- Email: [email protected]
Development of an SCRFQ heavy-ion linac for RI beams
Nuclear Instruments and Methods in Physics Research B70(1992) 414-420 North-Holland
HHH B
Beam Interactions with Materials"Atoms
Development of an SCRFQ heavy-ion linac for RIL beams S. Arai, A . Intanishi, T. Moritnoto, E . Tojyo and N . Tokuda Institute for Nuclear Study, University of Tokyo, Tanasht, Tokyo 188, Japan
S . Shibuya
Graduate University for Advanced Studies, Tsukuba, Ibaraki 305, Japan A split coaxial RFQ (SCRFQ is being developed to accelerate RI beams from I to 170 keV/u as part of the Japanese Hadron Project (JHP). Our SCRFQ is equipped with modulated vanes. On the basis of the studies on a cold model and a proton accelerating one, a 25 .5 MHz prototype for the JHP SCRFQ has been constructed. The prototype, consisting of three module cavities, is 2.1 m in length and 0.9 m in inner diameter, and accelerates ions with a charge-to-mass ratio (q1A) greater than 1/30 from I to 45.4 keV/u . The unloaded Q-value of the cavity is 6400, corresponding to about 84% of the calculated value, and the field imbalance between vanes is within ±0 .6% . The designed intervane voltage of 109 kV for ions with qIA= 1/30 is achieved with a 70 kW peak power . By using ions of three species, N,, N' and Ne + , acceleration tests are conducted . The transmission efficiency attained with a N + beam is better than 80% at normalized intervane voltages higher than 1.2 . 1 . Introduction The acceleration of RI beams extracted from an isotope separator on-line (ISOL) is planned in the Japanese Hadron Project (JHP) [1,2]. A heavy-ion linac complex composed of a split coaxial RFQ (SCRFQ), interdigital-H (IH) linacs, Alvarez linacs, and single-gap cavities has been designed so as to accelerate ions with a charge-to-mass ratio (q/A) greater than 1/60 from 1 keV/u to 6.5 MeV/u [3]. As R&D for the JHP heavy-ion linac, an SCRFQ with modulated vanes is being developed intensively, because the performance of the front-end linac is important to the acceleration of such ions. The RFQ has the merit that it can accelerate very low energy ions with a good transmission efficiency . However, for accelerating ions with such a small q1A, the RFQ must be operated at a low rf frequency of 10-30 MHz . Then, a rational cavity structure for the low frequency was required to keep the size of the cavity as small as possible. To reduce the cavity diameter, the idea of using a split coaxial resonator to the RFQ cavity was proposed by Milller at GSI during the development of a high-intensity RFQ for research of heavy-ion inertial fusion [4] . The GSI SCRFQ working at 13.5 MHz has the following features: electrodes like drift tubes with focusing fingers are used to generate accelerating and focusing fields, and each of the four inner conductors forming a quadrupole line is supported only at one point on the end wall of the cavity, as mentioned in section 3 .
On the other hand, our SCRFQ has the following features : modulated-vane electrodes are used to generate ideal accelerating and focusing fields; and a multimodule cavity structure is employed to support vane electrodes precisely and firmly at several points with stems. Our studies were initiated from the fabrication of a cold model in 1984 . The cold model, installed with flat vanes, was used to examine the mechanical and rf characteristics of the cavity. Through the studies on the cold model, it was verified that precise vane alignment, good mechanical stability and the required field distribution were achieved [5 ;. An equivalent circuit analysis was developed to explain theoretically the rf characteristics of the resonator [6]. Next, a proton accelerating model working at 50 MHz was constructed to evaluate the overall performance of the INS-type SCRFQ . The cold model was converted into a proton model by replacing the flat vanes with modulated ones. The acceleration tests with a low-current beam confirmed that the output energy, the energy spread, the transmission efficiency and the output-beam emittance agree approximately with the design values [7] . On the basis of the results obtained with the two models, a 25.5 MHz prototype for the JHP SCRFQ was constructed to obtain the know-how for practical use. The prototype, 2.1 m in length and 0.9 m in inner diameter, accelerates ions with a q1A greater than 1/30 from 1 to 45.4 keV/u . The cavity successfully underwent low- and high-power tests [8,9]. We are now conducting acceleration tests by using ions such as Nz , N + and Ne + .
0168-583X/92/$05.00 C 1992 - Elsevier Science Publishers B.V . All rights reserved
415
S. Arai et aL /Development of an SCRFQheavy-ion linac
This paper describes the R&D program for the JHP heavy-ion linac, the principle of the INS-type SCRFQ, and the development of the 25.5 MHz prototype . 2. R&D program for the JHP heavy-ion linac For the construction of the JHP heavy-ion linac, R&D of the SCRFQ and the III linac are essential. The final dimensions of both machines should be determined by means of the model tests. So far, we have put emphasis on the development of an SCRFQ because of budgetary and manpower limitations. The technical know-how to realize the JHP SCRFQ will be soon obtained from the many experimental results of the 25 .5 MHz prototype . Then, to carry forward the R&D into the next stage, a test facility forthe RI-beam acceleration is being planned at INS. The layout of the test facility is shown in fig . 1. Radioisotopes, produced by bombarding a thick target with a 40 MeV proton beam from the SF cyclotron, are ionized in an ion source, mass-analyzed through an ISOL, and transported to a post-accelerator. The accelerators will be operated with a duty factor higher than 10%. Ions with a q1A greater than 1/30 will be accelerated from 1 to 800 keV/u . The ions accelerated by the SCRFQ up to 170 keV/u are charge-stripped, accelerated by two IH linacs up to 420 or 670 keV/u, and their energy is continuously varied by a cavity in the range 170-800 keV/u. We will begin designing a model of the IH linac at the end of 1991 . 3. Principle of the INS-type SCRFQ The structure of the 25 .5 MHz prototype SCRFQ is shown in fig . 2. The whole cavity comprises three module cavities . The vanes are bolted onto spearshaped back plates, which mechanically stabilize the vanes and improve the Q-value. Coupling rings, one pair per module, electrically short opposing electrodes. FI
s-(-)L 1
The vertical vanes are fixed to the cavity cylinder with horizontal stems at two positions, and the horizontal ones are fixed with vertical stems at two other positions. With the above structure, the vane electrodes are firm enough. The principle of the split coaxial resonator proposed by Müller is understood from fig. 3, illustrating how the 2 x A/4 TEM cavity is transformed into the split coaxial cavity. The voltage difference between adjacent electrodes is almost flat along the beam axis, and the generated electric field is four-pole symmetric. The most important feature of the split coaxial resonator is that it has a larger inductance compared with other structures with the same cavity radius, such as the four-vane structure or the 0-mode A/4 structure . The concept of the multimodule split coaxial cavity has been created during the development of an SCRFQ with modulated vanes. The split coaxial resonator has a structural defect in that the long electrodes are supported only at one point on the end wall of the cavity, as seen in fig. 3. Therefore, if vanes are mounted to the electrodes, they are so heavy that they become mechanically unstable. This problem has been solved by connecting more than three shorter split coaxial resonators and supporting the electrodes at more than two points, as illustrated in fig. 4. This structure is Sp(It CoaxiaL CovIty
2xl/4 TEM COVITY
POST ACCELERATOR
670 keV/u SF CYCLOTRON
Fig. 2. Structure of the 25 .5 MHz prototypeSCRFQ.
V1 (z)
170-800 keV/u
Fig. 1 . Layout of the test facility for the RI-beam acceleration.
Fig. 3. Principle of the split coaxial resonator. VI . RI BEAM ACCELERATION
416
S. Amiet aL / Development of an SCRFQheavy-ion linae
Ailutti-module Split Coaxial Cavity
of the JHP machine except the minimum q/A. When the prototype designed under this condition is constructed, most of the experimental results will be applicable to the JHP machine, because the rf characteristics are the same as those of the JHP machine. The prototype comprises three module cavities as
z Fig. 4. Conceptual diagram of the multimodule split coaxial cavity resonator. called a multimodule cavity structure : the whole cavity comprises several module cavities, and the vanes running through the whole cavity are supported at several points with good mechanical stability.
4. Development of a 25.5 MHz prototype model The main parameters of the prototype model are summarized in table 1 in comparison with those of the JHP SCRFQ. Issues to be studied are summarized as
follows: (1) to examine the dynamic range of the intervane voltage, (2) to examine the cooling efficiency of the cavity in relation to the power level and the duty factor, (3) to evaluate the performance of the vanes machined by means of a two-dimensional cutting technique, and (4) to improve the cavity structure of the
shown in fig . 2. The material of the tanks is mild steel, and its inner wall is plated with copper to a thickness
of 100 Wm; that of the inner structure except the vanes is oxygen-free copper. The vanes are made of chrome-copper alloy. Compared with the proton model, the cavity structure was improved at the following points : (1) water channels are installed for the high power operation (about 70 kW in peak, 10% in duty); (2) the stems have been replaced with stem-flanges. The stem-flanges supporting the electrodes, consisting
of the vanes and spear-shaped back plates, were arranged at equal distances by four spacing rods . Hence the inner structure was assembled precisely and firmly before installation into the cavity tanks . The module
length was fixed at 70 cm since the droop of the vanes due to gravity was estimated to be less than 35 wm . The diameter of the cavity was designed so that the
resonant frequency of 25 .5 MHz is located between frequencies given by two cases: all the windows are completely closed or open . The main geometrical and rf parameters are summarized in table 2. As for the rf parameters, design values for the two cases are compared with measured values after tuning of the cavity .
proton model for easier assembling and for high-power operation,
4.2. RF aspects of the cavity
4.1. Construction of the cavity
tuned to 25 .45 MHz by adjusting the area of the stem-flange windows. Fine tuning of the frequency to 25.50 MHz was performed by using piston tuners of
The beam dynamics design was performed under the condition that the operation frequency, injection beam energy, beam emittance, maximum intervane voltage and mean bore radius were the same as those
The resonant frequency of the cavity was roughly
aluminum blocks (188 mm in diameter). The unloaded Q-value measured at 25.5 MHz was 6400, corresponding to about 84% of the calculated value.
Table 1 Main parameters of the prototype model in comparison with those of theJHP SCRFQ Frequency (f)[MHz] Charge-to-mass ratio (q/A) Kinetic energy (T)[keV/u] Normalized emittance (e )[,r mm mrad] Vane length (L)(m] Number of cells Kilpatrick factor (ftc) Intervane voltage (V) [kV] Mean bore radius (rn) [cm] Min. bore radius (a .) [cm] ; . Margin of bore radius (am;n /abeam) Focusing strength (B) Limiting current (him) [mAl
JHPmachine
Prototype
25 .5 z 1/60 1-170.2 0.6 22.3 537 2.2 109.3 0.946 0.618 1.15 3.0 3.0
25 .5 z1/30 1-45.4 0.6 2.135 136 2.2 109.3 0.946 0.521 1 .20 6.0 2.5
S. Arai et al. / Development ofan SCRFQ heavy-ion linac
Table 2 Geometrical and rf parameters of the prototype model
Resonant frequency [MHz] Number of modules Tank length [cm] Inner diameter of tank [cm] Vane thickness [cm] Radius of inner electrode [cm] Total capacitance [pF] Total inductance [nHj Resonant resistance [k12] Unloaded Q-value Power loss (at 110 kV) [kW1
Design values for twocases Closed windows Open windows 27 .7
24.0
3
417
Measured values after tuning 25 .5
210 .0
90 .0 3.0
18 .0 434 76.3 142 10700 43
We measured the distributions of electric field strengths in the cavity by means of a perturbation method . The azimuthal field balance was measured by moving a dielectric perturbator, 10 mm in length and 20 mm in diameter, along two vanes used as a guide, as shown in fig. 5. Four curves corresponding to intervane fields in the quadrants are drawn in the figure and they almost overlap one another because the error of the field balance is very small. The resulting azimuthal imbalance was within f0.6%. We have achieved an intervane voltage of 114 kV under a pulse operation with a duty factor of 4% and a peak power of 75 kW . This voltage is higher than 109 kV, the design voltage for q/A =1/30. The rf conditioning of the cavity was conducted under pulse operations with duty factors of 0.6-3%; the input power was increased step by step so that the vacuum in the cavity might be kept less than 7 x 10 -6 Torr . The rf conditioning is necessary to overcome multipactoring and to increase the upper limit of the applicable voltage determined by the spark discharge . Fig. 6 shows the
m m rn m v
101 7600 53
achieved intervane voltage as a function of the aging time (operation time x duty factor). To exceed some multipactoring levels, we needed longer aging times. The three plateaus in fig. 6 indicate severe multipactoring levels. From the result of the rf conditioning, we found that the applicable intervane voltages are at present between 6 and 114kV . Therefore the RFQ can accelerate ions with a q/A between 1/2 and 1/30 . The spark discharge occurs gradually at intervane voltages higher than 110 kV. However, the frequency of the sparking was reduced with time when the cavity was conditioned. We expect that the applicable intervane voltage will be raised to more than 114 kV by further conditioning. When the cavity was excited at an average power of 7 kW, the resonant frequency increased from 25 .5 MHz by about 100 kHz. Since the amount of the frequency shift compensated with three piston tuners is about 100 kHz, the duty factor is limited to 10% in the case of pulse operation with a peak power of 70 kW, required for q/A =1/30.
Y
200
0
100
0
m
v
t L
454 86 87 6400 70
116
50
C7
Ç
NN O L a
y m
L
Axial Distance (cm)
Fig. 5 . Azimuthal field balance andthe position of the pertur bator .
Q
10 5
0
50 100 Aging Time (minutes)
150
Fig. 6. Attained intervane voltage as a function of aging time (operation time xduty factor). VI . RI BEAM ACCELERATION
418
S. Arai et aL / Development of an SCRFQheavy-ion 6nac Ouadrupole Magnets
Turbomolecular Pump
Turbomolecular Pump (1500 I/s)
Bendina Magnet
Einzel Lens 3 / StiD eerng elector f Einzel Lens 2 Ion Source Einzel Lens 1 Îon Separator
Emittonce Monitor Faraday Cup 1 L. ... i . 0
.I lm
Fig. 7. Setupof the acceleration test stand. 4.3. Bears acceleration tests
100
Acceleration tests were performed under the pulse operation at a repetition rate of 50 Hz . The pulse
.ô
widths of the rf and the beams were 600 and 300 Rs, respectively. The setup of the test stand is shown in fig. 7. Ions were produced in a compact ion source of ECR type . Peak currents of the extracted beams were 180
NZ ,
RA for 35 RA for N+, and 120 AA for Ne +. The low-energy beam transport from the ion source to the RFQ comprises four einzel lenses, an ion separating magnet, an electrostatic deflector for beam steering, a beam monitoring system, and a vacuum pumping system. The current and emittance of the input beam are
t=
Horizontal
100
50 .N ... 0 rf/R.I
X -50
â 50 0
É
T-50
Vertical
us ral."Oh,
-100 NO -100 -10 -5 0 5 10 -10 -5 0 5 10 x (mm) y (mm) . Fig. 8 Emittonce profiles of the input beam (N' ions). The bars indicate measured profiles cut off at a threshold level, 5% of the maximum density, and the ellipses are the design emittances.
measured with a Faraday cup and an emittance monitor, which are set at the entrance of the RFQ. The high-energy beam transport for diagnosis of the output beam comprises an emittance monitor, three Faraday cups, a magnetic-quadrupole doublet, and a vacuum pumping system .
100
The beam transmission depends sensitively on the emittance of the input beam. Hence we matched the emittance to the RFQ acceptance as well as possible . The N+ beam emittances matched before the transmission measurements are shown in fig . 8 as a typical example. Transmissions of NZ , N+ and Ne + beams were measured as functions of the intervene voltage. Trans-
mission efficiency is defined by the current ratio of the accelerated beam to the input beam. The result is shown in fig. 9. In the figure, we use the normalized intervane voltage Vn, defined by the intervene voltage divided by the design value. The current of accelerated
ions was measured with the Faraday cup located downstream of the magnetic-quadrupole doublet . Though
unaccelerated ions are transported through the RFQ, most of them diverge in the quadrupole magnets and do not reach the Faraday cup. The measured transmis-
v
m
.
N
Y ~
r rr
80 60
s
ir
40
.waa""""'aw;
e
Accelerated Beams N+ Experiment Ne+ Experiment . ° N2+ Experiment _ ----- PARMTEQ
20 0 0 .5
v
1 .0
1 .5
2.0
2.5
Normalized intervane voltage Fig. 9. Transmissions measured as a function of the intervane voltage.
S. Arai et aL / Developmentofan SCRFQheavy-ion linac Horizontal
Vertical
x (mm)
y (mm)
Fig. 10. Emittance profiles of the output beam (N' ions). The bars indicate measured profiles cut off at a threshold level, 5% of the maximum density, and the ellipses are the design emittances . sion efficiency of the N+ beam exceeds80% at V > 1.2 . The transmission increases rapidly around the design voltage. However, a beam simulation by the PÀRMTEQ code predicts a steeper increase as shown in fig. 9. The output emittances of N+ beam were measured at V = 1.28. The result is shown in fig. 10. By comparing figs . 8 and 10, it was confirmed that there is no emittance growth in the RFQ. By using a magnetic spectrometer with a bending angle of 45°, it was confirmed that the output energy was accelerated up to the designed energy of 45 .4 keV/u. The energy resolution of the spectrometer was
419
t 0.34% when the slit width was set at t3 mm. Energy spectra of N' ions were measured at several values of the intervane voltage. The obtained energy spectra are shown in fig . 11 . The full energy spread varied from t 3.3% to t 6.0% in the range 0.96 < V < 2.08. The minimum value, t 3.3%, was obtained at V = 1.12, and agreed with the design value of t 3.3%. 5. Concluding remarks In the R&D of the 25 .5 MHz SCRFQ we have obtained the following results : (1) the rf characteristics of the cavity agree with the design values: (2) the field imbalance among quadrants is within ±0 .6%; (3) the intervane voltages between 6 and 114 kV are applicable at a duty factor of 4% ; (4) the transmission efficiency exceeds 80% at normalized intervane voltages, V > 1.2, and the energy and spread of the output beam agree approximately with design values; and (5) the rapidly increasing rate of transmission measured around the design intervane voltage is lower than predicted by PARMTEQ . The result of (2) means that the accuracy of the vane alignment before installation into the tanks is maintained even after the installation . As for the duty factor of (3), the inner structure or its cooling must be improved to make the duty factor higher than 10%. A probable cause of the discrepancy of (5) is an effect of the accelerating field reduction due to the two-dimensional cutting in the vane fabrication on the beam performance . In order to investigate this effect in detail, we must measure the transmission more precisely by improving the input emittance. Acknowledgements The authors wish to express their thanks to M. Kihara for his encouragement. Development of the SCRFQ is supported by the Accelerator Research Division, High Energy Physics Division and Nuclear Physics Division of INS. The prototype was constructed at Sumitomo Heavy Industries, Ltd. The computer work was carried out on a FACOM M780 of the INS computer facility. Studies of the prototype are supported by a Grant for Scientific Research of the Ministry of Education, Science and Culture. References
40
42
44
46
48
Beam energy (keV/u)
50
Fig. 11 . Energy spectra of theoutput beam at several values of the intervane voltage .
[1] T. Yamazaki, INS Report 763 (1989). [2] T. Nomura, INS Report 780 (1989). [3] S. Arai, Proc . 18th Int. Symp. on Physics with High-Intetr sity Hadron Accelerators (World Scientific, 1990) p. 433. VI . RI BEAM ACCELERATION
420
S. Arai et al. / Development ofan SCRFQ heavy-ion linac
[4j R.W. Miller, GSI Report 79-7 (1979). [5j S. Arai, T. Fujino, T. Fukushima, E . Tojyo, N. Tokuda and T . Hattori, IEEE Trans. Nucl . Sci . NS-32 (1985) 3175. [6j S . Arai, INS-T-464 (Accelerator-4) (1986). [7] N. Tokuda, S. Arai, T. Fukushima, T. Morimoto and E. Tojyo, INS Report 743 (1989) .
[8j N. Tokuda, S . Arai, A. Imanishi, T. Morimoto, S . Shibuya and E. Tojyo, INS Report 794 (1989). [9j S . Shibuya, S. Arai, A. Imanishi, T. Morimoto, E . Tojyo and N . Tokuda, INS Report 843 (1990).
HHH B
Beam Interactions with Materials"Atoms
Development of an SCRFQ heavy-ion linac for RIL beams S. Arai, A . Intanishi, T. Moritnoto, E . Tojyo and N . Tokuda Institute for Nuclear Study, University of Tokyo, Tanasht, Tokyo 188, Japan
S . Shibuya
Graduate University for Advanced Studies, Tsukuba, Ibaraki 305, Japan A split coaxial RFQ (SCRFQ is being developed to accelerate RI beams from I to 170 keV/u as part of the Japanese Hadron Project (JHP). Our SCRFQ is equipped with modulated vanes. On the basis of the studies on a cold model and a proton accelerating one, a 25 .5 MHz prototype for the JHP SCRFQ has been constructed. The prototype, consisting of three module cavities, is 2.1 m in length and 0.9 m in inner diameter, and accelerates ions with a charge-to-mass ratio (q1A) greater than 1/30 from I to 45.4 keV/u . The unloaded Q-value of the cavity is 6400, corresponding to about 84% of the calculated value, and the field imbalance between vanes is within ±0 .6% . The designed intervane voltage of 109 kV for ions with qIA= 1/30 is achieved with a 70 kW peak power . By using ions of three species, N,, N' and Ne + , acceleration tests are conducted . The transmission efficiency attained with a N + beam is better than 80% at normalized intervane voltages higher than 1.2 . 1 . Introduction The acceleration of RI beams extracted from an isotope separator on-line (ISOL) is planned in the Japanese Hadron Project (JHP) [1,2]. A heavy-ion linac complex composed of a split coaxial RFQ (SCRFQ), interdigital-H (IH) linacs, Alvarez linacs, and single-gap cavities has been designed so as to accelerate ions with a charge-to-mass ratio (q/A) greater than 1/60 from 1 keV/u to 6.5 MeV/u [3]. As R&D for the JHP heavy-ion linac, an SCRFQ with modulated vanes is being developed intensively, because the performance of the front-end linac is important to the acceleration of such ions. The RFQ has the merit that it can accelerate very low energy ions with a good transmission efficiency . However, for accelerating ions with such a small q1A, the RFQ must be operated at a low rf frequency of 10-30 MHz . Then, a rational cavity structure for the low frequency was required to keep the size of the cavity as small as possible. To reduce the cavity diameter, the idea of using a split coaxial resonator to the RFQ cavity was proposed by Milller at GSI during the development of a high-intensity RFQ for research of heavy-ion inertial fusion [4] . The GSI SCRFQ working at 13.5 MHz has the following features: electrodes like drift tubes with focusing fingers are used to generate accelerating and focusing fields, and each of the four inner conductors forming a quadrupole line is supported only at one point on the end wall of the cavity, as mentioned in section 3 .
On the other hand, our SCRFQ has the following features : modulated-vane electrodes are used to generate ideal accelerating and focusing fields; and a multimodule cavity structure is employed to support vane electrodes precisely and firmly at several points with stems. Our studies were initiated from the fabrication of a cold model in 1984 . The cold model, installed with flat vanes, was used to examine the mechanical and rf characteristics of the cavity. Through the studies on the cold model, it was verified that precise vane alignment, good mechanical stability and the required field distribution were achieved [5 ;. An equivalent circuit analysis was developed to explain theoretically the rf characteristics of the resonator [6]. Next, a proton accelerating model working at 50 MHz was constructed to evaluate the overall performance of the INS-type SCRFQ . The cold model was converted into a proton model by replacing the flat vanes with modulated ones. The acceleration tests with a low-current beam confirmed that the output energy, the energy spread, the transmission efficiency and the output-beam emittance agree approximately with the design values [7] . On the basis of the results obtained with the two models, a 25.5 MHz prototype for the JHP SCRFQ was constructed to obtain the know-how for practical use. The prototype, 2.1 m in length and 0.9 m in inner diameter, accelerates ions with a q1A greater than 1/30 from 1 to 45.4 keV/u . The cavity successfully underwent low- and high-power tests [8,9]. We are now conducting acceleration tests by using ions such as Nz , N + and Ne + .
0168-583X/92/$05.00 C 1992 - Elsevier Science Publishers B.V . All rights reserved
415
S. Arai et aL /Development of an SCRFQheavy-ion linac
This paper describes the R&D program for the JHP heavy-ion linac, the principle of the INS-type SCRFQ, and the development of the 25.5 MHz prototype . 2. R&D program for the JHP heavy-ion linac For the construction of the JHP heavy-ion linac, R&D of the SCRFQ and the III linac are essential. The final dimensions of both machines should be determined by means of the model tests. So far, we have put emphasis on the development of an SCRFQ because of budgetary and manpower limitations. The technical know-how to realize the JHP SCRFQ will be soon obtained from the many experimental results of the 25 .5 MHz prototype . Then, to carry forward the R&D into the next stage, a test facility forthe RI-beam acceleration is being planned at INS. The layout of the test facility is shown in fig . 1. Radioisotopes, produced by bombarding a thick target with a 40 MeV proton beam from the SF cyclotron, are ionized in an ion source, mass-analyzed through an ISOL, and transported to a post-accelerator. The accelerators will be operated with a duty factor higher than 10%. Ions with a q1A greater than 1/30 will be accelerated from 1 to 800 keV/u . The ions accelerated by the SCRFQ up to 170 keV/u are charge-stripped, accelerated by two IH linacs up to 420 or 670 keV/u, and their energy is continuously varied by a cavity in the range 170-800 keV/u. We will begin designing a model of the IH linac at the end of 1991 . 3. Principle of the INS-type SCRFQ The structure of the 25 .5 MHz prototype SCRFQ is shown in fig . 2. The whole cavity comprises three module cavities . The vanes are bolted onto spearshaped back plates, which mechanically stabilize the vanes and improve the Q-value. Coupling rings, one pair per module, electrically short opposing electrodes. FI
s-(-)L 1
The vertical vanes are fixed to the cavity cylinder with horizontal stems at two positions, and the horizontal ones are fixed with vertical stems at two other positions. With the above structure, the vane electrodes are firm enough. The principle of the split coaxial resonator proposed by Müller is understood from fig. 3, illustrating how the 2 x A/4 TEM cavity is transformed into the split coaxial cavity. The voltage difference between adjacent electrodes is almost flat along the beam axis, and the generated electric field is four-pole symmetric. The most important feature of the split coaxial resonator is that it has a larger inductance compared with other structures with the same cavity radius, such as the four-vane structure or the 0-mode A/4 structure . The concept of the multimodule split coaxial cavity has been created during the development of an SCRFQ with modulated vanes. The split coaxial resonator has a structural defect in that the long electrodes are supported only at one point on the end wall of the cavity, as seen in fig. 3. Therefore, if vanes are mounted to the electrodes, they are so heavy that they become mechanically unstable. This problem has been solved by connecting more than three shorter split coaxial resonators and supporting the electrodes at more than two points, as illustrated in fig. 4. This structure is Sp(It CoaxiaL CovIty
2xl/4 TEM COVITY
POST ACCELERATOR
670 keV/u SF CYCLOTRON
Fig. 2. Structure of the 25 .5 MHz prototypeSCRFQ.
V1 (z)
170-800 keV/u
Fig. 1 . Layout of the test facility for the RI-beam acceleration.
Fig. 3. Principle of the split coaxial resonator. VI . RI BEAM ACCELERATION
416
S. Amiet aL / Development of an SCRFQheavy-ion linae
Ailutti-module Split Coaxial Cavity
of the JHP machine except the minimum q/A. When the prototype designed under this condition is constructed, most of the experimental results will be applicable to the JHP machine, because the rf characteristics are the same as those of the JHP machine. The prototype comprises three module cavities as
z Fig. 4. Conceptual diagram of the multimodule split coaxial cavity resonator. called a multimodule cavity structure : the whole cavity comprises several module cavities, and the vanes running through the whole cavity are supported at several points with good mechanical stability.
4. Development of a 25.5 MHz prototype model The main parameters of the prototype model are summarized in table 1 in comparison with those of the JHP SCRFQ. Issues to be studied are summarized as
follows: (1) to examine the dynamic range of the intervane voltage, (2) to examine the cooling efficiency of the cavity in relation to the power level and the duty factor, (3) to evaluate the performance of the vanes machined by means of a two-dimensional cutting technique, and (4) to improve the cavity structure of the
shown in fig . 2. The material of the tanks is mild steel, and its inner wall is plated with copper to a thickness
of 100 Wm; that of the inner structure except the vanes is oxygen-free copper. The vanes are made of chrome-copper alloy. Compared with the proton model, the cavity structure was improved at the following points : (1) water channels are installed for the high power operation (about 70 kW in peak, 10% in duty); (2) the stems have been replaced with stem-flanges. The stem-flanges supporting the electrodes, consisting
of the vanes and spear-shaped back plates, were arranged at equal distances by four spacing rods . Hence the inner structure was assembled precisely and firmly before installation into the cavity tanks . The module
length was fixed at 70 cm since the droop of the vanes due to gravity was estimated to be less than 35 wm . The diameter of the cavity was designed so that the
resonant frequency of 25 .5 MHz is located between frequencies given by two cases: all the windows are completely closed or open . The main geometrical and rf parameters are summarized in table 2. As for the rf parameters, design values for the two cases are compared with measured values after tuning of the cavity .
proton model for easier assembling and for high-power operation,
4.2. RF aspects of the cavity
4.1. Construction of the cavity
tuned to 25 .45 MHz by adjusting the area of the stem-flange windows. Fine tuning of the frequency to 25.50 MHz was performed by using piston tuners of
The beam dynamics design was performed under the condition that the operation frequency, injection beam energy, beam emittance, maximum intervane voltage and mean bore radius were the same as those
The resonant frequency of the cavity was roughly
aluminum blocks (188 mm in diameter). The unloaded Q-value measured at 25.5 MHz was 6400, corresponding to about 84% of the calculated value.
Table 1 Main parameters of the prototype model in comparison with those of theJHP SCRFQ Frequency (f)[MHz] Charge-to-mass ratio (q/A) Kinetic energy (T)[keV/u] Normalized emittance (e )[,r mm mrad] Vane length (L)(m] Number of cells Kilpatrick factor (ftc) Intervane voltage (V) [kV] Mean bore radius (rn) [cm] Min. bore radius (a .) [cm] ; . Margin of bore radius (am;n /abeam) Focusing strength (B) Limiting current (him) [mAl
JHPmachine
Prototype
25 .5 z 1/60 1-170.2 0.6 22.3 537 2.2 109.3 0.946 0.618 1.15 3.0 3.0
25 .5 z1/30 1-45.4 0.6 2.135 136 2.2 109.3 0.946 0.521 1 .20 6.0 2.5
S. Arai et al. / Development ofan SCRFQ heavy-ion linac
Table 2 Geometrical and rf parameters of the prototype model
Resonant frequency [MHz] Number of modules Tank length [cm] Inner diameter of tank [cm] Vane thickness [cm] Radius of inner electrode [cm] Total capacitance [pF] Total inductance [nHj Resonant resistance [k12] Unloaded Q-value Power loss (at 110 kV) [kW1
Design values for twocases Closed windows Open windows 27 .7
24.0
3
417
Measured values after tuning 25 .5
210 .0
90 .0 3.0
18 .0 434 76.3 142 10700 43
We measured the distributions of electric field strengths in the cavity by means of a perturbation method . The azimuthal field balance was measured by moving a dielectric perturbator, 10 mm in length and 20 mm in diameter, along two vanes used as a guide, as shown in fig. 5. Four curves corresponding to intervane fields in the quadrants are drawn in the figure and they almost overlap one another because the error of the field balance is very small. The resulting azimuthal imbalance was within f0.6%. We have achieved an intervane voltage of 114 kV under a pulse operation with a duty factor of 4% and a peak power of 75 kW . This voltage is higher than 109 kV, the design voltage for q/A =1/30. The rf conditioning of the cavity was conducted under pulse operations with duty factors of 0.6-3%; the input power was increased step by step so that the vacuum in the cavity might be kept less than 7 x 10 -6 Torr . The rf conditioning is necessary to overcome multipactoring and to increase the upper limit of the applicable voltage determined by the spark discharge . Fig. 6 shows the
m m rn m v
101 7600 53
achieved intervane voltage as a function of the aging time (operation time x duty factor). To exceed some multipactoring levels, we needed longer aging times. The three plateaus in fig. 6 indicate severe multipactoring levels. From the result of the rf conditioning, we found that the applicable intervane voltages are at present between 6 and 114kV . Therefore the RFQ can accelerate ions with a q/A between 1/2 and 1/30 . The spark discharge occurs gradually at intervane voltages higher than 110 kV. However, the frequency of the sparking was reduced with time when the cavity was conditioned. We expect that the applicable intervane voltage will be raised to more than 114 kV by further conditioning. When the cavity was excited at an average power of 7 kW, the resonant frequency increased from 25 .5 MHz by about 100 kHz. Since the amount of the frequency shift compensated with three piston tuners is about 100 kHz, the duty factor is limited to 10% in the case of pulse operation with a peak power of 70 kW, required for q/A =1/30.
Y
200
0
100
0
m
v
t L
454 86 87 6400 70
116
50
C7
Ç
NN O L a
y m
L
Axial Distance (cm)
Fig. 5 . Azimuthal field balance andthe position of the pertur bator .
Q
10 5
0
50 100 Aging Time (minutes)
150
Fig. 6. Attained intervane voltage as a function of aging time (operation time xduty factor). VI . RI BEAM ACCELERATION
418
S. Arai et aL / Development of an SCRFQheavy-ion 6nac Ouadrupole Magnets
Turbomolecular Pump
Turbomolecular Pump (1500 I/s)
Bendina Magnet
Einzel Lens 3 / StiD eerng elector f Einzel Lens 2 Ion Source Einzel Lens 1 Îon Separator
Emittonce Monitor Faraday Cup 1 L. ... i . 0
.I lm
Fig. 7. Setupof the acceleration test stand. 4.3. Bears acceleration tests
100
Acceleration tests were performed under the pulse operation at a repetition rate of 50 Hz . The pulse
.ô
widths of the rf and the beams were 600 and 300 Rs, respectively. The setup of the test stand is shown in fig. 7. Ions were produced in a compact ion source of ECR type . Peak currents of the extracted beams were 180
NZ ,
RA for 35 RA for N+, and 120 AA for Ne +. The low-energy beam transport from the ion source to the RFQ comprises four einzel lenses, an ion separating magnet, an electrostatic deflector for beam steering, a beam monitoring system, and a vacuum pumping system. The current and emittance of the input beam are
t=
Horizontal
100
50 .N ... 0 rf/R.I
X -50
â 50 0
É
T-50
Vertical
us ral."Oh,
-100 NO -100 -10 -5 0 5 10 -10 -5 0 5 10 x (mm) y (mm) . Fig. 8 Emittonce profiles of the input beam (N' ions). The bars indicate measured profiles cut off at a threshold level, 5% of the maximum density, and the ellipses are the design emittances.
measured with a Faraday cup and an emittance monitor, which are set at the entrance of the RFQ. The high-energy beam transport for diagnosis of the output beam comprises an emittance monitor, three Faraday cups, a magnetic-quadrupole doublet, and a vacuum pumping system .
100
The beam transmission depends sensitively on the emittance of the input beam. Hence we matched the emittance to the RFQ acceptance as well as possible . The N+ beam emittances matched before the transmission measurements are shown in fig . 8 as a typical example. Transmissions of NZ , N+ and Ne + beams were measured as functions of the intervene voltage. Trans-
mission efficiency is defined by the current ratio of the accelerated beam to the input beam. The result is shown in fig. 9. In the figure, we use the normalized intervane voltage Vn, defined by the intervene voltage divided by the design value. The current of accelerated
ions was measured with the Faraday cup located downstream of the magnetic-quadrupole doublet . Though
unaccelerated ions are transported through the RFQ, most of them diverge in the quadrupole magnets and do not reach the Faraday cup. The measured transmis-
v
m
.
N
Y ~
r rr
80 60
s
ir
40
.waa""""'aw;
e
Accelerated Beams N+ Experiment Ne+ Experiment . ° N2+ Experiment _ ----- PARMTEQ
20 0 0 .5
v
1 .0
1 .5
2.0
2.5
Normalized intervane voltage Fig. 9. Transmissions measured as a function of the intervane voltage.
S. Arai et aL / Developmentofan SCRFQheavy-ion linac Horizontal
Vertical
x (mm)
y (mm)
Fig. 10. Emittance profiles of the output beam (N' ions). The bars indicate measured profiles cut off at a threshold level, 5% of the maximum density, and the ellipses are the design emittances . sion efficiency of the N+ beam exceeds80% at V > 1.2 . The transmission increases rapidly around the design voltage. However, a beam simulation by the PÀRMTEQ code predicts a steeper increase as shown in fig. 9. The output emittances of N+ beam were measured at V = 1.28. The result is shown in fig. 10. By comparing figs . 8 and 10, it was confirmed that there is no emittance growth in the RFQ. By using a magnetic spectrometer with a bending angle of 45°, it was confirmed that the output energy was accelerated up to the designed energy of 45 .4 keV/u. The energy resolution of the spectrometer was
419
t 0.34% when the slit width was set at t3 mm. Energy spectra of N' ions were measured at several values of the intervane voltage. The obtained energy spectra are shown in fig . 11 . The full energy spread varied from t 3.3% to t 6.0% in the range 0.96 < V < 2.08. The minimum value, t 3.3%, was obtained at V = 1.12, and agreed with the design value of t 3.3%. 5. Concluding remarks In the R&D of the 25 .5 MHz SCRFQ we have obtained the following results : (1) the rf characteristics of the cavity agree with the design values: (2) the field imbalance among quadrants is within ±0 .6%; (3) the intervane voltages between 6 and 114 kV are applicable at a duty factor of 4% ; (4) the transmission efficiency exceeds 80% at normalized intervane voltages, V > 1.2, and the energy and spread of the output beam agree approximately with design values; and (5) the rapidly increasing rate of transmission measured around the design intervane voltage is lower than predicted by PARMTEQ . The result of (2) means that the accuracy of the vane alignment before installation into the tanks is maintained even after the installation . As for the duty factor of (3), the inner structure or its cooling must be improved to make the duty factor higher than 10%. A probable cause of the discrepancy of (5) is an effect of the accelerating field reduction due to the two-dimensional cutting in the vane fabrication on the beam performance . In order to investigate this effect in detail, we must measure the transmission more precisely by improving the input emittance. Acknowledgements The authors wish to express their thanks to M. Kihara for his encouragement. Development of the SCRFQ is supported by the Accelerator Research Division, High Energy Physics Division and Nuclear Physics Division of INS. The prototype was constructed at Sumitomo Heavy Industries, Ltd. The computer work was carried out on a FACOM M780 of the INS computer facility. Studies of the prototype are supported by a Grant for Scientific Research of the Ministry of Education, Science and Culture. References
40
42
44
46
48
Beam energy (keV/u)
50
Fig. 11 . Energy spectra of theoutput beam at several values of the intervane voltage .
[1] T. Yamazaki, INS Report 763 (1989). [2] T. Nomura, INS Report 780 (1989). [3] S. Arai, Proc . 18th Int. Symp. on Physics with High-Intetr sity Hadron Accelerators (World Scientific, 1990) p. 433. VI . RI BEAM ACCELERATION
420
S. Arai et al. / Development ofan SCRFQ heavy-ion linac
[4j R.W. Miller, GSI Report 79-7 (1979). [5j S. Arai, T. Fujino, T. Fukushima, E . Tojyo, N. Tokuda and T . Hattori, IEEE Trans. Nucl . Sci . NS-32 (1985) 3175. [6j S . Arai, INS-T-464 (Accelerator-4) (1986). [7] N. Tokuda, S. Arai, T. Fukushima, T. Morimoto and E. Tojyo, INS Report 743 (1989) .
[8j N. Tokuda, S . Arai, A. Imanishi, T. Morimoto, S . Shibuya and E. Tojyo, INS Report 794 (1989). [9j S . Shibuya, S. Arai, A. Imanishi, T. Morimoto, E . Tojyo and N . Tokuda, INS Report 843 (1990).