CHARACTERISTICS OF A HIGH-CURRENT, MULTI-BUNCHED BEAM

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1 Particle Accelerators, 1990, Vol. 27, pp Reprints available directly from the publisher Photocopying permitted by license only 1990 Gordon and Breach, Science Publishers, Inc. Printed in the United States of America CHARACTERISTICS OF A HIGH-CURRENT, MULTI-BUNCHED BEAM YUJIRO OGAWA, TETSUO SHIDARA, HITOSHI KOBAYASHI, yun OTAKE National Laboratory for High Energy Physics(KEK), Tsukuba, 305 Japan and GEN'ICHI HORIKOSHI Tsukuba College oftechnology Abstract The energy spectrum ofa high-current, multi-bunched beam at the KEK positron generator linac was measured and compared with a numerical analysis. INTRODUCTION A systematic study of the characteristics of a high-current, multi-bunched beam is considered to be one of the most important items related to the R&D of a future e+/elinear collider l near the TeV region. However, there have so far been few studies 2 on this subject. The positron generator linac 3 at KEK has a high-intensity primary electron beam with a pulse width of 2 ns and a peak current of 10 A, which is bunched and accelerated to 250 MeV by an RF field of frequency 2856 MHz. The resulting bunch number amounts to 5-6 and the electric charge on each bunch becomes a few nano Coulomb. The energy spectrum of this high-current, multi-bunched beam was measured at the energy-analyzing station in front of the target and compared with a numerical analysis of the beam dynamics. In the following, the experimental set-up, results, and a numerical analysis are presented. Some interesting results that can not be simply explained by calculations are also discussed. EXPERIMENTAL SET-UP The primary electron section of the KEK positron generator linac is schematically illustrated in figure 1. An electron gun 4 emits an intense electron beam of 150 key with a peak current of 10 A and a pulse width of 4.2 ns, which is longitudinally compressed to a 2-ns pulse with a peak current of about 15 A by a Sub-Harmonic Buncher(SHB)5 with a modulation frequency of 119 MHz and a subsequent drift space of about 3 m. The [379]/133

2 134/[380] Y. OGAWA ET AL. electron beam is then split into several bunches by a prebuncher and a buncher of 2856 MHz and finally accelerated to 250 MeV by accelerating guides of the same frequency. The energy-analyzing station6(figure 2) is set up in front of the target. It consists of collimators, a bending magnet, beam profile monitors, a wall current monitor, a slit?, and a bunch monitor of the strip-line type8. In a measurement of the energy spectrum, the energy resolution is set to about 0.15 % by using a slit width of 1 mm, expecting to obtain a bunch structure of the spectrum due to a transient beam-loading effect. The bunch monitor is utilized to identify each expected peak as a corresponding bunch of the beam. In order to experimentally estimate the transient beam-loading effect 9, a semi-long electron beam10 ofabout 40 ns is also accelerated and analyzed by the same station MHz ACCGUIDES 2856 MHz ENERGY ANALYZING STAnON FIGURE 1 Layout ofkek positron generator linac. POSITRON PRODUCTION TARGET e- l e+ I I WCM BUNCHMONJIUR P-TAna1 FIGURE 2 Energy-Analyzing Station(EAS) at 250 MeV in front of the target. RESULTS The energy spectrum of an intense multi-bunched electron beam of 250 MeV with a peak current of 10 A and a pulse width of 2 ns was measured using the above-mentioned

3 HIGH-CURRENT, MULTI-BUNCHED BEAMS [381]/135 energy-analyzing station. Figure 3 shows the energy spectra obtained as a parameter of the relative accelerating phase. It indicates several peaks which were experimentally identified as being bunches by the bunch monitor. It was found that the peaks are disposed in order of time and that the order is fixed, even if the accelerating phase is changed. The full width of the energy spread is about 9 %. > E ~~ e- Short (2ns) 200 I I, ~ u 3= > ~~ e- Short (2ns) ;: 200 E I MAG (A) FIGURE 3 Energy spectra at 250 MeV. ~t/j is a relative accelerating phase. The accelerating-phase dependence of the energy of each bunch is derived from Figure 3 and given in Figure 4, where the size of points shows the energy spread of each bunch. Fitted curves nearly express cosine functions, but the optimum accelerating phase ofeach bunch is not identical and is shifted by about a few degrees at maximum.

4 136/[382] Y. OGAWA ET AL. The transient beam-loading effect was experimentally inferred by an examination of the dynamical energy spectrum of the semi-long pulse beam(40 ns, 2.3 A). Figure 5 shows the relation between the energy and the timing of the semi-long pulse, from which the transient beam-loading per unit charge was calculated approximately to be 0.2 %/nc. 43, ::: ~ep FIGURE 4 Accelerating-phase dependence of the energy ofeach bunch. FIGURE 5 Dynamical energy spectrum of a semi-long beam of about 40 ns. NUMERICAL ANALYSIS A numerical calculation using a one-dimensional disk model with a space-charge effect was performed for a comparison with the experimental results. The electron beam pulse emitted from the gun(150 kv) was divided into 240 disks in the longitudinal direction and

5 HIGH-CURRENT, MULTI-BUNCHED BEAMS [383]/137 the dynamical motion of each disk was tracked in every section of the SHB(50 kv), the prebuncher, the buncher and the accelerating guide. The transient beam-loading effect was taken into account by using the experimental value obtained above in the accelerating guide. A compression of the pulse width at the SHB section, the formation of bunches at the PB/B section and the energy spectrum at each section were examined with various parameters. As a result, the energy spectrum at 250 MeV is illustrated in Figure 6 for an optimum accelerating phase. It explains the experimental result(figure 3) to the extent of one order of magnitude, though details are different between Figures 3 and 6; especially, the measured total energy spread is nearly twice as large as the calculated one, where the major cause of the energy spread is supposed to be due to the transient beam-loading effect. The energy difference and the energy spread ofeach bunch could not be estimated due to a lack of numerical precision in the calculation. 15 FIGURE 6 Calculated energy spectrum at 250 MeV ENERGY(MEV) FIGURE 7 Formation of bunches at the output of the buncher. 5 Ol..--.l.----J.l.l..~-4...I..---L.--U---U.-.l..._..J,-1-U-I-----U- ' LU_...1 L_...L..._L..._.U. J4._' I PHASE(DEGREE) o 500

6 138/[384] Y. OGAWA ET AL. The behavior of the formation of bunches at the output of the buncher is shown in Figure 7. All phase intervals of bunches are the same as one another and almost 360 degrees within the calculation precision, which does not agree with the fact that the measured optimum accelerating phase is shifted by about a few degrees at maximum. DISCUSSION We found several interesting facts concerning the energy spectrum of a high-current, multi-bunched beam such as a large total energy spread, an energy difference and spread of each bunch, and unequal phase intervals of bunches. These can not be simply interpreted by the usual numerical calculations based on a one-dimensional disk model with a transient beam-loading effect. It is pointed out that the effect of a longitudinal wake field may already appear regarding the phenomena mentioned above. In order to clarify the obtained facts, we plan to gather more elaborate data and to perform more sophisticated numerical analyses. At the same time, we will try to find a theoretical prediction derived from a calculation of the longitudinal wake field of a multi-bunched beam. REFERENCES 1. G. Horikoshi, Y. Kimura, and T. Nishikawa, KEK Plans for a Linear Collider R&D Proc. ofthe 1988 LinearAcceleratorConference, Williamsburg, VA, USA, Oct.3 7, For instance, see papers presented ibid.. 3. A. Asami & the Electron Linac Group, ProfUess of Positron Generator at KEK ibid.. 4. S. Ohsawa, Y. Ogawa, Y. Otake, M. Yokota, S. Fukuda, Y. Saito, A. Enomoto, O. Azuma, H. Iwata and A. Asami, Progress ofelectron Gun Systems for the e-ls:t.+ Linac at KEK ibid.. 5. A. Asami, Y. Ogawa, S. Ohsawa, S. Anami, S. Fukuda, A. Enomoto, N. Kaneko, Y. Otake, Y. Saito, T. Shidara, I. Abe, I. Sato and J. Tanaka, Injector of the Positron Generator Proc. of the 1986 Linear Accelerator Conference, SLAC, Stanford, CA, USA, Jun. 2-6, 1986, p T. Shidara, T. Ogoe, Y. Ogawa, S. Ohsawa, Y. Otake, K. Kakihara, N. Kamikubota, H. Kobayashi, H. Hanaki, K. Furukawa, A. Enomoto, T. Urano and G. Horikoshi, Improvements on Monitor System in the KEK 2.5-GeV Linac KEK Report 88-11, Jan A. 7. K. Kakihara et al. in preparation. 8. Y. Otake et al. in preparation. 9. J.E. Leiss, Beam Loading and Transient Behavior in Traveling Wave Electron Linear Accelerators in Linear Accelerators, edited by P.M. Lapostolle and A.L. Septier (North-Holland Publ. Co., Amsterdam 1969), B1.2, p S. Ohsawa & the Electron Linac Group, Positron Production and Acceleration Proc. of the Symposium on the Accelerator Technology for the High Brilliance Synchrotron Radiation Sources, Tokyo, Japan, Sep. 5-6, 1988, p.567.