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Proceedings ArticleDOI

Coaxial higher-order mode damper employing a high-pass filter

12 May 1997-Vol. 3, pp 2932-2934

Abstract: Two different types of coaxial higher-order mode (HOM) dampers have been investigated for the Advanced Photon Source (APS) storage ring cavities: e-probe dampers and h-loop dampers. Realization of the h-loop dampers has been difficult because the loop antenna couples not only to the HOMs but also to the accelerating mode and results in loss of Q at the fundamental frequency. Previously, a first-order fundamental rejection filter was tested with unsatisfactory rejection characteristics. This problem can be overcome by using a higher-order high-pass filter between the loop and the matched load. Prototype dampers have been fabricated and tested in a storage ring single-cell cavity and the damping characteristic was analyzed.
Topics: Damper (56%), Coaxial (53%), High-pass filter (53%), Loop antenna (50%)

Summary (2 min read)

A bstracr

  • Two different types of coaxial higher-order mode (HOM) dampers have been investigated for the Advanced Photon Source (APS) storage ring cavities: e-probe dampen and h-loop dampers.
  • Realization of the h-loop dampers has been difficult because the loop antenna couples not only to the HOMs but also to the accelerating mode and results in loss of Q at the fundamental frequency.
  • Previously, a first-order fundamental rejection filter was tested with unsatisfactory rejection characteristics.
  • This problem can be overcome by using a higher-order high-pass filter between the loop and the matched load.
  • Prototype dampers have been fabricated and tested in a storage ring single-cell cavity and the damping characteristic was analyzed.

1 IXTRODUCTION

  • The fundamental frequency rejection is achieved by the cutoff characteristic of the waveguide.
  • A proper fundamental frequency rejection scheme must be implemented.
  • E-probe dampers can be used in the cavity equatorial plane without the rejection filter.
  • Since the radial component of the TMoI-like electric field is zero in the midplane.
  • Since the poor rejection characteristic increased the fundamental mode power loss.

2 LOOP COUPLED DAMPER

  • H-loop type coaxial damper with high-pass filter, also known as Figure I.

DISCLAIMER

  • Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or pmcess disclosed, or represents that its use would not infringe privately owned rights.
  • Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
  • The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
  • Qesistance and provide greater damping due to greater over-coupling in general.
  • The damping of the higher frequency HOMs are sensitive to the loop sizes, since the loop inductance may approach or exceed the quarter wavelength at these frequencies.

2.2 High-Pass Filter

  • For the rejection at 352 MHz in a compact package, a 9-th order coaxial Chebyshev-type filter was chosen [5].
  • Figure 3 shows the measured frequency characteristics of the high-pass filter.
  • All coaxial transmission line sections have a 1" outer diameter and a 50-R characteristic impedance.
  • The crossed tuning stubs support the center conductor of the filter rn hich consists of four capacitors and five copper pieces.
  • The capacitors are made with kapton dielectric and copper sheets around the dielectric.

2.3 Rf Absorber

  • For the dampers, matched load terminations ar used.
  • Water cooling is needed for the high-power couplers and the load.
  • Therefore, in an E-probe damper with water cooling through the center conductor of the coaxial structures, an aluminum nitride (AIN) ceramic absorber was used.
  • High power resistor-type rf loads can be used.
  • The mechanical and electrical properties of this termination are shown in Table 1 .

3 MEASUREMENT

  • Each cavity will have two E-probe dampers and two H-loop dampers when the dampers are ready.
  • In the present measurements, only two H-loop dampers with 9-th order Chebyshev high-pass filters have been used.
  • The coupling loops have an area of 3.7 in'.
  • Q-factors are measured using a vector network analyzer and compared with the computed results.
  • In mode designation, 0 and 1 denote the monopole modes and the dipole modes, respectively.

DISCUSSION

  • For the coaxial HOM dampers, it was desired to have lower deQing at the fundamental frequency while having higher damping at other harmful HOMs.
  • Fundamental deQing still exists.
  • Since the filter characteristic is measured for a 50-R system, the low resistance of the loop cannot have a high rejection ratio.
  • The dampers are compact and will dissipate low power at the fundamental frequency.
  • Compared with the deQing requirement of the HOMs in the A P S storage ring, the results of measurements show that many HOMs can be damped successfully with the coaxial dampers.

5 ACKNOWLEDGMENT

  • The authors thank Arther Vetter of Boeing Defense and Space Group for helpful information on the damper with high-pass filter.
  • The authors also thank W. Yoder and D. Bromberek for their effort in prototype fabrication and measurement of the dampers.
  • This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. W-3 1-109-ENG-38.

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.-
\I
COAXIAL
HIGHER-ORDER
MODE
DAMPER
e
EMPLOYING
A
HIGH-PASS
FILTER
Y.
W.
Kang and
X.
Jiang
Advanced Photon Source, Argonne National Laboratory
9700
South
Cass
Avenue, Argonne, Illinois
60439
USA
A
bstracr
Two different
types
of coaxial higher-order mode
(HOM)
dampers
have been investigated for the Advanced
Photon Source
(APS)
storage ring cavities: e-probe damp-
en
and
h-loop dampers. Realization of the h-loop dampers
has
been difficult because the
loop
antenna couples not
only
to
the HOMs but
also
to the accelerating mode and
results in loss of
Q
at the fundamental frequency. Previ-
ously, a first-order fundamental rejection filter was tested
with unsatisfactory rejection characteristics. This problem
can be overcome by using a higher-order high-pass filter
between the loop and the matched load. Prototype dampers
have been fabricated and tested in a storage ring single-cell
cavity and the damping characteristic
was
analyzed.
1
IXTRODUCTION
The single-cell cavities being used
in
the
APS
storage
ring are optimized sphericaI type cavities whose funda-
mental accelerating mode frequency is
352 MHz. Each
cavity frequency is adjusted with a mechanical plunger
tuner.
The
cavities are manufactured to have a slight elon-
gation with an increment
in
the lengths for a staggered fre-
quency characteristic at the HOMs. The effect of the
staggering cavity HOhI frequencies was studied and the
instability growth rates and the deQing requirement on
higher-order modes of the APS storage ring were reported
[I,
21.
For the
APS
storage ring cavities, dampers with
E-
probe and H-loop couplers have been developed to damp
HOMs in 352-MHz single-cell cavities for the storage ring
of the APS. The H-loop dampers employ high-pass filters
for fundamental frequency rejection. The fundamental fre-
quency rejection filter has a very steep rejection response
and minimum deQing at the fundamental frequency.
A
similar H-loop coupled coaxial damper with a higher-order
high-pass filter has been used for
333-MHz
cavities of an
accelerator
for
free-electron laser application [3].
For
HOM
dampers. broadband damping and less
fun-
damenta! mode damping are important. In the waveguide
dampers. the fundamental frequency rejection
is
achieved
by the cutoff characteristic of the waveguide. For coaxial
HOM dampers. a proper fundamental frequency rejection
scheme must be implemented. E-probe dampers can be
used
in
the cavity equatorial plane without the rejection
fil-
ter.
since the radial component of the TMoI-like electric
field is
zero
in
the midplane. Since the H-loop dampers are
used
in
the equatorial plane of the cavity. the loop plane
must
be
perpendicular to the fundamental mode H-field to
Zouple
uith the man) transverse magnetic
HOMs.
and
a\
fundamental frequency rejection filter is needed. Previ-
ously, H-loop dampers with
a
half-wavelength short stub
in
parallel or a quarter-wavelength short stub
in
series have
been tested
141.
The rejection characteristics of the single
stub designs were poor and also rejected some HOM fre-
quencies around the even and odd multiples of fundamen-
tal frequency. Since the poor rejection characteristic
increased the fundamental mode power
loss.
a damper
with a better rejection filter has been investigated.
2
LOOP COUPLED
DAMPER
Figure
1
shows
the loop coupling coaxial damper
assembly. The H-loops are used
in
the cavity midplane.
The loop plane is perpendicular to the
TMol
modeH-field.
Two dampers are
used
to damp the dipole modes. Both the
size of the loop and the location
in
the cavity determine the
coupling
to
specific mode fields and thus the damping
ratio.
Figure
I
:
H-loop type coaxial damper with high-pass filter.
2.
I
Coupling
Loop
Two H-loop type dampers are used on the
6"
ports
in.
the cavity midplane
with
an angular separation
of
90".
The loop plane
in
this position strongly couples to the
TM,, mode field. Therefore.
the
measured input imped-
ance
of
the coupling loop sh0u.s low impedance values
for
overcoupling. The coupling
loop
is
made
of
0.5"-diameter
copper tube.
Figure
2
shows
the measured input impedance
of
the
damper coupling loop
of
various sizes. The
low
impedance
of
the loop
at
a resonant frequenq shows that the mode
of
the frequenc?.
can
be
damped
b>
the
resistive termination
whose resistance
value
is greater
than
the input resistance.
The different coupling
gives
varied darnping at other mode
frequencies.
Greater
IOOD
sizes
:?>ye
lo\s.er
loop
input
..

DISCLAIMER
This report was prepared
as
an account of work
sporrsored
by an agency of the United
States
Government. Neither the United States Government
nor
any agency thereof,
nor
any
of
their employees, make any warranty, express or implied,
or
assumes
any
legal
liabili-
ty
or
responsibility for the accuracy, completeness,
or
usefulness
of
any
information, appa-
ratus,
product, or pmcess
disclosed,
or represents that its
use
would
not
infringe privately
owned
rights. Reference herein to any specific
commercial
product, process, or
service
by
trade
name,
trademark, manufacturer,
or
otherwise does not
necessarily
constitute
or
imply its endorsement,
recommendation,
or
favoring by the United
States
Government
or
any agency thereof. The views and opinions
of
authors expressed herein do not
necessar-
ily
state
or
reflect
those
of the United States Government or any agency thereof.


qesistance and provide greater damping due to greater
over-coupling in general. However, the damping
of
the
higher frequency
HOMs
are sensitive
to
the
loop sizes,
since the loop inductance may approach
or
exceed the
quarter wavelength at these frequencies.
For
the higher
fre-
quenoy
HOMs,
the high loop input impedance may result
in ander-coupling which gives
poor
damping.
For
the low
-bop impedance, the fundamental frequency rejection filter
needs
to
have low input resistance in order to have smaller
fundamental frequency damping. From the measurement,
it
can
be
seen
that
50-R
load resistance is adequate for
damping the
HOMs.
+
Tmpedance
150
Q
nominal
!
Power handling capabilit!
1150
W
CW.
1
OOOW peak
Thermal resistance
/0.3-
cnv
01%
COP
0.:
Figure
2:
Input impedance
of
the coupling loop of various
sizes:
(1)
5
inch’,
(2)
3.7
inch’,
(3)
2.4 inch’.
2.2
High-Pass
Filter
A
fundamental frequency rejection is necessary to
minimize the
loss
of
the accelerating field using
a
band-
stop or a high-pass filter. For the
APS,
an H-loop damper
design employing a high-pass filter has been investigated
to obtain a good fundamental frequency rejection. For the
rejection at
352
MHz in a compact package, a 9-th order
coaxial Chebyshev-type filter was chosen
[5].
The filter
has a 0.5-dB pass-band ripple and
70
dB rejection at
350
MHz. and a corner frequency
of
480
MHz.
Figure
3
shows
the measured frequency characteristics
of
the high-pass
fil-
ter. This frequency response is very close to
the
predicted
response from calculation. The construction
of
the filter is
shown
in
Figure
4.
All
coaxial transmission line sections
have
a
1”
outer diameter and
a
50-R
characteristic imped-
ance. The crossed tuning stubs support the center conduc-
tor
of
the filter
rn
hich consists
of
four
capacitors and five
copper pieces. The capacitors are made with kapton
dielectric and copper sheets around the dielectric. Sliding
shorts are used on the stubs to adjust the stub inductances
for the desired frequency response. The input and output
impedances are chosen to be
50
R
for simplified measure-
ment and near optimum Q-factor
of
the transmission lines.
The circuit component values
of
the filter elements are
shorn
n
in
Table
I.
Figure
3:
Measured frequency response
of
the filter.
Figure
4: Coaxial high-pass filter structure.
2.3
Rf
Absorber
For the dampers, matched load terminations
ar
used.
The load must be able to dissipate a certain amount of
power with good vacuum properties
if
no vacuum barrier
is
used. Water cooling is needed for the high-power cou-
plers and the load. Since reliable water cooling with metal
tubing always forms a short circuit for the
rf
signal, a
lossy
dielectric-type load is desirable unless a noninterfering
water passage is provided. Therefore,
in
an E-probe
damper with water cooling through the center conductor
of
the coaxial structures, an aluminum nitride
(AIN)
ceramic
absorber was used. However,
in
the H-loop damper
design, the center conductors of the coaxial stubs can pro-
vide water passage, and
so
compact broadband. high
power resistor-type
rf
loads can be used. This type of load
is made of
a
carbon resistor on a beryllium oxide (BeO)
ceramic substrate. The mechanical and electrical proper-
ties
of
this termination are
shown
in
Table
1.
The measured
frequency characteristic
of
the coaxial termination is
shown
in
Figure
5.
Tabie
1
:
Termination Specification

Figure
5:
Frequency response
of
the
50-R
resistive
termination.
3
MEASUREMENT
Each cavity will have two E-probe dampers and two
H-loop dampers when the dampers
are
ready. In the
present measurements, only two H-loop dampers with 9-th
order Chebyshev high-pass filters have been used. The
measured deQing ratios are shown in Table
2.
The cou-
pling loops have an area of 3.7
in’.
The listed modes are
the most dangerous HOMs
in
the single-cell cavity being
used in the
APS
and can cause beam instability. Q-factors
are measured using a vector network analyzer and com-
pared with the computed results. In mode designation,
0
and
1
denote the monopole modes and
the
dipole modes,
respectively.
M
and
E
denote the boundary conditions with
magnetic and electric walls in the cavity equatorial plane,
respectively. The result shows that most dipole modes
are
damped effectively; the fundamental mode is damped little
while the first dipole mode
is
damped significantly. Note
that some monopole modes can also be damped with the
H-loop dampers since the loop is offset from the cavity
midplane.
Table
2:
Measured Q-factors for the Higher-Order Modes
with Two Dampers with High-Pass Filters
I
1
UnloadedQ
1
DampedQ
1
Frequency
1
(MHd
I
Mode
!
4
DISCUSSION
For
the coaxial HOM dampers, it was desired to have
lower deQing at the fundamental frequency while having
higher damping at other harmful
HOMs.
The fundamental
mode deQing is due to the finite
Q
of
the input coupler and
the high-pass filter at the frequency. Although the rejection
ratio
of
the filter alone is measured to be
-70
dB which
provides almost no power transmission, fundamental
deQing still exists. Since the filter characteristic is mea-
sured
for
a
50-R
system, the low resistance
of
the loop
cannot have
a
high rejection ratio. The intrinsic resistance
of the copper parts in the loop and the filter input contrib-
ute to a
loss
that is not negligible. The next step
is
brazing
an annular alumina ceramic window between
the
coupling
loop and the filter input in order to eliminate the exposure
of the filter circuit to the vacuum. High power testing
of
the dampers will be performed with the ceramic window.
The measurements
show
that the coaxial dampers can
provide good performance
for the cavities without provi-
sion for special waveguide-type damper ports. The mea-
sured fundamental frequency deQing factor is about 9%
with two dampers. This
is
a
great improvement
f’rm
previ-
ous studies and may be one of the lowest values with the
HOM damping. The dampers are compact and will dissi-
pate low power at the fundamental frequency. Compared
with the deQing requirement of the HOMs in the
APS
stor-
age ring, the results of measurements show that many
HOMs can be damped successfully with the coaxial damp-
ers. This result is promising €or
use of the design in the
storage ring of the
APS.
5
ACKNOWLEDGMENT
The authors thank Arther Vetter of Boeing Defense
and Space Group for helpful information on the damper
with high-pass filter. The authors also thank
W.
Yoder and
D.
Bromberek
for
their effort in prototype fabrication and
measurement of the dampers. This work was supported by
the
U.S.
Department of Energy, Office of Basic Energy
Sciences, under Contract No. W-3
1-
109-ENG-38.
6
REFERENCES
[
11
[2]
L.
Emery, “Coupled-Bunch Instabilities in the APS Ring,” Proc.
of
the 1991 Particle Accelerator Conference, pp. 1713-1715, 1991.
L.
Emery. “Required Cavity
HOM
deQing Calculated from Proba-
bility Estimates
of
Coupled Bunch Instabilities
in
the APS Ring.”
Proc. of the 1993 Panicle Accelerator Conference. pp. 3360-3362,
1993.
A.
M.
Vetter.
T.
L.
Buller. and
T.
D.
Hayward.
D.
R.
Smith. and
V.
S.
Starkovich. “APLE Accelerator Prototype Cavity Fabrication
and Low Power Tests.” Proc. of the 1993 Panicle Accelerator Con-
ference.
pp.
1075-1077. 1993.
P.
J.
Matthews.
Y.
W.
Kang. and
R.
L.
Kustom. “Storage Ring Cav-
ity Higher-Order Mode
Dampers
for
the Advanced Photon
Source.“ Proc.
of
the 1995 Panicle Accelerator Conference. pp.
1684-
1886. I995
C.
Mathei.
L.
Young.
and
E.
.M.
T.
Jones.
.Microwuve
Fii’lters.
impedunce- J4archin.g.
.Vent.cirirc.
and
Coup1in.q
Srrucrures.
Anech
HOUSK.
1980.
[3]
[4]
[SI
Citations
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12 May 1997-
Abstract: The Advanced Photon Source (APS) is a 7-GeV full energy positron storage ring for generating synchrotron radiation with an injector. The booster synchrotron RF system consists of a single 1-MW klystron which drives four five-cell cavities at 352 MHz. The storage ring cavities consist of four groups of four single cells powered by two 1-MW klystrons for 100-mA operation. An overview of the operation of the APS 352-MHz RF systems is presented.

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12 May 1997-
Abstract: The higher-order modes (HOMs) of APS storage ring (SR) RF cavities and waveguides were measured under various operating conditions. The HOMs of the 352-MHz RF cavity can be one of the major contributors to the coupled bunch instability. The distribution of HOMs under various conditions of beam current, cavity temperature, cavity tuning, single-bunch and multi-bunch operation, and fill patterns, are presented. The HOMs' shunt impedance of the loaded cavities were also measured. The effect of stagger tuning of the 16 cavities and their waveguide system is compared, and the HOM dampers are examined.

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Abstract: A disk-type input coupler for the Pohang Light Source (PLS) storage ring cavity was fabricated and tested up to 60 kW A coaxial adapter is required for this system which should have an exit for the loop coupling tubes Since this input coupler can extract HOMs better than the cylinder-type one out of the cavity, a specially designed HOM absorber was implemented in the design of the adapter A characteristic of the system is simulated by the HFSS code The result of the simulation and a preliminary measurement are presented

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1 citations


01 Jan 1998-
Abstract: The higher-order modes (HOMs) of APS storage ring (SR) rf cavities and waveguides were measured under various operating conditions. The HOMs of the 352-MHz rf cavity can be one of the major contributors to the coupled bunch (CB) instability. The distribution of HOMs under various conditions of beam current, cavity temperature, cavity tuning, single-bunch and multi-bunch operation, and fill patterns, are presented. The HOMs' shunt impedance of the loaded cavities were also measured. The effect of stagger tuning of the 16 cavities and their waveguide system is compared, and the HOM dampers are examined.

References
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Proceedings ArticleDOI
Louis Emery1Institutions (1)
06 May 1991-
Abstract: A study of coupled bunch instabilities for the APS storage ring is presented. The instabilities are driven by the higher-order modes of the fifteen 352-MHz single-cell RF cavities. These modes are modeled using the 2-D cavity program URMEL. The program ZAP is then used to estimate the growth time of the instabilities for an equally spaced bunch pattern. The cavity modes most responsible for the instabilities is singled out for damping. >

13 citations


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Proceedings ArticleDOI
P.J. Matthews1, Y.W. Kang1, R.L. Kustom1Institutions (1)
01 May 1995-
Abstract: Coaxial, mode selective higher-order mode dampers for the Advanced Photon Source storage ring cavities have been fabricated and tested. Two types of dampers will be employed. Electric field probe dampers are positioned in the equatorial plane of the cavity so as to not couple to the fundamental TM/sub 01/ accelerating mode. Additionally, two magnetic field probe dampers with quarter-wavelength stub rejection filters are positioned in the cavity equatorial plane 90 degrees apart to facilitate dipole mode damping. Both damper types use a vacuum compatible, aluminum nitride (AlN) ceramic RF absorber as the matched load. Measurements were made to optimize the frequency response of the tapered absorber. The design eliminates the need for a ceramic vacuum window.

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Abstract: The first 5-cell, 433-MHz accelerator cavity for the APLE free electron laser experiment at Boeing has been assembled and tuned. Low power RF measurements indicate that the critical electrical parameters (Q/sub 0/, f, and /spl beta/) for the accelerator and TM/sub 110/-like modes are consistent with expectations based on measurements made on an 800-MHz model cavity. The cavity is being readied for its initial vacuum bakeout, during which the constituents of the residual gas will be monitored. First application of high power RF is scheduled for late summer of 1993. >

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