JAERI-Conf 95-021

2p2-2
DEVELOPMENT OF
A VARIABLE-FREQUENCY RFQ LINAC FOR THE RILAC
Osamu KAMIGAITO, Akira GOTO, Yoshitoshi MIY AZAWA, Toshiya CHIBA,Masatake HEMMI, Shigeo KOHARA, Masayuki KASE,Yuri BATYGIN, and YasushigeY ANO
The Institute of Physical and Chemical Research (RIKEN)
Wako-shi, Saitama 351-01, Japan
Abstract
Development of a variable-frequency RFQ
linac which will be used for a new injector for the
RIKEN heavy-ion linac (RILAC) is under progress. The
RFQ will accelerate ions with mass-to-charge ratios of 6to 27 at up to 450 keV per charge by varying its
operational frequency from 17.7 to 39.2 MHz. The RFQ
resonator has a folded-coaxial structure and the resonant
frequency is changed by a movable shorting plate. After
low- and high power tests of the real RFQ structure, theacceleration tests have been performed using Argon andOxygen beams at the frequencies ofl7.7, 26.1, 34.4, and39.2 MHz. The maximum transmission efficiency
obtained from the first tests was 85 %.

In addition to the equipment described above, adetachable stem is installed underneath tl1e conductor
tube. This stem is, however, pulled in only when highfrequency operation is required, and is in electric contact
with both the conductor tube and tl1e ba<>e plate while itis detached below the base plate in low-frequency
operations. Because at installation, tl1e rf electric current
is shared by the detachable stem and the upper stem, thepower consumption is expected to be less than that when
only the upper stem is used.3)

1. Introduction
A new injector for the RIKEN heavy ion linac(RILAC) 1) has been developed, which consists of an18GHz-ECRIS and a variable-frequency RFQ Iinac, inorder to meet the growing demand for more intensity ofheavy ion beams. As for the RFQ linac, a real structure
has recently been constructed based on the study of tl1e rf
characteristics using a half-scale model. In this paper we
describe recent results obtained from various tests as wellas the outline of the RFQ resonator.

ShortingPlateFeeder
UPPER PART~-- -?-----

LOWER PART

Conductor-+---+-Tube2. RFQ resonator

Detachable StemThe RFQ resonator is based on a folded-coaxialstructure. The distinct features of this RFQ are that it canbe operated in a low frequency region and the frequency
range is quite large.2)
Figure 1 shows a schematic layout of the RFQ
resonator. Horizontal vanes are held by front and rear
supports fixed on the base plate. Vertical vanes are fixed
on the inner surfaces of a rectangular tube which
sunounds the horizontal vanes. This tube is supported byfour ceramic pillars placed on the base plate. A stem
suspended from the ceiling plate is in electric contact
with the rectangular tube. A shorting plate placed aroundthe stem can be moved vertically, which varies tl1e
resonant frequency. Radio-frequency power is capacitively
fed through the side wall. A capacitive tuner is set on theother side and two capacitive pick-up monitors are on thebase plate.

Vane

Ceramics
PillarPick-up VerticalMonitor Vane

Figure 1. Schematic drawing of the RFQ resonator.
The inner volume of the resonator is about 1700mm (lengtl1) x 700 mm (widtl1) x 1000 mm (height).
The resonator is separable into upper and lower parts, as
shown in Fig. 1. The horizontal vanes and therectangular tube with the vertical vanes are rigidly fixed
inside the lower part. The upper part containing t11e stemand the movable shorting plate can be removed as a unit.
This separable structure pennits accurate alignment of t11e
vanes and easy maintenance.
The cooling pipes are placed by taking the resultof the heat analysis into account. Cooling water for t11e
horizontal vanes is supplied through the front and rear
supports of tl1e vanes. That for the vertical vanes and therectangular tube is supplied through the inside of theupper stem. The total water flow is 155 1/min of 7 atm.

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JAERI-Conf 95-021

Two turbo-molecular pumps (1500 lis) are fixed on bothsides of the resonator.
The vanes have been three-dimensionallymachined within the accuracy of ±50 Jlm. The vaneparameters have been modified from the output values of
the P ARMTEQ program because decrease of thetransmission efficiency has been observed by a numericalsimulation. 4 )MAFIA calculations overestimate the measured values byabout 50 %. This is considered to result from the factthat the calculation does not realistically treat theroughness of the wall surface and the imperfection of theelectric contact. The power losses estimated from theshunt impedances are 6 kW at 17.7 MHz and 26 kW at
39.2 MHz for the designed intervane voltage of 33.6 kV
in the cw operation.
12000

3. Recent results

10000

3.1 Low power tests
The resonant frequencies, the Q-values and theshunt impedances were measured in the same manner as
in our previous paper. 2) Figure 2 shows the measured
resonant frequency along with the values calculated by
the MAFIA program. The horizontal axis represents thegap distance between the top surface of the conductor
tube and the bottom surface of the shorting plate. The
resonant frequency varies from 17.7 to 39.2 MHz bychanging the position of the shorting plate by a stroke of790 mm.

6000
4000
2000
180;-----------------------~

150

90

60

00;---~----~--~----r---~1535::r:-0o o---C:_ ----

020

2530

3540

Frequency ( MHz )Detachable Stem - in:::s 30

0Rs (ki2).... • •1203040,-.._N

8000.........-Meas.

Figure 3. Measured Q-values and the shunt
impedances along with the MAFIA calculations.
The data of symbols and curves are obtained under
the same conditions as in Fig. 2.

Calc.

>.

(.)

E3 25::l0'~~20
Detachable Stem - out_.-Meas.
............ Calc.

154-----r---~----~----~

0

800
400
600
200
Distance Tube - Short (mm)Figure 2. Measured resonant frequency along withthe MAFIA calculations. The closed circles andthe solid curve represent the measured and thecalculated values, respectively, when thedetachable stem is out of the resonator. The opencircles and the dashed curve represent themeasured and the calculated values, respectively,
when the stem is in the resonator.
Figure 3 shows the measured Q-values and shunt
impedances. The corresponding MAFIA-calculation
curves are shown in the figures as well. The shunt
impedance Rs is defined by v2; (2P), where Pis the rf
power consumption and V is the intervane voltage. As
expected, above 30 MHz the measured Q-values andshunt impedances with the detachable stem installed arelarger than those without the detachable stem. The

3.2 High power testsThe rf power source with an Eimac 4CW 50000E
has a cw power of 40 kW at maximum between 16.9
MHz and 40 MHz.
As a result of the high power tests, thefollowing results have been obtained: [1] The RFQ canbe stably operated when the intervane voltage is between
20 kV and 30 kV. The vacuum stays in a range of 1 - 3x IQ-7 Torr at a pump head. No temperature rise has
been detected during the operation. [2] Emissions of bluewhite glow are observed on the ceramic pillars below theintervane voltage of 15 kV. [3] Emissions of red-whitelight are observed above the intervane voltage of 35 kV.
The reasons of [2] and [3] described above areconsidered to be the multipactoring on the ceramics
pillars and the heating due to the dielectric losses aroundthe metal screws fixing the pillars to the conductor tube,respectively. Alternative structure of the pillars are underfabrication so as to avoid these drawbacks.
3.3 Acceleration tests
Acceleration tests have been performed using
beams from an 18GHz-ECRIS. Figure 4 shows theschematic drawing of the beam line. The extracted bemn-237-JAERI-Conf 95-021

from the ion source is focused by Einzel lenses and are
bent by a bending magnet. The bending magnet also has
a focusing function by the slant pole edges. The beam is
focused again by a solenoid lens before entering theRFQ. 5) The diagnostic boxes contain Faraday cups,
profile monitors, and slits. Three capacitive pick-up
probes are used for the TOF measurement of the beam
velocity.
The accelerated ions so far are Ar3+, Ar6+,
Ar11+, and o5+ at the frequencies of 17.7, 26.1, 34.4,
and 39.2 MHz, respectively, at the intervane voltage ofabout 20 kV. The maximum transmission efficiency was
85 % with the beam intensity of 10 - 50 eJ.lA. The
velocity of the output beam is in agreement with thedesigned value within 1 %.
In these tests the emittance of the input beam
was not measured. The measurement is in preparation.
Acceleration tests with full-intensity beams from theECRIS are also to be performed.

5. Acknowledgment
The authors are grateful to Dr. N. Tokuda andDr. S. Arai at INS for the usage of vane-culling prognun
as well as for the fruitful information about the vane
design. The resonator was fabricated by Sumitomo Heavy
Industries, Niihama Work, and the rf power source byDenki Kogyo.
6. References
1.

2.

4. Summary
A variable-frequency RFQ which will be used
for a new injector of the RIKEN heavy ion linac(RILAC) has been constructed. Acceleration tests havebeen performed using beams from the 18GHz-ECRIS
after low- and high power tests of the resonator. The
maximum transmission efficiency obtained from the firsttests was 85 %.
Alternative shapes of the ceramic pillars areunder fabrication so as to avoid the drawbacks observed inthe high power tests. Measurements of the beamemittance and the acceleration tests with full-intensity
beams from the ECRIS are in preparation.
Power Amp.
16.9-40.0 MHz -+-+-~
40 kW (cw)

Faraday Cup
Profile Monitor
TOP Pick-up Probe

01m

3.

4.
5.

M. Odera, Y. Chiba, T. Tonuma, M. Hemmi, Y.
Miyazawa, Y. Inoue, T. Kambara, M. Kase, T.
Kubo, and F. Yoshida, "Variable Frequency
Heavy-Ion Linac, RILAC," Nucl. lustrum. andMethods, 227, 187 (1984).
0. Kamigaito, A. Goto, Y. Miyazawa, T. Chiba,M. Hemmi, M. Kase, and Y. Yano, "Design of aNew Type of a Variable-Frequency RadioFrequency Quadrupole Linac with a FoldedCoaxial Resonator," Jpn. J. Appl. Phys. 33(1994) L537.
.
0. Kamigaito, A. Goto, Y. Miyazawa, T. Chiba,
M. Hemmi, S. Kohara, M. Kase, and Y. Yano,"Improvement of Frequency Variability of theFolded-Coaxial Radio-Frequency Quadrupole Linacby Installing a Detachable Stem in Its Resonator,"to be published in Jpn. J. Appl. Phys.
Y. Batygin, A. Goto, 0. Kamigaito, and Y.
Yano: RIKEN Accel. Progr. Rep. 28 (1994)
p.l72 and in this proceeding.
N. Inabe, M. Kase, 0. Kamigaito, Y. Miyazawa,M. Hemmi, T. Chiba, A. Goto, Y. Batygin, <md
Y. Yano: RIKEN Accel. Progr. Rep. 28 (1994)
p.l66.

Faraday Cup
Profile Monitor
Slit

90° Analyzing Magnet

Solenoid Lens,.---- RFQ
Frequency: 17.7- 39.2 MHz
Mlq: 6-27
Input Energy: 10 keV/q
Output Energy: 450 keV/q
Vane Length : 142 em
Intervane Voltage: 33.6 kVPower: 6- 26 kW (cw)

18GHz
Klystron

1.5kW

Figure 4. Schematic drawing of the beam line.

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