The PEFP 20-MeV Proton Linear Accelerator

Transcription

1 Journal of the Korean Physical Society, Vol. 52, No. 3, March 2008, pp Review Articles The PEFP 20-MeV Proton Linear Accelerator Y. S. Cho, H. J. Kwon, J. H. Jang, H. S. Kim, K. T. Seol, D. I. Kim, Y. G. Song and I. S. Hong Proton Engineering Frontier Project, Korea Atomic Energy Research Institute, Daejeon Y. H. Kim Samsung Electronics, Yongin (Received 21 September 2007) The Proton Engineering Frontier Project (PEFP), promoted by the Korean Government since 2002, has the goal of developing a 100-MeV high-current proton linear accelerator and utilizing its beam for scientic, industrial and medical applications. A 20-MeV proton linac has been developed as the front end of the 100-MeV proton linac. The 20-MeV proton linac consists of a 50-keV proton injector, a 3-MeV radio frequency quadrupole (RFQ), a 20-MeV drift tube linac (DTL) and radio frequency (RF) systems. In addition, the technology for each component has been developed. The accelerator has been installed and operated to supply an average current of 100 na at the Korea Atomic Energy Research Institute (KAERI) test stand. PACS numbers: W Keywords: Proton accelerator, PEFP, RFQ, DTL, RF system I. INTRODUCTION The Korean Government launched the Proton Engineering Frontier Project (PEFP) in 2002 to help realize potential applications of high-power proton beams. The primary goal of the project is to develop a highpower proton linear accelerator to supply 100-MeV proton beams and to construct user beam line facilities, whose users can utilize proton beams with a wide range of energies and currents for their research and development programs [1]. In addition, the 100-MeV accelerator can be used as a proton injector for the next-stage high-power accelerators with higher energy, such as a high-energy linac or rapid cycling synchrotron for GeV proton beam applications, such as the spallation neutron source [2]. Based on a user demand survey on proton beam applications, 100-MeV and 20-MeV proton beams will be extracted and distributed to a maximum of ve users simultaneously by using AC magnets employing a programmable current power supply, as shown in Figure 1. The total power of the 100-MeV beam will be 160 kw and the total power of the 20-MeV beam will be 96 kw. We will control the beam energy stepwise by using an RF ON/OFF switch for each DTL tank. To control the beam energy continuously, we will place energy degraders and energy lters in the beam lines for special applications [3]. A 20-MeV proton linear accelerator has been developed as the front end of the 100-MeV accelerator, which choys@kaeri.re.kr; Fax: consists of a 50-keV proton injector, a 3-MeV RFQ, a 20-MeV DTL and RF systems, as shown in Figure 2. II. THE 20-MEV PROTON LINEAR ACCELERATOR 1. The Proton Injector [4] The injector includes a duoplasmatron proton source and a low-energy beam transport (LEBT). The beam current extracted from the source reached a current of 50 ma. The extracted beam has a normalized emittance of 0.2 mm-mrad from a 90 % beam current, where the proton fraction is >80 %. To achieve pulsed operation, a high-voltage switch has been installed in the high-voltage power supply, whose rise and fall times are <50 ns. The pulse length and the repetition rate can be easily changed using this semiconductor switch. As shown in Figure 3, the LEBT consists of two solenoid magnets that can lter the H + 2 ions and two steering magnets that can control the beam's position and angle at the entrance of the RFQ. Figure 4 shows the beam signal measured using a Faraday cup. 2. The 3 MeV RFQ [5,6] The PEFP RFQ is designed to accelerate a 20-mA proton beam using a voltage from 50 kev to 3 MeV and has -721-

2 -722- Journal of the Korean Physical Society, Vol. 52, No. 3, March 2008 Fig. 1. Schematic diagram of the PEFP accelerator and user beam lines. Fig MeV proton linear accelerator. Fig. 3. Ion source and LEBT. the usual four-vane-type design. The entire structure is separated into two segments that are resonantly coupled for eld stabilization. The RF power is fed into the cav- ity through two iris couplers in the third section. Table 1 shows the main parameters of the RFQ. A 3-MeV, 350-MHz PEFP RFQ has been fabricated,

3 The PEFP 20-MeV Proton Linear Accelerator { Y. S. Cho et al Table 1. The PEFP RFQ parameters. Frequency Input/output energy Transmission rate Total length Peak surface eld Output emittance (normalized rms) Type 350 MHz 50 kev/3 MeV 98.3 % cm 1.8 Kilpatrick 0.22 mm-mrad (0.11 deg-mev) Four-vane type resonant coupling Fig. 6. RF signals after high-power RF conditioning. Key: Ch 1 = forward, Ch 2 = reverse, Ch 3 = cavity and Ch 4 = klystron reverse. Fig. 4. Beam signal at the LEBT exit. The horizontal scale = 100 ms/div and the vertical scale = 10 ma/div. Fig. 7. RFQ current. Key: Ch 1 = ACCT (located at the RFQ upstream) current (15 ma/div) and Ch 2 = RFQ output current (10 ma/div). the LEBT and the RF parameters. Figure 7 shows a typical beam signal detected during the beam tests. 3. The 20-MeV DTL Fig. 5. PEFP 3 MeV RFQ. tuned, installed and tested. The low-power eld tuning satis ed the design requirements. A test stand was installed at the KAERI test facility and the RFQ was conditioned up to the designed power level by using a low duty factor (Figure 5). High-power RF conditioning experiments for the RFQ were carried out up to a peak power of 450 kw, a pulse length of 80 s and a repetition rate of 1 Hz. The time required for this conditioning was about 8 h. The RF signals shown in Figure 6 are the signals detected after the conditioning, which were very stable. Beam tests were carried out by adjusting The PEFP 20-MeV DTL consists of four tanks that accelerate the 20-mA proton beam from 3 MeV to 20 MeV. The total length of the DTL is about 20 m. The PEFP DTL structures were designed for a beam duty of 24 % and the FFDD lattice con guration has a magnetic eld gradient of 5 kg/cm and an e ective eld length of 3.5 cm. The DTL parameters are shown in Table 2. The DTL was fabricated using electroplating technology for the tanks and e-beam welding technology for the drift tube. A laser tracker was used to align the drift tubes in the tanks. Figure 8 shows the inside of a tank. The tuning goals for the PEFP DTL were such that the deviation in frequency was less than 5 khz from the design value and the eld distribution was less than 2

4 -724- Journal of the Korean Physical Society, Vol. 52, No. 3, March 2008 Table 2. Summary of the PEFP 20-MeV DTL. Resonant frequency Klystron operation Beam operation Maximum peak current Maximum pulse width Maximum repetition rate Maximum beam duty Maximum average current 350 MHz DC Pulse 20 ma 2 ms 120 Hz 24 % 4.8 ma Fig. 9. RF signals inside the four tanks of the DTL 1. Key: Ch 1 = Tank 1, Ch 2 = Tank 2, Ch 3 = Tank 3 and Ch 4 = Tank 4. Fig. 8. Inside a DTL tank. % throughout a tank with a tilt sensitivity against perturbations of less than 100 %/MHz. For the 20-MeV DTL, a single klystron drives four DTL tanks simultaneously. For this multicavity driving concept, temperature control systems and mechanical phase shifters were installed in each tank. High-power RF tests were carried out. The peak RF power to each tank was 150 kw. The waveforms of the cavity eld at each tank are shown in Figure 9. Current transformers were installed at the entrance of Tank 1 and at the exits of Tanks 2 and 3. In addition, a Faraday cup was installed at the exit of Tank 4. The beam transmission through the DTL tanks was measured using these beam diagnostic devices for tuning the machine parameters. A typical beam signal is shown in Figure The RF System [7] The accelerator facilities at the KAERI test stand include the 20-MeV accelerator itself, two sets of 1-MW, 350-MHz RF systems, two sets of {100-kV, 20-A DC high-voltage power supplies for the klystron, two sets of 2-MW cooling systems for the cavity and the RF system. The design duty of the 20-MeV accelerator was 24 % and two 1-MW, 350-MHz klystrons were used to drive the 20-MeV accelerator: one was for the RFQ and the Fig. 10. Beam signal of the 20-MeV DTL. Key: Ch 1 = ACCT (located at the RFQ upstream) current (15 ma/div), Ch 2 = RFQ output current (10 ma/div) and Ch 3 = DTL output current (5 ma/div). other was for the DTL. All the other ancillary facilities, such as the klystron power supply and cooling system, were designed for an operational duty of 100 %. During the low-duty operational tests at the KAERI test stand, the RF system operated such that the electron beam of the klystron was in the continuous wave (CW) mode and only the input RF signal was modulated for low-duty pulse operation. TED Model TH2089F klystrons (350 MHz, 1 MW CW, Thales Electron Devices) were used as the RF source for the 3-MeV RFQ and for the 20-MeV DTL. Two high-voltage power supplies and two modulating anode power supplies were fabricated and tested for the klystrons. In addition, iris-type input couplers were developed and installed in the RFQ and the DTL.

5 The PEFP 20-MeV Proton Linear Accelerator { Y. S. Cho et al Fig. 11. Schematic drawing of the RF system. Fig. 12. FPGA hosted in the VME board. Figure 11 shows a schematic drawing of the RF system with a low-level RF (LLRF) for the 20-MeV proton linac. The digital LLRF system was developed and the stability requirements of the RF eld were: amplitude = 1 % and phase = 1. Our digital feedback control system was based on a commercial eld-programmable gate array (FPGA) card hosted on a virtual machine environment (VME) board, as shown in Figure 12. A control logic based on feedback and feed-forward control was implemented in the FPGA by using a very high-speed integrated circuit hardware description language (VHDL). Figure 13 shows the pulse-to-pulse stability of the RF Fig. 13. Pulse-to-pulse RF amplitude and phase variation (DTL). amplitude and phase in the DTL tank. 5. Target Station for 20-MeV Proton Beam Users The operational license for the 20-MeV proton linac installed at the KAERI site was issued in April 2007 by the Korea Institute of Nuclear Safety (KINS). According to our operational license, a 20-MeV beam with an

6 -726- Journal of the Korean Physical Society, Vol. 52, No. 3, March 2008 III. CONCLUSIONS Fig. 14. Target station for the 20-MeV proton beam users. We have developed the technology required for the construction of a proton linac. Using this technology, a 20-MeV proton linac was developed and installed at the KAERI site in Daejeon, Korea. It has been tested at a low duty and has been used to supply a 20-MeV beam to users with an operational license to supply an average current of 100 na. After the construction of a new site in Gyeongju, Korea, the 20-MeV linac will be moved and installed as the front end of a 100-MeV proton linac. Final performance tests at the designed duty will be carried out at this time. ACKNOWLEDGMENTS This work was supported by the 21C Frontier R & D program of the Ministry of Science and Technology (MOST) of the Korean Government. REFERENCES Fig. 15. The beam pro le at the target station (MD-55 Gafchromic lm). average current of 100 na can be supplied to users for their applications. At the exit of the 20-MeV DTL, a target station has been prepared to supply beams to the user, as shown in Figure 14. The 20-MeV proton beams are extracted into air through an aluminum beam window and the beam pro les are measured using MD-55 Gafchromic lm, as shown in Figure 15. [1] B. H. Choi, in Proceedings of Particle Accelerator Conference 2005 (Knoxville, 2005), p [2] Y. Y. Lee, Nuc. Eng. Tech. 37, 433 (2005). [3] M. Baba, Nuc. Eng. Tech. 38, 319 (2006). [4] Y. S. Cho and K. Y. Kim, J. Korean Phys. Soc. 48, 721 (2006). [5] H. J. Kwon, H. S. Kim, K. T. Seol and Y. S. Cho, J. Korean Phys. Soc. 48, 726 (2006). [6] H. J. Kwon, H. S. Kim, J. H. Jang and Y. S. Cho, J. Korean Phys. Soc. 50, 1450 (2007). [7] H. S. Kim, H. J. Kwon and Y. S. Cho, J. Korean Phys. Soc. 48, 732 (2006).