The Nanosecond Bunching System at KIGAM Tandem Accelerator

The Nanosecond bunching system at KIGAM Tandem Accelerator

G. D. Kim, W. Hong, H. W. Choi, H. J. Woo, and J. K. Kim, Korea Institute of Geoscience, and Mineral Resources, Taejon 305-350, Korea

J. H. Chang Korea Atomic Energy Research Institute Taejon 305-600, Korea

Abstract KIGAM has developed a nanosecond bunching system to obtain the reliable neutron capture cross section data. The pulsed beam is obtained by a set of deflector electrode and slit. Also the bunched beam is obtained through the three stage electrodes, to which double bunching principle applies. The specification of neutron bunching beam is designed to be that the repetition rate is 8 MHz, the width is about 2 ns, and duty factor is about 20 %.

I. Introduction The nuclear data project as one of the nation-wide nuclear R&D programs was launched by KAERI in 1999, Its main goals are to establish a nuclear data system, to construct the infrastructure for nuclear data production and evaluation, and to develop a highly reliable nuclear data system. KIGAM is interested in neutron cross section data of neutron energies range from 1 MeV to 2 MeV, which is corresponded with field of fusion reaction. So we have measured neutron capture cross sections of a few materials, such as 65Cu, and 186W, by MeV continuous neutron beam from 3T(p,n)3He reaction and a neutron activation method. We have large errors to other`s experimental data.[1,2] So we plan to measure the prompt gamma ray following neutron capture reaction to obtain the accurate nuclear data. We need pulsed neutron beams to remove the background gamma rays and the beam associated background gamma rays. A neutron pulsing and bunching system of KIGAM 1.7 MV Tandem accelerator has been designed and is now under fabrication. The pulsed beam is obtained by a set of deflector electrode and slit. Then bunched beam is obtained through the three stage electrodes, to which the double bunching principle applies. The specification of neutron bunching beam is designed to be that the repetition rate is 8 MHz, the width is about 2 ns, and duty factor is about 20 %. A program was made to show the bunching profile and compression ratio of proton beam. Applied alternative voltage, size and intervals of electrodes, and slit size were determined by a simulation by the home-made program and a calculation by superfish code. A 45o bending magnet of C type to guide the charged particle beam to a new neutron experimental room was fabricated. Also power supplies for beam bunching system are being fabricated. The positions of A beam profile monitor, slits, faraday cup and magnetic quadruple doublet lens was determined to confirm an optimum pulsed beam shape. The beam dynamic for each component was calculated by NEC made beam transport program.

Ⅱ. Structure of beam bunching system The Nanosecond bunching system at KIGAM tandem accelerator consists of three stage ; (1) beam pulsing and bunching system like fig. 2; (2) beam bending system into neutron scattering room like fig. 3 ; (3) beam guiding system into neutron generating chamber like fig. 6. The total size of the beam pulsing and bunching system is 1100 mm, The interval from the end electrod of deflector and bunch slit is 283 mm, deflector electrods length

500 cm m c

0 5 6 detetctor Neutron chamber collimator Gate Value

wall BPM Neutron Steerier chamber feedthrufaraday Implantation m cup c

chamber

5 ERD .

6 Steerier chamber 0

270 cm 1 R =58.95 cm 45o TMP bending slit 150 cm magnet Rough Scanner SteerierSlit RF Source pump Acceleration Tank feedthru einzel o Faraday BPM lense TMP 30 buncher slit SF gas cup

º§·ê·Î¿ìÁî(50 mm) 4 º§·ê·Î¿ìÁî 6 4 5 78.2 R 5.5 1

1 13 3 3 3 1 S US 1 BPM 278.3 5 2.5 22.

1 entrance 1 0 9 .7 10. 3 3 R 5.25 59.0 MQD 142 R 5.25 20 22 R 3 1 11.1 22.4 93.6 84.2 259.6 SF gas exit ionsource vaccuum Faraday gauge Analyzing magnet cup Magnet SNICS source

Fig. 1 KIGAM tandem accelerator with a nanosecond beam bunching system

and its width is 80 mm and 30 mm respectively. Buncher length is 265 mm considering the flight length of ion beam during half period time of buncher. beam pulsing frequency is 4 MHz, and beam bunching frequency is 8 MHz. The Sweeping slit size is the diameter of 1.5 mm. The einzel lense is positioned before deflector to have a good sweeping. The linear motion facility of slit have been constructed for using a continuous beam. the diameter of first bunching electrod is 30 mm. The beam width of pulsing system can be calculated by this formula.[3] E denotes beam energy in keV. d is the interval of deflector in cm. Δy is slit size. Charge is coulomb. Vo means the applied electrostatic potential in volt. w is sweeping frequency in Hz. l1 is the length of deflector, l2 means the length from deflector's electrod end to slit. When E is 28 keV, d is 3 cm , Δy is 0.2 cm, Charge is 1.6 x 10-19 coulomb, Vo is 81 V, w is 4 MHz, l1 is 8 cm, and l2 is 28.3 cm, Beam width is 64 ns

2 Δt = 4 E d Δy /{ (q Vo w)(l1 +2 l1 l2 )} KIGAM BUNCHER SYSTEM Slit mover

Buncher chamber

Gate valve

283 mm

Einzellense Slit Buncher Deflector

Fig. 2. Beam pulsing and bunching system at KIGAM

The electronics is designed in the method of applying a high voltage with MHz frequency both to deflector electrod and to buncher electrod, synchronically. The frequency of deflector is 4 MHz, and that of buncher is 8 MHz. The applied voltage is first controlled by two amplifier. The deflector amplifier gain is 25 times larger than original voltage and that of buncher is 25 times, respectively. Applied voltage is controlled by the LCR resonance. The available deflector voltage is from 0.5 kV to 0.5 kV, and that of buncher is from 0 kV to 5 kV with connecting the one part of transformer to ground. Buncher

input LCR resonance AC output -5V ~ + 5 V AC 0 V ~ 5 kV 300 W 8 MHz Pulse Delay Amplifier generator 36 ns 4 MHz

50 W output AC Amplifier -0.5 k V ~ + 0.5 kV input AC -5V ~ + 5 V

LCR resonance Deflector

Fig. 3. Circuit of beam pulsing and bunching system

Also the available maximum magnetic field of 45o bending magnet with a water cooling is about 1 Tesla. Total beaning degree is 75o with respect to beam direction. The bending magnet is fabricated to guide a proton and a deutron considering of stigmatic effect. The curvature of bending magnet is about 59 cm. Fig. 4 shows the 45o bending magnet facility. The maximum current is 75 A, which is applied to a bending magnet. [4]

BR = 144 (MT/z2)0.5 kgauss-cm

B is Magnet field in kgauss, R is curvature of bending magnet in cm, T is the incident particle energy in MeV, M denotes the mass of projectile particle, and Z is the atomic number of projectile particle.

Fig.4. 45o bending magnet 600

) 40 A T m

d l e i f

450 c i t e

n 400 30 A g a M 350

300 0 2 4 6 8 10 12 Point

Fig. 5. magnetic field in the center of bending magnet.

Fig. 5 shows the magnetic field in the center of 45o bending magnet. We can see the good stability of magnetic field within 0.1 % error with respect to magnet center. The magnet field at both end side( point 1 and point 10 ) is smaller than that of the other side. The beam optics is controlled by two magnetic dipole lenses after bending a beam. The beam profile monitor and faraday cup was positioned to show the beam shape and the beam quantity respectively. Also vacuum chamber was fabricated to pump an air from beam line and to obtain the start signal of TOF. The neutron chamber was made to position a neutron target and to cool the neutron target with a fron collant. The length of total beam line after beam bending is about 564 cm. The neutron scattering room is constructed to decrease the background neutron scattering. The vertical and horizontal room size is 600 cm and 500 cm respectively. The shielding distance between the original experimental room and neutron scattering room is minimum 65 cm. The shielding material is mainly a concreat, a high density polyethylen, and a styroform. 158 70 1152 700 1180 352 1882 150 2 1 Gate valve 4 Steering magnet Insulator 3 TOF chamber FC BPM Y X Gauge

vaccum exit

TMP 6 3 8 8 5 7

Rotary pump 7

Fig. 6 Beam line after bending magnet Fig. 7 Design of neutron scattering room

Ⅲ. Simulation

The beam sweeping and bunching is performed by an electric field with a MHz frequency. If the applied electric field were perpendicular to incident beam, this field could move the incident beam and the pulsed beam could be obtained through slit, which was positioned at a constant distance. Also in buncher if electric field paralleled the incident beam. and first electrod and last electrod must be grounded, The different acceleration on the pulsed beam would happen at between first electrod and second electrod, the different deceleration on the pulsed beam would happen at between second electrod and third electrod. Double bunching would be performed. The length from buncher end electrod to target is 16,305 mm. The total flight time of this length is about 1.24 us. The average velocity after deflecting and bunching is 0.231 cm/ns(flight length =135cm), and the velocity after acceleration is 2.5455 cm/ns(flight length=1087.5cm). The average velocity during 3.4 MeV acceleration is 1.800 cm/ns (length=408 cm). A program was encoded to show the pulsing and bunching shape like Fig,9. deflector double bunching system

4MHz 8 MHz slit

Fig. 8. The principle of pulsing and bunching Fig. 9. The simulation of showing a pulsing and a bunching shape

The potential in this program can be calculated by Superfish. The arrival time of this calculation at a neutron target can be obtained by recalculating the moving distance with average velocity of 0.231 cm/ns. The moving distance is 286.15 cm. The beam width of bunching beam at target is about 1.43 ns. Fig. 10 and table 1 show the bunching beam shape at about target. The width of beam at target is about 1.43 ns Also the beam dynamic for each component was calculated by a beam transport program. The bunched beam size in neutron target is a 6.9 mm in vertical and a 6.2 mm in horizental 250

Bunch 2 z(move) vs Hist 2 Bunch 1 z(move) vs Hist 1 Bunch 3 z(move) vs Hist 3 200 m c

0 150 . 0

c 100 i t r a P

0 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Relative position (cm)

Fig. 10. The bunching beam shape at about neutron target

Table 1. The beam width at target by a simulation

No. Z (cm) Bunch width (ns) * 1 314.71 5.61 2 286.15 1.43 3 257.29 5.71 4 228.08 18.05 5 199.45 27.75 6 170.41 36.41 7 141.55 51.39 8 112.76 73.68

Ⅳ. Conclusion

The specification of neutron bunching beam was designed to be that the repetition rate of this system is 8 MHz, the width is about 2 ns, duty factor is about 20 %, and proton energy dispersion is about 6 keV. Also beamline, each electronics, bending magnet for new proton beam port, and new neutron scattering experimental room were designed. Reference [1] G. D. Kim, W. Hong, H. W. Choi, H. J. Woo, J. K. Kim, and J. H. Chang, Proceedings of 2000 workshop on nuclear Pro. and Eva, 1(2001) 65. [2] G. D. Kim, T. K. Yang, Y. S. Kim, H. J. Woo, H. W. Choi, W. Hong, and J. H. Chang, Proceedings of 2001 workshop on nuclear Pro. and Eva, 1(2002) 53. [3] J.B. Marion and J.L.Fowler, "Fast Neutron Physics", chapter 4, Interscience publisher, New York (1960). [4] W.D. Bygrave, et al, " Accelerator Nuclear Physics " Highvoltage Engineering Corporation (197).