The ATLAS accelerator facility is based on superconducting radio frequency resonator technology developed at Argonne over the past twenty years. The facility provided its first beams for research using this new technology in 1978. Since then over 45,000 hours of beams have been provided for research in nuclear, atomic and other fields of study. Ion beams of essentially any atomic species are now available from the facility, although safety rules tightly control the delivery of beams lighter than mass number 12.
In 1992 the newest major addition to the ATLAS accelerator was commissioned - the Positive Ion Injector (PII) - providing ATLAS with the capability of accelerating the very heaviest of atomic species, including uranium to energies above the coulomb barrier. The ATLAS accelerator system now consists of three major components:
A rebunching/debunching resonator is provided for experimental areas III and IV. This device allows the experimenter to manipulate the longitudinal phase ellipse for optimized timing or energy resolution on target. Additionally a beam sweeper is available for control of the beam duty cycle. These major components are described more in the following paragraphs. An overview of the ATLAS facility is shown in Figure II.1.
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The Positive-Ion Injector comprises three major subsystems - an electron cyclotron resonance (ECR) ion source and high-voltage platform, a 12-MHz beam bunching system, and a 12-MV superconducting linac accelerator. A second ECR ion source system is under construction and will be available for research by the end of FY 1997.
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The PII ECR ion source is a 10GHz electron cyclotron resonance ion source optimized for high charge-state heavy ion production. The source is the first ECR source to be mounted on a high-voltage platform and was designed for the lowest power consumption consistent with high charge-state operation. The source and its system are mounted on a high-voltage platform capable of sustaining up to 350 kV of bias. This system provides ion beams with charge-to-mass ratio in excess on 0.1 and a velocity of approximately 1% the speed of light required by the PII linac. Examples of beams from the source used by the facility are 40Ar11+ and 238U26+. Figure II.2 shows a cross-section view of the PII ECR ion source.
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In order for the linac to provide acceleration without introducing significant energy spread to the beam, the injected beam must be bunched into narrow time packets as they enter the first resonator of any linac section. The initial bunching of the beam is accomplished with a three-stage bunching system.
A three-stage bunching system compresses 60-70% of the dc beam from the source into ~0.25-ns wide bunches for injection into the PII linac. The first stage of the bunching system consists of a four-harmonic buncher on the high-voltage platform. This buncher operates at a fundamental frequency of 12.125 MHz creating a beam pulse train with a period of 82.5 ns.
The second stage of the bunching system is a room-temperature chopper which removes approximately 30% of the DC. beam not properly bunched. This chopping procedure is necessary in order to avoid producing components of the beam which have significantly different energies and times than the main bunch.
The last stage of bunching refocuses the nanosecond wide beam bunch to produce a time focus just upstream of the first linac resonator with a FWHM of <250 ps. This last bunching stage for the PII is a normal conducting two-gap spiral resonator operating at 24.25 MHz approximately 2 meters in front of the PII linac.
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The PII linac consists of 18 niobium superconducting resonators providing a total equivalent voltage of approximately 12 MV. The PII resonators are of the quarter-wave resonator type but have an unusual split drift-tube assembly which produces a four-gap accelerating structure in each resonator. Four different 'matched velocity' designs are required in order to efficiently accelerate the low-velocity beam provided by the PII ECR source up to a velocity sufficient (0.06c) for injection into the remainder of ATLAS. Figure II.3 shows a cutaway view of the quarter-wave resonators used in PII along with the original ATLAS split-ring resonator used in the remainder of the linac. The arrangement of resonators and solenoids in the PII cryostats is shown in Figure II.4
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The tandem electrostatic injector consists of three major subsystem - a negative ion source injector, a 12 MHz bunching system, and an FN electrostatic tandem accelerator.
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The negative-ion source system is based on a commercial NEC SNICS II negative-ion source. This source is of the inverted cesium-sputter source type. The high-voltage platform is designed to operate at up to 300 kV, but generally operating voltages do not exceed 175 kV.
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A three-stage bunching system similar to that described above associated with the PII linac except that the last bunching stage is accomplished by a single superconducting resonator which operates at 97 MHz after the beam has been accelerated through the tandem. This superconducting bunching resonator is also used in conjunction with PII acceleration for rebunching into the first stage of the ATLAS linac.
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The FN tandem electrostatic accelerator which is usually operated at a terminal voltage of 8.5 MV. The terminal foil-stripping and the maximum voltage of 9 MV limit the tandem injector to ion acceleration for A<82. The tandem uses a thin carbon foil stripper in the terminal of the machine. Carbon stripping foils 2-5 micrograms/cm2 thick are used for all beams in order to achieve good transmission and emittance characteristics.
The beams produced by either of these injectors possess good transverse and longitudinal emittance. The transverse emittance properties are necessary in order to obtain good transmission efficiency through the system and to minimize any energy resolution degradation due to transverse dependence of the accelerating fields in the resonators. A small value of the longitudinal emittance is important in order to achieve acceleration with minimum distortion of the phase ellipse and make possible good bunching of the beam pulse. Beam pulses must be injected into the ATLAS linac with width of the order of 100-200 ps so that the acceleration process does not increase the beam energy width significantly. The longitudinal emittance from the PII is somewhat better than that from the tandem for similar beams. This is due to the elimination of stripping at low and moderate energies for lighter ions and also due to improved beam transport optics for the PII system.
After acceleration through the appropriate injector the beam may be passed through a stripper foil in order to raise the charge state prior to injection into the main ATLAS linac. For the tandem injector, the second stripper is located upstream of the tandem 90deg. analyzing magnet. Foil thicknesses of 5 micrograms/cm2 and 10 micrograms/cm2 are most often used at this location. Beams from the PII linac are stripped either at the entrance to the main ATLAS linac or after partial acceleration in the '40 degree bend' region.
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The final stage of acceleration occurs in the split-ring resonator superconducting linac accelerator. This portion of the linac is divided into two major sections as shown in Figure II.1. The first section is colloquially known as the "booster" linac while the latter section is identified as the "ATLAS" linac. The "booster" linac consists of 24 independently-phased superconducting split-ring resonators and the "ATLAS" linac contains 18 additional resonators. The first twelve of the resonators are constructed so that particles with velocities of 0.06c are synchronous with the accelerating field. The remaining 30 resonators in the booster and "ATLAS" linac have a synchronous velocity of 0.105c. These resonators are tuned to operate at a frequency of 97 MHz.
A cutaway view of one of these resonators is shown in Figure II.2. These three-gap structures are known as "split-ring" resonators; so named because the supporting arms for the drift tubes are formed, conceptually, by splitting and then deforming a ring. The drift tubes and supporting arms are formed from pure niobium. The outer housing and end plates are made from a composite material of niobium explosively bonded to copper. A resonator is cooled to 4.5 K by liquid helium which flows in at the base and through both support arms and drift tubes.
Superconducting solenoids are used as focusing elements in all parts of the linac. These solenoids are capable of peak fields of 7-8 Tesla. The linac uses a total of 33 solenoids interspersed among the resonators.
The resonators and solenoids of the split-ring linac are grouped into a total of 7 cryostats. Figure II.3 shows the internal arrangement of one of the ATLAS cryostats which is typical of the entire linac.
The helium cryogenic system is driven by three refrigerators, a CTI-1650 and two CTI-2800's. In 1995 these refrigerators were improved by the addition of "wet" expansion engines for each unit. The improved system now has a total refrigeration capacity of approximately 750 watts at 4.6 K.
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The tightly-bunched beam emerging from the linac will drift into a bunch of 1.0 to 3.0 ns FWHM on target due to the energy spread in the beam if no additional operations are performed on the bunch. For those experiments which need to make use of the potentially good time structure of the beam, a single superconducting resonator is located after the linac just upstream of the first switching magnet. This resonator is intended to rebunch the beam to produce a time waist on target for best timing conditions. An alternative mode of operation is to use the resonator to "debunch" the beam. This mode rolls the phase ellipse so that it has a minimum projection on the energy axis, producing a minimum in the energy spread of the beam.
The rebuncher is controlled from the linac control console. Setting this resonator is performed by manual adjustment, minimizing the resulting time width or energy width on target as measured in the experimenter's detectors. Programs are available to calculate the amplitude which is needed for rebunching and debunching.
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A non-resonant electrostatic beam sweeper is available for control of the beam duty cycle. The sweeper is capable of operating at frequencies from DC. up to approximately 4 MHz. The controller for the beam sweeper can be operated in a variety of modes. These include:
An individual pulse removal mode of operation is available when the controller operates in mode 1 above. Therefore the experimenter may have transmitted onto target a fixed fraction of the beam pulses provided by the bunching system from 1/2 down to a few bunches as desired - even to zero. This results in a corresponding loss of intensity but allows complete flexibility in beam period for experiments that need longer pulse periods than are provided by the bunching system.
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