The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Improveme nt of Bridge Coupler for Disk-and-W asher Structure
Hiroyuki AO, Yoshihisa. !WASHITA,Akira NODA Hiromi OKAMOTO, Toshiyuki SHIRAI and Makoto INOUE
Nuclear Science R~search Facility, Institute for Chemical Research; Kyoto University,Gokanosho, Uji, Kyoto 611 JAPAN

Abstract
A high power model of the DAW cavity for electron acceleration is under fabric().tion. In this model,
two separate accelerating tubes are connected by a coaxial bridge coupler. The accelerating mode frequ:ency(fa)and the coupling mode frequency(!c) of the bridge coupler were calculated by SUPERFISH and MAFIA andtuned to the operating frequency (2857MHz). We adopt
a choke termination at the supports of the bridge coupler. It is effective for adjusting both frequencie_s. Basedon these results, the final dimensions were fixed by coldmodel measurement. This modification brings muchhigher RF coupling between cavities with the bridge
coupler, so-that the electric field distribution becomes
uniform and the tilt sensitivity is improved.

Washer/Eviicuation PortSupport.

Fig. 2 Conceptual design of the bndge coupler

1 Introduction
The DAW( disk-and-washer) linac structure has highshunt impedance, good vacuum properties, and high stability. These advantages are suitable for a high betaaccelerator. The mode overlapping problem that was.found in early studies have been overcome by adopting
the biperiodic structure.[!]A high power model of the biperiodic 1-support
DAW(see Fig.l) for an electron acceleration is under fabrication and test. The operating frequency is designed at2857MHz which is. the same as the disc-loaded waveguideaccelerating tubes used in our laboratory. The injectionenergy for the acceleration test is intended to be about
60MeV. In this model, two separate 1.2m long accelerating tubes are connected by a coaxial bridge coupler
which has an RF coupler, a vacuum port, and three frequency tuners on its outer wall. Figure 2 sho:ws theconceptual design of the bridge coupler. A spool Is fixedby four supports. It replaces the two washers so that
one accelerating gap is filled with the spool.

pportwasherFig. 1 DAW with biperiodic L-support

2 Accelerating Mode and Coupling ModeTwo RF mode frequency, accelerating mode and coupling mode, must be adjusted to the operating frequency
in the DAW structure.
The accelerating mode generates the strong electric field at the accelerating gaps (see Fig.3). The frequency of the high power RF supplied from a klystronis 2857MHz which is as same as the disc-load linac. So,
the frequency of the accelerating mode must be adjusted
to the operating frequency (2857MHz) exactly.
On the other hand the coupling mode produces coupling between acceler~ting cavities (see Fig.4). Thismode redistributes stored energy and recover the flatness of the accelerating field when the field flatness isdisturbed.
Although there are the two modes at the same frequency, only the accelerating mode is excited bec.auseof the boundary condition at the end plate of the hn.ac.
Only when the field flatness is disturbed, the ~ouphng
mode is excited. It i_s not necessary to adJust thecoupling mode frequency to the accele~ating mode ?nestrictly, because the DAW has a very h1g~ RF couphng
between the cavities. But, if the couphng mode frequency is located far from the accelerating mo~~ one,this frequency difference breaks flatness or stab1hty of
the accelerating field. So, it is desirable to .keep thecoupling mode frequency close to the acceleratmg modeone.

3 Tuning of the Accelerating Mode Frequency
Firstly, we estimate the effect of the supports o~ thefrequency. The dimensions of the spool are determmedby the SUPERFISH calculation so that the accelerating mode frequency(fa) matches to about 2857 MHz.
Then, the frequency is re-calculated for both cases--176-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

with supports and without supports-b y MAFIA. The
cross-section of the support is treated as square in theMAFIA calculation because of its mesh characteristics.
By comparing the results of both cases, the support effect on the frequency is estimated as about lOMHz increase for the accelerating mode. The correction of thedimension is calculated by SUPERFISH so as to compensate the support effect. And then, the frequency isre-calculated including the supports by MAFIA. Thesesteps are repeated until the frequency converges to thedesigned value. The final results are shown in Table 1.
The electric field plot obtained by SUPERFISH is shownin Fig.3,4.

""'"

Table 1
The results of the accelerating mode frequency(!a)
calculation.

SUPERFISH
MAFIA( without support)
MAFIA( with support)
Fig. 5ethe support.

make an RF insulation between the outer wall and thesupport. Fig.5 shows the mesh for the MAFIA calculation. The accelerating mode frequency(fa ) and thecoupling mode frequency(fc) are calculated by MAFIA
changing the length of the coaxial area. The cross section of the outer conductor is treated as square again,
The results are listed in Table 2. It decreases the coupling mode frequency drastically, while the accelerating
mode frequency is not changed much.
Fig. 3

Table 2
The results of the MAFIA calculation. fa is calculated
for Omm and 24.8mm only.
coaxial length [mm]
023.5
24.8
25.5

fa [MHzJ
2855.652852.12

fc [MHz]3242.862917.312855.142824.67

5 Cold model measurem ent

Fig. 44 Tuning of the Coupling Mode Frequency

As the electric fields(Fig.3,4) show, the support isperpendicul ar to the electric field of the acceleratingmode, while it is parallel to that of the coupling mode.
In order to recover the support effect on coupling fre-Because approximati on errors cannot be avoided in
the calculations, the final dimensions should be determined by cold model measurement. The cold model ofthe bridge coupler with choke termination supports wasfabricated. Then it was connected with the cold modelof regular cells. The frequencies of the two modes andthe electric field distribution were measured.

5.1 Electric field distribution

quency, we adopt the ~ coaxial choke termination to-177-

It is expected that the high RF coupling gives more

The 11th Symp?sium on Accelerator Science andTechnology, Harima Science Garden City, 1997

flat distribution of the electric field. And it reduces theeffect from the fabrication error too. Two types of coldmodel are measured. One has the choke termination,and the other does not. Fig.6 and Fig. 7 show the results. The bridge coupler is installed at the center( cellnumber 0) of the accelerating tube. Two figures showsthat the improvement of the coupling mode frequency
brings the high RF coupling constant and high stabilityof the electric field.

TS is defined as follows:

TS = (x 100 [%/MHz]

where E;t and E;; are the on-axis electric field of the celln in the first and second measurement. Fig.8,9 shows TS
measurements.

0.50.11 1

Q-·· TS(%)0.5MHz

40

0.0-8- TS(%)1.0MHz

l

N' 20

0.40

J:
~

c~

0.30
0.20!,t - ~~En +En l:::,.fl-±ll±l-l±l-14i+I=FR+.-----------r ·---..!-~---------·--·-+------ --+ -t- ------ ·--20}--40cell number151015--·---- .---- -~+- ··-- _·10 ·5100 .5 10_lf.L20Fig. 8 TS measurement without choke termination

cell number

Fig. 6 Field distribution of no choke termination(f a=285 7[MHz] ,fc=3029 [MHz])
10N s.oJ:~ 0.0Cf<.

"iii..... .s.o·10·15

20cell number·10·50

cell number

Fig. 7 Field distribution of choke(fa=2854[MHz],fc=2857[MH z))

Fig. 9 TS measurement with choke termination

10termination

5.2 Tilt sensitivity
Tilt sensitivity (TS) is an index of the field stability.
The effect from fabrication errors is estimated.
The field stability was investigated as follows : Twofield distributions with different detuning of the end cells
are carried out :6 Conclusion

The cold model with the choke termination was fabricated and measured. The frequencies of accelerating
and coupling modes were tuned to the operating frequency. This improvement brings the high RF coupling
which leads to the flat field distribution and the low tilt
sensitivity.
In a high power model, a vacuum, cooling and RF
contact must be considered .. The design of high powermodel is proceeded on the basis of these results.
Acknowledgment

1. The accelerating mode frequency is lowered bysticking perturbation rods into both the end cells.
2. One rod is pushed in more to lower the frequency
by llf.
3. The other rod is pulled out to restore the frequency.

The version of the MAFIA used here is release 2.04.
The version of SUPERFISH is Release 4.12. Computation time of MAFIA was provided by the Supercomputer
Laboratory, Institute for Chemical Research, Kyoto University.
·4. Measure the field distribution by the bead-pullmethod.

References

5. Measure the distribution with perturbation in opposite direction, which tends to tilt the field opposite
direction.

[1) Y. !washita, "Disk-and-washer structure with biperiodic support", Nuclear Instruments and Method inPhysics Research A348, pp. 15-33 (1994)-178-