A Circularly Polarized Beam Deflector for
Direct Measurement of Ultra Short Electron
Bunches*
J. Haimson, B. Mecklenburg, G. Stowell and B. Ishii
Haimson Research Corporation
3350 Scott Blvd., Building 60, Santa Clara, CA 95054-3104, USA
Abstract. Described herein is an accurate means of directly measuring the longitudinal phase
space of ultra short electron bunches typically produced by advanced design short wavelength
accelerators and photoinjectors. A circularly polarized RF beam deflector provides a highly
amplified image display having electron bunch phase and energy distributions projected in
orthogonally separated azimuthal and radial directions, respectively. Combining such a
microwave deflector with an emittance limited focused beam offers a simple on-line diagnostic
that avoids the complexity, sensitivity and a priori assumptions associated with electro-optic
frequency domain methods, and also avoids RF kicker bunch length measurement ambiguities
caused by longitudinal mixing of the particle energy and phase distributions. Accurate self
calibration, and the avoidance of pulse jitter by directly coupling the deflector power from the
linac RF source, are additional attractive features of this technique. Design parameters and
phase orbit characteristics are presented for an on-line diagnostic system having a 1/4%
momentum resolution, a 56 femtosecond time resolution and a 16 femtosecond sampling gate,
for evaluating the short bunch longitudinal phase space performance of the 17 GHz linac at the
MIT Plasma Science and Fusion Center.
INTRODUCTION
The use of advanced injector configurations, near optimum RF system designs, and
superconducting accelerator concepts have resulted in the progressive improvement of
particle accelerator beam quality; and further improvement of performance remains
dependent on the continuing reduction of beam transverse and longitudinal phase
space. Marked reduction in transverse phase space has been demonstrated with the
recent development of high-brightness, relativistic electron sources [1,2,3]; and
ongoing efforts to reduce electron bunch widths to approximately 100 fs, by charge
compression [4] and by direct formation using high frequency accelerator systems [5],
have succeeded in also reducing the longitudinal phase space.
The formation and acceleration of very short electron bunches is of considerable
interest for a variety of new applications, including the generation of intense coherent
radiation, free electron lasers, wakefield accelerators and future linear colliders.
Because the existing fast diagnostic benchmark instrument - the streak camera - is not
capable of accurately resolving such ultra short signals, it has become increasingly
* Work supported by U.S. Department of Energy SBIR Grant No. DE-FG03-01ER83247.
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
810
important to develop alternate means of measuring very short bunches, and preferably,
to also provide an on-line diagnostic to assist in optimizing the electron bunch
characteristics.
Alternate short bunch measurement techniques have included far-infrared
interferometry [4] to analyze the coherent radiation from the bunch, and electro-optic
sampling using laser/beam interactions. The assumptions of longitudinal charge
distribution (bunch form factor) associated with these frequency-domain techniques,
the derivation of bunch phase data by reconstruction [6], and the difficulties associated
with laser timing and data acquisition have all encouraged the pursuit of time-domain
techniques for the definitive measurement of short RF bunches.
A time domain technique using an RF circularly scanned deflected beam was first
applied in the measurement of electron bunches in high current linac injectors [7], and
using a traveling wave deflector for linearly scanning the beam from an S-band linac
to measure the longitudinal charge distribution of 2 degree wide RF bunches [8]. In
the latter case, a calibrated phase shifter at the RF input of the structure was varied,
causing the beam to be linearly scanned across a slit; and the transmitted current
collected by a Faraday cup was recorded as a function of the phase shifter position.
More recently, half picosecond resolution has been achieved at S-band with linear
deflection [9]; and time/energy transformation techniques at lower RF frequencies
have been used to measure bunch length by off-phase acceleration [10,11]. In the
latter case, the bunch traverses the accelerator structure at a phase in close proximity
to the RF zero crossing, and an energy analyzing magnet is used to display the beam
dispersion on a viewing screen as the polarity of the structure gradient is altered.
The above linear deflection and off-phase techniques have the disadvantage,
however, of producing longitudinal mixing of the particle energy and phase
distributions that result in bunch width measurement ambiguities.
The short electron bunch time domain measurement technique described herein,
avoids the above disadvantage, and holds promise of demonstrating a high phase
resolution, limited only by the minimum beam spot size and the maximum beam
deflection that can be achieved at the analyzing plane.
TECHNICAL APPROACH
During the traversal of a circularly polarized, beam deflecting RF structure, the RF
bunched beam can be made to interact with two quadrature phased, orthogonally
polarized transverse magnetic deflection fields such that particles of equal energy
populating a sequential array of thin slices cut transversely through the bunch will
experience identical transverse momenta impulses but at incrementally changing
(rotating) radial directions that depend only on the RF phase of the cavity fields at the
time of traversal of each slice. Moreover, within any one slice, particles having
different energies will be deflected in the same radial direction but at different
deflection angles.
Thus, after drifting an extended distance beyond the RF deflector, a highly
expanded spatial distribution of particles will be projected in the transverse plane with
the electron bunch phase and energy distributions presented in orthogonally separated
811
azimuthal and radial directions. Since this technique displays a two-dimensional time
and energy distribution that reveals the longitudinal phase space of the electron bunch,
it promises to offer a simple, on-line diagnostic for the definitive measurement of very
short RF bunches.
An outstanding feature of this time-domain technique is the precision selfcalibrating capability of the RF deflector. Due to the inherent spatiotemporal
characteristic of the RF deflector, by varying the phase of the RF input power, the
imaged electron bunch will trace a circular path having a circumference corresponding
exactly to the RF periodicity of the deflector.
For the 17 GHz linac at MIT, to ensure imaging the electron beam with a minimum
cross-section, a low aberration, strong lens assembly shall be located immediately
upstream of the RF deflector to produce an emittance limited spot at the analyzing
plane.
For a normalized transverse beam emittance of 5n mm-mradian and beam energies
in the range of 10 to 20 MeV, with a 2 mm diameter beam at the RF deflector, the
beam focusing system will produce an emittance limited beam spot of sub-mm
dimensions at an analyzing plane located approximately 2 m downstream of the lens.
The RF deflector design parameters were selected to match the drift distance and spot
size, so that the transverse momentum imparted to the beam would provide the radial
deflection (%) necessary to achieve the desired phase resolution. For example, to
achieve sub-degree resolution with a sub-mm focal spot, an azimuthal scan path (2;i%)
of 1 mm/RF degree is required, and this defines the radial deflection % as 57.3 mm.
Thus, for a 15 MeV beam and a 2.1 m drift space, the transverse momentum imparted
to the beam by the RF deflector must be 423 keV/c (0.828 in units of moc).
Since the spot size at the analyzing plane (r) is given by
r = Sz/rQ
(1)
where S = drift distance, s = transverse beam emittance and ro = beam radius at the
deflector, and the beam deflection angle (0) is given by the ratio of transverse to
longitudinal momentum
(2)
the minimum resolvable phase [360r/(2;i%)] can be expressed in radians as
(3)
where sn = Py as the normalized transverse beam emittance, and p_L is the transverse
momentum imparted to the beam (in units of m0c).
The minimum resolvable time is simply c^n divided by the angular frequency (co)
of the RF deflector; and, if the normalized emittance is conserved during the
acceleration process, it can be noted [R. H. Miller, private communication] that the
achievable phase, or time, resolution is independent of the beam energy.
812
r
rx*
YJ
„
0B
LINAC
SYSTEM
BEAM
FOCUS
ELEMENTS
CIRCULAR
RF SCANNING
BEAM
DEFLECTOR
VIDEO
CAMERA
DIAGNOSTIC
CHAMBER
WITH
ADJUSTABLE
SLITS
/
EVACUATED
FARADAY
CUP
FIGURE 1. Diagnostic system for the direct measurement of ultra short bunches showing the
circularly polarized RF deflector with the azimuthal scan phase shifter (A) and the set quadrature phase
shifter (B).
A conceptual layout of the microwave diagnostic system for the direct
measurement of ultra short electron bunches is illustrated in Figure 1. It can be noted
that, to conserve the inherent high resolution of this diagnostic technique, and to avoid
measurement errors due to RF pulse amplitude and phase jitter, the RF deflector input
power will be coupled directly from the same high power klystron used to bunch and
accelerate the beam.
The diagnostic chamber, located downstream at the analyzing plane, is designed so
that the deflected beam can be video imaged for qualitative evaluation; and precision
measurements of the bunch phase length and longitudinal charge distribution can be
obtained by azimuthally scanning the image across a radially oriented, narrow
analyzing slit. For the above discussed 17 GHz application using an azimuthal scan of
1 mm/degree, a slit width of 0.1 mm will provide a sampling gate of 16.2
femtosecond. Moreover, by adjusting the radial length of the narrow analyzing slit, or
using an orthogonal slit assembly, it will be possible to record the energy-phase
distribution of the RF bunch.
CIRCULARLY POLARIZED RF BEAM DEFLECTOR
During the initial planning of this project, the intention was to extend a technique
that proved highly successful with the circularly polarized, 499 MHz beam deflecting
cavities developed for the Thomas Jefferson National Accelerator Facility. With that
technique, the deflector cavities were arranged to simultaneously support two
quadrature-phased transverse magnetic, dipole-like modes that were strongly polarized
in orthogonal planes. Special mode tuners were used to achieve the extremely high
isolation required to conserve the amplitude and phase independence of each mode
while sharing the same beam loading and boundary wall conditions. While two sets of
dipole mode frequency tuners and mode isolation tuners can be readily accommodated
in a low frequency cavity, this technique presents a formidable engineering challenge
at 17 GHz. Thus, for this application, we elected to use two separate, orthogonally
813
oriented, magnetic dipole cavities connected by a short drift tube to provide greater
than 60 dB mode isolation.
A symmetric, dipole-like transverse magnetic field mode, TMno (cylindrical) or a
TEio2 (rectangular), was most favored for this application because the combined effect
of both the transverse H and transverse E field interactions produce low aberration
beam deflections, and because of favorable past experience in designing, fabricating
and high power beam testing a wide variety of these cavities.
An early design consideration was to ensure that the overall operational stability of
the 17 GHz deflection system, and also the response of the higher order mode RF
fields to frequency and temperature variations, would be consistent with that of the
existing injector and linac systems [5]. Thus, because the high electric field
prebuncher cavity in the linac's 500 kV injection system operates at a loaded Q of
1800 with a near critical coupling value, the beam deflector cavities were specified to
have unloaded Q values of approximately 3600.
When operating with low current, relativistic beams, deflector cavities can be
designed with small beam apertures and a large transit angle (71); and although this has
the advantage of increasing the transverse shunt impedance and reducing beam
aberrations, the unloaded Q values tend to be undesirable high. Typically, a copper
cavity operating at 17 GHz in the TMno mode, and having a transit angle of n, will
have an unloaded Q of approximately 8800. For this 17 GHz application, however, it
was possible to retain a high transit angle (163°) and still achieve an unloaded Q of
3500, by fabricating the dual cavities from a common copper body and using low
carbon stainless steel end walls.
Initial design studies of the deflector cavity geometry, field distributions and
particle phase orbits made use of traditional analytical procedures and a well
established 3D PIC code [12]; and the cavity geometry was subsequently refined using
an accurate 3D EM code to evaluate the sidewall coupling parameters, the mode
polarization features, and the coupling response to cavity de-Q-ing (stainless steel end
walls). A conventional thick septum, sidewall aperture was chosen for coupling RF
power directly from the incoming WR62 RWG feed into the TMno cavity. The length
of the rectangular coupling aperture was made equal to the full height of the cavity,
thereby minimizing the transverse dimension of the aperture and the associated field
asymmetry with respect to the beam centerline. To provide a means of positively and
simultaneously locking the orientation of each orthogonal dipole field pattern, the dual
cavity assembly was designed with a thick common wall having off-axis holes
interconnecting the two cavities. This technique enables discrete field perturbing
zones to be located at strategic mode locking positions, and improves the inter-cavity
pumping conductance for this narrow beam tunnel assembly. The polarizing holes
have generous wall-blending radii and are located at 30% peak electric field regions.
The cavity fabrication dimensions and tolerances included allowance for the
presence of brazing alloy, final matching and dressing of the coupling aperture, and
tuning alignment with the operating temperature and frequency of the linac. A cutaway view through the 17 GHz dual cavity assembly, showing the orientation of the
common wall mode polarizing holes, the orthogonal RF feeds and the sidewall
coupling apertures, is illustrated in Figure 2.
814
FIGURE 2.
2. Cut-away
Cut-away view
view through
through the
the dual
dual cavity
cavity beam
beam deflector
FIGURE
deflector showing
showing orientation
orientation of
of the
the mode
mode
polarizing common
common holes,
holes, the
the orthogonal
orthogonal RF
RF feeds
feeds and
and the
the sidewall
polarizing
sidewall coupling
coupling apertures.
apertures.
The 17
17 GHz
GHz beam
beam deflection
The
deflection system
system design
design parameters
parameters summarized
summarized in
in Table
Table 11
indicate that
that each
each cavity
cavity will
will require
require an
indicate
an input
input peak
peak RF
RF power
power of
of approximately
approximately
3/4 MW
MW to
to achieve
achieve the
the desired
desired 57
57 mm
mm beam
beam deflection
3/4
deflection at
at the
the analyzing
analyzing plane.
plane. ItIt can
can
be noted
noted that,
that, while
while the
the peak
peak electric
electric field
field value
value is
be
is high,
high, itit is
is less
less than
than the
the 100
100 MV/m
MV/m
TABLE 1.
1. Design
Design Parameters
Parameters of
of the
the 17
TABLE
17 GHz
GHz Circularly
Circularly Polarized
Polarized Beam
Beam Deflection
Deflection System
System
Deflector Cavity
Cavity Operating
Operating Mode
Mode
TM
1100
Deflector
TMn
Number of
of Cavities
Cavities (orthogonally
(orthogonally oriented)
oriented)
22
Number
Orthogonal Mode
Mode Isolation
Isolation
63
63 dB
dB
Orthogonal
Beam Tunnel
Tunnel Diameter
Diameter
3.96
mm
Beam
3.96mm
Operating Dipole
Dipole Mode
Mode Frequency
Frequency
17.136
17.136 GHz
GHz
Operating
Degenerate Dipole
Dipole Mode
Mode Frequency
Frequency
17.238
Degenerate
17.238 GHz
GHz
Nominal Beam
Beam Energy
Energy
15
MeV
Nominal
15MeV
Transverse Momentum
Momentum Imparted
Imparted to
to the
the Beam
Beam [82%(H
420
y)]
Transverse
[82%(HXx),
), 18%(E
18%(Ey)]
420 keV/c
keV/c
RF Deflection
Deflection Angle
Angle
27
RF
27 mradian
mradian
Deflector Drift
Drift Distance
Distance to
to Analyzing
Analyzing Plane
Plane
2099
mm
Deflector
2099mm
Beam Deflection
Deflection at
at Analyzing
Analyzing Plane
Plane
57
mm
Beam
57mm
Deflector Cavity
Cavity Transit
Transit Angle
Angle
0.906
Deflector
0.906 π7i
Beam Coupling
Coupling Coefficient
Coefficient -– Transverse
Transverse Magnetic
Magnetic Field
0.497
Beam
Field
0.497
Beam Coupling
Coupling Coefficient
Coefficient -– Transverse
Transverse Electric
Electric Field
0.805
Beam
Field
0.805
Deflector Cavity
Cavity Unloaded
Unloaded Q
Q
3500
Deflector
3500
Deflector Cavity
Cavity Loaded
Loaded Q
Q
1700
Deflector
1700
Peak RF
RF Input
Input Power
Power to
to Each
Each Cavity
Cavity
734
kW
Peak
734
kW
6
Cavity
Peak
Electric
Field
6 V/m
89
×
10
Cavity Peak Electric Field
89xl0 5 V/m
Cavity Peak
Peak Magnetic
Magnetic Fie
Field (Hφ )
2.04
2.04 ×x 10
105 A/m
A/m
Cavity
2560
(Bφ)
2560 gauss
gauss
(Bo)
Frequency Control:
Control: Individual
Individual tuners,
tuners, and
-− Frequency
and water
water cooling
cooling channels
channels machined
machined into
into the
the cavity
cavity
body and
and the
the end
end walls.
walls.
body
Mode Polarization:
Polarization: Common
Common wall
wall apertures.
apertures.
-− Mode
De-Q-ing
: Achieved
Achieved with
with stainless
-− De-Q-ing_____:_
stainless steel
steel end
end walls.
walls.
815
peak
peak operating
operating fields
fields existing
existing in
in the
the high
high power
power cavities
cavities of
of the
the 17 GHz relativistic
klystron
klystron [13]
[13] that
that provides
provides the
the 200
200 ns
ns wide
wide high
high power
power RF
RF pulses
pulses to
to the
the linac.
linac. ItIt can
can
also
be
noted
that
the
potentially
troublesome
degenerate
dipole
mode
frequency
also be noted that the potentially troublesome degenerate dipole mode frequency has
has
been
been separated
separated from
from the
the operating
operating frequency
frequency by
by aa safe
safe margin
margin of
of approximately
approximately 10
resonance
resonance widths.
widths.
Deflector
Deflector Cavity
Cavity Field
Field Characteristics
Characteristics
The
The transverse
transverse momentum
momentum imparted
imparted to
to the
the beam
beam by
by aa magnetic
magnetic dipole
dipole deflecting
deflecting
cavity
cavity includes
includes contributions,
contributions, not
not only
only from
from the
the main
main interaction
interaction with
with the
the transverse
transverse
magnetic
magnetic field
field (H
(Hxx)) in
in the
the body
body of
of the
the cavity,
cavity, but
but also
also from
from transverse
transverse electric
electric field
field
(E
(Eyy)) interactions
interactions that
that occur
occur at
at the
the cavity
cavity entrance
entrance and
and exit
exit beam
beam apertures.
apertures. The
The
simulated
simulated TMno
TM110 mode
mode cavity
cavity H
Hxx field
field distribution,
distribution, taking
taking into
into account
account the
the presence
presence of
the
the coupling
coupling aperture,
aperture, mode
mode polarizing
polarizing holes
holes and
and beam
beam apertures,
apertures, is
is shown
shown plotted
plotted in
in
Figure
3
on
the
x-z
plane
cut
through
the
axis
at
y
=
0.
Similarly,
Figure
4
shows
Figure 3 on the x-z plane cut through the axis at y = 0. Similarly, Figure 4 shows the
the
x-z
x-z plane
plane distribution
distribution of
of the
the EEyy transverse
transverse electric
electric field,
field, and
and indicates
indicates that
that the
the E
Eyy field
field
typically
typically extends
extends along
along the
the drift
drift tubes,
tubes, has
has peak
peak values
values at
at the
the entrance
entrance and
and exit
exit of
of the
the
cavity,
cavity, and
and aa polarity
polarity reversal
reversal at
at the
the midplane
midplane of
of the cavity.
cavity.
Type
== H-Field (peak)
Mon i tor
Mode Z
Component
X
Plane at y
e
Frequency
17131.5
Phase
270 degrees
FIGURE
modetransverse
transversemagnetic
magneticfield
field(H
(Hx)x)distribution
distributionon
onthe
thex-z
x-zplane
planeatatyy==0.0.
FIGURE 3.
3. TM
TM110
110mode
816
mode transverse
transverseelectric
electricfield
field(Ey)
(Ey)distribution
distributionon
onthe
thex-z
x-zplane
planeataty y==0.0.
FIGURE 4.
4. TMno
TM110 mode
FIGURE
For the
the maximum
maximum deflection
deflection case,
case, an
an incoming
incoming particle
particle approaching
approachingthe
thecavity
cavity
For
entrance will
will interact
interact with
with both
both the
the peak
peak of
of the
the electric
electric deflecting
deflectingfield,
field,and
andaalow
low
entrance
intensity, decaying
decaying transverse
transverse magnetic
magnetic field
field that
that initially
initially produces
produces an
an opposing
opposing
deflection.
deflection. Just
Just prior
prior to
to the
the particle
particle entering
entering the
thecavity,
cavity,the
themagnetic
magneticfield
fieldwill
willhave
have
reversed;
reversed; and when
when the
the particle
particle arrives
arrives atat the
the cavity
cavitymidplane,
midplane,the
themagnetic
magneticdeflecting
deflecting
field
field will be
be at
at a peak
peak value,
value, and
and the
the transverse
transverse electric
electricfield
field(having
(havingdecayed
decayedtotozero
zero
both in space
space and
and time)
time) will
will be
be reversing
reversingpolarity.
polarity. Thus,
Thus,during
duringthe
theremainder
remainderofofthe
the
trajectory,
trajectory, the
the particle
particle will
will continue
continue to
to be
be deflected
deflectedininthe
thesame
samedirection
directionby
bythe
thenow
now
reversed and
and growing
growing transverse
transverse electric
electricfield
fieldand
andby
bythe
thedecaying
decayingtransverse
transversemagnetic
magnetic
field,
the particle
particle will
will receive
receive two
two cumulative,
cumulative, unidirectional
unidirectional transverse
transverse
field, i.e.,
i.e., the
momentum
momentum kicks
kicks due
due to
to Ey.
Ey. The
The above
above described
describedfield
fieldinteractions
interactionsare
areillustrated
illustratedinin
the Figure
Figure 5 plots
plots of
of the
the TMno
TM110 cavity
cavity H
Hxx and
and EEyypeak
peakfield
fielddistributions
distributionscompared
comparedtoto
the field
field values
values experienced
experienced by
by 15
15 MeV
MeV electrons
electrons traversing
traversing the
the deflector
deflector atat two
two
different
different radial
radial positions,
positions, namely,
namely, on
on axis
axis(x
(x==0,0,yy==0)
0)and
and11mm
mmoff-axis
off-axisininthe
theplane
plane
of deflection
deflection (x
(x == 0,
0, yy == 1).
1). The
The EEyy field
field reversal
reversalduring
duringthe
theparticle
particletrajectory
trajectoryresults
results
in
in aa high
high beam
beam coupling
coupling coefficient
coefficient (0.805),
(0.805),whereas
whereasthe
theHHxxextended
extendedfield
fielddistribution
distribution
(>p
(>βeeA,/2)
λ/2) results
results in
in aa relatively
relatively low
low coupling
coupling coefficient
coefficient (0.497)
(0.497)due
duetotothe
theopposing
opposing
polarity
polarity transverse
transverse momenta
momenta kicks
kicks atatthe
thecavity
cavityentrance
entranceand
andexit.
exit.
The
The Figure
Figure 55 field
field integral
integral comparisons
comparisonsof
ofthe
thetwo
tworadial
radialorbits
orbitsshow
showthat
thatthe
the11mm
mm
off-axis
off-axis particle
particle experiences
experiences an
an EEyy interaction
interaction 15.8%
15.8% higher
higher and
and an
an HHx x interaction
interaction
2.7%
net
2.7% lower
lower than
than the
the on-axis
on-axis particle.
particle. However,
However, since
since the
the ratio
ratio ofof HHx x toto EEy ynet
momenta
contributions
is
4.6:1
for
this
cavity
geometry,
momenta contributions is 4.6:1 for this cavity geometry, the
the inherent
inherent field
field
compensation
compensation of
of this
this mode
mode results
results in
in aatransverse
transversemomentum
momentumvariation
variationofofless
lessthan
than1%
1%
between
the
two
orbits.
Further
interaction
studies
at
other
orbit
locations
within
between the two orbits. Further interaction studies at other orbit locations withinthe
the
beam
beam cross
cross section
section have
have yielded
yielded similar
similar low
low aberration
aberrationresults
resultsfor
forparticles
particlestraversing
traversing
817
7
2x10
2x10'
Field
Field seen
seen by
by particle:
particle:
E
Ey(t)
(t) at
at xx == 0,
0, yy =1
=1 mm
mm
y
E
at x == 0,
y == 00 _ _ _ _ .
Ey(t)
(t)atx
0,y
7
Ey (V/m)
1x10
1x10'
0
Integral E
.Integral
Eyy(t)
(t) dz
dz ==.
74.5
keV
at y == 0
—— 74.5keVaty
~ - - 86.3
86.3keV
keVat
at yy == 11 mm
mm
7
-1x10
-1x10'
Ey at
-E
atxx =
= 0,
0, yy == 00 ---E
Eyyy at
at xx == 0,
0, yy == 11 mm
mm
i
i
7
-2x10
-2x10'
5
2x10
2x10°
H
at
Hxx at
x=0,
x=0, y=0
y=0
Integral
integral H
Hxx(t)
(t) dz
dz ==____
A at y == 00
—— 921.4
921.4Aaty
- - • 896.7
896.7 A
A at
at yy ==11mm
mm
Hx at
x=0, y=1 mm
Hx (A/m)
5
-£ 1x10
1x105
^
0
Field seen by particle:
Hx(t) at x = 0 and y=0 & 1 mm
0
0.005
0.005
0.010
0.010
0.015
0.015
zz(m)
(m)
0.020
0.020
0.025
0.025
0.030
0.030
FIGURE
FIGURE 5.
5. Transverse
Transverse EEyy and
and H
Hxx fields
fields experienced
experienced by
by 15
15 MeV
MeV electrons
electrons during
during traversal
traversal of
of the
the
deflector
deflector cavity
cavity at
at radial
radial positions
positions xx == 0,
0, yy == 00 and
and xx == 0,
0, yy == 11 mm,
mm, compared
compared to
to the
the cavity
cavity peak
peak field
field
distributions.
distributions.
the
the cavity
cavity during
during the
the time
time of
of maximum
maximum deflection
deflection when
when the
the induced
induced beam
beam energy
energy
spread
spread tends
tends to
to zero.
zero.
The
The maximum
maximum induced
induced beam
beam energy
energy spread
spread occurs
occurs when
when particles
particles enter
enter the
the
magnetic
minimum
magnetic dipole
dipole cavity
cavity at
at aa time
time ωt
ooto0 =
= π−θ
TI—0x/2
corresponding to
to the
the minimum
T/2 corresponding
deflection
particles on
deflection case,
case, where
where θ0x
is the
the cavity
cavity transit
transit time.
time. For
For this
this case,
case, off-axis
off-axis particles
on
T is
opposite
opposite sides
sides of
of the
the centerline
centerline will
will experience
experience continuous
continuous accelerating
accelerating and
and retarding
retarding
E
Ezz fields
fields during
during the
the full
full transit
transit time,
time, as
as the
the deflecting
deflecting magnetic
magnetic field
field decays
decays to
to zero
zero
and
and grows
grows in
in reverse
reverse polarity.
polarity. The
The maximum
maximum beam
beam energy
energy spread
spread (∆V)
(AV) induced
induced by
by
the
the RF
RF deflector
deflector cavity
cavity can
can be
be expressed
expressed simply
simply as,
as,
∆V = 2πp ⊥ d / λ 0
(4)
(4)
where
with aa 17
where dd is
is the
the beam
beam diameter
diameter in
in the
the cavity.
cavity. Thus,
Thus, with
17 GHz
GHz deflector
deflector cavity
cavity
designed
beam, an
designed to
to impart
impart aa transverse
transverse momentum
momentum of
of 420
420 keV/c
keV/c to
to the
the beam,
an induced
induced
energy
energy spread
spread of
of 302
302 keV
keV can
can be
be expected
expected during
during aa net
net zero
zero deflection
deflection traversal
traversal of
of the
the
cavity
cavity when
when operating
operating with
with aa 22 mm
mm diameter
diameter beam.
beam.
818
SHORT BUNCH
BUNCH MEASUREMENT
MEASUREMENTTECHNIQUE
TECHNIQUE
SHORT
The phase and beam cross section related induced beam energy spread is
The phase and beam cross section related induced beam energy spread is
determined
by the
the choice
choice of
of bunch
bunch entry
entry phase
phase into
into the
the first
first cavity
cavityand
andthe
therotation
rotation
determined by
sense
of
the
second
cavity.
A
unique
feature
of
the
circularly
polarized
deflector,
sense of the second cavity. A unique feature of the circularly polarized deflector,
however, is
is that
that any
any beam
beam energy
energy spread
spread contribution
contributionintroduced
introducedby
bythe
thedeflector
deflectorcan
can
however,
be
readily
identified
using
a
simple
measurement
technique
described
as
follows.
The
be readily identified using a simple measurement technique described as follows. The
phase
shifter A
A (Figure
(Figure 1)
1) isis used
used toto set
set the
the azimuthally
azimuthallyscanned
scannedbeam
beamimage
imageatata a
phase shifter
circumferential
location
corresponding
to
the
maximum
deflection
of
one
the
circumferential location corresponding to the maximum deflection of one ofof the
cavities,
e.g.,
the
9
o'clock
position
given
by
the
horizontal
scan
cavity;
and
the
radial
cavities, e.g., the 9 o’clock position given by the horizontal scan cavity; and the radial
and azimuthal
azimuthal charge
charge distributions
distributions are
are recorded.
recorded. The
The vertical
verticalscan
scancavity
cavityisisthen
thendedeand
energized
to
produce
a
stationary
deflected
beam
image,
and
the
charge
distributions
energized to produce a stationary deflected beam image, and the charge distributions
are again
again recorded.
recorded. The
The change
change in
in the
the radial
radial charge
charge distribution
distributionofofthe
thetwo
twoimages
images
are
represents the
the energy
energy spread
spread introduced
introduced by
by the
the deflector.
deflector. For
Forthe
theparameters
parameterslisted
listedinin
represents
Table 1,
1, the
the longitudinal
longitudinal momentum
momentum (radial)
(radial) dispersion
dispersion isis1.75%
1.75%per
permm.
mm. Also,
Also,toto
Table
ensure correct
correct beam
beam alignment
alignment and
and to
to take
take into
into account
account any
anysystem
systemasymmetries,
asymmetries,the
the
ensure
above
measurements
should
be
repeated
with
the
image
repositioned
at
the
3
o'clock
above measurements should be repeated with the image repositioned at the 3 o’clock
location.
location.
The stationary
stationary cross-sectional
cross-sectional image
image of
of an
an idealized
idealized (monochromatic)
(monochromatic) oblate
oblate
The
spheroidal
shaped
linac
bunch,
having
a
1/2
mm
maximum
diameter
and
a
length
spheroidal shaped linac bunch, having a 1/2 mm maximum diameter and a length ofof
36.4 micron
micron (3/4°),
(3/4°), is
is shown
shown transformed
transformed in
in Figure
Figure 66 toto aa 750
750 micron
micronlong
longprolate
prolate
36.4
spheroidal image by
by the
the 1°/mm
l°/mm azimuthal
azimuthal scan
scan produced
produced by
by excitation
excitation ofof the
the
orthogonal, quadrature
quadrature phased
phased second
second cavity.
cavity. Particles
Particles having
having lower
lower and
and higher
higher
energies will be deflected
deflected and
and distributed
distributed above
above and
and below
belowthe
themedian
medianenergy
energypath,
path,
respectively. For
For example,
example, the
the dotted
dotted profile
profile superimposed
superimposed on
on the
the Figure
Figure 6 6
azimuthally scanned
scanned image
image illustrates
illustrates the
the charge
charge distribution
distribution ofof an
an electron
electronbunch
bunch
tail correlated
correlated energy
energy characteristic.
characteristic.
having a head to tail
--— (122 fs at 17.136 GHz)
(a) FULLY DEFLECTED FOCUSED IMAGE
USING SINGLE CAVITY EXCITATION
(V-AV)
0 MAX = 1/2 mm
1 ° per mm -*—
(b) AZIMUTHALLY SCANNED
FOCUSED IMAGE USING
DUAL CAVITY EXCITATION
CHARGE
DISTRIBUTIONS
FIGURE
FIGURE 6.
6. Electron
Electron bunch
bunch image
image and
and charge
charge distributions
distributionsfor
for(a)
(a)the
thefully
fully deflected
deflectedbunch
bunchusing
usinga a
single
single excited
excited cavity
cavity to
to produce
produce aa stationary
stationary image
image prior
prior toto exciting
exciting the
thequadrature-phased
quadrature-phasedsecond
second
cavity,
cavity, and
and (b)
(b) the
the azimuthally
azimuthally extended
extended bunch
bunch image
imageproduced
producedby
byswitching-on
switching-onthe
thesecond
secondcavity
cavitytoto
display
display the
the longitudinal
longitudinal phase
phase space
space of
of the
the RF
RFbunch.
bunch.
819
CONCLUSIONS
The circularly polarized deflector described herein for the direct measurement of
ultra short electron bunches has the advantage of sharing the same RF source as the
linear accelerator to provide natural synchronization with the RF bunches; has an
inherent accurate calibration defined simply by the operating frequency and the linear
deflection at the analyzing plane; and can be conveniently integrated into existing
systems to provide an on-line diagnostic that can be readily interpreted and controlled
by accelerator operators without extensive training. A proof of principle experiment
has been planned, and fabrication of a circularly polarized RF deflector system has
commenced for on-line evaluation of the short bunch longitudinal phase space
performance of the 17 GHz linac at the MIT Plasma Science and Fusion Center.
ACKNOWLEDGMENTS
This work was sponsored by the U. S. Department of Energy, Division of High
Energy Physics.
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820