Table Of ContentSubmittedto ’Chinese Physics C’
A method for designing the variable-way high-power cavity combiner*
Yong-Tao LIU (刘勇涛) Gui-Rong HUANG (黄贵荣)1)
Lei SHANG (尚雷) Hong-Xiang LIN (林宏翔) Bai-Ting DU (杜百廷)
6 NationalSynchrotronRadiationLaboratory,UniversityofScienceandTechnology ofChina,Hefei230029, China
1
0
Abstract: Thecavitycombinerwasputforwardforhighpowercombiningduetoitsadvantagesoflargercombining
2
ability, variable input ways and less power loss. For the high power cavity combiner, it is better to keep the power
n loss ratio in a reasonable range, because much power loss would lead to strict requirements on cooling system. The
u
combiner with variable input ways is convenient for outputting different powers according to practical demands. In
J
this paper, a method for designing a variable-way high-power cavity combiner was proposed, based on the relation
1
between input and output coupling coefficients obtained by analyzing the equivalent circuit of the cavity combiner.
] Themethodcanmakethedesignedcavitycombinerinamatchingstateanditspowerlossrateinareasonablerange
h asthenumberofinputwayschanges. Asanillustratingexample,acavitycombinerwith500MHzandvariableinput
p
ways from 16 to 64 is designed, and the simulation results show that our proposed method is feasible.
-
c
c Key words: cavity combiner, variable ways, equivalent circuit, coupling coefficient
a
. PACS: 29.20.-c, 07.57.-c
s
c
i
s 1 Introduction efficients and then propose the method for design the
y
cavity combiner to achieve proper combining efficiency
h
p and coupling coefficient range. In order to demonstrate
The high power Radio Frequency (RF) Solid State
[ the method, numerical study on a cavity combiner as
Amplifier (SSA) has been widely employedas the power
an example of the application of the proposed method
3 supplyinacceleratorsinthepastovertenyears[1,2]. At
v is done using the Computer Simulation Technology Mi-
presentthe SSA commonly adopts the multi-stage coax-
9 crowave Studio (CST MWS) [7].
ialorwaveguidecombiningstructure,whosecapabilityin
9
6 accommodatinginputportsonperstageispoor,andthe
2 Equivalent circuit of a cavity combiner
5 number of inputs is not more than 12 in coaxial or 4 in
0 waveguidenormally. Inthe casewithhighoutput power
. The cavity combiner can be considered as a cavity,
1 of 100 kW or even more, the SSA’s combining tree will
coupledwithnidenticalinputportsandoneoutputport
0 be very complicated, and therefore it is inefficient and
[8, 9]. The simple way to analyze such a RF network is
6
expensive.
1 totaketheoutputportasthesourceend,andtheequiv-
To resolve this problem, the cavity combining tech-
: alent circuit in this case is shown in Fig. 1, where I is
v niquethatcanaccommodatedozensorevenhundredsof
the driving current, R is the impedance ofoutput port,
Xi inputports,wasproposedbytheESRF,andisbeingde- R isthecavityshunt0impedance,R istheimpedanceof
veloped in recent years [3–5]. The condition of an ideal S i
r input port, and n , n are the transformer ratios of the
a lossless combiner N ∗β =β is obtained in the 1 2
loop loop output cavity to input, output coupler, respectively. The cou-
design and calculation of the cavity combiner [6]. But
plingcoefficientsofinputandoutputcouplersaredefined
in our opinion this approximation ignoring the cavity
as
dissipation is, in fact, probably unsuitable. The cavity
R R
loss should be considered at first when designing a cav- βi=n2RS ,β0=n2RS . (1)
itycombiner,sinceitdeterminesthecombiningefficiency 2 i 1 0
andthenecessitytousethecoolingsystemforoutputting WecansimplifytheequivalentcircuitbynormalizingR ,
i
high power. In the paper we will analyze the character- and the normalized equivalent circuit is shown in Fig.
istic of the cavity combiner with power loss, derive the 2, where the input, output impedances and the shunt
relationship between the input and output coupling co- impedance of the cavity are represented by 1, β /β and
i 0
Received21Junurary2016
∗SupportedbyNationalNaturalScienceFoundationofChina(11079034)
1)E-mail:[email protected]
(cid:13)c2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of
ModernPhysicsoftheChineseAcademyofSciences andIOPPublishingLtd
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Submittedto ’Chinese Physics C’
β , respectively. Referring to the normalized circuit in The following derivations are based on the precondi-
i
Fig. 2, the impedance of the load end is tion of matching. Therefore, we have
1β β β0=nβi+1, (7)
Z = n i = i , (2)
L0 1+β nβ +1 (n−1)β +1
n i i Γ =− i . (8)
i nβ +1
i
and the reflection coefficient of output port is
According to the equivalent network theory, the trans-
mission coefficients are derived as
Γ =ZL0−ββ0i =β0−nβi−1. (3)
0 ZL0+ββ0i β0+nβi+1 Si0=snββi+i 1, (9)
Similarly, specifying an arbitrary input port i as the
β
source end, the load impedance is S = i (i6=j), (10)
ij nβ +1
i
1 β where i and j ports are different input ports. It can be
Z = = i , (4)
Li n−1+β1i+ββ0i β0+(n−1)βi+1 apnrodvtehdetphoawtetrhedisrseilpaatitoendbinetcwaeveitnytPheisoutput power P0
c
and the reflection coefficient of input port is P
β = 0. (11)
0 P
c
Z −1 β +(n−2)β +1
Γ = Li =− 0 i . (5) Thenwecandefinethepowerlossratioη andcombining
i Z +1 β +nβ +1 l
Li 0 i efficiency η of the combiner as
c
Ingeneral,theoutputportofcombinermustbematched
P 1
to the transmission line, which means η = c = , (12)
l P β
0 0
Γ =0. (6) 1
0 η =1− . (13)
c β
0
This is the fundamental characteristic of the perfect
multi-way combiner or splitter. 3 Design method
Asignificantadvantageofthecavitycombineristhat
the input couplers can be designed and manufactured
in a removable way. So according to the actual output
power requirement, we can adjust the amount of input
ways, and meanwhile maintain the RF amplifier mod-
ules that are connected to the input ways operating in
the efficientstate. Eventhoughonthe occasionthatthe
SSA’s output power and the number of the input ways
ofthecavitycombineraredetermined,theremovablein-
put coupler is still suggested. The reason is that on the
designandteststageofthecombiner,aprototypeisusu-
Fig. 1. Equivalent circuit of the n-way cavity combiner.
allyneededtobemanufactured. Itwillbeconvenientfor
carryingoutthe RFmeasurementandthepowertesting
oftheprototype,whichiscoupledpartialinputcouplers.
We redefine the input and output coupling coeffi-
cients as β (n) and β (n), respectively. Their relation
i 0
is
β (n)=nβ (n)+1, (14)
0 i
wherenisthenumberofinputs. Ifthemaximumamount
of input ways is set to N, we have
β (N)=nβ (N)+1. (15)
0 i
Whenoutputtinghighpower,thecombiningefficiencyis
important for the cavity combiner. For example, in the
Fig. 2. Normalized circuit of then-way cavity combiner. case of 100kW output, it is better to set the loss rate η
l
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to be less than 1%. Because in this case the air condi- theacceptrangebychoosingaproperinputcouplingco-
tioningcanwellmeetthecavitycoolingrequirements,or efficient. The amount of inputs n may vary from one to
else the water-cooling system probably has to be used. dozens, and thus the output coupler must be tunable in
According to Eq. (12), η is inversely proportion to a largerange. So the waveguidecoupler shouldbe taken
l
β . The larger β is, the higher the combining efficiency into consideration [10, 11].
0 0
η becomes. Butiftheβ islargerthan200,itisdifficult Onthebasisoftheaboveanalysis,comparedwiththe
c 0
toberealizedduetoastrongperturbationoftheelectro- firstmethod,thesecondmethodismoreconvenientwhen
magnetic field caused by the output coupler. Therefore, the amount of input couplers changes. Therefore, the
the β (N) is suggestedin the rangeof 100 to 200,which second method is adopted for designing the high power
0
corresponds to the η in the range of 0.5%-1% for a cav- cavity combiner.
l
ity combiner with 100 kW or more output power. In The relation between the input and output coupler
addition, β (N) < 1 (under-coupling state) is also un- coupling coefficients that are calculated with different
i
suitable. In this case the geometrical dimensions of the values of power loss ratio η varying from 1%-5% is pre-
l
input coupler are small, so it is hard to maintain the sented in Fig. 3. From this figure, it can be clearly seen
coupling coefficient stable, due to that the coupling co- that for a givenconstantβ , we canobtain the rangesof
i
efficient is easy to be changed even the coupler with a combining number and power loss ratio, which are con-
small mechanical deviation or deformation. Therefore, venient for designing the cavity combiner.
when designing a N-way cavity combiner with the max-
imum output power of 100kW or more, the range of the
10.5
parameters are suggested as l=1%
9.0 l=2%
100≤β (N)≤200
0 . (16) 7.5 l=3%
( βi(N)≥1 l=4%
6.0
The upper limit of N is i l=5%
4.5
β (N)−1 200−1
N= 0 ≤ =199. (17)
β (N) 1 3.0
i
1.5
If the number of the input ways decreases from N to n,
there are two methods to maintain the cavity combiner
0.0
under the matching condition. 0 16 32 48 64 80 96 112 128 144
The first method is to keep β (n) = β (N). This n
method has the advantage of maint0aining a0stable com- Fig. 3. (color online) The change of βi with input
ways n at different power loss ratios η.
l
bining efficiency of the cavity combiner. But the input
couplingcoefficientwillincreaseasthe amountofinputs
n decreases, 4 Application
N
β (n)= β (N). (18)
i n i To demonstrate the method, we carried out numer-
ical research on a 500 MHz cavity combiner prototype
Duetothatalargeamountofinputsaremountedonthe
design using CST MWS. The coefficient β is set to 1.5,
cavity, it will take a lot of work to tune the input cou- i
referring to Fig. 3, which corresponds to the ranges of
plers when the input amount decreases. Besides, it will
n and β are 16-64, 20-100, respectively. The specific
increase the electromagnetic field perturbation, because 0
parameters of the cavity combiner prototype are listed
the geometricaldimensions ofloops arevery largeif n is
in Table 1.
small.
Thesecondmethodistokeepβi(n)=βi(N),andthe Table 1. The parameters of the cavity combiner
output coefficient is changed as prototype.
β (n)=nβ (N)+1. (19) frequency/MHz 500
0 i
max. output power/kW 80
The advantage of this method is that we can only tune inputpowerperway/kW 1.5
the output coupler to maintain the cavity combiner rangeoftheinputwaysn 16,32,48,64
matching. With this approach, it will change the com- inputcouplingcoefficientβi 1.5
bining efficiency of the cavity combiner since it is deter- outputcouplingcoefficientβ0 20-100
mined by β (n). But the loss power can be maintain in
0
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4.1 Resonant cavity and couplers design Fig. 4. The coupling coefficient can be adjusted by at-
tachingarotatableflangeoncoupler. Thedimensionsof
Thelayoutofourdesignedcavitycombineroperating
theloopthatcanbeestimatedbytheanalyticalequation
at 500 MHz is shown in Fig. 4. The cavity is made of
in the reference [13] and then the coupling coefficient of
aluminium and the cross-section of the cavity is regular
the input couplers is optimized to 1.5.
hexadecagon. The sixteen side walls of the cavity are
AWR 1800waveguidetransitionto 6 1/8 inchcoax-
assembled and disassembled, eight of which will totally
ial used as the output coupler is shown in Fig. 4. The
employ 64 input couplers. The TE mode is chosen
010 shortportofthewaveguideisshiftable,whichhasalarge
as the operating mode and the radius of the incircle of
effect on tuning the coupling coefficient β . To guaran-
16 polygon is R =2.405∗λ /(2∗π) [12]. To make sure 0
0 0 tee that the coupling coefficient can be varied between
thatthereisenoughbandwidthbetweenTE andother
010 15 and 110, which needs some allowance, the length of
modes, the height of the cavity must be set to a proper
probe and the shiftable short port of the waveguide are
value.
calculated and chosen by CST MWS.
4.2 Simulation results
The numerical study on the cavity combiner is car-
ried out from 16 to 64 input ways, which increases in
step of 16.
There are 16 same input couplers and output cou-
pler optimized above that are symmetrically mounted
on the cavity by columns, and each side wall employed
with 8 couplers. Before simulation, we define the out-
put waveguideas 1 port, the input couplers as 2-17port
by columns. Due to the perturbation caused by cou-
plers, the resonant frequency of the cavity may deviate
from the design value, so the tuner is needed, which is
inserted into the bottom of the cavity as shown in Fig.
4. The calculated results of the S parameter of 16 ways
cavitycombineraregiveninthe Fig. 5. We canseethat
Fig. 4. The layout of thecavity combiner.
thescatteringcoefficientS isbetterthan-30dBat500
11
Magnetic-coupling loops mounted on the cavity are MHz,whichindicatesthecombinerismatchedbytuning
usedtofeedenergyintothecavitywhichisalsoshownin the short port of the waveguide and the tuner.
Fig. 5. (color online) The S parameters of 16 ways matching cavity combiner.
Besides, the cases of 32, 48, 64 input couplers sym- which means that the combiner achieves at the match-
metricallymountedonthecavityarestudiedinthesame ing condition. As compared with 16 ways, in the case
wayasin the caseof16input couplersabove. By means of 64 ways the short way of the waveguideis shifted 100
of adjusting the short port of the waveguide and the mm along the waveguide transmission direction under
tuner, the S of 32, 48 and 64 ways cavity combiner is the matching condition. The matching scattering coef-
11
tuned less than -30 dB at resonant frequency 500 MHz, ficients of 64 ways cavity combiner are given in Fig. 6.
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Fig. 7 shows the change of the maximum, minimum dB.This isbecausethe TE fieldstrengthsatdifferent
010
input reflectance coefficients in the simulation and the couplingloopsareslightlydifferent. Wecanalsoseethat
theoretical reflectance coefficients, which are calculated the theoretical Γ always keep in the range between the
i
byEq.(8),withtheamountofinputways. Arepresenta- simulated maximum and minimum Γ , which illustrates
i
tive of transmit coefficient S and its theoretical values alltheinputwayswithgoodRFcoherenceandthesimu-
15
calculatedbyEq.(9)inthesimulationvaryingalongthe lationresultswereconsistentwiththetheoreticalresults.
amount of input ways is shown in Fig. 8. We can see from the Fig. 8 that the simulated transmit
ItcanbeseenfromFig. 7thatthedifferencebetween coefficientS arealsoconsistentwiththetheoreticalval-
15
thesimulatedmaximumandminimumΓ islessthan0.1 ues.
i
Fig. 6. (color online) The S parameters of 64 ways matching cavity combiner.
Allthesimulationresultsabovedemonstratethatthe Fig. 8. (color online) The change of S with the
15
cavitydesignedusing the proposedmethod is capableof amount of input ways.
combining 16-64 same input ways, just needing to tune
theshortportofthewaveguideandtunerwhenthenum-
5 Conclusion
ber of input ways changes, and the results are in good
agreementwiththeproposedmethod, whichmeansthat
Inthis paper,we firstanalyzedthe equivalentcircuit
the design method of cavity combiner is feasible.
ofthe cavitycombiner,andderivedthe relationbetween
the input and output coupling coefficients, as well as
-0.1
thescatteringparametersunderthematchingcondition.
Based on the obtained relation and practical applica-
-0.2
tion, it is suggested that the range of power loss rate
B) Min for the maximum outputting power is 0.5%-1% and the
d -0.3 Max
( numberofinputwaysofthe highpowercavitycombiner
/ i Theoretical is less than 200. Then we proposed a method for de-
-0.4
signing the variable-way high-power cavity combiner, in
which the input coupling coefficient is constant and the
-0.5
output coupling coefficient is tunable. The method was
appliedtothedesignofacavitycombinerwith500MHz
-0.6
and variable input ways from 16 to 64, and the simula-
16 32 48 64
Fig. 7. (color online) The cnhange of Γi with the tion results agree well with the theoretical RF parame-
amount of input ways. ters, thus demonstrating the reliability of the proposed
method. In addition, to experimentally test and verify
thesimulationresults,aprototypeisbeingdesignedand
-11
Simulated will be manufactured.
-12 Theoretical
-13
)
B
d -14
(
S / 15 -15
-16
-17
-18
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-19
16 32 48 64
n
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