MEASUREMENTS ON THE POST STABILIZED LINAC
Shinj i MACH IDA
Department of Physics, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113, Japan
Takao KATO and Sadayoshi FUKUMOTO
National Laboratory for High Energy Physics
Oho-machi, Tsukuba-gun, Ibaraki-ken, 305, Japan
Main parameters of the model tank

Table·ABSTRACT

Number of cells
Frequency
Tank Length
Inside diameter
Outer diameter
Drift tube
Bore diameter
EnergyBeta

The Alvarez linac tank that is the model of thesecond tank at KEK is designed and fabricated to conA
firm the field stabilization by post couplers.
change of the on-axis electric field distribution due
to the post insertion shows strong evidence of thestabilization. When the post mode overlaps the accelerating one, the on-axis field is distorted drasticalInvestigating the stabilization, some parameters
ly.
It is
to find the stabilized point are obtained.
possible to tune post couplers accurately using theseparameters. Tab rotation at the top of a post coupler
has a great effect on the field. We can use them tomake a tilt on the field.

15
401.5 MHz2.51 m0.45 mem
8.0em
1.5
20.60 - 28.78 MeV0.2062- 0.2421

The calculated value and the measured one
diagram.
agree with each other within 0.07 %.
BOO~~INTRODUCTION700

At KEK, the second linac tank is designed to accelerate negative hydrogen ions from 20 MeV to 40 MeV.
For the second tank above, post couplers are to beinstalled to stabilize the field. One of the reasonsfor installing post couplers is that the mechanicalerrors at fabricating the tank and the beam loading
effect can be reduced by them. Post couplers make anenergy flow in the tank to compensate a local disturbIt is necessary
ance. Another reason is as follows.
to adjust the resonant frequency to that of the firsttank, and then two motor-driven tuners are provided.
This tuning also causes a change of the field distribution, thus a field stabilizer is necessary.
The Alvarez linac with fifteen cells is fabricatedand its characteristics are measured. To determine theparameters of the model tank, computer code, SUPERFISH
and PARMILA, are used. The field distribution is measured by a standard bead perturbation method.

~N~

600

6"" 500()~o TE21o TMOlo TEll(j);:l0'Q)h"""

400

300L-------------------~--------~010
Mode Number

Fig.l Dispersion curve without drift tubes
Second, Fig. 2 shows the Brillouin diagram withdrift tubes. The resonant frequency of TMOlO differsfrom the calculated value by SUPERFISH about 507 kHz or
0.1 %. This calculation is done with double precision.
The difference between the measured value and the calculated one is small enough.

MECHANISM OF THE STABILIZATION

ff

the stabilization by post couThe mechanism
plers is as follows 1 • Usually an Alvarez linac operates at a lower edge of a dispersion curve. We callThis operation has a disthis zero mode operation.
advantage that mechanical errors of a tank and a beamloading effect directly affects a field distribution,
because RF group velocity is zero. When post couplers
are inserted into a tank, the capacitance between apost coupler and a drift tube and the inductance arounda post coupler make an RF resonant circuit. All post
couplers form a chain of oscillators and a new dispersion curve of post couplers arises. Then the length ofpost couplers is tuned to couple the post mode with an
As a result of a confluence, theaccelerating one.
dispersion curve near the operating point has a finite
slope and the mode spacing between an accelerating modeand higher modes becomes large. The slope of a dispersion curve represents RF group velocity. This means
the existence of an energy flow in the tank and it becomes harder to excite higher modes. In this way thestabilization of a field is obtained.

600 ~--------------------,----------,

~

500

~

N::r:

6400

""

300()

~(j)o TMOlo TEllo STEM;:l0'Q)

h200"""G D D D 0 D 0 D D D1000

D D D El

10Mode Number

Fig.2 Dispersion curve with drift tubes

CHARACTERISTICS OF THE MODEL TANK
Furthermore, considering the dependence of aSUPERFISH calculation on its mesh size, this difference
can be reduced. Figure 3 shows the dependence of theresonant frequency on the mesh size. In this figure,"A" points out the value we use. Taking account ofthe difference between "A" and "B", which points outthe ideal value of infinitesimal size; the measured

The radius of the model tank is reduced to half
the second tank, so the resonant frequency becomes 401MHz. It has fifteen unit cells, corresponding to the20.60 MeV to 28.78 MeV section. Main parameters of thetank are shown in Table 1.
First, the characteristics of the tank without
drift tubes are measured. Figure 1 shows its Brillouin
83value approaches the calculated. one, and 507 kHz reduces about 100 kHz. On the other hand, the mechanicalerror of the radius is within 0.1 mm. Since the resonant frequ-ency of TM010 is in proportion to the inverseof the radius, the frequency error due to the mechanical error becomes about 100 kHz. This shows the calculation is quite accurate.

12r---------.---------.---------.-.
o unperturbed"' perturbed ll5mm
o - and stabilized
10404r-------.--------.-------.-------.
8'N 403

e::r:>.

CJ

6 ' - - - - - _ L_ _ _ __.___ _ ____L_J
051015402~BC1):::10"
C1)f.<

401!':..

~Cell NumberA!

Fig.4 Field distribution
before and after post stabilization-rn...,

400'------~-------L-------L------~

0.25

00.5

Mesh Size

0.75-~(em)

Fig.3 Mesh size dependence in SUPERFISH:::1>.

10t1l

8f.<

6f.<

f.<
......
;8"'

EFFECT OF POST COUPLERS

t1l..........

To observe the effect of post couplers, intent'ional perturbation of moving both of the end plates
toward the same direction is given. It causes a changeof the resonant frequency in first and last cells.
Table 2 shows the detail of it.

"d:§-4<a2<0Three kinds of perturbation to the end platesCase 2

Case 3ll1 mmfirst celllast cellll3 mmfirst celllast cell

Increase ofcell length

Change offrequency- 1 mm+ 1 mm- 1.5 MHz+ 1.0 MHz+ 3 mm

+ 3.5 MHz- 3 mm

- 3.0 MHz- 5 mm+ 5 mm>< distorted at 12.5 em

o stabilized at 13.5 em

105Cell Number0Fig,5 Post couplers effecton the field distribution

To represent the flatness fumerically, a distortion parameter Dx is introduced2 •Dx = I:iIF.1. - F.1. I

In Ungrin's paper, Fi is defined in each cell, but here
F. is taken as the average of fifteen cells. Figure
61 shows the dependence of Dx on the length of a postliS mm

first celllast cell~!

-~Table 2

Case 1

12- 6.0 MHz+ 4.0 MHz
5In each case, Brillouin diagram, shift of 'operating
frequency, and field distribution are measured.
In this experiment, the introduced frequency shift
as perturbation is about 1 to 6 MHz. In the case of
the second tank, it is about 0.9 MHz.

-rn......

4:::1>.

~o unperturbed
o endplate ll1mm

stabilized

0-~+f.<

Field distribution
Figure 4 shows the measured field distribution before and after the stabilization. In this figure, "unperturbed" expresses the condition that the end plates
are not perturbed and the tank is not stabilized, "perturbed" expresses that the end plates are perturbed ofcase 1 and the tank is not stabilized, and "perturbedand stabilized" expresses that the end plates are perturbed and the tank is stabilized. Without the seabilization, the field decreases as a cell number increases when the resonant frequency of first cell decreases and that of last cell increases. Post couplers
have a strong effect on such a field distribution, andwe can make a flat field with the stabilization. On
the other hand, post couplers also have an effect todistort the field when the post mode overlaps the accelerating one. Figure 5 shows the distorted field.
When the perturbation is ll1 mm or ll3 mm, the same results are obtained.

t1lf.<......
;82f.<~><Ci00510Post Coupler (em)

15Fig.6 Distortion'parameter definedby Ungrin, ·el aL
coupler in .three cases of perturbation. In each case,
we can observe a peak due to the distortion of a field
at 12.5 em. At 13.5 em a dip exists and it shows thefield is stabilized. Using this parameter, we can determine a.stabilized point accurately.

84

Brillouin diagram
Figure 7 shows the dependence of Brillouin diagramon the post length. As post couplers are inserted into
the tank, the capacitance and inductance increase andits resonant frequency decreases. When the post lengthis 13.5 em, dispersion curve near the operating point
has a finite slope and the mode spacing between
operating point and next one becomes large.
600
o no post
0 post 12.5cm
013.5cm
14.5cm

500~

N~

2§_

400:>-.
0>::

300<!)

Fig.9 The model tank with post copulers;:J0""<!)........

200
15,---------,---------,---------~

100

100

Mode Number
Fig. 7 Dispersion curve, changing post length

10Resonant frequency
Figure 8 shows the dependence of the resonant frequency on the post length. There is a critical transition point at 12.5 em. This frequency transition corresponds to the field distortion as shown in Figs. 5and 6. At the left of this point, the frequency decreases because the post mode pulls down an acceleratingone. At 13.5 em, the stabilized point, the frequencies
for three kinds of perturbation agree.

5o tab pointed up
pointed right<>
o pointed left
105
Cell Number
Fig.lO Field distribution vs. tab rotation

402.0~

No unperturbed
o endplate <'>lmm
0ll3mmll5mm"'

401.5~

charge, we select large tabs for the second tank. Inaddition, large tabs effectively influence the tilt on
the field when they are rotated.

6CONCLUSION:>-. 401.0()>::<!);:J0""<!)The stabilization by post couplers is achieved.
As shown in Fig. 4, it is possible to correct the field
distribution disturbed by mechanical errors. However,
we should note that post couplers distort the field
when the post mode overlaps the accelerating one.
Since there is a little distance between the stabilized
point and the distorted one, it is necessary to adjustSome information about thispost couplers carefully.
problem is obtained as follows.
It is confirmed that the distortion paramter isuseful to determine the stabilized point of a post
As shown in Fig. 6, there is the sharp dipcoupler.
that represents the stabilized point, so we can tune
the length of a post coupler accurately.
At the stabilized point'· the reso.nant frequency
remains constant for every kinds of perturbation. Thissuggests the possibility that the frequency can be used
as a parameter to tune the post length as well as thedistortion parameter.

"

400.5........
400.0

1505Post Coupler

1015(em)

Fig.8 Frequency shift by insertion of post,
taking endplate perturbation for parameter
TAB ROTATION AND ITS SIZEAt the top of a post coupler, a tab is attached as
When all tabs point to the same
shown in Fig. 9.
direction, up or down, symmetry of a post coupler and aExperiments mentioned above
drift tube is conserved.
Then these tabs areare done on that condition.
rotated to point right or left to change the coupling
Asymmetry of this geometry
between unit cells.
influences an energy flow in the tank and makes a tilt
Figure 10 shows the field distribution
on the field.
in three cases. The field decreases in the directionthat post couplers point to.
To determine the tab size, two kinds of tabs withareas l. 7 and 3. 0 times a post coupler cross section
are prepared. The field stabilization is achieved witheach tab and in the case of no tab the field is stabilized as well. The post length at the stabilized pointis 13.5 em for large tabs, 14.4 em for small tabs andTo prevent a post coupler dis15.9 em for no tab.

ACKNOWLEDGMENTS
The authors are indebted to Dr. K. Oide for thedevelopment of a computer program and all members of
the Injector group for their help.
REFERENCES
1)
2)

85

E.A. Knapp, B.C. Knapp and J.M. Potter, Rev. Sci.
Instr., 39, 652, 1968.
J. Ungri~ S.O. Schriber and R.A. Vokes, IEEE
Trans. Nucl. Sci., NS-30, No.4, 3013, 1983.