P. R. Forman

Linear birefringence effects on fiber-optic current sensors

The occurrence of linear birefringence is inevitable when dealing with fiber optics. Intrinsic birefringence can be minimized, but deploying the fiber on an experiment will introduce stress birefringence due to bending and pressure. We have studied the effects of this extraneous linear birefringence on the measurement of current-induced circular birefringence in a fiber which also has a strong twist circular birefringence bias. Orienting the analyzing polarizer by a proscribed procedure gives minimum error. Quantitative error limits on the current for given fiber coil radii and winding tensile stress are calculated. Additional restrictions on the fiber lead-in and lead-out sections are discussed.

Fiber-optic heterodyne phase-shift measurement of plasma current

By combining twisted optical sensing fiber and heterodyne phase detection of circular birefringence we have (a) overcome the distortion problem caused by residual linear birefringence in the Faraday rotation method of measuring enclosed current and (b) used only a single output detector without requiring intensity normalization. Resolution of 400 ampere-turns has been obtained in the hostile electromagnetic environment of a working thermonuclear fusion research device. The fiber was simply wound around the existing machine. The measured values are in excellent agreement with those of the electrical Rogowski coil installed when the machine was built.

Self-pumped phase conjugation in BaTiO 3 at 1.06 μm

We report the first observation to our knowledge of self-pumped phase conjugation in BaTiO(3) at 1.06 microm. An essential feature is the lower threshold external ring configuration rather than total internal reflection geometry. Although the product of incident power (1 W/cm(2)) and formation time (600 sec) is much larger than the corresponding values at 0.80 microm (0.15 W/cm(2); 120 sec), the effect was observed in two crystals purchased more than a year apart and poled at separate facilities.

Pulsed holographic interferometry at 10.6 μm

Double‐exposure holographic interferograms using a 10.6‐μm pulsed CO2 laser as the radiation source are shown. Results using two recording media, thin bismuth films, and cuprous mercuric iodide are reported.

DYNAMIC STABILIZATION OF A LINEAR z PINCH BY A MAGNETIC QUADRUPOLE.

The effect of high‐frequency linear quadrupole magnetic fields on the m = 1 kink instabilities of a z pinch, as proposed by Osovets, is investigated. Short‐wavelength instabilities of 5‐12 cm are observed to be dominant and are not suppressed by the stabilization fields. This is contrary to the reported result of others that the short‐wavelength instabilities do not develop if the long wavelengths of the order of the interelectrode distance are stabilized. For the experimental conditions used, magnetohydrodynamic theory predicts stabilization of wavelengths of 20‐80 cm and reduced growth rate for the shorter wavelengths. The observed instabilities have a reduced growth rate in agreement with theory.

Time‐resolved pump depletion in n‐InSb Raman spin‐flip laser

We have observed a strong transient optical absorption in n‐InSb (n=1016 cm−3) induced by an intense (2 MW cm−2) 10.6‐μm laser beam at 2 K. Detailed studies of the time history of the transmitted 10.6‐μm beam and the spin‐flip Raman laser pulse produced in the InSb show that the 10.6‐μm absorption lasts for ≳100 ns, and that spin‐flip lasing occurs only at the beginning of the intense portion of the 10.6‐μm pump pulse and after the absorption has disappeared.

Double-pulsed time differential holographic interferometry

A unique method for double-pulsed holographic interferometry has been developed and employed in a meter-long theta-pinch experiment. Q-switching of two orthogonally polarized ruby laser beams, using the same lasing medium but different laser cavities, allows pulse separations of 0.2-100 microsec. The double-pulse operation with short pulse separation avoids many temporal aberrations seen in single-pulse systems and allows time differential interferograms of transient phenomena such as seen in theta-pinch plasmas. Orthogonal polarization of the two beams permits background fringe construction without the need for a mechanically introduced planar phase shift.

Two-Dimensional Interferometry with a Pulsed 10.6-μm Laser

We report the use of a simply constructed CO2 TEA laser for making space resolved end-on interferograms of the refractivity due to the free electron density of a short theta pinch plasma at late times during the discharge. For 10.6-μm radiation a phase shift of 2π only requires 2 X 10 electrons/cm, a factor of 15 increase in sensitivity over interferograms made with a ruby laser as the light source. A number of materials tha t have been found useful for recording the interferograms are described. The laser was designed to have a spatially uniform output with pulse energies in excess of 1 J. An electrode configuration employing resistor limited discharges was chosen because of its simplicity and consistent operation. About 2700 0.5-W 1000-Ω carbon resistors were inserted into the holes of Vector board (2.5/5 -mm grid). Consequently seven tightly packed rows of resistive pins were formed with a width of 1 cm and a length of 180 cm. The resistors were encapsulated in Dow Corning 182 Sylgard. The laser power supply is a 0.l-μF capacitor with a triggered spark gap. The optical cavity is formed by two 10-m radius mirrors. One of these is gold coated beryllium copper, and the other is a 6 5 % reflecting germanium output mirror. Output energy has been measured with a calibrated Scientech disk calorimeter. With the capacitor charged to 40 kV and a mixture of 90% He: 7 % CO2: 3 % N2 a t local atmospheric pressure (650 Torr) the output energy is 2.4 ± 0 .1J . For a 9 2 % He: 8% CO2 mixture the output is 0.9 ± 0.05 J. The removal of nitrogen has the previously observed effect of both reducing the amplitude of the gain switched giant pulse and removing the slower (1-2 μsec) low-power secondary pulse. Since interferometry of theta pinch plasmas requires a submicrosecond pulse to prevent smearing out of the fringes, the work reported here required the use of the 0.9-J pulses without N2 in the gas mixture. These have a 0.2-μsec half-width.

Quantitative evaluation of phase-conjugate novelty filters

A Michelson interferometer whose signal arm is terminated by a self-pumped BaTiO(3) phase conjugator records phase-change distributions in times short compared with the conjugator rewrite time that correspond quantitatively to a single (not a double) traversal of the phase disturbance. The proffered explanation, which is experimentally substantiated, also places limitations on the pictorial accuracy of intensity-change novelty filters.

Two‐dimensional interferometric imaging of moving plasma disturbances in the ZT‐40M reversed‐field pinch, using time‐delayed correlation techniques

An eight‐chord interferometer array, together with a single‐chord interferometer separated toroidally by 90°, shows propagating global plasma density disturbances in the final 1 ms prior to discharge termination in ZT‐40M. A time‐delayed correlation technique applied to the nine‐chord data allows reconstruction of the shape of the propagating disturbance and discriminates against competing random density fluctuations.