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ANL/CP--76982 DE92 040728
First operational experience with the positive-ion injector of ATLAS L. M. Bollinger, R. C. Pardo, K. W. Shepard, P. J. Billquist, J. M. Bogaty, B. E. Clifft, R. Harkewicz, K. Jch, P. K. Markovich, F. H. Munson, G. Zinkann, and J. A. Nolen Physics Division, Argonne National Laboratory, Argonne, IL 60439 USA
Abstract A Positive-Ion Injector (PII) designed to enable ATLAS to accelerate all stable nuclei has been completed and successfully tested. This new injector system consists of an ECR source on a 350-kV platform coupled to a 12-MV superconducting injector linac formed with four different types of independently-phased 4-gap accelerating structures. The injector linac is configured to be optimum for the acceleration of uranium ions from 0.029 to =1.1 MeV/u. When ions with q/A > 0.1 are accelerated by PII and injected into the main ATLAS linac, CW beams with energies over 6 MeV/u can be delivered to the experimental areas. Since its completion in March 1992, PII has been tested by accelerating ^Si7*, ^Ar11"1", l32Xe13+, and ^Pb 24 *. For all of these, transmission through the injector linac was =100% of the pre-bunched beam, which corresponds to =60% of the DC beam from the source. The accelerating fields of the superconducting resonators were somewhat greater than the design goals, and the whole system ran stably for long periods of time.
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Figure 1 Layout of the positive-ion injector (PII) of ATLAS.
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The submitted manuscript has bMn authored bv e contractor of the U. S. Government under contract No. W-31-109-ENG-3S. Accordingly, the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U. S. Government purposes.
Figure 2 Photograph of one of the three cryostats of the PII linac. The superconducting resonators and the superconducting solenoids are suspended from the top plate and are being lowered.
1. Introduction Until recently, the heavy-ion accelerator ATLAS1 consisted of a 9-MV tandem injector coupled to a superconducting linac with niobium resonators. This system has been in operation, in various stages of development, since 1978 and has provided experimenters with a total of >45,000 hours of beam on target. In 1983 a search was begun for some kind of improved injector that could ultimately replace the tandem in order to increase the mass range of ATLAS to uranium and increase beam intensities for all masses. After considering various possibilities a positive-ion injector with the layout shown in Fig.l was chosen. The basic concept for this injector is an ECR ion source on an open-air voltage platform followed by a. superconducting drift-tube linac2. Fig.2 is a photograph of one of the three PII linac cryostats with 5 of its 6 resonators and focussing solenoids being lowered into the vacuum box. The Positive-ion Injector (PII) was completed in March 1992\ and a ^'Pb beam was accelerated to over 1 GeV total energy in April. By late summer ^'U beams at energies above 6 Mev per nucleon should be developed. More details on the concepts, design, and early performance are available in the references3*12.
2. Description of the positive-ion injector The layout of 'he system4"6 and its main components are shown in Fig.l. The ECR ion source7 is mounted on a 350-kV platform with beam from the source being magnetically analyzed and bunched on the voltage platform. The beam is then accelerated to ground potential, analyzed with better resolution in Analyzer #2, and transported toward the injector linac through the achromatic beamline. A chopper on the linac-injection line removes unbunched ions, and a second buncher at the input to the injector linac rebunches the beam so as to form a narrow beam pulse (0.3-0.5 nsec FWHM) at the first accelerating structure in the linac. In a little more detail, the ECR ion source was designed in 1986 and its characteristics are typical of other ECR sources of that time7. RF power is provided by a 10 GHz klystron. The ion-extraction voltage for the source is in the range 12-18 kV. The source and other equipment on the platform are powered (140 kW) by isolation transformers, and apparatus on the platform is cooled by deionized water circulating from ground potential. The original set of isolation transformers proved to be unsatisfactory and had to be replaced by units manufactured by a different company. For the heaviest ions, it is necessary to strip the beam at the output to PII in order to obtain the high charge state required for the beam to be accelerated effectively by the main ATLAS linac. Hence, high currents of charge states in the 24-28 range are particularly important for ions such as lead and uranium. The injector linac consists of 18 superconducting resonators housed in 3 large cryostats, one of which is shown in Fig.2. About 225 W of LHe cooling at 4.6 °K is available for the linac. The heavy beams, with q/A-0.1 are accelerated by the ECR platform to velocities -0.008c, so the first resonator structure of the PII linac is particularly critical. In this cavity, the beam energy typically doubles (in 10 cm path length). Section views of all four low-beta superconducting structures developed for this application are shown in Fig.38"10. A plot of the calculated velocity increase through the first 4-gap structure (type II) is shown in Fig.4. The effective accelerating gradient for this structure is 4.5 MV/m, while the three larger types typically run at 3 MV/m. RESONATORS FOR POSITIVE -ION INJECTOR
2. Early beam tests of PII As mentioned above, four ions, including 208Pb, were successfully accelerated through PII this spring, and at the present time a uranium beam is being developed. The heavy beams are especially needed for the positron-electron experiment (APEX) beginning this Fall. Beam transmission of the bunched beam through the PII linac is not far from 100%, while about 60% of the DC beam from the ion source is captured by the bunching system. The lead and silicon beams were accelerated through ATLAS to check system stabilty and
Figure 3 Section views of the four types of Nb resonators developed for the low ion velocities of PII.
beam quality in both transverse and longitudinal phase space. The results are that qualitatively the transverse emittances are similar to those with the tandem injector, while the longitudinal emittances are significantly better. The stability and reproducibility of tunes are also quite good. The combination of excellent if time structure and good intensity heavy ion beams will be very useful for a wide variety of nuclear physics investigations. Currently a project to optimize the ATLAS velocity profile for the mass 200 region is in progress, and all fast tuners are being upgraded with a much better design.
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Figure 4 Calculation of the ion velocity, beta, vs. distance through the first 4-gap accelerating structure of PII. The energy more than doubles in 10 cm.
This work was supported by the U.S. Department of Energy, Nuclear Physics Division, under Contract W-31-109-Eng-38. References 1.
L. M. Bollinger, Ann. Rev. Nucl. Part. Sci. 36 (1986)475. See for numerous references concerning ATLAS. 2. L. M. Bollinger and K. W. Shepard. in Proc. 1984 Linear Accel. Conf., Seeheim, Fed. Rep. of Germany, May 7-11, 1984, GSI-8411 (1984)217. 3. L. M. Bollinger, et al.. Proc. 6th Int. Conf. Electrostatic Accel, and Assoc. Boosters, June, 1992, to be published. 4. R. C. Pardo, L. M. Bollinger, and K. W. Shepard, Nucl. Instr. and Meth. B24/25 (1987) 746. 5. L. M. Bollinger, R. C. Pardo, and K. W. Shepard, in Proc. 1986 Linear Accel. Conf., Stanford Univ., Cal. June 2-6, SLAC Report -303 (1986) 266. 6. P. K. Den Hartog et al., Nucl. Inst. Meth. in Physic Research 840/41(1989) 900. 7. R. C. Pardo and P. J. Billquist in Proc. 1989 Part. Accel. Conf., March 20-23, 1989, Chicago, IL, Cat. No. 89CH2669-0, Vol. 1 (1989) 319. DISCLAIMER
8.
9. 10.
11.
12.
K. W. Shepard, in Proc. 1985 Part. Accel. Conf., Vancouver, B. C , Canada, May 13-16, 1985, IEEE Trans. Nucl. Sci.. NS-32, No. 5(1985) 3574. K. W. Shepard, in Proc. 1987 IEEE Part. Accel. Conf., March 16-17, 1987, IEEE Cat. No. 87CH2387-9 (1987) 1812. K. W. Shepard, P. K. Markovich, and G. P. Zinkann, in Proc. 1989 IEEE Part. Accel. Conf., March 20-23, 1989, Chicago, Illinois, IEEE Cat. No. 89CH2669-0 (1989) 976. J. M. Bogaty, R. C. Pardo, and B. E. Clifft, in Proc. 1990 Linear Accel. Conf., Sept. 10-14, Albuquerque, New Mexico, Los Alamos Report LA-12004-C (1990)465. L. M. Bollinger, P. K. Den Hartog, R. C. Pardo, K. W. Shepard. R. Benaroya, P. J. Billquist, B. E. Ciifft, P. K. Markovich, F. H. Munson Jr., J. M. Nixon, and G. P. Zinkann, in Proc. 1989 IEEE Part Accel. Conf., March 20-23, 1989, Chicago, Illinois. IEEE Cat. No. 89CH2669-0 (1989)1120.
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