Current measurements by Faraday rotation in single mode optical fibers

Development of techniques for measuring magnetic fields and currents by Faraday rotation in single-mode optical fibers has continued. We summarize the results of attempts to measure the toroidal plasma current in the ZT-40 Reversed-Field-Pinch using multi-turn fiber coils. The fiber response is reproducible and in accord with theory, but the amount and distribution of the stress-induced birefringence in this case are such that prediction of the sensor response at low currents is difficult if not impossible. The low-current difficulty can be overcome by twisting the fiber to induce a circular birefringence bias. We report the results of auxiliary experiments with a fiber that has been twisted with 15 turns per meter and then re-coated to lock the twist in place

New Phase-coherent Measurements of Pulsar Braking Indices

Pulsar braking indices offer insight into the physics that underlies pulsar spin-down. Only five braking indices have been measured via phase-coherent timing; all measured values are less than 3, the value expected from magnetic dipole radiation. Here we present new measurements for three of the five pulsar braking indices, obtained with phase-coherent timing for PSRs J1846-0258 (n=2.65+/-0.01), B1509-58 (n=2.839+/-0.001) and B0540-69 (n=2.140+/-0.009). We discuss the implications of these results and possible physical explanations for them.Comment: 7 pages, 5 figures. To be published in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface" (April 24-28, 2006, London, UK), eds. D. Page, R. Turolla, & S. Zan

Primer on laser scattering diagnostics

The theory of laser scattering is presented in abbreviated format, with emphasis on physical interpretation, followed by sections on laser sources, practical considerations in designing experiments, and current developments in extending the techniques to multispace and multitime point measurements

Lasers as a tool for plasma diagnostics

Lasers can be used as non-perturbative probes to measure many plasma parameters. Plasma refractivity is primarily a function of electron density, and interferometric measurements of phase changes with either pulsed or CW lasers can determine this parameter with spatial or temporal resolution over several orders of magnitude sensitivity by using laser wavelengths from the near uv to the far infrared. Laser scattering from free electrons yields the most fundamental electron temperature measurements in the plasma parameter range where individual scattering events are uncorrelated in phase and ion temperature or plasma wave and turbulence structure in the opposite limit. Laser scattering from bound electrons can be many orders of magnitude larger if the laser is matched to appropriate resonance frequencies and can be used in specialized circumstances for measuring low-ionized impurity or dominant species neutral concentrations and velocities

High-density-plasma diagnostics in magnetic-confinement fusion

The lectures will begin by defining high density in the context of magnetic confinement fusion research and listing some alternative reactor concepts, ranging from n/sub e/ approx. 2 x 10/sup 14/ cm/sup -3/ to several orders of magnitude greater, that offer potential advantages over the main-line, n/sub e/ approx. 1 x 10/sup 14/ cm/sup -3/, Tokamak reactor designs. The high density scalings of several major diagnostic techniques, some favorable and some disadvantageous, will be discussed. Special emphasis will be given to interferometric methods, both electronic and photographic, for which integral n/sub e/dl measurements and associated techniques are accessible with low wavelength lasers. Reactor relevant experience from higher density, smaller dimension devices exists. High density implies high ..beta.., which implies economies of scale. The specialized features of high ..beta.. diagnostics will be discussed

Far infrared Faraday-rotation measurement on a reversed-field-pinch plasma

The integral n/sub e/B/sub theta/.dl is measured on the Los Alamos reversed field pinch plasma (ZT-40) by the rotation of the linear polarization of the 185 ..mu..m emission of a CO/sub 2/-pumped CH/sub 2/F/sub 2/ laser. The essence of the method is to monitor phase distortions of the regular beat frequency produced when two counter-rotating circular polarizations, frequency shifted with respect to each other and sent along a common off-center chord through the plasma, are detected behind a linear polarization analyzer. Rotations are measurable to the order of 0.01 cycles. The same frequency shifted beams have been used with linear polarizations in a heterodyne interferometer configuration to give integral n/sub e/ dl with high sensitivity. Work is in progress to make both measurements simultaneously

Proposal to develop techniques using magneto-optic and electro-optic effects in optical fiber for CTR diagnostics

We discuss the developing technology of measuring electric and magnetic fields with optical fibers using the Faraday and Kerr effects, magnetostriction, and Sagnac interferometry. We review the measurement of induced birefringence in the presence of natural birefringence. We propose the use of these effects in making measurements in the fusion research program, with ZT-40 as an example

Current sensing in magnetic fusion experiments by Faraday rotation in single-mode optical fibers

We find that sensors exploiting the Faraday effect in single-mode optical fibers are practical means of measuring large currents in the MFE environment. Work still needs to be done to overcome the effects of linear birefringence. We have seen distortion caused by dynamic stress-induced birefringence and shown the importance of physically eliminating it because of the difficulty of treating it analytically