EFFECTS OF GAPS AT LONGITUDINAL VANE JOINTS ON RFQ FIELD
N. Ueda, T. Nakanishi
Institute for Nuclear Study, University of Tokyo
Midori-cho, Tanashi, Tokyo 188, JapanandK. Inoue
Tsurumi Works, Toshiba Corporation
Suehiro-cho, Tsurumi-ku, Yokohama 230, JapanOn each end flanges four capacitive tuners aremounted. The tuner is a copper rod of 25 mm indiameter. On the cy 1i nder wa 11 there are 32 ho 1es of14 mm dia. for a perturbing metal block, rf feed andThepickup. The cavity is excited with a loop.
resonant frequency of TE210 mode was predicted at296.5 MHz with SUPERFISH calculation.
The vanes are assembled within an error of 0.1 mm
in the transverse plane. With no spacer there is:clearance below 0.1 mm between the vanes owing tomachining and assembly errors.

Summary
Effects of gaps at longitudinal vane joints on
the RFQ field were studied by using a model cavity offour vane structure. The model consists of twocavities joined with a ring spacer between their outerconductors. With gap width of 0.1 to 2 mm, field
distribution, resonant frequency and Q value were
measured for TE210 mode. The resonant frequency was296.5 MHz. The shift was within 30 kHz namely 0.01
percent. Significant effect on the field distribution
was not observed. The Q value was 4000 for no spacer,and 3600 for 2 mm spacer.

Field Measurement
Experimental setup

Introduction

The field strength was measured with perturbation
method. It is we 11 known that the res on ant frequency
shift due to a perturbation is proportional to thesquare of the field strength at the perturbator
position. On this experiment a shift of the phasedifference between the input and pickup signals wasmeasured with a vector vo 1tmeter. The phase shift isproport ina 1 to the resonant frequency shift when theperturbation is small. On resonance, the electric
length of the rf pick up cable was adjusted to give nophase difference between the input and pickup signals.
An analog output which is proportional to the phasedifference was recorded on a strip chart or read by adesktop computer through a GP-IB digital voltmeter.
A brass rod was inserted from side holes tomeasure the magnetic field. Aluminum perturbing ballsof 2 mm and 6 mm in diameter were used to measure theelectric field around the vane tops.

On a construction of a long RFQ linac, forexample, 7 m long, the vanes should be made byconnecting several sections, because it is very
difficult to make the vanes in a piece. Considering
assembly procedure and transportation, it is practicalto make the RFQ by connecting several four vane
cavities each of which is assembled separately. At thejoints it is desirable to make gaps between the vanes,in order to tolerate machining errors and unequal
thermal elongation. The gaps, however, may lower Q
value, give a jump to the vane voltage and enhancesurface electric field strength around the gaps. Theneffects of the gaps on the RFQ field were studied.
Mode 1 Cavity
The cavity is made by connecting two sections(Figs.1,2). They are joined with a ring shaped copper
spacer of various thickness between the cylinders.
Each section has an inner diameter of 258 mm and alength of 490 mm. The cylinders are made of copper
plated mild steel. The vanes were made by separating
the straight vanes of the INS RFQ cold model-2 1).
Each has aThe vanes are made of pure copper.
thickness of 35 mm and 470 mm length. The vanes aremounted to the cylinder by using fixing flanges. The
rf contact is assured with copper braid having rubber
string inside. The vane top is approximated by acircular arc of 14.3 mm radius and tangential lines.
The aperture radius r 0 is 14.3 mm. Both sides of thecavity are covered with end flanges made of a 1umi num
alloy. The distance between the vane end and theflange is 20 mm.

TE210 mode
With no spacer, the end tuners were adjusted togive uniform field within a few percent error. The
measured resonant frequency was 296.5 MHz for theTE210 mode. Then the field distribution, Q value andresonant frequency shift were measured for spacers
with thickness of 0.2 to 2 mm without retuning the endtuners.
490

490

ring

Fig.2. Schematic drawing
Dimensions are in mm.

Fig.1. View of the model cavity.
86ofthemodelcavity.

The magnetic field distributio ns are shown for nospacer and 2 mm spacer (Fig.3). The azimuthal
distributio n varied slightly, but it can be recovered
easily. The electric field distributio n between thevanes with 2 mm spacer is shown in Fig.5. Serious
effect of the gaps on the distributio n was notobserved. The resonant frequency varies only within 30
kHz namely 0.01 percent. The Q value lowered from 4000
for no spacer to 3600 for 2 mm spacer. The electricfield distributio n very near the vane tops is shown inFig.4. A depression of 12 percent was observed at thecenter, but no enhancement was detected around thegaps.

Conclusion
It was confirmed that gaps below 2 mm do notgive serious effect on the RFQ field. In this model
the aperture radius of the vane tops is 14.3 mm andconsiderab ly larger than a radius of a real RFQ which
will have a radius around 5 mm. By assuming that theeffect of the gap can be scaled in a ratio of gap to
aperture radius, the 2 mm gap is equivalent to 0.7 mm
on a real machine. It is not difficult to controle
the gap width below this value on manufacturing of themachine.
Beam loss due to alignment error of the beam axis
at the joints and enhancement of surfar;:e field near
the gaps are discussed in another paper.2J

Quadrupole field tilted along the axis
In order to estimate the vane voltage jump whenthe electric field is not uniform along the axis, aquadrupole field of tilted distributio n was excited.
The end tuners on one end were touched to the vanes
and ones on another end were set 20 to 7 mm apart fromthe vanes to give azimuthally uniform field. The end
tuning gives the steepest field distributio n.
In Fig.6 the electric field between the vanes isshown for no spacer, 0.1 mm and 2 mm spacer. Jumps ofthe intervane field, Q value and resosnant frequency
shift for spacers of various thickness are shown inFig.?. The ratio of the jump to the maximum fieldstrength is 16 percent in phase shift, namely, 8
percent in electric field for 2 mm spacer.

Acknowledgement
The authors express their thanks to Mr. S. Itoh,Tsurumi Works, Toshiba Corporatio n, who supported themanufacturing of the model cavity. They also thanks toS. Watanabe who helped us on the ex peri mental setup.
They are grateful to S. Yamada for helpful discussion.
References
1) T. Nakanishi et al., INS Report NUMA-30 (1982).
2) T. Nakanishi et al., contributed to this symposium.

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input, pickup~

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Fig.3. Magnetic field distributio n measured with aperturbing rod inserted through side holes. Verticalaxis is phase shift (deg.). Left : no spacer. Right2 mm spacer.

==i=i=-=R=j 4--.

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7(deg.)
687(deg.)
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Fig.5.
Electric field distributio n between theadjacent vanes with 2 mm spacer. The perturbing ballis 2 mm in diameter. Vertical axis is phase shift(arbitrary scale).

Fig.4. Field distributio n near the vane tops. The
perturbing ball diameter is 2 mm. Vertical axis isphase shift (deg.).
874000Q value

3000__ , _________ 0....-\

2000v/306 [(11Hz)
resonant frequency

305
304

~~ spacer 0.1 mm~~-chamber 4.

+150(%)~+10·~·L::·:_~:n~---~~=-=====:~~

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open end

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0=1spacer thickness (mm)2

short end
Fig.?. Q values, resonant frequency and jump of theintervane field vs. spacer thickness for the tilted
mode. The jump of the phase shift is shown in theratio to the shift of the maximum field (percent).

Fig.6. Field distribution of a quadrupole field tilted
along the axis. The perturbing ball diameter is 6 mm.
The ball position is the same as Fig.4. Vertical axis
is phase shift (arbitrary scale).

88