Issue 76

Beam Dynamics Newsletter

3.5 Status of the FFA at Kyushu University

Yujiro Yonemura, Hidehiko Arima, Nobuo Ikeda,
Faculty of Engineering of Kyushu University, Fukuoka, Japan.
Yoshiharu Mori, Kyoto University, Kyoto, Japan.

Introduction

A Center for Accelerator and Beam Applied Science has been established to promote activities in all the related scientific, medical, engineering and educational fields at Kyushu University  [1]. To realize the purpose of the center, Kyushu University has decided to construct a new facility merging three institutes - the Cockcroft-Walton Accelerator Laboratory of Faculty of Engineering, the Institute for Irradiation and Analysis of Quantum Radiations and Kyushu University Tandem Accelerator Laboratory of Faculty of Sciences - on its new campus (Ito Campus)  [1].

Figure 1 shows an overview of the new accelerator facility at Kyushu University. The facility consists mainly of a 10 MeV proton cyclotron, an 8 MV tandem accelerator and a 150 MeV FFA (150 MeV Fixed Field Alternating Gradient Accelerator) as a replacement for the Cockcroft-Walton accelerator.

The construction plan of the facility was divided into three stages. A building of the first stage was built in 2008, and the 150 MeV FFA was constructed between 2009 and 2011. In the second stage, the building was constructed as an extension to the first stage one, and construction of the tandem accelerator has been completed in 2014. A beam extraction line of the 150 MeV FFA and a beam injection line from the tandem accelerator to the 150 MeV FFA were constructed between 2015 and 2018. In third stage, we are planning to construct a new beam lines for experiments using beams from the 150 MeV FFA in the high energy experimental room.

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Figure 1: Overview of the accelerator facility at Kyushu University.

3.5.1 150 MeV FFA

The 150 MeV FFA was developed at High Energy Accelerator Research Organization (KEK) as a prototype of a proton FFA for various applications such as proton beam therapy. The construction of the 150 MeV FFA started in September 2002 at the east counter hall in KEK, and the beam extraction with 100 Hz operation was successfully demonstrated in November 2005  [2]. The FFA was disassembled in June 2006, transported by land from KEK to Ito Campus at Kyushu University in March 2008.

The re-construction of the FFA at the Center for Accelerator and Applied Beam Science at Kyushu University started in 2008. Beam commissioning started in December 2011, and beam acceleration was successfully demonstrated in 2013  [3]. The extraction kicker and the extraction magnetic septum were commissioned in 2017. The beam extraction line was constructed between 2016 and 2018. Figure 2 shows a schematic overview of the 150 MeV FFA. The main parameters of the 150 MeV FFA are summarized in Table 1.

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Figure 2: Overview of the 150 MeV FFAs.

Table 1: Design parameters of the 150 MeV FFA

Energy 10 – 125 MeV (proton)
Type of magnet Triplet radial (DFD)
Number of Cells 12
Average radius 4.47 – 5.20 m
Betatron tune (injection energy) 3.62 (Horizontal)
1.42 (Vertical)
Magnetic field Focus: 1.63 T
Defocus: 0.78 T
Revolution frequency 1.5 – 4.2 MHz
Repetitionrate 100 Hz/ 2 Cavity
Beam current 1.5 nA (In the 1st stage)
Injectors

As an injector of the 150 MeV FFA, a cyclotron which accelerates protons to 10 MeV is employed. The cyclotron was originally developed by Japan Steel Works for positron emission tomography and material irradiation. The main parameters of the cyclotron are summarized in Table 2.

Table 2: Cyclotron parameters

Energy 10 MeV (proton)
Type AVF Cyclotron
Ion Source Internal PIG (LaB6 cathode)
RF Dee voltage 40 kV
Extraction Radius 300 mm
Magnetic Field Max. 1.54 T
RF Frequency 47 MHz (2nd hamonic acceleration)
Mean Beam Current 2 \( \mu \)A (In the 1st stage)

The tandem accelerator will be employed as a heavy ion injector to the 150 MeV FFA. The tandem accelerator has been employed independently for AMS, student experiments, RI beam production and low-energy nuclear-physics experiments. The parameters of the tandem accelerator are summarized in Table 3.

Table 3: Tandem accelerator parameters

Accelerator Type Horizontal Tandem Van de Graaff
Model NEC Pelletron (8UDH)
Terminal Voltage 8 MV
Accelerator Tank Diameter: 3.0 m
Length: 13.6 m
Insulation Gas SF6 (pressure: 0.6 MPa)
Ion Source Sputter Ion Source: NEC MC-SNICS
RF Ion Source: NEC Alphatross
Injection Voltage -70 kV
Beam p, d, H.I.
Current 1 nA (in the first stage)
Terminal Stripper C Foil and N2 gas
Charging Device Double pellet Chains
(Current: 150 \( \mu \mathrm {A}\times 2 \))
Injection and Extraction Systems

The injection system consists of an injection magnetic septum, an injection electric septum and a pair of injection bump magnets. An injected beam is deflected by the injection magnetic septum, and its position and angle are adjusted by the injection electric septum. The bump magnets make o bump orbit in the septa. The extraction system consists of an extraction kicker magnet and an extraction septum magnet.

One of the technical difficulties in the operation of the 150 MeV FFA was the closed orbit distortion caused by a large fringing field of the magnets of the main ring  [4]. The injection bump magnets and the extraction kicker magnet installed in the straight section couple to the fringing field, which distorts the closed orbit. In order to overcome the problem, a new type of bump magnet and kicker magnet have been developed.

The bump magnet consists of an air core coil to avoid coupling with the fringing field. The coil consists of 49 wires with a diameter of 0.6 mm to reduce the skin effect and heat generation. The measured inductance of the magnet is 5.6 \( \mu \)H. The switching power source supplies a half-sinusoidal pulse current to the bump magnet. The output peak current and pulse width are 2000 A and 10 \( \mu \)s, respectively [5].

The kicker magnet is composed of three air core coils. These coils are electronically connected in parallel in order to reduce the total inductance of the kicker magnet. The measured inductance of the magnet is 1.1 \( \mu \)H. The switching power supply consists of E2V CX1175 thyratron and PFN network. The voltage and current of the power supply are designed to be 40 kV and 5100 A, respectively, maximum. The rise time of the current is expected to be about 190 ns (0-96%) [6].

Acceleration System

The rapid cycling acceleration of the 150 MeV FFA was successfully achieved with the MA (Magnetic Alloy) cavity developed at KEK. However, the cooling system for the MA cores had a technical difficulty in terms of the thermomechanical reliability. Since the efficiency of the heat cooling was low, the temperature on the inner surface of the core reached over 150\( \circ \)C  [7].

To resolve this problem, a new type of the RF cavity with a high-efficiency cooling system has been developed  [1]. The cavity consists of two MA cores, and the water cooled plates are attached to one side of the cores. Most part of the inside area of the cooling plate is covered by the coolant passage so as to significantly increase the contact area between the plate and cooling water. The measured maximum shunt impedance was 200 \( \Omega \). The measured resonance frequency and the quality factor of the cavity were 2.7 MHz and 0.43, respectively. The measured acceleration voltage is 4.0 kV, which was the voltage required to achieve the rapid acceleration of 100 Hz with two RF cavities  [8].

Beam Monitor

Figure 3 shows the schematic layout of the beam diagnostic devices installed in the 150 MeV FFA. Beam probes are installed for the observation of the beam profile in a radial direction by the current detection with a picoammeter and controlled by PLC with LabVIEW  [9]. Non-destructive beam monitoring based on an electrostatic pick-up is desirable to prevent radioactive contamination and it allows continuous measurement during acceleration. Therefore, a horizontal beam monitor with trapezoidal-shaped electrodes is used for detection of the horizontal beam position and tune measurement and a vertical one with a square plate for the vertical beam position and tune measurement. The radius of the beam orbit shifts from 4.47 m to 5.20 m during RF acceleration at a revolution frequency of 1.5 to 4.2 MHz. Therefore, beam monitors that cover a wide range of the horizontal area are required for the beam diagnostics. A new beam position monitor with five-segmented triangular electrodes has been developed  [10]. The calibration test of the prototype monitor was performed before installation and the results have shown an excellent position linearity except for the edge regions.

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Figure 3: Schematic layout of beam diagnostics of the 150 MeV FFA.

Tune Correction

In the scaling FFA, the betatron tune is theoretically constant, but in actual ring it is not. Adjusting the focusing force is useful for avoiding resonances due to undesirable tune variation and allowing the FFA to have the required versatility. In the 150 MeV FFA, additional iron plates for adjustment of the vertical focusing force are attached on both outer sides of the defocusing magnets of the DFD triples and give the desired correction of the vertical tune compared with the case of no additional plates  [11]. Furthermore, the procedure for adjustment of the horizontal focusing force by installing multi-stepped coils and/or changing the geometry of the pole surfaces has also been examined  [12].

Summary

The construction of the new accelerator facility has been completed at the Center for Accelerator and Beam Applied Science at Kyushu University. Beam commissioning of the 150 MeV FFA has successfully been demonstrated. In parallel to the construction and the beam commissioning of the 150 MeV FFA, a new type of extraction kicker magnet, the injection bump magnet, beam monitors, the tune correction system and the RF cavity have been developed. The experiments with extracted beam are now in preparation.

References
  • [1]  Y. Yonemura et al., “Status of Center for Accelerator and Beam Applied Science of Kyushu University”, Proceedings of the 11th European Particle Accelerator Conference, Genoa, Italy (2008).

  • [2]  M. Aiba et al., “Beam extraction of 150 MeV FFAG”, Proceedings of the 10th European Particle Accelerator Conference, Edinburgh, Scotland (2006).

  • [3]  Y. Yonemura, “Current status of beam commissioning of FFAG accelerator at Kyushu University”, Int. FFA Workshop 2013 (2013).

  • [4]  Y. Yonemura et al., “Commissioning of 150 MeV FFAG synchrotorn”, Proceedings of 9th European Particle Accelerator Conference, Lucerne, Switzerland (2004).

  • [5]  T. Matsunaga et al., “Development of the extraction kicker for FFAG accelerator at Kyushu University”, Proceedings of the 7th Annual Meeting of Particle Accelerator Society of Japan, pp.593-595 (2010).

  • [6]  S. Kuratomi et al., “Multi-turn injection the FFAG accelerator at Kyushu University”, Proceedings of the 9th Annual Meeting of Particle Accelerator Society of Japan, pp.403-406 (2012).

  • [7]  Y. Yonemura et al., “Development of RF acceleration system for 150 MeV FFAG accelerator”, Nuclear Instruments and Methods in Physics Research, Section A, 576(2-3), 294-300 (2007).

  • [8]  Y. Yonemura, “Beam commissioning of 150 MeV FFAG accelerator at Kyushu University”, Int. FFA Workshop 2012 (2012).

  • [9]  Y. Yonemura et al., “Present status of beam commissioning of FFAG accelerator at Kyushu University”, Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan, pp.852-854 (2013).

  • [10]  S. Mochizuki et al., “Development of non-destructive large-aperture beam monitor”, Progress in Nuclear Science and Technology Vol.1, pp.328-331 (2003).

  • [11]  N. Motohashi, Int. FFAG Workshop 2015 (2015); Y. Waga, Int. FFA Workshop 2018 (2018).

  • [12]  M. Ueda, “Correction of focusing force with pole surface coils for the radial sector type FFAG accelerator”, Int. FFA Workshop 2017 (2017).