Time-Space Resolved Experimental Diagnostics of Theta-Pinch Plasma by Faraday Rotation of Infrared He-Ne Maser Radiation

Abstract

We report an experimental method for timespace resolved measurements on a dense, extreme-temperature, plasma in a theta pinch by observation of Faraday rotation of 3.39jU infrared He-Ne maser radiation. Since the rotation is proportional to B, this method affords a powerful technique for measuring B in pure, fully ionized, hydrogenic plasmas as occur in controlled fusion research when electron density n is determined independently. A unique feature of the method is the successful utilization of a low-power (2-mW) CW gas-phase maser to extreme temperature plasma diagnostics. Seven degrees of rotation along the central axis are observed at the 3.5-jLtsec, 50000-gauss, peak field compressing fully ionized deuterium at an initial electron density of 6xl0 15 /cm 3 , and immersed in a 4500-gauss forward bias field. The results confirm earlier considerations 1 ' 2 and expectations that the coherence, monochromaticity, and low beam divergence properties of infrared and optical masers could be effectively utilized for magnetoplasma diagnostics.The experimental arrangement, found to be highly versatile and accurate, is shown in Fig. 1. 2 mW of 3.39/n linearly polarized radiation is delivered in the output beam of the He-Ne gas- SPHERICAL MIRRORphase maser which utilizes a metal-coated spherical mirror and a silicon flat for the infrared resonator. A first pair of flat and spherical mirrors focuses the beam to less than 1-mm diameter at the center of the theta-pinch plasma tube. A second pair of flat and spherical mirrors directs the emerging beam in turn through a 3.39-IR narrow band pass filter, and iris, and focuses it onto a polarization analyzer. The second spherical mirror provides compensation for beam refraction that is observed to occur due to radial electron-density variations. A time-resolving infrared polarization analyzer for the 2-to 4-micron range was developed for this investigation. 3 The instrument employs two Brewster-angle, air-germanium interfaces positioned such that the parallel and perpendicular components of the polarized radiation are directed onto two separate uncooled InAs photovoltaic detectors. Angular resolution of approximately 0.5° for polarization oscillations as high as 1 Mc/sec is obtained for 0.5 mW into the analyzer. In this application the analyzer is positioned at 45° relative to the nonrotated polarization to provide equal intensity onto the detectors. The preamplified detector outputs are both added and subtracted, with the sum S(t) and difference D(t) simultaneously displayed on the dual-beam oscilloscope. As Faraday rotation is effected by the plasma, the angular rotation 0 r is given by (t)=Urcsin[D(t)/S(t)] r a)FIG. 1. Faraday-rotation arrangement for IR maser diagnostics of theta-pinch plasma.Deuterium is initially fully ionized within the 20-cm length, 5-cm ID, coil by super high-power 50 MW preheating at 1.5 Mc/sec. 4 A forward or reverse B 0 bias field to 10 kG is provided. magnetic compression is achieved by 3.5-jusec rise to a peak as high as 75 kG. 5 156

Plasma spectroscopy

“…(4) Measurement of the Faraday rotation of light (usually from a gas laser) enables magnetic fields in the plasma to be determined (see Dougal et al 1964, Falconer et al 1965.…”

Noise and Bell's Inequality

“…Now, the electric field of any polarized wave in an arbitrary polarization direction can still be decomposed into two orthogonal components, say E V and E H , such that the sum of the squares of these two components is equal to the square of the field of the polarized wave. In fact, this is the normal manner in which the polarization is measured; the wave is projected onto detectors on these two orthogonal axes, and the polarization angle inferred from the detected amplitudes with simple geometry [17,18,19]. With the principle axes of the detector used to designate the "vertical" and "horizontal" coordinates, we may write the vector electric field as…”

U.S.A. National Committee Report, Fifteenth URSI Assembly, Munich, September 1966: Radio Electronics; Progress in Radio Electronics

“…The scattering of laser radiation by plasmas is being used to measure density and temperature and to study collective effects in plasm~s [e.g., Ramsden and Davies, 1966]. Still another diagnostic application of lasers to plasmas is the determination of electron density by interferometry [Gerardo, Verdeyen, and Gusinow, 1965] and Faraday rotation [Dougal, Craig, and Gribble, 1964]. In addition to its use as a measurement tool, laser radiation has been proposed as a measure of producing dense plasmas [Dawson, 1964].…”