R. Benaroya

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.

Tests on superconducting helix resonators

A niobium helix‐resonator cavity close to the frequency suitable for a heavy‐ion accelerator has been subject to an 8‐week test, including 323 h at full power (axial electric field Eax>2.5 MV/m). The cavity, with an anodically deposited protective film of Nb2O5, had a high Q and supported high fields without deterioration over the whole period during which it was subject to a moderate internally generated x‐ray dose. Properties were found to be stable against considerable thermal cycling and exposure to air. Measurement of emitted x‐ray maximum energy allowed an independent corroboration of the field measurement calibration.

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.

Ultra-Short Pulies of Heavy Ions

The bunching requirements for a heavy-ion tandemlinac accelerator are defined and a bunching system to satisfy these requirenents is outlined. This discussion introduces an experimrent on the bunching of 45 MeV 16O ions by means of a ¿/2 superconducting-helix resonator. The measured ion-bunch width is 64 psec, a value daminated by the resolution width of the ion detector. By correcting for the detector-resolution width one infers that the ion bunch itself is <40 psec wide.

Tests of a niobium split-ring superconducting heavy-ion accelerating structure

A niobium split-ring accelerating structure designed for use in the Argonne superconducting heavy-ion energy booster has been successfully tested. The superconducting resonator has a resonant frequency of 97 MHz and an optimum particle velocity β = 0.11. Ultimate performance is expected to be limited by peak surface fields, which in this structure are 4.7 E a electric and 170 E a (Gauss) magnetic, where E a is the effective accelerating gradient in MV/m. RF losses in two demountable superconducting joints severely limited performance in initial tests. Following independent measurements of the rf loss properties of several types of demountable joints, one demountable joint was eliminated and the other modified. Subsequently, the resonator could be operated continuously at E a = 3.6 MV/m (corresponding to an energy gain of 1.3 MeV per charge) with 10w rf input power. Maximum field level was limited by electron loading. The mechanical stability of the resonator under operating conditions is excellent: vibration induced eigenfrequency noise is less than 120 Hz peak to peak, and the radiation pressure induced frequency shift is Δf/f = 1.6 × 10-6E a 2.

APS storage ring vacuum system

The advanced Photon Source synchrotron radiation facility, under construction at the argonne National Laboratory, incorporates a large ring for the storage of 7 GeV positrons for the generation of photon beams for the facility’s experimental program. The Storage Ring’s 1104 m circumference is divided into 40 functional sectors. The sectors include vacuum, beam transport, control, acceleration and insertion device components. The vacuum system, which is designed to operate at a pressure of 1 nTorr, consists of 240 connected sections, the majority of which are fabricated from an aluninum alloy extrusion. The sections are equipped with distributed NeG pumping, photon absorbers with lumped pumping, beam position monitors, vacuum diagnostics and valving. The detaileds of the vacuum system design, selected results of the development program and general construction plans are presented.

The Superconducting Heavy-Ion Linac at Argonne

The design, status, and performance of the first operating superconducting heavy-ion accelerator, a linac used to boost the energies of beams from a 9-MV tandem, is summarized. When completed in 1981, the linac will consist of 24 independently-phased splitring niobium resonators operating at 97 MHz. This linac is designed to provide 29 MV of acceleration. Because of the modular character of the system, the linac has been operable and useful since mid-1978, when a beam was accelerated through 2 units and the first nuclear-physics experiments were performed. Now, 16 resonators are in use, and a beam has been accelerated for ~ 6000 hr. Resonator performance has been remarkably stable, in spite of vacuum accidents, and the linac as a whole operates reliably without operators in attendance during nights and weekends. The ease and speed with which the beam energy can be changed is proving to be unexpectedly valuable to users.

A high-performance Nb helical cavity

A 92-MHz superconducting-Nb helix resonator of exceptional quality has been thoroughly tested under a variety of conditions. The unit is a full-scale λ/2 structure with dimensions appropriate for heavy-ion acceleration. When operated at a temperature of 1.8K and with bare (not anodized) Nb surfaces, the low-field Q is 9.4 × 109, equivalent to a surface resistance of 5 × 10-10ohms. The maximum surface magnetic field is 1200 G and the maximum surface electric field is 37 MV/m, which corresponds to a traveling-wave axial accelerating field of 4.6 MV/m. These characteristics set new performance standards for helix resonators. A systematic study of the effects of various surface treatments, including abuses of the cavity, are described. The tests consist of 24 liquid helium cooldowns, at 4.2K and 1.8K, of the cavity with bare and anodized Nb surfaces which at various times were electropolished, oxypolished and heat treated. RF and helium conditioning are discussed as techniques to get through multipactoring barriers and extend the maximum obtainable electric field.

Development and Operation of a Prototype Superconducting Linac for Heavy-Ion Acceleration

A prototype superconducting-helix accelerator is described and design considerations are discussed. The results obtained during 120 hours of beam acceleration are given. These include a wealth of practical engineering experience, the demonstration of stable operation with external phase control, and measurements of various kinds of accelerator-physics data.

Superconducting low-velocity linac for the Argonne positive-ion injector

A low-velocity superconducting linac has been developed as part of a positive-ion injector system, which is replacing a 9-MV tandem as the injector for the ATLAS accelerator. The linac consists of an independently phased array of resonators and is designed to accelerate various ions over a velocity range 0.008<v/c<0.06. The resonator array is formed by four different types of superconducting interdigital structures. The linac is being constructed in three phases, each of which will cover the full velocity range. Successive phases will increase the total accelerating potential and permit heavier ions to be accelerated. Assembly of the first phase was completed in early 1989. In initial tests with beam, a five-resonator array provided approximately 3.5 MV of accelerating potential and operated without difficulty for several hundred hours. The second phase is scheduled for completion in late 1989 and will increase the accelerating potential to more than 8 MV.<<ETX>>