Wakefield Acceleration in Structures
Manoel E. Conde
Argonne National Laboratory
High Energy Physics Division
9700 S, CassAve. Bldg. 362
Argonne, IL 60439
Abstract. Wakefield acceleration in dielectric loaded structures is discussed in this paper. We
present a description of the dielectric wake field accelerator concept, comparing some features of
the collinear and the two beam accelerator configurations. The Argonne Wakefield Accelerator
Facility (AWA) is discussed in detail, including major upgrades that are presently taking place.
The basic features and capabilities of the facility are presented, and the dielectric wakefield
acceleration results are briefly summarized. Possible variants of the two beam accelerator
configuration are discussed, and work on planar dielectric structures in various institutions is
presented. We conclude this report mentioning prospective achievements of dielectric wakefield
accelerating structures.
INTRODUCTION
The High Energy Physics accelerator community has no clear path to design linear
colliders with energies higher than a few TeV. Therefore, new technology needs to be
developed in order to make these higher energies accessible. The new technology
needs to address the following three issues: (a) the energy source from which energy
will be transferred to the beam; (b) the structure or medium that will support high
accelerating gradients; (c) a means to transport and distribute the energy to the beam.
We believe that electron beam driven accelerating schemes can make important
contributions to these three issues.
Electron beam driven accelerators can be based on structures or plasmas. The latter
is discussed in another paper in these proceedings [1]. This paper will report on the
status of the development of dielectric based wakefield accelerators. Wakefield
accelerators based on metallic structures (e.g. CLIC project at CERN [2]), and the use
of active media for energy storage [3] will not be discussed here.
DIELECTRIC WAKEFIELD ACCELERATOR CONCEPT
Dielectric wakefield accelerators are dielectric loaded waveguides that are excited
by the passage of electron beams. The wakefields generated by the "drive" bunches
correspond to the Cherenkov radiation emitted by the electrons as they traverse the
structures with a speed higher than the radiation group velocity supported by the
structures. These devices are typically constructed as hollow cylindrical dielectric
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
63
tubes
tubes inserted
inserted into
into cylindrical
cylindrical metallic
metallic waveguides,
waveguides, or
or as
as dielectric
dielectric slabs
slabs attached
attached to
to
the
is
the walls
walls of
of rectangular
rectangular metallic
metallic waveguides.
waveguides. The
The simplicity
simplicity of
of these
these devices
devices is
expected
expected to
to make
make them
them relatively
relatively inexpensive.
inexpensive. The
The absence
absence of
of irises
irises should
should allow
allow
operation
the
operation atat high
high accelerating
accelerating gradients
gradients without
without electrical
electrical breakdown
breakdown problems
problems in
in the
structures.
structures.These
These dielectric
dielectric structures
structures compare
compare favorably
favorably with
with conventional
conventional iris
iris loaded
loaded
structures
structuresininterms
termsof
ofdamping
damping of
of higher
higher order
order modes
modes [4].
[4].
Figure
Figure 11 shows
shows the
the longitudinal
longitudinal cross
cross section
section of
of aa cylindrical
cylindrical dielectric
dielectric loaded
loaded
passage of
an
accelerator.
accelerator. The
The intensity
intensity of
of the
the accelerating
accelerating field
field W
Wzz generated
generated by
by the
the passage
of an
electron
bunch
of
charge
Q
and
bunch
length
σ
is
given
approximately
by
electron bunch of charge Q and bunch length <JzZ is given approximately by
é æ π σ ö2 ù
z
WZ ≈ 3 exp
expê − 2 ç
ç λ ÷÷ ú
2
/2
ê
n
è
ø úû
a
ë
where
the n-th
n-th
where aa isisthe
the inner
inner radius
radius of
of the
the dielectric
dielectric tube,
tube, and
and λkn is
is the
the wavelength
wavelength of
of the
mode
high
modesupported
supportedby
by the
the structure.
structure. The
The expression
expression shows
shows that
that itit is
is desirable
desirable to
to have
have high
charge
charge bunches
bunches traversing
traversing small
small diameter
diameter structures.
structures. An
An example
example of
of aa dielectric
dielectric
structure,
structure,including
includingthe
the beam
beam parameters
parameters and
and resulting
resulting wakefield
wakefield is
is given
given in
in Table
Table 1.
1.
Q
2b
2b 2a
ε
Q
Cu
FIGURE
FIGURE1.1. Longitudinal
Longitudinalcross
crosssection
section of
of aa cylindrical
cylindrical dielectric
dielectric loaded accelerator.
TABLE1.1. Example
Exampleof
ofcylindrical
cylindricaldielectric
dielectric loaded
loaded structure.
structure.
TABLE
Innerdiameter
diameterof
ofdielectric
dielectric(2a)
(2a)
6 mm
Inner
Outerdiameter
diameterof
ofdielectric
dielectric(2b)
(2b)
10 mm
Outer
10
Dielectricconstant
constant((s)
4.6
Dielectric
ε)
Chargeofofelectron
electronbunch
bunch(Q)
(0
100 nC
nC
Charge
100
Bunchlength
length((<r
1 mm
mm
Bunch
σz)z)
RFpower
powergenerated
generated(P)
(P)
400 MW
MW
RF
400
RFfrequency
frequency (f(f))
19 GHz
RF
19
Acceleratinggradient
gradient(W
(W
MV/m___________
Accelerating
z) z)_________________________9292 MV/m
bunchtrain
train of
of four
four 100
100nC
nCbunches
bunches or
or eight
eight 50
50 nC
nC bunches
bunches traversing
traversing the device
AAbunch
describedininTable
Table11would
would generate
generate an
an accelerating
accelerating gradient
gradient of over 300 MV/m.
described
Dielectric wakefield
wakefield accelerators
accelerators can
can be
be operated
operated in
in the collinear
collinear configuration,
Dielectric
where the
the same
same structure
structure isis used
used to
to decelerate
decelerate the
the drive
drive bunches
bunches and
and to accelerate the
where
probe
bunches.
That
is,
an
electron
bunch
traverses
the
dielectric
loaded structure,
probe bunches. That is, an electron bunch traverses
emitting
radiation
and
establishing
the
wakefield
in
the
structure;
this
drive bunch is
emitting radiation and establishing the wakefield in
followed
by
another
electron
bunch
(probe
or
witness
bunch)
which
is
accelerated by
followed by another electron bunch (probe or witness
the
wakefield
present
in
the
structure.
the wakefield present in the structure.
64
Alternatively,
accelerators can
can be
be operated
operated inin the
the two
twobeam
beam
Alternatively, dielectric
dielectric wakefield
wakefield accelerators
accelerator
where two
two dielectric
dielectric loaded
loadedstructures
structuresare
areused
used(Fig.
(Fig.
accelerator configuration
configuration (TBA),
(TEA), where
2).
the drive
drive beam
beam isis coupled
coupled out
out of
of the
the decelerating
decelerating
2). The
The RF
RF power
power generated
generated by
by the
structure
loaded structure,
structure, where
where the
the witness
witnessbeam
beamisis
structure and
and into
into aa second
second dielectric
dielectric loaded
accelerated.
accelerated.
Schematic Diagram of Argonne Wakefield Step-Up Transformer
High Amplitude Wave (Compressed)
\ Low Amplitude Wave (uncompressed)
Low Diel. Const. Material
High Diel. Const. Material
I—>
Intense Drive Beam
Accelerated Beam
FIGURE 2.
2. Schematic
Schematic of
of aa dielectric
dielectric wakefield
FIGURE
wakefield two
two beam
beam accelerator,
accelerator, also
also known
known asas step-up
step-up
transformer.
transformer.
The two
two beam
beam accelerator
accelerator scheme
scheme offers
The
offers several
several advantages
advantages over
over the
the collinear
collinear
mode of
of operation.
operation. In
In the
the collinear
collinear configuration,
mode
configuration, the
the so-called
so-called transformer
transformerratio
ratio
cannot have
have aa value
value greater
greater than
cannot
than two
two ifif the
the longitudinal
longitudinal distribution
distribution ofof charge
chargeisis
symmetric with
with respect
respect to
to the
the center
symmetric
center of
of the
the bunch;
bunch; that
that is,
is, for
forsymmetric
symmetricbunches
bunchesthe
the
rate
at
which
the
witness
beam
gains
energy
cannot
exceed
twice
the
rate
at
rate at which the witness beam gains energy cannot exceed twice the rate atwhich
whichthe
the
drive beam
beam loses
loses energy.
energy. This
This limitation
limitation is
drive
is easily
easily avoided
avoided in
inthe
thetwo
twobeam
beamaccelerator
accelerator
scheme,
where
the
RF
pulse
can
be
compressed
in
the
second
structure,
scheme, where the RF pulse can be compressed in the second structure,both
bothininthe
the
transverse plane (essentially by having the second structure with smaller dimensions)
transverse plane (essentially by having the second structure with smaller dimensions)
and in the longitudinal dimension (by using a higher dielectric constant material in the
and in the longitudinal dimension (by using a higher dielectric constant material in the
second structure, with the corresponding lower group velocity for the radiation). The
second structure, with the corresponding lower group velocity for the radiation). The
possibility of RF pulse compression, with the consequent higher accelerating gradient
possibility of RF pulse compression, with the consequent higher accelerating gradient
in the second structure, is the reason why this configuration is also known as "step-up
in the second structure, is the reason why this configuration is also known as “step-up
transformer".
transformer”.
Both configurations can make use of multiple drive bunches, in the form of a bunch
Bothallowing
configurations
make
usethe
of multiple
drivethe
bunches,
in the
formway,
of a bunch
train,
for the can
build
up of
fields inside
structure.
In this
both
train,
allowing
for
the
build
up
of
the
fields
inside
the
structure.
In
this
way,
both
schemes can achieve higher accelerating gradients; the collinear case, however,
is still
schemes
can
achieve
higher
accelerating
gradients;
the
collinear
case,
however,
is
still
subject to the transformer ratio limit of two.
subject
to
the
transformer
ratio
limit
of
two.
Both schemes can make use of multiple stages of acceleration (and deceleration) to
Bothhigher
schemes
make use
multiple
stages
of acceleration
(and(ordeceleration)
reach
finalcan
energies.
Thisofwould
require
several
drive bunches
several driveto
reach
higher
final
energies.
This
would
require
several
drive
bunches
(or
several
drive
bunch trains) to traverse several structures. While this is possible in the
collinear
bunch trains) to traverse several structures. While this is possible in the collinear
65
configuration, it
it is
is certainly
certainly simpler
simpler in
in the
the two
two beam
beam accelerator
accelerator case,
case, because
because of
of the
the
configuration,
greater flexibility
flexibility in
in terms
terms of
of the
the beam
beam optics.
optics.
greater
ARGONNE WAKEFIELD
WAKEFIELD ACCELERATOR
ACCELERATOR FACILITY
FACILITY (AWA)
(AWA)
ARGONNE
The Argonne
Argonne Wakefield
Wakefield Accelerator
Accelerator Facility
Facility (AWA)
(AWA) was
was designed
designed to
to study
study
The
wakefield acceleration.
acceleration. The
The facility
facility uses
uses aa high
high charge
charge drive
drive beam
beam to
to excite
excite
wakefield
wakefields, and
and aa low
low charge
charge witness
witness beam
beam to
to probe
probe the
the wakefields.
wakefields. These
These two
two beams
beams
wakefields,
can be
be made
made to
to propagate
propagate through
through the
the same
same structure,
structure, to
to study
study collinear
collinear acceleration,
acceleration,
can
or can
can travel
travel to
to two
two separate
separate structures
structures (Fig.
(Fig. 3).
3).
or
FIGURE 3.
3. Schematic
of the
the Argonne
Argonne Wakefield
Wakefield Accelerator
Accelerator beamlines.
beamlines.
FIGURE
Schematic of
The drive
drive beam
beam is
is generated
generated by
by aa half-cell
half-cell photocathode
photocathode RF
RF gun
gun running
running at
at 1.3
The
1.3
GHz.
The
bunch
charge
can
be
varied
from
10
to
100
nC,
with
a
bunch
length
of
to
GHz. The bunch charge can be varied from 10 to 100 nC, with a bunch length of 15
15 to
35 ps
ps FWHM.
FWHM. The
The drive
drive gun
gun is
is followed
followed by
by two
two linac
linac tanks
tanks (1.3
(1.3 GHz)
GHz) that
that bring
bring the
the
35
beam energy
energy up
up from
from 22 MeV
MeV to
to 15
MeV.
beam
15 MeV.
The 44 MeV
MeV witness
witness beam
beam is
is generated
generated by
by aa 66 1½
cell photocathode
photocathode RF
RF gun.
gun. The
The
The
A cell
bunch charge
charge is
is typically
typically 0.1
0.1 nC
nC with
with aa bunch
bunch length
length of
of 88 ps
ps FWHM.
FWHM. Both
Both guns,
guns, as
as
bunch
well as
as the
the linac
linac tanks,
tanks, are
are powered
powered by
by aa single
single 30
30 MW
MW L-band
L-band klystron.
klystron.
well
The laser
laser system
system consists
consists of
of aa dye
dye oscillator
oscillator (496
(496 nm)
nm) followed
followed by
by aa dye
dye amplifier
amplifier
The
and
an
excimer
amplifier;
a
doubling
crystal
is
followed
by
a
second
excimer
amplifier
and an excimer amplifier; a doubling crystal is followed by a second excimer amplifier
(248 nm),
nm), bringing
bringing the
the final
final pulse
pulse energy
energy to
to 88 mJ,
mJ, with
with aa pulse
pulse duration
duration of
of 66 to
to 88 ps
ps
(248
FWHM.
FWHM.
Over the
the past
past few
few years,
years, several
several experiments
experiments have
have been
been successfully
successfully carried
carried out
out at
at
Over
the
AWA
facility
in
both
the
collinear
and
the
two
beam
accelerator
configurations
the AWA facility in both the collinear and the two beam accelerator configurations
[5].
All these
these experiments
experiments used
used dielectric
dielectric loaded
loaded cylindrical
cylindrical waveguides,
waveguides, with
with
[5]. All
operating
frequencies
ranging
from
7
to
20
GHz.
The
dielectric
materials
are
typically
operating frequencies ranging from 7 to 20 GHz. The dielectric materials are typically
Magnesium-Calcium-Titanate based
based ceramics
ceramics (MCT),
(MCT), with
with dielectric
dielectric constants
constants
Magnesium-Calcium-Titanate
spanning from
from 44 to
to 40.
40. Accelerating
Accelerating fields
fields of
of about
about 15
MV/m have
have been
been measured
measured in
in
spanning
15 MV/m
collinear acceleration
acceleration experiments.
experiments. A
A two
two beam
beam accelerator
accelerator experiment
experiment operating
operating at
at
collinear
7.8 GHz
GHz demonstrated
demonstrated that
that more
more than
than 90
90 %
% of
of the
the RF
RF power
power generated
generated by
by the
the drive
drive
7.8
66
beam
yielding an
an accelerating
acceleratingfield
fieldofof
beam(4(4MW)
MW)was
wascoupled
coupled into
into the
the second
second structure,
structure, yielding
77MV/m.
Figure
4
shows
the
energy
modulation
of
the
witness
beam
as
it
enters
the
MV/m. Figure 4 shows the energy modulation of the witness beam as it enters the
dielectric
loaded
structure
at
different
values
of
the
RF
phase.
dielectric loaded structure at different values of the RF phase.
FIGURE4.4. Plot
Plotshowing
showingthe
thewitness
witness beam
beam energy
energy as
as aa function
FIGURE
function the
the delay
delay beam
beam the
the witness
witnessand
andthe
the
drivebeams.
beams.
drive
Plasmawakefield
wakefieldacceleration
acceleration experiments
experiments have
have also
Plasma
also been
been carried
carried out
outatatthe
theAWA
AWA
facility.
Electron
beam
focusing
and
acceleration
in
the
underdense
(blowout)
facility. Electron beam focusing and acceleration in the underdense (blowout)regime
regime
werefirst
firstobserved
observedwith
withthe
theAWA
AWA beam
beam [6].
[6].
were
Much
higher
accelerating
gradients
can
be achieved
achieved if
Much higher accelerating gradients can be
if the
the quality
quality of
ofthe
thedrive
drivebeam
beam
increases.
Smaller
emittances
allow
the
operation
of
smaller
diameter
increases. Smaller emittances allow the operation of smaller diameter structures,
structures,
whichyield
yield much
much higher
higher accelerating
accelerating fields.
fields. Shorter
which
Shorter electron
electron bunches
bunches would
would also
also
permit operation at higher RF frequencies. The field superposition resulting from the
permit operation at higher RF frequencies. The field superposition resulting from the
operation with longer bunch trains would allow the RF fields to build up to higher
operation with longer bunch trains would allow the RF fields to build up to higher
levels as well. For these reasons the AWA facility is currently undergoing several
levels as well. For these reasons the AWA facility is currently undergoing several
major upgrades.
majorAupgrades.
new photocathode RF gun has been built to replace the old drive beam gun. The
newhad
photocathode
RF gun
built
to 2replace
theRF
oldpower
drive was
beamavailable.
gun. The
oldAgun
been designed
and has
builtbeen
when
only
MW of
old
gun
had
been
designed
and
built
when
only
2
MW
of
RF
power
was
available.
This limited amount of RF power had led to the construction of a half-cell gun,
This
limited
amount soft
of RF
power
had led
to at
thetheconstruction
of athehalf-cell
gun,
yielding
a relatively
2 MeV
electron
beam
gun exit, with
consequent
yielding
a
relatively
soft
2
MeV
electron
beam
at
the
gun
exit,
with
the
consequent
high emittance and long bunch length. The new 1 ½ cell gun is expected to produce a
high
emittance
long
The new
1 VT.bunch
cell gun
is expected
to produce
7.5 MeV
beamand
with
12 bunch
MW oflength.
RF power.
Similar
charges
(10 – 100
nC) willa
7.5
MeV
beam
with
12
MW
of
RF
power.
Similar
bunch
charges
(10
100
will
be generated by the new gun, but with shorter bunch lengths (2 – 5 ps rms) andnC)
much
belower
generated
by
the
new
gun,
but
with
shorter
bunch
lengths
(2
5
ps
rms)
and
much
emittances (30 – 200 π mm mrad). The new gun (Fig. 5) has been conditioned
lower
(30 - 200
mm mrad).
new gun (Fig.beam
5) has
been the
conditioned
up to emittances
12 MW of power,
andnproduced
the The
first photoelectron
during
last few
uphours
to 12ofMW
of
power,
and
produced
the
first
photoelectron
beam
during
the lastonce
few
operation of the old laser system. The beam will be fully diagnosed
hours
of operation
of the
oldsystem
laser starts.
system. The beam will be fully diagnosed once
operation
with the new
laser
operation
with the
newnew
laserlaser
system
starts.
Installation
of the
system
was completed a few days before the start of
Installation
of The
the new
system
was of
completed
daysTsunami
before the
start of
this
workshop.
new laser
system
consists
a Spectraa few
Physics
oscillator
this
workshop.
The newregenerative
system consists
of aandSpectra
Physics Tsunami
oscillator
followed
by a Spitfire
amplifier
two Ti:Sapphire
amplifiers
(TSA
followed
by a Spitfire
twolength
Ti: Sapphire
50). It produces
1.5 mJregenerative
pulses at 248amplifier
nm, with aand
pulse
of 6 to 8amplifiers
ps FWHM(TSA
and
50).
It produces
mJ to
pulses
at 248
pulse lengthweofhave
6 to had
8 psrunning
FWHMthis
and
a repetition
rate1.5
of up
10 pps.
Thenm,
verywith
briefa experience
a repetition rate of up to 10 pps. The very brief experience we have had running this
67
new
that its
its power
power stability
stability and
and beam
beam profile
profile quality
qualityare
areindeed
indeedmuch
much
new system
system confirms
confirms that
better
the old
old laser
laser system
system could
could provide.
provide.
better than
than what
what the
FIGURE 5. Picture of
of the
the new
new AWA
AWAdrive
drivegun
gununder
underinstallation.
installation.
The third item being upgraded
upgraded is
is the
the photocathode
photocathode material.
material. The
The old
old drive
drive gun
gun
operated routinely with magnesium
photocathodes,
being
able
to
produce
up
to
magnesium photocathodes, being able to produce up to100
100nC
nC
of charge
charge with
with 55 mJ
10~−44).). The
The new
new drive
drive
of
mJ of
of UV
UV laser
laser beam
beam (quantum
(quantum efficiency
efficiencyof
of 11 x× 10
beam has
has been
beam
been commissioned
commissioned with
with aa copper
copper photocathode,
photocathode, but
but that
that will
will soon
soon be
be
replaced by
replaced
by aa cesium
cesium telluride
telluride photocathode
photocathode [7],
[7], which
which isis expected
expectedtotohave
haveaaquantum
quantum
efficiency close
close to
10%. We
efficiency
to 10%.
We need
need aa quantum
quantum efficiency
efficiency of
of about
about 1%
1% inin order
order toto
generate
64
bunches
of
50
nC
with
1.5
mJ
of
laser
energy
at
248
nm.
generate 64 bunches of 50 nC with 1.5 mJ of laser energy at 248 nm.
VARIANTS OF
VARIANTS
OF THE
THE TWO
TWO BEAM
BEAM ACCELERATOR
ACCELERATOR SCHEME
SCHEME
There are
are two
There
two possible
possible variants
variants of
of the
the dielectric
dielectric two
twobeam
beam accelerator
acceleratorscheme:
scheme:(a)
(a)
the
structure
where
the
drive
beam
loses
energy
can
be
used
as
a
power
the structure where the drive beam loses energy can be used as a power source
source for
for
metallic accelerating
accelerating structures;
metallic
structures; (b)
(b) aa dielectric
dielectric loaded
loaded structure
structure can
canbe
bepowered
poweredby
byaa
conventional
RF
source.
conventional RF source.
Both of
of these
Both
these options
options are
are currently
currently being
being pursued
pursued by
by the
the AWA
AWA group
group inin
collaboration
with
other
research
institutions.
Duly
Research,
in
collaboration
collaboration with other research institutions. Duly Research, in collaboration with
with
CERN and
and ANL,
CERN
ANL, is
is building
building 21
21 GHz
GHz dielectric
dielectric loaded
loaded structures
structurestotobe
betested
testedasaspower
power
sources using
sources
using the
the CTF
CTF II
II electron
electron beam
beam at
at CERN
CERN as
as the
the drive
drive beam
beam [8].
[8]. These
These
structures
are
expected
to
generate
more
than
150
MW
of
RF
power
at
21
GHz.
structures are expected to generate more than 150 MW of RF power at 21 GHz.
The AWA
The
AWA group
group has
has built
built 11.4
11.4 GHz
GHz dielectric
dielectric loaded
loaded prototype
prototype structures
structurestotobebe
tested
at
NRL
using
their
X-band
magnicon
as
the
RF
power
source
[9].
tested at NRL using their X-band magnicon as the RF power source [9].
68
PLANAR DIELECTRIC WAKEFIELD STRUCTURES
W-band dielectric loaded structures have been built and tested using the NLCTA
electron beam at SLAC [10]. These planar dielectric structures were constructed by
inserting two alumina slabs into a rectangular waveguide section. The transverse
dimensions of the structures are of the order of hundreds of micrometers, with a 720
um gap for the passage of the 300 MeV electron beam. One of these devices was
configured in a ring resonator circuit, where the measurements indicated that 200 kW
of circulating power generated an accelerating field of 20 MV/m.
A collaboration between Yale, Columbia and Omega-P is developing planar
dielectric loaded structures with transverse dimensions of the order of tens of
micrometers. They plan to chop the electron beam from the accelerator LACARA
[11], in order to make 1 um long bunches of 1 pC charge. A bunch train composed of
ten of these small bunches, each spaced by 20 jim, is expected to generate an
accelerating field of about 600 MV/m.
CONCLUSION
Substantial progress is being made in the development of dielectric loaded
structures as an alternative to the use of metallic structures. The AWA upgrades will
make it a powerful tool for the study of electron beam driven accelerators and the
exploration of higher accelerating gradients. The new AWA beam will be capable of
demonstrating gradients of the order of 200 to 300 MV/m in dielectrics, in both
collinear and step-up transformer structures. This intense beam will be able to generate
hundreds of megawatts of RF power with frequencies in the range of 30 to 100 GHz,
with pulse lengths of tens of nanoseconds, assuring a prominent place for beam driven
schemes in the quest for multi TeV colliders.
ACKNOWLEDGMENTS
This work was supported by DOE, High Energy Physics Division, Advanced
Technology Branch, under Contract No. W-31-109-ENG-38.
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