WE6RFP102
Proceedings of PAC09, Vancouver, BC, Canada
PROGRESS TOWARDS A 9.37GHZ HYBRID DIELECTRIC-IRIS-LOADED
STRUCTURE FILLED WITH LOW LOSS DIELECTRIC*
Xiaodong He#, Cong-Feng Wu, Sai Dong, Yuanji Pei
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei,
P. R. China
Abstract
One of the major concerns in the development of
hybrid dielectric-iris-loaded structure is the performance
of the used dielectric. The previous dielectric is
machinable but the loss tangent is slightly high. So we
adopt the new dielectric (Mg-Ca-Ti-O) with loss tangent
of about 2e-4. Because of its high hardness and brittleness,
the machining technology and methods are attempted.
Here we present some research results of the structure
with the new dielectric. Especially, the coupler for the
structure with this dielectric is designed with Frequency
domain solver of the Microwave Studio (MWS) based on
the Kyhl method. This calculation method can save much
more time under the same precision than other methods.
The experimental results show that the SWR˘1.02 at the
operation frequency and SWR˘1.1 at arrange of 20 MHz
frequency bandwidth.
INTRODUCTION
There are obvious advantages for the X-band
accelerating structure compared with the S-band one.
First, the shunt impedance per unit length of X-band is
higher than that of S-band. Second, the maximum
permissible electric field strength is also higher.
The hybrid dielectric-iris-loaded structure may have
lower ratio of peak surface electric field at the iris to axial
accelerating electric field by optimizing the geometric
parameters, while r/Q of the new structure being
comparable to iris-loaded accelerating structure.
Using machinable dielectric whose dielectric constant
being 5.81, the model cavities and coupler were
investigated by large numbers of experiment with the help
of MAFIA code and MWS T-Domain Solver [1], [2]. But
the loss tangent of this dielectric is slightly high. We
studied the hybrid dielectric-iris-loaded structure with the
new dielectric of its loss tangent being about 2e-4 instead
of the above dielectric.
The RF coupler is an important component for an
accelerator structure. Originally the coupler design is
performed experimentally by Kyhl method [3]. This cost
one too much more time. Some numerical calculation
methods based on 3D-code such as MAFIA had been
discussed in detail [4, 5]. However, by these means it is
inconvenient to model a complicated 3D structure. The
calculation time for getting the results is long with time
domain method in MAFIA. By using the PBA (Perfect
Boundary Approximation) and TST (Thin Sheet
___________________________________________
* Work is supported by the National Nature Science Foundation of
China, Grant No. 10375061, 10375060 and 10675116
#
[email protected]
Technology) for mesh generation and F-Domain Solver in
MWS, we can get the same accuracy quickly compared
with the one in MAFIA.
In general, to iris-loaded waveguide structure, the
position of the short plunger for detuning is in the centre
of the coupler cavity. Large numbers of calculations for
the new structure show that the coupler cavity cannot be
detuned when the plunger is set in the centre of the cavity.
The investigations of the simulation and experiment for
the new coupler are made. The above results are
presented.
RF COUPLER DESIGN
The operating frequency is chosen to be 9.37GHz. The
dimensions of the regular cells in our model have been
designed for that frequency at the 2π/3 phase shift per cell.
The sketch of the accelerator cavities are shown in figure
1. The RF Properties of new structure is given in Table 1.
The ratio of peak surface electric field at the iris to axial
accelerating electric field is equal to 1.1.
Figure 1: Sketch of regular accelerator cavities. In the
figure, region I is vacuum; region II is ceramic with
dielectric constant Ԗ୰ ; region III is copper, a is the disk
radius, b is the outer radius, and h is the beam hole radius.
t is the thickness of the disk, and d is the length of one
cell.
Table 1: The Properties of the New Structure With
t=1.5mm, d=10.665mm and λ =32.017 mm
a
(mm)
4.5
b
(mm)
6.376
h
(mm)
2.5
߳௥
6.45
Es/
Ea
1.1
R
(MΩ/m)
50.78
Q
vg (c)
5068
0.048
Because of high hardness and brittleness of the new
dielectric, diamond tools were used for machining. The
structure consisted of two accelerating cavities and two
half of cavities located at both sides is used to measure
the operating frequency. The reflection parameter S11 vs
the frequency was got through Network Analyzer
Hp8722D, which is shown in Fig. 2. The 2π/3 mode
frequency is 9.36325GHz. Considering the influence of
the coupling probe, temperature and humidity, the actual
frequency is close to 9.37GHz.
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WE6RFP102
the new struucture show that the possition of thee
plunger shouuld be apart from the co
oupler cavityy
centre (3mm forward the electric gun) for detuning..
To iris-loadedd waveguide structure, the position is inn
the centre of the
t cavity.
f
acceleraating cavity, calculate thee
2) Detune the first
phase of ˆ୫ୣୟ୬ and change 2b till to οɔ୫ୣୟ୫
ൌ ͳͺͲ଴ .
୫
3) Keep the pluunger in the first accelerrating cavity,,
calculate thee phase of ˆమಘ and chan
nge h till too
οɔమಘ ൌ ʹͶͲ଴ Ǥ
Figure 2: Thee measuremennt result of 2π
π/3 mode frequuency
v S11.
vs.
The idea of
o “the phase numerical
n
sim
mulation calcullation
for matchingg RF coupler” is inspired from the experiiment
design, i.e., Kyhl
K
method [3]. Accordingg to the impeddance
analysis, an important
i
bassis for the optimal matchingg and
tuning of couupler is shownn in Fig.3 by Smith-chart. Three
T
frequencies ( ˆπΤଶ ǡ ˆଶπΤଷ ǡ ˆ୫ୣୟ୬ ) shoulld be taken into
account. Heere ˆ୫ୣୟ୬ ൌ ሺˆπΤଶ ൅ ˆଶπΤଷ ሻȀȀʹ . Detunee the
coupler cavity with thee usual metaallic plunger, and
determine thhe “detuned short” posittion in the input
regular wavveguide. Detuune the folloowing cavity,, and
determine thee positions off ˆ୫ୣୟ୬ , ˆπȀଶ annd ˆଶπȀଷ .
Figure 3: Sm
mith chart ploot, first cavity properly couppled
annd tuned.
య
య
4) Repeat steps 1, 2 and 3, tilll the coupler is tuned and
matched. Fig..7 shows the S
S-parameter polar plot
simulated by MWS after tuuning and mattching the
coupler cavityy.
mulated by
Figure 5: Detunning the coupling cavity sim
MWS
S.
ure 6: S-param
meter polar ploot by detuning
g the coupling
Figu
caavity simulatedd by MWS.
Figure 4 shows the skketch of the coupler.
c
Therre are
three dimenssions to be determined:
d
t coupler cavity
the
c
radius b, the coupler apertuure h and its thickness
t
t.
Figure 7: S-pparameter polaar plot after tu
uning and
matching thee coupler cavitty simulated by
b MWS.
Figure 4: Skketch of the cooupler.
In numericcal simulationn design the cooupling and tuuning
status of RF
F coupler caan be realizedd from the phase
p
relations of the several modes as deescribed beloow to
determine thhe optimizatioon direction of
o the key sizzes of
RF coupler.
1) Detune the couplingg cavity by a short meetallic
plunger (Fig.5), andd calculate thhe phase of ˆ୫ୣୟ୬
whose result is shownn in Fig. 6 byy MWS F-Doomain
E
larrge numbers of calculationns for
solver. Especially,
A 4-cell modell is generateed by MWS.. The modell
conssists of one input
i
couplerr cavity, two
o acceleratingg
cavitties and one output
o
couplerr cavity as sh
hown in Fig.8..
Thesse two tapereed rectangularr waveguides are uniform..
The simulation reesult by MWS
S T-Domain Solver showss
that the (accelerrating) TM01 mode hass the centerr
frequ
uency of 9.37 GHz, where the reflection coefficient iss
less than -45dB and
a the transm
mission coeffficient is overr
99%
% as shown in Fig.
F 9.
With
W
the helpp of the M
MWS Field Monitor,
M
thee
ampllitude and phaase of the axiaal field are giv
ven in Fig. 10..
From
m this figure, we can find tthat: the axiall electric fieldd
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WE6RFP102
Proceedings of PAC09, Vancouver, BC, Canada
in the coupleer cavity is low
wer than the accelerating
a
caavity,
the phase shhift of the fouur cells is aboout 3600, the phase
p
variation in each coupleer cavity is nearly
n
zero across
a
forward halff the cavity and
a totals to 600 for the whole
w
coupler cell, the phase shhift of two reegular accelerrating
cavities is 24400, this meanns the coupler is consist of half
h a
standing caviity and half a travelling cavvity.
Figure 11: The
T assembledd structure forr cold testing.
Figure 8:
8 4-cell modeel for T-Domaain calculationn.
Figure 9: S-parameter
S
prroperties of thhe X-band hybbrid
dielectricc-iris-loaded structure
s
(MW
WS simulationn)
Figure 122: The measurrement result of
o SWR vs.
freqquency.
he different between caalculation ressult and thee
Th
expeeriment one abbout the couppler aperture iss 0.2mm. Thee
otherr sizes keep unchanged. T
This means that
t
the Kyhll
meth
hod can give an effective theoretical in
nstruction forr
RF coupler dessign of hybbrid dielectrric-iris-loadedd
struccture. Thereeby manufaccture processs and thee
expeeriment are greeatly simplifieed.
SUMMA
ARY
We
W have develloped the X-bband hybrid dielectric-irisd
loaded structure based
b
on the nnew dielectricc. The couplerr
desig
gn was madde quickly aand accurately by usingg
Freq
quency Domaiin Solver of M
MWS. Cold testing
t
showss
good
d transmissionn over the operation frequen
ncy band. Thee
deep
p experimentaal investigatioons on these issues are inn
prog
gress.
REFERENCES
Figure 10: Plots
P
of ampplitude and phase
p
of the axial
electric field in the centre of acceleratinng structure (M
MWS
simulation). The field magnitudes
m
aree normalized to 1
Watt RF pow
wer.
E
EXPERIM
ENT RESU
ULTS
Fig. 11 shows the assem
mbled structurre for cold testing.
f
acceleratting cavities and two cooupler
There are five
cavities in thhis structure. The
T voltage sttanding wavee ratio
(SWR) is measured
m
withh Network Analyzer
A
Hp87722D
under the coondition of the coupler cavvity radius b being
b
held 5.551 mm
m , w beinng 7.6mm.andd h being 1.55mm.
SWR˘1.02 at the operatiion frequencyy and SWR˘1.1 at
2 MHz freqquency bandw
width are achhieved
arrange of 20
(shown in Fig. 12).
[1]C
Cong-Feng Wu,
W
Lin H
Hui, Wang Lin, etc.,,
“Modelcavity
“
investigationss and calculations on HOM
M
for
f
an X-band
X
hybrrid dielectrric-iris-loadedd
accelerating
a
sttructure”, Pacc07, 2007, 06.
[2] Xiaodong
X
Hee, Cong-Feng Wu, Sai Do
ong, “Couplerr
design
d
and teest for X-Bannd hybrid loaded circularr
waveguide
w
acccelerator”, ICM
MMT2008, 2008,
2
04.
[3]E. Westbrook,, Microwave Impedance Matching off
Feeed Waveguide to the Disk-loaded Acceleratorr
Sttructure Operaating in the 22π/3 mode, SLAC-TN-63S
10
03, December, 1963.
[4] Derutyer
D
C K Ng H, Ko K
K, Numerical simulation off
coupler
c
Cavities of Linacss: SLAC-PUB
B-6220, Aprill
1993.
1
[5] K.
K Jin, Nuclearr Instruments and Methods in Physics
Research
R
A 5339 (2005) 1000–104.
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