Beam Generation, Conditioning and Monitoring:
Working Group VII Report
X. J. Wang1 and Kami Kishek2
1
National Synchrotron Light Source
Brookhaven National Laboratory
Upton,NY11973, USA
2
Institute for Research in Electronics and Applied Physics
University of Maryland University of Maryland
College Park, MD 20742, USA
Abstract. The work presented at the working group VII: Beam Generation, Conditioning and
Monitoring, is briefly summarized in this report. The challenges of the working group VII is to
produce and preserve the high-brightness electron beam suitable for future second generation
laser and other advanced accelerators. Topics presented in the working group covered wide
range of subjects; facilities for high-intensity beam physics and X-ray generation, highbrightness femto-second beam generation, coherent transition radiation (CSR) and micro-bunch
instability in the magnetic chicane bunch compressor, beam instrumentation for both transverse
and longitudinal phase space characterization are the major topics discussed in the working
group.
INTRODUCTION
The challenges of the working group VII of the 10th advanced accelerator concepts
workshop (AAC) is to examine the latest development in high-brightness beam
generation, conditioning and monitoring, and discuss the critical issues for future
R&D. We will present a brief summary of the talks presented at the working group
VII in this report. Due to large number of talks presented, it is almost impossible to
cover all subject discussed, we apology for those colleagues whose work we could not
comment on.
The presentation and discussion in the working group VII covered quit wide range
of topics; there are total 26 talks from 10 institutions presented in our working group
sessions. Large number of talk presented in our group came from graduate students
(about one third of the total talks). We also held joint session with three other working
groups on special subjects. With computational accelerator physics group (group I), a
joint session on simulation techniques for particle sources was held. A session on
exotic beam generation, such as positron and proton generation from laser plasma
interaction, muon cooling etc, was jointly sponsored with High Energy Density
Physics & Exotic Acceleration Schemes group (group II). A dedicated session on the
injectors for the second-generation plasma accelerator was held with Laser-Plasma
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
175
Acceleration group (group VI). Here is the brief list of the subjects presented in our
working group:
High intensity beam physics
High-brightness electron beam generation.
Magnetic bunch compressor and CSR.
Surface roughness wake field.
Femto-second bunch length and timing jitter measurement techniques.
Electron beam and laser beam control (shaping).
Beam instrumentation.
Femto-seconds kilo-ampere bunch generation from photoinjector.
HIGH-BRIGHTNESS ELECTRON SOURCES
With the successful demonstration of the all basic laser plasma acceleration
schemes, improvement of the efficiency and quality of laser accelerators now become
more urgent [1]. It was generally recognize that, electron beam with 108 particles,
normalize emittance less than 1 mm-mrad and bunch length less than 50 fs (FWHM)
is required for the so call second-generation laser accelerators [2].
There are many proposals around to produce such electron beam. Inverse FreeElectron Laser (IFEL) is the only technique has experimentally demonstrated that, it
can produce microbunched electron beam for another IFEL laser accelerator [3,4].
Since IFEL only produce a train of microbunched electron beam, it lacks the
flexibility and transverse beam quality for other laser accelerators.
It will be very instructive to look back the history when we explore the new
technologies for producing femto-second, kilo-Ampere high-brightness electron beam
for laser accelerator applications. The introduction of the photocathode RF gun twenty
years ago [5] brought at least two orders of magnitude improvement in electron beam
brightness. This improvement comes from two aspects. Higher field gradient on the
cathode and terminal beam energy is one of them. Field gradient for a traditional
thermionic gun is about 1 to 10 MV/m and beam energy less than 500 KeV. The Sband RF gun has a field gradient of 100 MV/m and beam energy about 5 MeV. The
factor of ten improvements in field gradient and energy is critical in space-charge
effect reduction. And it leads to the second factor in beam brightness improvement,
that is, electron pulse length shortened from ns to ps, and peak current from 10
Ampere to 100 Ampere. We are now facing the similar challenges to produce femtoseconds electron beam for the second-generation laser accelerators, that is, we need
an order of magnitude improvement in both field gradient and energy for the next
generation electron sources for femto-second kilo-Ampere electron beam generation.
Further more, transverse emittance and energy spread must also be small in order to
preserve the femto-second electron beam. For example, for a 5 MeV electron beam
with 10 mrad divergence and 1% energy spread, after 1 meter drift space, the bunch
length increments due to the divergence and energy spread both are about 333 fs.
Pulsed DC gun [6-7] and optical plasma injectors [8-11] offer the possibility of
producing femto-second high-brightness electron beam directly with peak acceleration
176
field on the order of GV/m. Even with relative high field gradient, most pulse DC gun
can only produce a electron beam with energy 10 MeV or less (with RF boost cavity,
10 MeV is the possibility), this makes it extremely difficult to preserve electron beam
bunch length shorter than 100 fs.
The controversies around the optical plasma injector continue exist due to the lack of
experimental data on the beam characterization. Pitthan of SLAC and Fubiani of
LBNL presented extensive analysis and simulation of the optical plasma injectors.
Their analysis showed that, the large energy spread of electron beam produced by the
optical plasma injector is the major cause of bunch lengthening and emittance growth.
M.C. Thompson of UCLA presented simulation studies of possible high-brightness
electron source using plasma density transition trapping [12]. Table I summarized
possible beam parameter can be produced using this technique.
Table I: UCLA plasma density transition trapping electron source.
Peak Density
2xl013
2xl015
2xl017
3
3
cm"
cm"
cm"3
1.5 psec
150fsec
15 fsec
at, Diver
10 nC
InC
lOOpC
Qdrive
2.7psec
270 fsec
28 fsec
at, Captured
Q Captured
1.2nC
120 pC
12 pC
163 Amp
166 Amp
166 Amp
57
Sx,
normalized,Captured mm-mrad
Bn0rmalized,Captured
5 x 1010
5.9
mm-mrad
5 x 1012
0.6
mm-mrad
5 x 1014
IPeak,Captured
XJ. Wang of BNL gave an overview of the status of the photoinjector R&D [13].
The challenge of photoinjector R&D is to continue improving the beam performance
with the reliability and stability required for advanced accelerators and FEL
applications. One of the key capability of the photoinjector is that, it can be not only
optimized for transverse emittance, but also it can be optimized for longitudinal
emittance. The worldwide R&D effort in high-brightness CW photoinjector emerged
[14] after the triumph of energy recovery linac (ERL) at the JLAB [15]. The
challenges for CW photoinjector are brightness of the electron beam and heat load for
both cavity and laser system [14]. Court Bohn of N. Illinois Univ. updated the latest
development of the flat beam experiment at the FERMILAB AO [16]. A transverse
emittance ratio of approximately 50 was measured.
One of the most promising techniques capable of generating the electron beam
satisfy the above requirements is so call longitudinal emittance compensation
technique and off crest acceleration in the energy boost linac followed the
photocathode RF gun [17-18]. A 20 pC electron beam is produced by a 8 ps (FWHM)
frequency quadrupled Nd:YAG laser from a photocathode RF gun injector operating
at the longitudinal emittance compensation mode. The measured electron beam bunch
length at the RF gun exit is about 800 fs(rms), it is further compressed using RF
focusing in the 3 meter long linac down to 10 fs (rms) (fig.l). By adding a focusing
solenoid magnet in the middle of the energy boost linac, the emittance can be kept to
177
0.5
mm-mrad. By
0.5 mm-mrad.
By lunching
lunching the
the electron
electron beam
beam at
at the
the low
low RF
RF gun
gun phase
phase at
at aa field
field of
of 100
100
MV/m, we
we not
MV/m,
not only
only compress
compress the
the electron
electron beam
beam in
in the
the RF
RF gun,
gun, but
but also
also chirp
chirp the
the
electron
beam in
electron beam
in such
such way
way we
we can
can further
further compress
compress the
the beam
beam in
in downstream
downstream drift
drift
space
space and
and linac.
linac. The
The 10
10 fs
fs long
long beam
beam was
was achieved
achieved at
at the
the beam
beam energy
energy about
about 40
40 MeV.
MeV.
P. Musumeci
Musumeci presented
P.
presented sub-ps
sub-ps electron
electron beam
beam experimental
experimental results
results atat the
the UCLA
UCLA
Neptune lab
using the
Neptune
lab using
the similar
similar technique.
technique.
20pc,100Mv/m
,drift=3.05,12.0degree,8ps,R=
20pc,100Mv/m,drift=3.05,12.0degree,8ps,R=
0.75m
,Bf=0.70 sigm
a-z
0.75mm
m,Bf=0.70
sigma-z
1.40E+03
1.20E+03
1.00E+03
8.00E+02
6.00E+02
10. 8 fs
4.00E+02
2.00E+02
0.00E+00
0.00E+ 1.00E+ 2.00E+ 3.00E+ 4.00E+ 5.00E+ 6.00E+ 7.00E+ 8.00E+
O.OOE+ 1.00E+ 2.00E+ 3.00E+ 4.00E+ 5.00E+ 6.00E+ 7.00E+ 8.00E+
00
02
02
02
02
02
02
02
02
20pc,100Mv/m,drift=3.05,12.0degree,8ps,R=0.75mm,Bf=0.70
20pc,100Mv/m,dritt=3.05,12.0degree,8ps,R=0.75mm,Bf=0.70
energy
energy
45
40
35
30
25
20
15
10
5
0
0
100
200
300
400
500
600
700
800
FIGURE 1. Electron bunch length (top) and energy as the function of the distance.
FIGURE 1. Electron bunch length (top) and energy as the function of the distance.
HIGH-INTENSITY
HIGH-INTENSITY BEAM
BEAM PHYSICS
PHYSICS
Collective
Collective effect
effect of
of the
the high-intensity
high-intensity beam
beam plays
plays fundamental
fundamental role
role in
in many
many
accelerator
applications,
from
electron
and
ion
sources,
nuclear
fusion
to
high-energy
accelerator applications, from electron and ion sources, nuclear fusion to high-energy
colliders.
colliders. P.G.
P.G. O'Shea
O'Shea gave
gave an
an overview
overview of
of the
the Univ.
Univ. of
of Maryland
Maryland electron
electron ring
ring
(UMER)
and
its
experimental
program.
Table
1
summarized
the
basic
parameters
(UMER) and its experimental program. Table 1 summarized the basic parameters of
of
UMER. UMER designed to serve as a research platform for intense beam physics.
UMER.
UMER designed to serve as a research platform for intense beam physics.
178
One of the most striking features of the UMER is the wide range of beam physics it
covered, from emittance dominated to space-charge dominated beams. With large
amount of beam diagnostic devices installed, UMER is capable of providing reliable
experimental data to validate both theory and computer simulation.
Table II; UMER parameters.
Energy
PY
Current
Generalized perveance
Emittance, 4x rms, norm
Pulse Length
Bunch charge
Circumference
Lap time
Tune Depression (k/k0)
10keV-50keV
0.2
100mA
1.5x10-3
lOmm-mr
50 -100ns
5nC
11.52m
197ns
XU2
K. Bishofberger of UCLA gave a presentation of his thesis research at FERMILAB
on the Tevatron electron lens, which is critical for reducing the space-charge effect of
the proton and anti-proton beams, and increase the luminosity of the Tevatron. W.
Brown of LLNL discussed a high-brightness electron beam facility now under
commissioning at the LLNL for Thomson X-ray source.
One of the hot topics discussed in our group is the CSR effect of electron beam
during the magnetic bunch compression. Magnetic compressor now is the standard
technique to produce short electron beam for many applications, from eV linear
collider, X-ray FEL to injector for laser accelerators. There are two effects may play
important role when considering the magnetic compressor, one is the non-linearity,
from both the RF and beam transport line, and other is CSR. Recent experimental
results of magnetic bunch compressors from CERN CLIC, DESY TTF, LEUTL OF
ANL, and SDL OF BNL re-discovered of the importance of reduction in RF
nonlinearity. Latest design of LCLS and TESAL FEL bunch compressors incorporated
higher-harmonic RF cavity to minimize the beam break-up. CSR effect is known lead
to emittance growth due to the residue dispersion for many years [19]. Latest theory
and simulation predict electron beam microbunching due to the klystron like
interaction of the CSR effect [20-23]. One of the proposed techniques to suppress the
CSR effect is to artificially introduce energy spread using the undulator.
H. Loos of the Brookhaven DUV-FEL facility (fig.2) and J.B. Rosenzweig of UCLA
presented latest experimental results of beam break-up in both transverse and
longitudinal phase space of electron beam during the bunch compression. UCLA
experimental was performed at the relative low energy (11 MeV), transverse phase
bifurcation was observed due to the space-charge effect.
179
50m
50
50 m
m
NISUS
NISUS
10m
10m
Ben
Ben
Coherent
Coherent
IR
IR
Undulator
Undulator
Bunch
Bunch
compressor
compressor
Ben
Ben
Linac
Linac
75
75
Dum
Dum
Dum
Time
Time
domain
domain
Dum
Dum
1.6 cell
cell gun
gun
1.6
1.6
cell
gun
Iwith
copper
with
with copper
copper
cathode
cathode
cathode
210
210 MeV
MeV
55
30
30 mJ,
mJ, 100
100 fs
fs
Ti:Sapphire
Ti:Sapphire
FIGURE
facility
the BNL.
BNL.
FIGURE2.2. The
TheDUV-FEL
DUV-FEL facility
facility the
Electron
beam
Electron
was
compressed
at
the
about 70
70 MeV
MeV at
at the
the BNL
BNL DUV-FEL
DUV-FEL
Electron beam
beam was
was compressed
compressed at
at the
the about
facility,
transverse
facility,
emittance
growth
and
electron beam
beam microbunching
microbunching (fig.3)
(fig.3) was
was
facility, transverse
transverse emittance
emittance growth
growth and
and electron
observed.
possibilities
observed.
There
are
two
of
observing
microbunching at
at the
the DUV-FEL.
DUV-FEL.
observed. There
There are
are two
two possibilities
possibilities of
of observing
observing microbunching
The
The
first
one
isis that,
that,
the
longitudinal
structures
of the
the photocathode
photocathode RF
RF gun
gun driving
driving
Thefirst
first one
one is
that, the
the longitudinal
longitudinal structures
structures of
laser
laserlead
leadtotothe
themodulation
modulation of
of the
the electron
electron beam;
beam; and
and this
this modulation was amplified
during
the
magnetic
bunch
during
compression.
The
other reason
reason would
would be
be the
the so
so called
called
during the
the magnetic
magnetic bunch
bunch compression.
compression. The
The other
micro-bunch
micro-bunch
instability
due
to
the
CSR
effect.
Experiment now
is underway
underway to
to further
further
micro-bunchinstability
instabilitydue
due to
to the
the CSR
CSR effect.
effect. Experiment
now is
clarify
those
clarifythose
thoseeffects.
effects.
clarify
effects.
100
200
300
400
500
600
FIGURE3.
Microbunchingobservation
observation at
at the
the BNL
BNL DUV-FEL.
DUV-FEL.
FIGURE
FIGURE
3.3. Microbunching
Microbunching
observation
at
the
BNL
DUV-FEL.
Zhou of
of UCLA/BNL
UCLA/BNL presented
presented experimental
experimental results
results of
F.F. Zhou
Zhou
F.
of
UCLA/BNL
presented
experimental
results
of the
the surface
surface roughness
roughness
wake
field
experiment
carryout
at
the
BNL
ATF.
Surface
roughness
wake field
wake
field experiment
experiment carryout
carryout at
at the
the BNL
BNL ATF.
ATF. Surface
Surface roughness
roughness wake
wake field
field could
could
playimportant
important role
role in
in high-frequency
high-frequency (>30
(>30 GHz)
GHz) structure
structure based
based accelerators,
play
important
play
role
in
high-frequency
(>30
GHz)
structure
accelerators, linear
linear
collider and
and X-ray
X-ray FEL
FEL because
because of
of the
the small
small aperture
aperture of
of the
the beam
beam pipe
collider
X-ray
because
collider
and
FEL
of
the
small
aperture
of
pipe and
and short
short
bunch
length.
The
ATF
experiment
observed
for
the
first
time
reduction
of
bunch length.
the
bunch
length. The
The ATF
ATF experiment
experiment observed
observed for
for the
the first
first time
time reduction
reduction of
of the
the
synchronousmode
modefor
foraaarandom
randomdistributed
distributed surface
surface roughness.
roughness.
synchronous
mode
random
synchronous
for
distributed
surface
roughness.
180
BEAM CONTROL AND INSTRUMENTATION
BEAM CONTROL AND INSTRUMENTATION
Beam control and conditioning is another subject covered in our group. Cs. Toth of
Beam
control
andtitled
conditioning
is another
covered of
in our
Tóth of
LBNL
gave
a talk
"Skew and
Chirp: subject
shape-control
highgroup.
power,Cs.ultra-short
LBNLforgave
a talkelectron
titled “Skew
and Chirp:
shape-control
of high the
power,
ultra-short
pulses
optimal
acceleration
in plasmas"
. He discussed
major
issues in
pulses for and
optimal
electron
acceleration
in plasmas”
. He discussed(CPA)
the major
issuesboth
in
amplitude
phase
control
for chirped
pulse amplification
as well
amplitude
phase
control for
pulsethe
amplification
well group
both
passive
andand
active
technique.
By chirped
controlling
laser pulse(CPA)
shape,as LBL
passive and improvement
active technique.
controlling
the laser pulse
groupof
demonstrated
of theBylaser
plasma accelerator.
Prof. shape,
UesakaLBL
of Univ.
demonstrated
improvement
of
the
laser
plasma
accelerator.
Prof.
Uesaka
of
Univ.
of
Tokyo showed that, Sumitomo Heavy Industries (SHI) group demonstrated emittance
Tokyo
showed
that,
Sumitomo
Heavy
Industries
(SHI)
group
demonstrated
emittance
reduction of a factor of two by shaping the longitudinal laser distribution using spatial
reduction
of a factor
of two
liquid
modulator
(SLM)
[22].by shaping the longitudinal laser distribution using spatial
liquid
modulator
(SLM)
[22].presented his design study how to control the electron
R. Joel England of UCLA
R.
Joel
England
of
UCLA
his design
study how
to control
the electron
beam longitudinal distributionpresented
for electron
beam driven
plasma
accelerator.
Use of
beam
longitudinal
distribution
for
electron
beam
driven
plasma
accelerator.
of
sextupoles to reduce nonlinear effects, negative R56 compression to produceUse
sub-ps
sextupoles
to
reduce
nonlinear
effects,
negative
R56
compression
to
produce
sub-ps
ramped beams and improve the transformer ratio > 2. Hui Li of Univ. of Maryland,
ramped beams and improve the transformer ratio > 2. Hui Li of Univ. of Maryland,
discussed
beam control on UMER, the programmable magnets made it possible to
discussed beam control on UMER, the programmable magnets made it possible to
control not only the beam trajectory and size, but also the skew and other unexpected
control not only the beam trajectory and size, but also the skew and other unexpected
errors.
errors.
Both
transverse and longitudinal beam instrumentations were covered in our working
Both transverse and longitudinal beam instrumentations were covered in our working
group
presentations.
study on
on the
the scattering
scatteringeffect
effectofof
group presentations.J.J.Power
Powerpresented
presented his
his simulation
simulation study
the
pepper
port
and
slit
for
transverse
emittance
measurement.
Maryland
group
the pepper port and slit for transverse emittance measurement. Maryland group
presented
both
design
and
initial
experimental
results
of
the
retarding
field
energy
presented both design and initial experimental results of the retarding field energy
analyzer
eV resolution
resolutionwas
wasachieved
achieved
analyzerfor
forspace
spacecharge
chargedominated
dominated low
low energy
energy beam.
beam. 11 eV
forfora a55keV
electron
beam.
keV electron beam.
Femto-second
is aa very
very active
active R&D
R&Dsubject,
subject,
Femto-secondbunch
bunch length
length measurement
measurement technique
technique is
Prof.
out systematic
systematic study
studyon
onthe
theall
allmajor
major
Prof.Uesaka's
Uesaka’sgroup
groupof
of Tokyo
Tokyo Univ.
Univ. has
has carried
carried out
optical
summarized the
the major
major results
resultsofofthe
the
opticaltechniques.
techniques. T.
T.Watanabe
Watanabe of
of Tokyo
Tokyo Univ.
Univ. summarized
their
theirexperiments
experiments(table
(tableIII).
III).
Table
length measurement
measurementtechniques.
techniques.
TableIII:
III:Summary
Summaryof
offemto-second
femto-second bunch length
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UsingRF
RFkicker
kickercavity
cavity for
for GeV
GeV femto-second
femto-second bunch
bunch length
Using
length measurement
measurementhas
hasbeen
been
proposed
for
several
years
[23-25],
An
outstanding
feature
of
the
RF
kicker
technique
proposed for several years [23-25], An outstanding feature of the RF kicker technique
precisionself-calibrating
self-calibrating capability
capability and
and possible
possible slice
isisitsitsprecision
slice emittance
emittance measurement,
measurement,
which
is
critical
for
future
X-ray
FEL.
SLAC
recently
successfully
which is critical for future X-ray FEL. SLAC recently successfully experimentally
experimentally
181
demonstrated 100 fs resolution using a S-band RF kicker. Jack Haimson presented his
design of a 17 GHz circular polarized RF kicker. The circularly polarized deflector has
a major advantage over a linear kicker because the longitudinal charge and momentum
distributions are displayed in orthogonally separated azimuthal and radial directions,
respectively.
ACKNOWLEDGMENTS
We would like to first express out gratitude to the 10th AAC workshop organizers;
it would be chaos without their dedicated support. The patience of the editors of this
proceeding is greatly appreciated. It is the hard work of the participants and presenter
of the working group VII made it possible for this summery. We would to thank
following individual for their nice presentation at the working group VII; ANL: J.
Power, BNL: X.Y. Chang, Henrik Loos, XJ. Wang; Haimson Associate: Jack
Haimson; LBNL: Cs. Toth and Baut Marcelis; LLNL: W. Brown; MIT: Steve Korbly;
N. Illinois Univ.: Court Bohn; SLAC: Rainer Pitthan; UCLA: F. Zhou, James
Rosenzweig, M. Thompson, Joel England, P. Musumeci, K. Bishofberger ; Univ. of
Maryland: Patrick G. O'Shea, Hui Li, Yun Zou, Yupeng Cui and Jonathan Neumann;
Univ. of Tokyo: Mitsuru Uesaka and T. Watanabe. The cooperation from other
working group leaders, particular Drs Thomas Antonsen of University of Maryland,
Thomas Cowan of GA Technologies and Antonio Ting of NRL, is greatly
appreciated.
REFERENCES
1. P. Sprangle, "Laser Driven Plasma Accelerators Injection, Guiding and Staging", invited talk
presented at the 10th Advanced Accelerator Concpts, June 22-26, 2002, Mandalay Beach, CA .
2. T.C. Chiou and T. Katsouleas, Phys. Rev. Lett., 81, 3411 (1998).
3. Y. Liu, X. J. Wang, D. B. Cline, M. Babzien, J. M. Fang, J. Gallardo, K. Kusche, I. Pogorelsky, J.
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