C.T. Roche

Power and magnetic field‐induced microwave absorption in Tl‐based high Tc superconducting films

The increase in the microwave surface resistance Rs of high Tc superconductors at elevated microwave power levels is reported for both oriented and unoriented Tl‐based films as a function of rf magnetic field at 820 MHz and 18 GHz. The application of dc magnetic fields produces qualitatively similar increases in Rs and in the surface reactance Xs. The increase in Rs with dc field is shown to arise from simple decoupling of grains by intergranular magnetic flux. The increase in Rs with microwave power, on the other hand, is a consequence of hysteretic intergranular processes.

Measurements of the surface resistance of high-T c superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-Tc superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveTc. In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.

Design considerations for high-current superconducting ion linacs

Superconducting linacs may be a viable option for high-current applications such as fusion materials irradiation testing, spallation neutron source, transmutation of radioactive waste, tritium production, and energy production. These linacs must run reliably for many years and allow easy routine maintenance. Superconducting cavities operate efficiently with high cw gradients, properties which help to reduce operating and capital costs, respectively. However, cost-effectiveness is not the sole consideration in these applications. For example, beam impingement must be essentially eliminated to prevent unsafe radioactivation of the accelerating structures, and thus large apertures are needed through which to pass the beam. Because of their high efficiency, superconducting cavities can be designed with very large bore apertures, there by reducing the effect of beam impingement. Key aspects of high-current cw superconducting linac designs are explored in this context.<<ETX>>

Apparatus for measurement of surface resistance versus rf magnetic field of high- Tc superconductors

A coaxial niobium quarter‐wave resonant cavity has been fabricated and used for measurement of the surface resistance of disk‐shaped samples of high‐Tc superconductors at 820 MHz and 4.2 K, and at surface rf magnetic fields ranging to 300 G. The cavity can accommodate samples as large as 36 mm in diameter. A measurement procedure has been developed with which rf surface resistances less than 20 μΩ can be resolved for a 24‐mm‐diam sample at 820 MHz. This cavity is useful in determining the suitability of high‐Tc superconductors for high‐power rf applications.

Application of rf superconductivity to high-brightness ion-beam accelerators

Abstract A development program is underway to apply rf superconductivity to the design of cw linear accelerators for high-brightness ion beams. The key issues associated with this endeavor have been delineated in an earlier paper. Considerable progress has been made both experimentally and theoretically to resolve a number of these issues. In this paper we summarize this progress. We also identify current and future work in the areas of accelerator technology and superconducting materials which will confront the remaining issues and/or provide added capability to the technology.

A superconducting quarter-wave resonator for high-brightness ion beam acceleration

Abstract A niobium resonant cavity optimized for phase velocity β 0 = 0.15 was constructed based on a coaxial quarter-wave geometry adapted for high-brightness ion beams. This cavity, which resonates at 400 MHz in the fundamental mode, operated at an average (wall-to-wall) accelerating gradient of 12.9 MV/m under continuous-wave fields. This is the highest average accelerating gradient achieved to date in low-velocity structures designed for cw operation. At this gradient, a cavity Q of 1.4 × 10 8 was measured.

Beam characterization with video imaging systems at the ANL 50 MeV H/sup -/ beamline

Video imaging systems consisting of scintillators, charge-coupled device (CCD) cameras, and in-beam CCD detectors are being used to characterize beams at the Argonne National Laboratory (ANL) 50 MeV H/sup -/ beamline. The characterization technique consists of placing pinholes, slits, or wires in the beam and viewing the resulting images or shadows on a downstream scintillator or CCD. The images are digitally recorded using a frame grabber and stored in computer memory where they are analyzed to determine Twiss parameters and local beam divergence. Since many of the measurements involve low-intensity beams and require good position resolution, studies have been performed on scintillators to obtain sensitivity and resolution data. Various scintillators, including Rarex, CsI, and CaF/sub 2/, have been evaluated. An in-beam CCD imager has also been tested.<<ETX>>

NPBTS-overview and capabilities

The Neutral Particle Beam Test Stand (NPBTS) provides a versatile facility for scientific and engineering studies on large-diameter, low-divergence neutral and charged particle beams. It consists of a linac that accelerates H/sup -/ atoms to 50 MeV at 10-12 mA and two experimental areas. Typical pulse widths are 30-150 mu s at repetition rates of 0.5-30 Hz. A small RMS-emittance is achieved by using a series of collimators to shave the 1.6- pi -mm-mr emittance measured at the output of the linac. Typical current in the experimental areas is 500-600 mu A. Experimental area A has been used to study the physics of beam diagnostics and foil neutralization and to measure (p,n) reaction cross sections. Experimental area B has a series of quadrupole objectives built by Los Alamos National Laboratory to reduce beam divergence. Typical beam characteristics are RMS diameters of 10-20 cm and a full-angle divergence (RMS) of 12-24 mu r. The facility contains a wide variety of diagnostics including segmented Faraday cups, beam toroids, stripline beam-position monitors, and wire scanners. In addition, several new diagnostic systems for large-diameter beams have been developed by Argonne and Los Alamos National Laboratories.<<ETX>>

Si(Li)NaI(Tl) sandwich detector array for measurements of trace radionuclides in soil samples

An ultra-sensitive x-/..gamma..-ray detector system for assaying trace radioactivity in actinide contaminated soil and ash samples has been developed. The new system consists of an array of 6 large Si(Li) x-ray detectors sensitive on both faces and mounted on edge in a paddle-shaped cryostat with a 14 cm dia Be window on each side. The paddle, with a sample of the soil placed at each window, is sandwiched between 2 large NaI(Tl) scintillators which suppress the ..gamma.. background. With x rays being measured simultaneously from soil in 2 sample holders and background reduced by 50% using anticoincidence, the sensitivity of this detector is 4 times higher than that of conventionally mounted Si(Li) detectors. A soil sample containing 50 pCi/g /sup 239/Pu was measured in 5 min with an uncertainty of <20% and a sample containing 7 pCi/g was measured in 1 hr. With FWHM resolution of 400 eV at 17 keV, the UL..beta../sub 1/ and NpL..beta../sub 1/ x-ray peaks are resolved thus permitting measurement of trace Pu in the presence of Am-241. This is the most sensitive and selective detector known for nondestructive assay of radioactivity in soil and other samples. 15 refs., 8 figs.

Recent developments in high-current superconducting ion linacs

It is noted that recent experimental and analytical results in the areas of resonator geometry and beam physics are very encouraging for superconducting high-current ion accelerators. Niobium resonators fabricated for high-brightness ion acceleration have yielded continuous-wave (CW) average accelerating gradients as high as 18 MV/m. In a superconducting RFQ (radio-frequency quadrupole) geometry, high CW surface electric fields were sustained over surface areas of order 10 cm/sup 2/. Analyses of cumulative beam breakup in superconducting linacs consisting of decoupled, independently phased cavities were also encouraging. In addition to the construction and testing of a superconducting section, future work will include the development of superconducting cavities operating at higher frequencies and velocities. Fabrication of a 2-gap, 850 MHz, beta /sub 0/=0.28 spoke resonator is nearly complete.<<ETX>>