John P. Turneaure

Critical states in 2D disk-shaped type-II superconductors in periodic external magnetic field

Abstract Following the procedure of Mikheenko and Kuzovlev, we present analytical solutions of field and current patterns in thin film disk-shaped type-II superconductors in perpendicular time-varying periodic external magnetic fields. We also calculate the magnetic moment and effective susceptibility. The analysis is carried out within the framework of the critical state model assuming a constant critical current. Our results are compared to that of Mikheenko and Kuzovlev; and we discuss the discrepancies.

The gravity-probe-b relativity gyroscope experiment: Development of the prototype flight instrument

The Gravity-Probe-B Relativity Gyroscope Experiment (GP-B) will measure the geodetic and frame-dragging precession rates of gyroscopes in a 650 km high polar orbit about the earth. The goal is to measure these two effects, which are predicted by Einsteins General Theory of Relativity, to 0.01% (geodetic) and 1% (frame-dragging). This paper presents the development progress for full-size prototype flight hardware including the gyroscopes, gyro readout and magnetic shielding system, and an integrated ground test instrument. Results presented include gyro rotor mass-unbalance values (15–86 nm) due the thickness variations of the thin niobium coating on the rotor, interior sphericities (163–275 nm peak-to-valley) of fused-quartz gyro housings produced by tumble lapping, gyro precession rates (gyroscopes at 5 K) which imply low mass-unbalance components parallel to the gyro axis (23–62 nm), and demonstration of a magnetic shielding factor of 2×1010 for the gyro readout system with one shielding component missing (the gyro rotor). All of these results are at or near flight requirements for the GP-B Science Mission, which is expected to be launched in 1995.

Elimination on electron multipacting in superconducting structures for electron accelerators

Electron multipacting in a 2.8‐GHz TM010‐mode anodized niobium test cavity at 1.4 K has been studied experimentally. An electron multipactor simulation computer program, which we developed, successfully calculates the multipactor field levels and multipactor location observed experimentally and also provides additional details of the multipactor phenomena. With the multipactor simulation program, a sharp‐cornered TM010‐mode cavity has been designed in which multipacting is greatly reduced. A subsequent experiment on such a sharp‐cornered anodized niobium cavity has shown that electron multipacting is in fact eliminated for this cavity up through the highest field achieved. This work has shown that, with a simple modification in geometry, multipacting can probably be eliminated in the superconducting structure for the Stanford electron accelerator.

Low Temperature Aspects of a Cryogenic Accelerator

The object of this paper is to identify the many low temperature aspects of a cryogenic accelerator and to indicate how these are related to the operating characteristics which might ultimately be achieved.

The Gravity Probe B test of general relativity

The Gravity Probe B mission provided two new quantitative tests of Einsteins theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earths orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

A space-based superconducting microwave oscillator clock

Superconducting Cavity Stabilized Oscillators, SCSO, have produced the most stable clocks to date, achieving an Allen variance of 3×10−16 for integration times between 102 and 103 seconds. Cavity frequency variations are mainly caused by acceleration effects due to gravity and vibrations, temperature variations, and fluctuations in the energy stored in the cavity. We describe the status of a project aimed at building an improved cavity system suitable for use on the International Space Station, ISS. Primary experimental applications include the measurement, in conjunction with other types of clocks, of the dependence of fundamental constants on the gravitational potential, gravitational redshift measurements, and the measurement of the anisotropy of the velocity of light. A major secondary application is as a flywheel for the atomic clocks co-located on the ISS.

The Stanford Superconducting Recyclotron

With four orbits of recirculation, the HEPL superconducting recyclotron is expected to provide a high duty factor (> 70%) electron beam at 300 MeV by 1982. At present the beam has been recirculated twice and extracted. To date, maximum energy has been 156 MeV with best energy resolution (FWHM) 0.018%. The dependence of beam breakup starting currents on orbit optics has been investigated. Installation of the components for the third and fourth orbits will be completed this year.

Measurements on Superconducting Nb Prototype Structures at 1300 MHz

Twenty-four low temperature measurements have been made on six different superconducting Nb cavities at 1300 NHz. These cavities were fabricated using Nb sheet and electron-beam welding and were processed using ultra-high-vacuum firing. Unloaded Qs as high as 4 × 109 and peak surface electric fields as high as 16 MV/m have been achieved.

Development of the Gravity Probe B flight mission

Abstract Gravity Probe B is an experiment to measure the geodetic and frame-dragging precessions, relative to the “fixed” “stars”, of a gyroscope placed in a 650 km altitude polar orbit about the earth. For Einsteins general relativity, the precessions are calculated to be 6.6 arcsec/yr for the geodetic precession and 0.042 arcsec/yr for the frame-dragging precession. The goal of the experiment is to measure these precessions to better than 0.01% and 1%, respectively. This paper gives an overview of the experiment and a discussion of the flight hardware development and its status. This paper also includes an estimate of the geodetic and frame-dragging errors expected for the experiment.

Progress in the development of a position sensor for LISA drag-free control

We report on progress in the development of free-falling moving test-masses for LISA and for the related technology demonstration mission. We present simple formulae to evaluate the performance of the device as a function of the various design parameters, and we compare them with preliminary experimental results from a test prototype we are developing. Quantitative agreement is found. Finally, we present a control law, along with a performance simulation, for low-frequency electrostatic suspension of the test-mass with minimal perturbation of the motion within the measuring frequency band.