K.W. Shepard

Split Ring Resonator for the Argonne Superconducting Heavy Ion Booster

A split-ring resonator for use in the ANL superconducting heavy-ion linac was constructed and is being tested. The electromagnetic characteristics of the 98-MHz device are the same as the unit described earlier, but the housing is formed of a new material consisting of niobium sheet explosively bonded to copper. The niobium provides the superconducting path and the copper conducts heat to a small area cooled by liquid helium. This arrangement greatly simplified the cryogenic system. Fabrication of the housing was relatively simple, with the result that costs have been reduced substantially. The mechanical stability of the resonator and the performance of the demountable superconducting joints are significantly better than for the earlier unit.

An improved phase-control system for superconducting low-velocity accelerating structures

It is noted that microphonic fluctuations in the RF eigenfrequency of superconducting (SC) slow-wave structures must be compensated by a fast-tuning system in order to control the RF phase. The tuning system must handle a reactive power proportional to the product of the tuning range and the RF energy content of the resonant cavity. The accelerating field level of many of the SC cavities forming the ATLAS linac has been limited by the RF power capacity of the presently used p-i-n diode based fast-tuner. A new system has been developed, utilizing p-i-n diodes operating immersed in liquid nitrogen, with the diodes controlled by a high-voltage V-groove metal-oxide semiconductor (VMOS) FET driver. The system has operated at reactive power levels above 20 kVA, a factor of four increase over an earlier design. The increased capacity will permit phase stabilization of superconducting resonators at higher field levels. In particular, it has enabled the operation of very-low-velocity superconducting structures at gradients of more than 4 MV/m in the ATLAS positive-ion injector linac.<<ETX>>

The positive-ion injector of ATLAS: Design and operating experience

Abstract The recently completed positive-ion injector for the heavy-ion accelerator ATLAS is a replacement for the tandem injector of the present tandem-linac system. Unlike the tandem, the new injector provides ions from the full range of the periodic table. The concept for the new injector, which consists of an ECR ion source on a voltage platform coupled to a very-low-velocity superconducting linac, introduces technical problems and uncertainties that are well beyond those encountered previously for superconducting linacs. The solution to these problems and their relationship to performance are outlined, and experience in the operation of ATLAS with its new injector is discussed.

The ATLAS Positive-Ion Injector project

The goal of the Argonne Positive Ion Injector project is to replace the ATLAS tandem injector with a facility which will increase the beam currents presently available by a factor of 100 and to make beams of essentially all elements including uranium available at ATLAS. The beam quality expected from the facility will be at least as good as that of the tandem based ATLAS. The project combines two relatively new technologies — the electron cyclotron resonance ion source, which provides ions of high charge states at microampere currents, and rf superconductivity which has been shown to be capable of generating accelerating fields as high as 10 MV/m resulting in an essentially new method of acceleration for low-energy heavy ions.

RF properties of high-T/sub c/ superconductors

An investigation was conducted of the RF properties of high-T/sub c/ superconductors over a wide range of temperatures, frequencies and RF field amplitudes. Both bulk polycrystalline samples and thick films on silver substrates were tested. At 150 MHz and 4.2 K, surface resistances of 18 mu Omega at low RF field and 3.6 m Omega at an RF field of 270 G were measured. All samples showed a strong dependence of the surface resistance on RF field. However, no breakdown of the superconducting state was observed up to the highest field achieved (320 G). >

Development of Superconducting Resonators for the Argonne Heavy-Ion Linac

Recent developments include a method for conditioning resonators which has produced a significant increase in accelerating gradient and also a design for a split-ring resonator with an optimum particle velocity of 0.16 c. Results of using a 1500 watt rf source to condition superconducting split-ring resonators are described. By repetitively pulsing for a few msec to field levels as high as an 8 MV/m effective accelerating field Ea, electron loading at high field levels has been substantially reduced. After such conditioning, continuous operation at Ea > 6 MV/m, corresponding to a peak surface electric field of 30 MV/m, has been obtained. A split-ring resonator designed for an optimum particle velocity ß = v/c = 0.16 is also described. The 145.5 MHz resonator is contained in the same 16 inch diameter, 14 inch length housing used for the ß = 0.1 Argonne split-ring. In design of the split ring element, a 20% reduction in peak surface electric field has been achieved with no significant increase in surface magnetic field.

Development and production of superconducting resonators for the argonne heavy ion linac

The first six niobium split-ring resonators for the Argonne Heavy-Ion Energy Booster have been completed. The average performance at 4.2K is an accelerating gradient of 3.7 MV/m or an effective accelerating potential of 1.3 MV per resonator for an rf input of 4 W/resonator. The resonators are constructed in part of an explosively bonded Nb-Cu composite material which performs well for rf surface fields of at least 200 G. In initial tests, the resonators frequently exhibit thermal instability at E a < 3 MV/m because of several types of microscopic surface defects. The methods used for locating, identifying, and removing these defects are discussed.

Development of superconducting niobium accelerating structures for heavy ions

Results of tests of two new designs for superconducting niobium resonant cavities are presented. Both types resonate at 145.5 MHz and accelerate most efficiently for particle velocities β= v/c = 0.16. One resonator is of the split-ring type, but of a simpler design than a previously reported β=.16 unit. Although the surface fields are higher, the performance is somewhat better than for the earlier design: an accelerating field E a = 4.3 MV/m has been obtained at 4.2K with 4w of rf input, where E a is defined as the energy gain per unit charge for a synchronous particle averaged over the interior resonator length. The other resonator is an 8-inch OD tapered coaxial quarter-wave line terminated with a drift tube of 1.50 inch aperture. At 4.2K, this resonator exhibits a low-level Q of 2×109, and has achieved E a = 4.7 MV/m with 2.8w of rf input.

Superconducting Triple-Spoke Cavity for &#946;=0.5 Ions

This paper reports results of cold tests of a 345 MHz, three-spoke-loaded TEM-class superconducting niobium cavity being developed for the RIA driver linac and for other high-intensity ion linac applications. The cavity has a beam aperture of 4 cm diameter, an interior length of 67 cm, and the transit-time factor peaks at = v/c = 0.5. In tests at 4.2 k, the cavity could be operated cw above the nominal design accelerating gradient of 9.9 MV/m, which corresponds to peak surface fields of 27.5 MV/m electric and 850 gauss magnetic. At this gradient the cavity provides more than 6 MV of accelerating potential. Cavity Q at 9.3 MV/m exceeded the nominal performance goal of 7.3×108. Operation at the design gradient at 4.2 K causes substantial boiling and two-phase flow in the liquid helium coolant, with the potential for microphonic-induced fluctuations of the rf frequency. Total microphonic rf frequency fluctuations were less than 1 Hz RMS operating cw at 9.7 MV/m at 4.2 K

A post-accelerator for the US rare isotope accelerator facility

Abstract The proposed rare isotope accelerator (RIA) facility includes a post-accelerator for rare isotopes (RIB linac) which must produce high-quality beams of radioactive ions over the full mass range, including uranium, at energies above the Coulomb barrier, and have high transmission and efficiency. The latter requires the RIB linac to accept at injection ions in the 1+ charge state. A concept for such a post accelerator suitable for ions up to mass 132 has been previously described [1] . This paper presents a modified concept which extends the mass range to uranium. A high resolution separator for purifying beams at the isobaric level precedes the RIB linac. The mass filtering process will provide high purity beams while preserving transmission. For most cases a resolution of about m/ Δ m=20 000 is adequate at mass A=100 to obtain a separation between isobars of mass excess difference of 5 MeV. The design for a device capable of purifying beams at the isobaric level includes calculations up to fifth order. The RIB linac will utilize existing superconducting heavy-ion linac technology for all but a small portion of the accelerator system. The exceptional piece, a very-low-charge-state injector section needed for just the first few MV of the RIB accelerator, consists of a pre-buncher followed by several sections of cw, normally-conducting RFQ. Two stages of charge stripping are provided: helium gas stripping at energies of a few keV/u, and additional foil stripping at ∼680 keV/u for the heavier ions. In extending the mass range to uranium, however, for best efficiency the helium gas stripping must be performed at different energies for different mass ions. We present numerical simulations of the beam dynamics of a design for the complete RIB linac which provides for several stripping options and uses cost-effective solenoid focusing elements in the drift-tube linac.