR to
to UV
UV in
in nonlinear
nonlinear crystals
crystalsseems
seems
to introduce
introduce temporal
temporal structure
structure on
on the
the originally
originally smooth
smooth gaussian
gaussian infrared
infrared pulse.
pulse. To
to
to introduce temporal structure on the originally smooth gaussian infrared pulse.To
To
measure the
the temporal
temporal profile
profile of
of the
the UV
UV light,
light, a difference frequency
frequency generation scheme
measure
measure
the temporal
profile of
the UV
light, aa difference
difference frequencygeneration
generationscheme
scheme
is used
used as
as shown
shown in
in Fig.
Fig. 2.
2. A
A small
small fraction
fraction (0.5
nJ) of
of the TiiSapphire
Ti:Sapphire oscillator pulse
(0.5
isis used
as shown
in Fig.
2. A
small fraction
(0.5 nJ)
nJ) of the
the Ti:Sapphire oscillator
oscillatorpulse
pulse
µ
J)
of
the
UV
light
at 266
nm in
train
at
800
nm
is
superimposed
with
a
small
part
(4
train
at
800
nm
is
superimposed
with
a
small
part
(4
jUJ)
of
the
UV
light
at 266
266 nm
nminin
train at 800 nm is superimposed with a small part (4 µ J) of the UV light at
a 100
100 jUm
µ m thick
thick BBO
BBO crystal.
crystal. Cut
for
type II phase
phase matching,
the difference
frequency
at
Cut for
for type
type
matching,
frequency
aa400
100nm
µ misthick
BBO and
crystal.
Cut
I phasephotodiode.
matching, the
the difference
difference
frequencyatat
generated and
further detected
detected with
with aa photodiode.
The temporal
temporal resolution
resolution is
400
nm
is
generated
further
The
400 nm is generated and further detected with a photodiode. The temporal resolutionisis
Signal
(V)
Signal
(V)
2
2
1.5
1.5
1
1
0.5
0.5
0
0
40
40 30
30 20
20 10
10 0
Phase matching
matching0 _io
-10
Phase
2
Phase
0
anglematching
(mrad) -10-20
0
angle
(mrad)
-2
2
-2
-30
-4
-20
0
-4
angle (mrad)
-6
-8
-30-40 -10
Time-2(ps)
-4 Time
(ps)
-6
-40
-8
Time (ps)
-10
FIGURE
3.
UV
temporal
profiles
for
different
phase
matching
angles of
of the
the THG
THG BBO
FIGURE 3. UV temporal profiles for different phase matching angles
BBO crystal.
crystal. The
The
arrowindicates
indicates
thetemporal
flat top
top profile.
profile.
FIGURE
3. UV
profiles for different phase matching angles of the THG BBO crystal. The
arrow
the
flat
arrow indicates the flat top profile.
851
estimated to be 250 fs. The crossing beam geometry and a blue color glass filter provide
a nearly background free data acquisition. By scanning a delay line in the IR beam the
entire UV pulse shape is reconstructed.
The large influence of the phase matching angle of the second BBO crystal in the UV
light generation can be seen in Fig. 3. A flat top shape in the temporal profile - preferred
to avoid space charge effects in the electron gun - can only be achieved within a very
narrow bandwidth of the phase matching angle and it does not appear at the angle with
highest conversion efficiency. We emphasize that the time structure measured by this
method is not visible using a streak camera. It is, however, visible in the electron beam
temporal profile at low charge.
RF zero phasing
The temporal profiles of the electron beam are obtained by the RF zero phasing
method [5]. The longitudinal distribution of the electron beam after the chicane is
converted into a time-energy correlation by accelerating the beam in the linac tank 4
at a RF zero crossing (non-accelerating) phase. The projection on the energy scale
on the viewscreen behind the magnet spectrometer then represents the original time
distribution. The best temporal resolution at maximum accelerating gradient in tank 4
can be calculated to be as small as 8 fs [6]. Due to finite residual energy spread and beta
function in the spectrometer and limited viewscreen size, the achieved resolution is about
50 fs FWHM. Both zero phases of tank 4 are routinely measured to detect remaining
chirp after the chicane and to eliminate this chirp with proper off-crest acceleration in
tank 3.
To distinguish between time structure in the compressed electron beam originating
in laser structure and structure from coherent effects during compression, time profiles
1.53ps
0.85 ps
0.2
< 20
u
10
0
0.56 ps
1.28ps
< 40
¥
§20
U
0
-
-4
2 0 2
Time (ps)
-
2 0 2
Time (ps)
FIGURE 4. Time profiles of (a) UV Laser and of electron beam with (b) 10 pC, (c) 50 pC, and (d)
200 pC bunch charge at 75 MeV energy and no compression. The rms length is given on top of each part.
852
of the laser and the uncompressed electron beam are compared. Temporal profiles of
the UV laser and the electron beam with three different charges are shown in Fig. 4. At
low charge the electron beam shows structure similar to the laser beam. The decrease in
bunch length of the electron beam is due to ballistic compression at low energy in the
gun and the drift before the first linac tank. As the charge increases, space charge force
smooths the time distribution and increases the bunch length.
Previous measurements in the frequency domain with coherent radiation emitted from
the electron beam have already shown results similar to the RF zero phase measurements [7], showing that the structure observed by this type of measurement can unambiguously be attributed to the time distribution of the electron beam.
LONGITUDINAL STUDIES
The longitudinal dynamics during acceleration in the first two linac tanks are well
understood and the initial conditions after tank 1 can be determined accurately by energy
and energy spread measurements with the magnet spectrometer [8]. To compress the
electron beam in the magnet chicane the beam is first chirped by off-crest acceleration
in tank 2. Since gun, tank 1, and tank 2 are powered by the same klystron, only the
phase of tank 2 in respect to gun and tank 1 is changed. A tank 2 phase of up to —28
degrees from the crest has been used in compression studies. The chirped beam is then
compressed in the chicane. The chicane strength R56 can be varied from 0 to 10 cm with
a typical value for compression around 5 cm.
To determine the energy spread after the chicane, both tanks 3 and 4 are turned off.
To measure the time profiles, any remaining chirp after the chicane is removed with an
appropriate tank 3 amplitude at zero phase to obtain minimum energy spread. Tank 4 is
then used to introduce a known time-energy correlation. The measured parameters of
the uncompressed and compressed electron beam and properties of the magnet chicane
are summarized in Table. 2.
TABLE 2. Parameters of the DUVFEL electron beam and
magnetic bunch compressor.
Energy at compression
Charge
Peak current uncompressed
compressed
Pulse length uncompressed
compressed
Local energy spread *
Normalized slice emittance
Chicane bend angle
Dipole length
70
50-300
40-60
200-600
1.2
0.3
210~ 5
2
0-14
19
0-10
5
Beta function at chicane
* estimated
853
MeV
pC
A
A
ps
ps
degrees
cm
cm
m
rms
rms
rms
Ocm
1.5cm
2.7cm
4.3cm
6.3cm
139 keV
120 keV
87.9 keV
68.3 keV
72.4 72.8 73.2
Energy (MeV)
72.4 72.8 73.2
Energy (MeV)
•ill
151 keV
72.2 72.6 73
Energy (MeV)
•ill
72 72.4 72.8 72 72.4 72.8
Energy (MeV)
Energy (MeV)
FIGURE 5. In the upper part electron beam images at the magnet spectrometer with tank 2 phase at
—16 degrees for different chicane R56 values given on top. Corresponding energy spectra in the lower part
with rms energy spread above.
Energy spectra
Extensive measurements with the electron beam at a medium charge of 50 pC have
been conducted to study the energy spectra of the compressed beam. The initial chirp
with tank 2 phase and the chicane strength was varied. With no initial chirp the chicane
has a negligible effect on the energy spread. At small and moderate chicane strengths
from 0 cm up to 3 cm the energy spectra of the chirped beam at different tank 2 phases
also do not depend on the chicane. The energy spectra change, however, considerably at
R56 values of 5 cm and higher where a strong modulation and spikes from the originally
smooth distribution evolve.
A sample of the measurements is shown in Fig. 5 with tank 2 phase at —16 degrees
and the chicane strength varied from zero to 6.3 cm. With increasing strength the rms
energy spread drops from 150 keV at no compression to 70 keV at full compression. This
is more than a factor of two decrease in energy chirp due to bending in the chicane, not
from changing RF phase. Since there is no further acceleration in tank 3 or 4, the energy
spectra can only be modified in or after the chicane by a coherent radiation process such
as coherent synchrotron radiation in the chicane or wakefields from structures in the
beam pipe.
Time profiles
With the same set of accelerator settings as for the energy spectrum measurements,
zero phasing measurements of the temporal distribution where performed. The results
for a chicane strength between 4.1 cm and 5.2 cm with tank 2 phase varied from 0 to
-26 degrees are combined in Fig. 6. The charge is again 50 pC. With increasing initial
chirp from tank 2, not only is the bunch length decreasing, but also are apparently small
854
0.808 ps
0.461 ps
60
40
5 10
0
20
-2
-1
0
1
-2
-1
0.613 ps
0
1
°
. . j. i
-2
g 20
U 10
40
0
Time (ps)
-1
1
-2
-1
0
0
1
= -20
2
0.195ps
160 f
120
80
-1
2
ih
0.354 ps
g 30
0
0.327 ps
au
^ 30
<
"S 20
1
Time (ps)
°
-2
V
4>2 = -26
J
-1
0
1
;
2
Time (ps)
FIGURE 6. RF zero phasing time profiles of the electron beam with no initial chirp and chicane off in
(a) and chicane on in (b). Increasing initial chirp from (c) with tank 2 phase set to -8 degrees to (f) with
-26 degrees. The chicane strength was set between 4.1 cm and 5.2 cm. The RMS bunch length is given on
top of each profile.
modulations in the uncompressed beam amplified to peaks and a strong modulation in
the time profile. Therefore, the peak current is increased by more than a factor of 6, while
the bunch length is only reduced by a factor of 4. This kind of microwave instability
in bunch compression is commonly attributed to an effect of coherent synchrotron
radiation, which can amplify initial modulations in the charge density [9]. However,
recent simulations of the DUVFEL bunch compressor indicate that this effect requires
both a much higher bunch charge and higher compression factor than used in this
experiment [10].
Two pulse study
The effect of structure in the temporal distribution of the UV drive laser on the electron
beam in the chicane has been investigated in a preliminary study. The extreme case
of a laser pulse separated into two parts was chosen by introducing a mask into the
dispersive plane in the laser compressor which blocked the central part of the spectrum
in the chirped pulse. The resulting laser pulse, together with the temporal profile of the
electron beam, is shown in Fig. 7. The accelerator set-up is with the chicane operated
near maximum strength at an R56 of 9.5 cm, but only a small initial upstream chirp,
resulting in almost no compression. The bunch charge in the experiment is 50 pC. The
overall structure of the laser pulse is maintained, but a strong modulation in the time
distribution evolves with about 64 jum period. This modulation could not be observed
when the chicane was turned off. The measured intensity in both parts of the electron
bunch differs due to a transverse sensitivity gradient in the imaging system for the
electron beam. The observed modulation occurs mainly in the head of the bunch and
855
-1.5
-0.5
0
0.5
Time (ps)
1.5
FIGURE 7. Laser time profile with two pulses in the upper part and corresponding electron beam profile
from RF zero phasing measurement below. The leading electrons have lower time coordinates.
not in the tail, giving reason to assume that this is a CSR effect, i.e. that radiation
emitted by the tail of the bunch modulates the head. However, the calculation of the
gain for the CSR instability shown in Fig. 8 indicate that a much higher peak current of
several hundred amperes and strong compression is required to explain the modulation.
Another possible cause is coherent wakefields in a small diameter beampipe upstream
of the measurement. Further experiments and simulations are underway to explore this
physics.
IPeak = 200A
—- 8 = 2 Jim
50
100
150
Wavelength (jim)
200
FIGURE 8. CSR instability gain for the SDL chicane. The wavelength corresponds to an electron beam
current modulation at the chicane entrance. See also [10].
856
SUMMARY
It has been shown that the laser time distribution used to produce a photoinjector electron
beam often has substantial modulations that must be accounted for to reproduce the
observed phenomena in simulations. These modulations modify both the energy and
time profiles of the resulting electron beam, and affect the compressed beam time profile.
Sub-picosecond instrumentation for both the electron and laser beams is necessary to
measure the distributions. The laser modulations do not explain all of the observed
phenomena of the compressed beam, particularly the microbunching near maximum
compression. This microbunching is not yet reproduced by codes that estimate coherent
synchrotron radiation effects. Initial studies of ballistic compression at DUVFEL do not
produce microbunching [11].
The microbunching is a source of very bright THz radiation that may be scientifically
useful. Studies are underway to deliberately modulate the injector drive laser, both to
smooth the profile for best PEL performance, and to induce stronger microbunching that
will enhance the THz output. We will continue to model the compression process, and
the linac impedances downstream of the compressor, with the goal of reproducing the
microbunching in simulation.
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2.
3.
4.
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6.
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13.
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Graves, W. S. et al., "Measured Properties of the DUVFEL High Brightness, Ultrashort Electron
Beam," in [12], p. 2860.
Doyuran, A. et al., "Diagnostics System for the NISUS Wiggler and PEL Observations at the BNL
Source Development Lab," in [13], p. 802.
Wang, D. X., Krafft, G. A., and Sinclair, C. K., Phys. Rev. E, 57, 2283 (1998).
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2860.
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[12], p. 2608.
Shaftan, T. V. et al., "Bunch Compression in the SDL Linac," in [13], p. 834.
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in a photo-injector (2002), to be published.
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857