Inverse Free Electron Laser Experiment at the
Neptune Laboratory
P.Musumecia, C.Pellegrinia, J.B.Rosenzweiga,
A. Varfolomeevb, S.Tolmachevb, T. Yarovoib
Department of Physics, University of California at Los Angeles, 405 HilgardAve, Los Angeles, CA
90095, USA
b
Coherent Radiation Laboratory, RRC"Kurchatov Institute", Moscow 123182, Russia
Abstract. We present an Inverse Free Electron Laser accelerator proposed for construction at the
UCLA Neptune Laboratory. This experiment will use a 1 TW CO2 laser to accelerate through
two strongly tapered undulators an electron beam from 16 MeV up to 55 MeV. The scheme
proposed is the diffraction dominated IFEL interaction. The Raleigh range of the laser beam is
about 2 cm, much shorter than the interaction length (the undulator length is 50 cm). In this
regime adiabatic capture is possible in the first part of the undulator. In the focus region, we
propose a solution to the problem of the dephasing between electrons and photons due to the
Guoy phase shift. Ponderomotive effects and implications for tolerances are also studied
INTRODUCTION
Inverse Free Electron Laser schemes to accelerate particles have been proposed as
advanced accelerators since many years [1-3]. Particular interest in this acceleration
scheme comes from the high gradients in theory available and from the efficient
microbunching of the accelerated electrons. Proof-of-principle IFEL experiments [45], including the staging of two different IFEL sections [6] have been carried out
successfully. Modest energy gain has been achieved mostly because of the limitations
in the radiation power available. A lot of interest is currently in the control of the
longitudinal phase space beam structure: femtosecond microbunches and few percent
energy spread for the particles trapped in the accelerating bucket are theoretically
possible. The purpose of the UCLA experiment is to achieve a substantial energy gain
and investigate the longitudinal structure of the electron beam.
At the Neptune Laboratory at UCLA [7] there is the unique opportunity of having
a high power laser and a relativistic high brightness electron beam in the same
experimental facility. In the proposed scheme, the electron beam is coming from a
photoinjector-booster linac system with energy of 14.0 MeV, and the high power CC>2
laser is focused by a lens (f/18) with focal distance of 2.6 m to tight spot of few
hundreds microns. Because the Raleigh range is much shorter than the undulator
length, the interaction is diffraction dominated [8].
The other fundamental ingredient for the Inverse Free Electron Laser is the
undulator that provides the coupling between photons and electrons. Strong tapering
of both period and magnetic field amplitude is needed for high-gradient acceleration.
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
278
There are strict tolerances requirements on the magnetic field profile and on the
alignment precision in order to maintain phase synchronicity and preserve the
accelerating bucket along the accelerator, especially around the high laser field focal
region. The undulator magnet will be built in collaboration with Kurchatov Institute.
IFEL CONFIGURATION
CO2 laser system
At the Neptune Laboratory a two wavelength 1 TW CO2 laser is currently used to
drive a plasma beat-wave experiment. A standard master oscillator-power amplifier
approach is used to amplify 100 ps pulses up to 100 J as presented elsewhere [9]. The
optics configuration for the IFEL accelerator is designed to provide 400 GW of 10.6
jim CC>2 laser focused to a waist of 350 jim. Radiation diffraction effects are a critical
point in this configuration. The model adequate to describe the CC>2 beam, is to apply
the gaussian beam approximation. The electromagnetic fields driving the interaction
are not constant along the undulator like in the case of plane waves but increase to a
peak at the focus and then decrease back down. Special care in the undulator magnetic
field profile design has to be taken to preserve the synchronicity with the electrons
considering the 180 degrees Guoy phase shift that takes place in a small region around
the focus. Even when this problem is solved with careful undulator magnetic field
shaping, other effects related to the small laser beam size appear. The first study on the
IFEL [10] showed that spatial jitter intrinsic in the optical f/18 configuration together
with ponderomotive scattering effects could seriously degrade the electron
acceleration. The strong laser electric fields at the focus are predicted to kick
transversely the electrons. In order to increase the stability of matching the electron
trajectories and the synchronous phase with the photons in this critical region the laser
beam size was increased to 350 jim. The electromagnetic fields at the focus are
smaller and the transverse kick problem is less critical. Peak accelerating gradient is
lower, but the high field region (-Raleigh range) is longer and substantial energy gain
can still be achieved.
TABLE 1. Laser parameters.
Laser Power
400 GW
Laser wavelength
10.6 urn
Laser waist size(w0)
350 urn
___________Raleigh range________________________3.6 cm_____________
Undulator
In collaboration with Kurchatov Institute we are building a tapered undulator to
provide the necessary synchronism between electrons and photons [11]. The design is
a hybrid permanent magnet planar undulator tapered in both period and magnetic field
amplitude. Ramping up the magnetic field amplitude only is not practical because
279
material
reached. At
At
material and
and technology
technology limits
limits in
in the
the achievable
achievable magnetic
magnetic flux
flux are
are quickly
quickly reached.
the
too long
long for
for the
the space
space
the same
same time
time period
period tapering
tapering only
only makes
makes soon
soon the
the undulator
undulator too
available
available in
in the
the experimental
experimental area.
area.
TABLE
TABLE 2.
2. Undulator
Undulator parameters.
parameters.
Initial
Initial
1.5
1.5 cm
cm
0.12
0.12 T
T
0.2
0.2
12
mm
12mm
Undulator
Undulator period
period
Magnetic
Magnetic field
field amplitude
amplitude
KK parameter
parameter
Gap
Gap
Final
Final
5.0
cm
5.0cm
0.6
0.6 T
T
2.8
2.8
12
mm
12mm
Magnetic field in the undulator
8
6
4
B(kGauss)
2
0
-2
-4
-6
-8
0.0
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
0.5
Distance
Distance along
along the
the undulator
undulator (m)
(m)
FIGURE
FIGURE1.
1. Magnetic
Magnetic field
field profile
profile simulated
simulated with
with RADIA.
RADIA.
Beam Centroid Trajectory
Laser Beam size
0.002
transverse position (m)
0.001
0.000
-0.001
-0.002
-0.003
-0.004
0.0
0.1
0.2
0.3
0.4
0.5
Distance
Distance along
along the undulator
undulator (m)
FIGURE
FIGURE2.
2. Particle
Particle trajectory
trajectory in
in the
the undulator
undulator and
and laser beam size.
Magnetic fields
fields are
are specially
specially designed around the focus region to preserve
Magnetic
synchronism with
with the
the laser
laser light.
light. Undulator parameters are given in Table 2. The
synchronism
undulator field
field simulations
simulations are
are done
done with the 3-dimensional magnetostatic code
undulator
RADIA [12].
[12].
RADIA
280
Y
Figure 3: Model used in RADIA simulation of Kurchatov IFEL undulator. The magnetic field is
tapered in period and amplitude. Note the magnetic corrector in the middle to maintain the resonance
condition in the critical focal region.
Electron beam
To minimize the effects of ponderomotive scattering and more generally to sample
a uniform region in the radiation fields, the electron beam should be focused to a spot
size smaller than the laser beam size. This is possible because the transverse
geometrical emittance of the electron beam is smaller than the one of the laser beam.
The focusing properties of the undulator are negligible because the undulator-induced
(3-function is long. With a quadrupole triplet upstream of the interaction region, we
can focus the beam down to a spot of 150 jim. It is important that the initial electron
beam energy is bigger than 14.0 MeV to satisfy the synchronicity condition at the
beginning of the undulator and optimize the trapping.
TABLE 3. Initial electron beam parameters
Energy
Energy spread (rms)
Charge
Pulse length (rms)
Rms transverse emittance
Beam size at the focus
14.0 MeV
0.5%
300 pC
3ps
10 Jim
150 urn
BEAM DYNAMICS
Analysis of the IFEL performance is performed with the code TREDI [13]. The full
Lorentz equations are solved. It should be noted that the strong period tapering and
violent acceleration bring the approximations used to write the PEL equations
(averaging over one undulator period) for the 1-dimensional IFEL theory to the limit
of validity. For this reason a full Lorentz equation solver like TREDI was chosen to
simulate the experiment.
281
synchronization
synchronization curve
curve
synchronization curve
0.05
0.05
0.025
0.025
0
0
0.025
0.025
0.05
' 00
0.05
0
0.05
0.05 0.1
0.1 0.15
0.15 0.2
0.2 0.25
0.25 0.3
0.3 0.35
0.35 0.4
0.4 0.45
0.45 0.5
0.5
0.05 0.1 Distance
0.15 0.2 0.25
0.3 0.35 0.4 0.45 0.5
Distancealong
alongthe
theundulator
undulator (m)
(m)
ElectricDistance
field along the undulator (m)
™™"""" Electric field
Electric field
"""" • transverse
transversevelocity
velocity
transverse velocity
FIGURE
4: Resonance
curve
FIGURE
curve for
for electrons
electrons
FIGURE 4:
4: Resonance
Resonance curve
for
electrons
The
particles are
pushed through
the analytical
of
gaussian
The
analytical electromagnetic
electromagnetic field
field of
of aaagaussian
gaussian
The particles
particles are
are pushed
pushed through
through the
the analytical
electromagnetic
field
mode
representing
the
laser
beam
[14],
and
through
the
3-dimensional
magnetic
field
mode
representing
the
laser
beam
[14],
and
through
the
3-dimensional
magnetic
field
mode representing the laser beam [14], and through the 3-dimensional magnetic field
map
from
the
code
RADIA.
Fig.4
shows
the
synchronization
curve
for
the
accelerated
map
synchronization curve
curve for
for the
the accelerated
accelerated
mapfrom
from the
the code
code RADIA.
RADIA. Fig.4
Fig.4 shows
shows the
the synchronization
electrons.
Different
tapering
can
be
used
other
of
the
electrons.
used to
to improve
improve one
one or
or the
the other
other of
of the
the
electrons. Different
Different tapering
tapering can
can be
be used
to
improve
one
or
the
characteristics
of
the
accelerating
bucket.
Energy
spread
and
micro
bunch
width
could
characteristics
spread and
and micro
micro bunch
bunch width
widthcould
could
characteristics of
of the
the accelerating
accelerating bucket.
bucket. Energy
Energy spread
be
tapering
was
be
adjusted
depending
on
the
final
application.
In our
our case
case the
the choice
choice for
for tapering
tapering was
was
beadjusted
adjusteddepending
depending on
on the
the final
final application.
application. In
In
our
case
the
choice
for
to
the
trapped
particle
fraction.
The
characteristics
of
the
output
beam
from
totooptimize
optimize
the
trapped
particle
fraction.
The
characteristics
of
the
output
beam
from
optimize the trapped particle fraction. The characteristics of the output beam from
the
the
accelerator
are
reported
in Table
Table 4.
4.
theaccelerator
acceleratorare
arereported
reported in
Longitudinal
phase space
Longitudinal
Longitudinal phase
phase space
space
gamma
gamma
150
150
150
100
100
V *
50
50
50
00
00
22
phase
phase
44
4
66
FIGURE
FIGURE5:
5:longitudinal
longitudinal phase
phase space
space at
at
the exit
exit
of
the
undulator
FIGURE
5:
atthe
exit of
ofthe
theundulator
undulator
TABLE
TABLE4.
Outputelectron
electronbeam
beamparameters
parameters
TABLE
4.4. Output
Output
electron
beam
parameters
Energy
Energy
Energy
Energy
Energyspread
spread
Energy
spread
Microbunch
Microbunchlength
length(rms)
(rms)
Microbunch
length
(rms)
Peak
Current
Peak
Current
Peak Current
Rms
Rmstransverse
transverseemittance
emittance
Rms
transverse
emittance
Trapped
particles
Trapped
particles
Trapped particles__________________
282
55
MeV
55
MeV
55MeV
2.5
%
2.5
%
2.5%
33 fs
fs
3fs
33 kA
kA
3kA
10
µm
10
10µm
Jim
25 %
25%
DIAGNOSTICS
a.u.
In
In order
order to
to align
align the
the electrons
electrons and
and the photons we plan to use screens at the entrance
and
and exit
exit of
of the
the undulator
undulator and
and aa pop-in
pop-in screen
screen in
in the
the center.
center. A
A fluorescent
fluorescent phosphorous
screen
screen with
with some
some graphite
graphite on
on itit so
so that
that intense
intense CO
CO22 can
can produce aa spark,
spark, will be used
for
for detection
detection of
of the
the electrons
electrons and
and the CO
CC>22 and to align with the necessary accuracy
both
both beams
beams right
right in
in the
the most
most critical
critical point, the focus. The pop-in actuator in the center
of
of the
the undulator
undulator will
will have
have aa position
position in
in which
which aa small
small sample
sample of
of Germanium
Germanium is
is
inserted
inserted in
in the
the beam
beam path.
path. Scanning
Scanning with a delay line the time of arrival of the
electrons
electrons with
with respect
respect to
to the
the CO
CC>22 pulse
pulse we
we can
can synchronize
synchronize the two pulses looking
how
how the
the transmission of the
the CO
CC>22 through the Germanium sample changes depending
on
on the
the relative
relative time
time of
of arrival
arrival of
of the
the electrons
electrons and
and the
the photons. This effect is the
electron-beam-controlled
electron-beam-controlled transmission
transmission of
of 10
10 µm
jim radiation
radiation in
in semiconductors
semiconductors [15]
[15] and
aa cross
cross correlation
correlation timing
timing technique
technique based
based on
on it,
it, is
is being
being successfully
successfully applied
applied at
at the
the
Neptune
Neptune lab
lab in
in the
the context
context of the Plasma Beat Wave accelerator.
The
The output
output beam
beam is
is microbunched
microbunched with
with 10
10 µm
jim period
period so
so that
that any
any radiation
generated
generated by
by the
the beam has a spectrum peaked at this wavelength. On the other hand it
will
will be
be very
very hard
hard to
to distinguish
distinguish between beam generated radiation and the driving high
power
power CO
CC>22 laser
laser beam.
beam. Moreover
Moreover aa transition radiation screen
screen cannot be inserted in
the
the beam
beam line
line too
too close
close to the exit of the undulator because it would be damaged by
the
the high
high power
power driving
driving laser.
laser.
.10 55-10
32
00
11
22
Frequency
Frequency (units
(units of
of resonant
resonant frequency)
frequency)
33
4
—— Resonant
Resonant particle
particle spectrum
spectrum
FIGURE
FIGURE 6:
6: Calculated
Calculated single
single particle
particle radiation
radiation spectrum
spectrum from
from Kurchatov
Kurchatov tapered IFEL undulator.
The
The proposed
proposed solution
solution to
to detect
detect the microbunching is to look at coherent undulator
radiation.
radiation. This
This diagnostics
diagnostics looks
looks at
at the
the beam structure
structure inside the undulator so that
debunching
debunching is
is not
not an
an issue.
issue. The
The expected spectrum
spectrum from resonant electrons is given in
Figure
Figure 6.
6. IfIf we
we integrate
integrate in
in aa reasonable
reasonable collection
collection angle
angle we calculate ~10 nJ of 3.3
jim radiation
radiation on
on aa detector
detector 33 m
m far
far away
away from the exit of the undulator. The signal
signal for
µm
beam microbunched
microbunched on
on the
the scale
scale of
of the radiation wavelength is coherent and
aa beam
proportional to
to the
the square
square of
of the number of electrons.
proportional
Finally to
to get
get the
the electron
electron energy
energy spectrum,
spectrum, aa standard
standard imaging
imaging spectrometer
spectrometer will
Finally
be used.
used.
be
283
STUDY OF ACCEPTANCES AND TOLERANCES
In the study of the acceptances and tolerances of the accelerator, it was observed that
changing the initial parameters does not affect the maximum final energy reached by
the electrons. Because of the stability of the accelerating region in the phase space, for
small variation of the parameters around the design, some particles will still be trapped
and accelerated to the final energy. The most sensitive parameter is in this case the
number of accelerated electrons. The tolerances refer to cases in which the fraction of
captured particle is bigger than 20%. Table 5 shows the results of this analysis.
TABLE 5. Tolerances of IFEL accelerator.
Energy
14.0 +14.5 MeV
Laser Power
350 n-500 GW
Laser displacement
-100 + +100 \\m
Laser angle misalignment__________________________-1 ++1 mrad___________
CONCLUSIONS
The goal of the IFEL project at the UCLA Neptune Laboratory is to achieve a
substantial energy gain (-50 MeV), and to study the longitudinal phase space
characteristics of the output beam. In this sense, it can be considered one of the first
"second generation" advanced accelerator experiment where the main question is not
if the scheme will work in principle, but what can the accelerator deliver in terms of
beam quality and numbers of electrons per bunch. The other important points of the
experiment common to other advanced accelerator experiments are to address the
issue of increasing the interaction length for an high gradient accelerator dealing with
the limitations of laser diffraction and to increase the final energy gain by tapering of
the structure to maintain phase synchronism with the accelerating particles.
ACKNOWLEDGEMENTS
The authors wish to thank S. Totchisky and C. Clayton for useful discussion. This
work is supported by U.S. Dept. of Energy grant DE-FG03-92ER40693
REFERENCES
1. Palmer R., JAppl.Phys. 43, 3014, 1972.
2. Courant E., Pellegrini C., Zakowicz W., Phys,Rev A 32,2813,1985
3. Fisher A., Gallardo J. , Sandweiss J., Van Steenbergen A., Proc.Adv.AcceLConcepts, Port Jefferson,
NY, AIP 279, p.299, 1993
4. Wernick I. and Marshall T.C., Phys Rev, A 46, 3566 (1992)
5. Van Steenbergen A., Gallardo J. , Sandweiss J. , Fang J. , Babzien M., Qiu X., Skaritka J., Wang
X.J., Phys Rev Let, 77, 2690 (1996)
6. Kimura W., in Proc, Adv, Accel Concepts, Santa Fe NM, 2001
7. Clayton C.E., Joshi C., Marsh K., Pellegrini C., Rosenzweig J.B., PAC 97 Proc., 678, 1997
284
8. Musumeci P., Pellegrini C., in Proc. Adv. Accel. Concepts, Santa Fe, NM, 2001
9. Tochitsky S. Ya., et al. Optics Letters, 24, 1717 (1999).
10. Varfolomeev A.A., Tolmachev S.V., Yarovoi T.V., RRCKI, Int. report CRL 02-01, 2001
11. Varfolomeev A.A., Tolmachev S.V., Yarovoi T.V, RRCKI, Int. report CRL 03-01, 2001
12.Elleaume P., et al. PAC 97 Proc., 3509 (1997)
13.Giannessi L. et al., Nucl. Instr. Meth., 393, 434 (1997)
14. Siegman A., "Lasers", University Science Books (1986)
15. Corkum P., et al. Journal of Applied Physics, 50, 3079 (1979)
285