The Jefferson Laboratory's superconducting radiofrequency (srf) Continuous Electron Beam Accelerator Facility (CEBAF) provides multi-GeV continuouswave (cw) beams for experiments at the nuclear and particle physics interface. CEBAF comprises two antiparallel linacs linked by nine recirculation beam lines for up to five passes. By the early 1990s, accelerator installation was proceeding in parallel with commissioning. By the mid-1990s, CEBAF was providing simultaneous beams at different but correlated energies up to 4 GeV to three experimental halls. By 2000, with srf development having raised the average cavity gradient to 7.5 MV/m, energies up to nearly 6 GeV were routine, at 1-150 µA for two halls and 1-100 nA for the other. Also routine are beams of >75% polarization. Physics results have led to new questions about the quark structure of nuclei, and therefore to user demand for a planned 12 GeV upgrade. CEBAF's enabling srf technology is also being applied in other projects.

A two-coil mutual-inductance technique for measuring the complex ac response of a two-dimensional (2-D) superconductor to a weak ac magnetic field is described. Analytical and numerical methods are presented which allow extraction of the complex ac conductance of the superconductor from the signal voltage induced in the detection coil by the screening currents flowing in the sample. The method is illustrated by measurements of the ac conductance of a square network of aluminum wires from which the penetration depths of both the network and (granular) aluminum are deduced. It is shown that the method provides a powerful tool to observe characteristic features associated with critical phenomena in 2-D superconducting systems.

Impedance measurements performed on a weakly frustrated triangular array of Josephson junctions over a wide range of frequencies and at temperatures such that vortex pinning is irrelevant reveal that vortex dynamics in an ideal two-dimensional (2D) superconductor does not obey Drude's classical prediction for a 2D Coulomb gas of free and independent vortex charges. An analysis in terms of a complex vortex dielectric constant implies that the vortex mobility vanishes logarithmically in the limit of small frequencies, thereby pointing to anomalous vortex diffusion.PACS numbers: 74.60. Ge, 74.25.Nf, The concept of vortex is essential to understand the physics of two-dimensional (2D) superfluids [1], In neutral superfluids and, under appropriate conditions, in charged superfluids the interaction between two vortices depends logarithmically on their separation, a feature leading to a natural description of the vortex medium in terms of a 2D Coulomb gas analog [2]. Detailed insight into the physics of 2D superfluids emerging from this picture is provided by studies of their response to a timedependent perturbation. The Andronikashvili torsional oscillator has proven to be quite successful to investigate vortex dynamics near the Kosterlitz-Thouless (KT) transition of liquid-helium films [3]. A corresponding probe for 2D superconductors relies on a two-coil mutual inductance technique [4] which allows us to extract the dynamical properties of the vortices from measurements of the sample's complex sheet impedance [5,6]. However, the experiments performed so far on liquid-helium films could not systematically explore the response as a function of frequency, while the investigations carried out on superconducting films [7] were almost unvariably affected by pinning effects masking the intrinsic 2D Coulomb-gas properties of the vortex medium. In this Letter, we report a study of the complex dielectric constant sico) of a dilute system of vortices created by a small perpendicular magnetic field H in an almost pinning-free triangular array of Josephson junctions. Our data, taken over a wide range of driving angular frequencies co, reveal novel and unexpected aspects of the dynamics of vortex excitations in an ideal, i.e., pinning-free, 2D superconductor.Compared to superconducting films, Josephson junction arrays (JJA) prepared with modern microfabrication techniques provide nearly ideal systems in which vortex pinning due to ever present disorder can be kept at extremely low levels. Moreover, in triangular arrays intrinsic pinning effects resulting from the periodic nature of the system are much weaker than in other lattice structures and become totally irrelevant at temperatures T appreciably lower than the zero-field KT transition temperature TKT [8]. Thus, if T is not too far below TKT and H corresponds to small values (<0.05) of the frustration parameter /, defined as the number of flux quanta per elementary triangular cell, one would expect the vortex medium in a triangular JJA to behave as a 2D Coulomb gas of free (i.e....

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Experimental evidence is presented for a supercurrent oscillation arising from vortex motion in the flux-flow regime of superconducting films with periodically modulated thickness. An essential condition for detecting the rf electric field associated with the oscillation is matching of the vortex lattice to the periodic pinning structure represented by the thickness modulation.This Letter reports the observation of radiofrequency (rf) electric fields in the flux-flow regime of type-II superconducting films with periodically modulated thickness when vortex motion is driven by a dc transport current. These oscillating voltages are a manifestation of a Josephson-like supercurrent oscillation arising from coherent vortex motion in the one-dimensional periodic pinning potential represented by the thickness modulation. 1 Evidence for the oscillation is found when the value of the transverse magnetic field H-B corresponds to matching of the vortex lattice to the periodic film structure.Several years ago, Kulik 2 and Schwartz 3 suggested that there is a close analogy between the flux-flow state in type-II superconductors and the ac Josephson effect in superconducting weak: links. In fact, a moving vortex lattice can be thought of as a supercurrent density distribution oscillating both in space and time in a way very similar to that sometimes observed in weak links in connection with the ac Josephson effect. 4 According to this picture, one would expect the existence of electromagnetic radiation from type-II superconductors in the flux-flow regime. 2,3 As pointed out by Meincke, 5 however, the dramatic mismatch between the flux-flow velocity (or phase velocity of the supercurrent pattern) and the phase velocity of the electromagnetic field almost excludes the possibility of detecting ac Josephson effects in type-II superconductors. Meincke's argument, however, is only valid for uniform motion of an extended vortex lattice. Actually, by considering flux flow in the presence of pinning, Schmid and Hauger 6 have shown that the pinning potential introduces the necessary mechanism for coupling electromagnetic fields to the supercurrent oscillations. This coupling can be significant only when the effect of pinning results in a sufficiently coherent modulation of the vortex velocity. In this case the corresponding modulation of the supercurrent density distribution leads to a net supercurrent oscillation which can therefore interact with the electromagnetic field. This interaction was first demonstrated experimentally for weak random pinning by Fiory 7 and recently for a periodic pinning structure by Martinoli et al.,8 who observed quantum-interference phenomena when the flux-flow state was driven by superimposed dc and rf currents.At this point, the problem of the direct detection of the supercurrent oscillation associated with the moving lattice arises in a quite natural way. In this connection, we note that Clem 9 predicts the existence of characteristic structures, related to the oscillation, in the noise power spectrum. Suc...