Improvement in beam quality of the JAEA AVF cyclotron for focusing heavy-ion beams with energies of hundreds of MeV

Improvement in beam quality of the JAEA AVF cyclotron for focusing heavy-ion beams with energies of hundreds of MeV

NIM B Beam Interactions with Materials & Atoms Nuclear Instruments and Methods in Physics Research B 260 (2007) 65–70 www.elsevier.com/locate/nimb I...

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 260 (2007) 65–70 www.elsevier.com/locate/nimb

Improvement in beam quality of the JAEA AVF cyclotron for focusing heavy-ion beams with energies of hundreds of MeV Satoshi Kurashima a,*, Nobumasa Miyawaki a, Susumu Okumura a, Masakazu Oikawa Ken-ichi Yoshida a, Tomihiro Kamiya a,b, Mitsuhiro Fukuda c, Takahiro Satoh a, Takayuki Nara a, Takashi Agematsu a, Ikuo Ishibori a, Watalu Yokota a, Yoshiteru Nakamura a a

a,b

,

Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Gunma University, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan c Osaka University, 10-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan Available online 14 February 2007

Abstract In order to achieve a heavy-ion microbeam with an energy of hundreds of MeV applied to the research fields of biotechnology and materials science, the JAEA AVF cyclotron (K = 110) has been upgraded to provide a high quality beam with a smaller energy spread and a higher current stability. A flat-top (FT) acceleration system of the cyclotron, designed to produce ion beams with an energy spread of DE/E 6 0.02%, has been developed to reduce chromatic aberrations in the lenses of the focusing microbeam system. The FT acceleration system provides uniform energy gain of the beam by superimposing a fifth-harmonic voltage on the fundamental one. In addition, stabilization of the acceleration rf voltage and the phase were achieved to accelerate the high quality beam and to provide it stably to the microbeam system connected to a cyclotron beam line. In the latest experiment, we have succeeded to accelerate 260 MeV 20Ne7+ with an energy spread of 0.05% in FWHM using the FT acceleration system. Ó 2007 Elsevier B.V. All rights reserved. PACS: 29.20.Hm; 41.75.Ak; 42.60.Da Keywords: High quality beam; Microbeam; Heavy-ion; Flat-top acceleration; AVF cyclotron

1. Introduction In TIARA1, various heavy-ion beams are utilized for the research in biotechnology and materials science, e.g. bystander effect, apoptosis for biological cells and singleevent effect in IC chips [1,2]. TIARA was equipped with a variable-energy AVF cyclotron (K = 110) which provides various ions; 5 through 90 MeV protons and 2.5 through 27 MeV/u heavy-ions [3]. A heavy-ion microbeam single ion hit system with a collimating aperture for biological applications was developed on a vertical beam line of the *

1

Corresponding author. Tel.: +81 27 346 9636; fax: +81 27 346 9690. E-mail address: [email protected] (S. Kurashima). Takasaki Ion Accelerators for advanced Radiation Application.

0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.278

cyclotron to provide 5–10 lm beam spot size or hitting accuracy to individual cell irradiations in atmosphere [1,4]. Another microbeam formation system, which consists of a set of quadruplet quadrupole magnets as the focusing lens system and a multi-collimation system, was installed on a different vertical beam line to meet a requirement of a higher spatial resolution microbeam for biology and also for semiconductor technology [5]. As discussed in [5], an beam energy spread DE/E is required to be less than 0.02% in FWHM to reduce the chromatic aberration in the microbeam focusing system with a beam acceptance limited to a small value of less than 1 lm mrad. While an electrostatic accelerator, which is often used to form a microbeam with a spot size of 1 lm or less, can accelerate ion beams with the energy spread of the order of 104, a

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cyclotron beam generally has an energy spread of 0.3%– 0.5% due to the rf acceleration, the voltage of which changes in a sinusoidal waveform. A flat-top (FT) acceleration [6] system is being developed to reduce the energy spread down to DE/E = 0.02%. The uniform energy gain can be obtained by adding a harmonic frequency voltage to the fundamental one. Many operational parameters, such as isochronous magnetic field, acceleration dee voltage and the phase width, should be optimized to accelerate the ion beam by the FT system. Because even slight fluctuations of the parameters cause instabilities of the energy spread, beam mean-energy, beam intensity, beam orbit and so on, the cyclotron magnet and rf system were required to be improved for realization of a highly stabilized high quality beam. The high quality beam should be obtained only after solving such peculiar subjects to the cyclotron completely. This paper describes the detail of the system improvement, beam tuning to accelerate the high quality beam which is rare case for the cyclotron, and measurement of the energy spread of the beam. 2. Requirements for high quality beam production The specifications of the cyclotron which are required to reduce the energy spread to DE/E = 0.02% are shown in Table 1 [7] and briefly described below. 1. The FT acceleration is a technique to accelerate ion beams with nearly the same energy gain by flattening an acceleration voltage waveform within a tolerable period of the rf phase. High stabilities of the voltages and its phases are necessary to prevent fluctuations of the energy gain for achieving the uniformity. 2. A slight change of the cyclotron magnetic field, which causes small drift of ions in the acceleration rf phase at each revolution, results in the large drift of the phase at the extraction radius. Very high stability of the magnetic field is needed to keep the beam bunch excursion in

Table 1 Requirements of the cyclotron performance for achieving the energy spread of DE/E 6 0.02% in FWHM Requirement

Tolerance

1a. Stability of acceleration voltage

DV/V 6 0.02% in FW for fundamental DV/V 6 0.1% in FW for harmonic

1b. Stability of rf phase

D/ 6 0.2 deg in FW for fundamental D/ 6 0.2 deg in FW for harmonic

2. Stability of cyclotron magnetic field 3. Control of Beam phase width 4. Beam bunching

DB/B 6 0.002% in FW

5. Resolution of the beam energy spread measurement

D/ 6 10 deg in FW 80% compression of injected beam within 10 deg RF DE/E 6 0.01%

the uniform energy gain region. The required stability was achieved by precisely controlling the temperature of the cyclotron magnet [8]. 3. Although the beam bunch width can be controlled with the phase defining slits in the center region of the cyclotron, more precise control of the beam width within 10° in rf phase is required for the uniform energy gain. 4. Spatial restriction of the beam width, resulting in reduction of the beam current, hinders optimization of the cyclotron parameters for improving the beam quality and consequently for producing the microbeam. A beam buncher is needed to compress an injected DC beam into the restricted beam phase region of 10° for enhancing the beam current. 5. In order to measure the energy spread of the FT accelerated beam, a new measurement system with a resolution of less than 0.01% is required. 3. Flat-top acceleration The beam phase width must be limited within ±1.2° at the sacrifice of the beam current to obtain the energy spread of DE/E = 0.02% using an ordinary sinusoidal voltage waveform. Moreover, higher stabilization of the magnetic field to less than 106 and precise control of the beam phase width within ±1.2° are necessary; it is difficult to achieve them. The third- or fifth-harmonic of the fundamental frequency is generally used for the FT acceleration [9,10]. The amplitude of the nth harmonics superimposed on the fundamental waveform is 1/n2 times the fundamental voltage. For the third-harmonic frequency, the power consumption of the resonator and the wave length are 8 and 1.7 times as large as the fifth-harmonic frequency, respectively. We have adopted the fifth-harmonic frequency to save electric power and to reduce the resonator size. The phase width to obtain the energy spread of DE/E = 0.02% is expanded to ±6.8°. The main components of the cyclotron and an outline of the resonators are shown in Fig. 1. The cyclotron has a pair of quarter-wavelength (k/4) coaxial type resonators with a movable-short. The range of the fundamental frequency is 11–22 MHz. Acceleration harmonics h, the ratio of the rf frequency to the orbital frequency of ions, of 1, 2 and 3 are used. The fifth-harmonic frequency of 55–110 MHz is required to cover the whole range of the fundamental frequency. The design of the FT resonator was determined by cold tests with a model of 1/1 scale and simulations using the MAFIA [11] code to find an optimum structure, achieving smaller size and lower power consumption [7]. The FT system consisting of resonators, amplifiers and a low-level controller was installed to the cyclotron system. A power test at the fundamental frequency of 17.475 MHz was carried out, and the flat-top voltage waveform was successfully observed at the dee voltage pick-up as shown in Fig. 2. If the voltage distributions of the fundamental and the fifth-harmonic frequencies are the same in a whole

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Fig. 1. Upper: Main equipments of the JAEA AVF cyclotron. A magnet yoke and a main coil of upper side are omitted. The cyclotron has two rf resonators of movable-short type. Lower: A schematic diagram of the cyclotron resonators. The FT resonator is capacitively coupled to the main resonator. Span angle of the dee electrode and the maximum voltage are 86° and 60 kV, respectively.

region of acceleration gaps of the dee electrode, we should use the ‘‘flat-top waveform’’ to obtain the high quality beam. However, the resonator actually has position dependence of the voltage distribution, which varies with the resonant frequency. The voltage decreases from the tip of the dee electrode toward the outer radius and is always zero at the movable-short. For a higher frequency, the voltage falls rapidly because the variation of the voltage distribution depends on the wavelength of a standing wave axially generated in a main resonator. The voltage distributions along the acceleration gap calculated by the MAFIA code are

shown in Fig. 3. The voltage of the fifth-harmonic frequency has clear position dependence. On the other hand, the fundamental voltage has a little decrease. The fifth-harmonic amplitude, therefore, needs to be optimized at each frequency to uniform the overall energy gain at extraction of ions from the cyclotron. If the amplitude is ideally optimized, the energy spread of the beam is reduced and radial spread of the beam bunch becomes smaller than the ordinary acceleration without the FT acceleration. A turn separation with a clear split of the beam bunches can be observed by using a radial current probe.

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32

0.02

31.8

Voltage Stability ΔV/V(%)

0

31.6

-0.02

Initial change -0.04

31.4

-0.06

31.2

31

-0.08

0 Fig. 2. The upper figure is the flat-top waveform monitored by the pick-up capacitor of the dee electrode. The lower one is the waveform of the fifthharmonic frequency obtained by a high-pass filter. Voltage and frequency of the fundamental waveform are 25 kV and 17.475 MHz, respectively.

1.2

Voltage (a.u.)

1 0.8 0.6 0.4 11 MHz 55 MHz 21 MHz 105 MHz

0.2 0

0

200

400

600

800

10 00

Distance from the Center (mm) Fig. 3. Voltage distributions along the acceleration gap of the dee electrode calculated at the fundamental and 5th-harmonic frequencies by the MAFIA code. Ion beam is extracted at the mean radius of 923 mm from the cyclotron.

4. Stabilization of the acceleration voltage and phase The stabilities of the acceleration voltage and phase are extremely important to produce the high quality beam because the energy gain of the beam in the acceleration gap is directly influenced by them. The required voltage stability of DV1/V1 6 0.02% is one-tenth of the original specification. One of the reasons causing the low stability is a large variation of the temperature of the rf low-level controller. Since specific values of electronic parts, such as resistors, capacitors and inductors, change with the temperature, the actual values of the voltage and phase also

Temperature in theControl Rack (˚C)

68

2 4 Time (hour)

8

Fig. 4. Variations of the dee voltage and the temperature in the rf control rack after stabilization. The voltage stability is within the required value of 0.02%.

fluctuate even if seemingly controlled by a feed-back circuit. In order to improve the stability, we put the rf low-level controller in a temperature-controlled box. A cooler using a Peltier device had been attached to the rack for rf low-level controller. However, it was difficult to control the temperature precisely because of insufficient capacity of the cooler. The temperature varied between 20 and 30 °C through the cyclotron operation. The temperature controller was replaced with a new one which has enough capacity to maintain the temperature in the rack within ±0.5 °C. Heat insulation of the rack was reinforced as well. As a result of these improvements, the temperature change of the rf low-level controller can be kept within 0.3 °C, and the voltage stability of DV1/V1 6 0.02% has been achieved as shown in Fig. 4. The phase fluctuation has been reduced to D/1 6 0.2° as well. The stabilities required for the fifthharmonic frequency have been easily realized because of loose tolerance. 5. Precise control of the beam phase width and increase of the beam intensity It is necessary to control the beam phase width precisely with a pair of phase defining slit in the central region because the beam bunch should be accelerated only in the region of uniform energy gain. Since the original phase defining slits cannot satisfy this requirement, we newly designed the central region, which composed of a pair of phase defining slits, inflector and puller electrodes. In a beam acceleration test, the phase width was successfully controlled within 10° and a minimum width of 3° was achieved by fine-tuning of the phase defining slits. A DC ion beam produced by an ion source is compressed by a beam buncher, and then injected into the

S. Kurashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 65–70

6. Acceleration and transportation of the high quality beam

0.3

Beam Current (uA)

0.25

V1 = 36.502 kV V5 = 0.0 kV

FT OFF

0.2 0.15 0.1 0.05 0

0

10

20

30

40

50

60

70

Probe Position (mm) 70

V1 = 36.05 kV V5 = 1.48 kV

60

Beam Current (nA)

cyclotron. A saw-tooth voltage waveform is most suitable for highly efficient beam bunching. An approximate sawtooth voltage waveform is generally formed by combining the fundamental, second and third-harmonics of a sinusoidal voltage waveform. This method requires an individual voltage generator for each harmonic, which brings increase in costs and makes the beam tuning complicated because of many parameters for the three frequencies. Therefore, a beam buncher using a sinusoidal waveform of the fundamental frequency is popularly used. We have been developing a new type of the saw-tooth waveform beam buncher, designed by applying a transitional phenomenon of an electric LCR circuit [12]. The beam bunching test was performed using a 260 MeV 20Ne7+ ion beam. The bunching efficiency, the ratio of the beam current with the buncher to that without it, was 14.5 as a result of simultaneous operation of the sinusoidal and the saw-tooth waveform buncher, while the bunching efficiency was 9.3 with the sinusoidal waveform buncher alone. A beam current sufficient for the beam tuning by the FT acceleration was obtained with two buncher operation.

69

FT ON

50 40 30 20 10 0

0

10

20

30

40

50

60

70

Probe Position (mm)

We have been developing the 260 MeV 20Ne7+ beam with the FT acceleration system for producing a microbeam for biotechnology experiments. The acceleration harmonics and the number of revolutions of the beam are h = 2 and N = 265, respectively. It is impossible to obtain the beam energy spread under 0.1% when the beam bunches with different number of revolutions are simultaneously extracted from the cyclotron, namely in the case of multi-turn extraction. In the ordinary acceleration using the fundamental voltage waveform, a radial spread of the beam bunch DR in the extraction region is wider than the radial interval of the adjacent revolutions Dr due to large energy spread. In this case, a beam bunch is divided into several pieces by the multi-turn extraction. On the other hand in the FT acceleration, the DR is narrower than Dr, and the turn separation can be observed clearly by a scanning probe with a differential beam current detector. The whole of a single beam bunch is extracted at a time, which is called single-turn extraction that is an indispensable condition to obtain a high quality beam. For a well-centered beam, Dr is expressed with non-relativistic approximation as Dr ¼

DE 1 r¼ r 2E 2N

where r is the radius of the equilibrium orbit of the ion. It is easy to observe the turn separation for a small or medium scale cyclotron, since the number of revolutions is smaller than one hundred. But in the case of the JAEA cyclotron using ordinary acceleration, the turn separation could not be seen even if Dr was enlarged by means of orbital precession caused by a first-harmonic component of the magnetic

Fig. 5. Radial beam current distributions with and without the FT voltage measured by the scanning differential probe at the entrance of the deflector. The accelerated beam was 260 MeV 20Ne7+. Clear turn separation was observed in the case of FT acceleration. The last beam bunch can pass though the deflector with transmission over 95%.

field. However, we succeeded in observing the turn separation with the FT acceleration by using the differential beam current probe as shown in Fig. 5. The extraction efficiency at the deflector was increased from 60% with the multi-turn extraction up to 95% with the single-turn extraction due to little hit of the beam on the deflector electrode. In order to confirm the reduction of the energy spread with the FT acceleration, we have developed a micro-slits system to measure the beam spread caused by energy dispersion in the analyzing magnet [13]. We carried out a beam test of the energy spread measurement for the 260 MeV 20Ne7+ beam with and without the FT acceleration, and the results are 0.05% in FWHM and 0.1% in FWHM, respectively. In practical microbeam formation [14], stable beam extraction from the cyclotron and ideal beam transport to the microbeam system is important as well as the reduction of the beam energy spread. Past experiences in beam preparation showed a few problems for microbeam formation. For example, a change in the cooling water temperature of the ion source caused deterioration in the beam quality, and focusing into the desired image was restricted. Because extracted beams from the cyclotron are not always matched with the axis of the microbeam system, we must precisely tune the parameters in the beam transport line and in the microbeam system.

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7. Summary [4]

In order to form a heavy-ion microbeam with an energy of hundreds of MeV, various developments, such as flattop acceleration and stabilization of the acceleration voltage were carried out. We succeeded in observing the turn separation and realizing the single-turn extraction, which is very difficult for the large scale AVF cyclotrons. As a result of the beam development of the 260 MeV 20Ne7+ using the flat-top acceleration, the energy spread was reduced to DE/E = 0.05% in FWHM. The required energy spread is, however, 0.02% in FWHM to form the microbeam with a diameter of 1 lm. The energy spread will be decreased to 0.02% by fine-tuning of the accelerating voltage, phase and the magnetic field simultaneously with measuring the energy spread. In addition to reduction of the energy spread, these techniques are very useful to stabilize the beam mean-energy and the beam intensity. We are going to accelerate the high quality beam with various ion species needed for the experiments with the microbeam.

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