Stray Laser Light Research Articles

Proof-of-principle of a diagnostic for D-T fusion-product alpha particles (abstract)

Results are presented for an alpha-particle diagnostic proof-of-principle test based on CO2 laser small-angle Thomson scattering. Because the test was performed on a nonburning plasma (Advanced Toroidal Facility), which had no energetic alpha particles, scattering measurements were made on the plasma electrons. The diagnostic was configured to duplicate the requirements for an alpha-particle measurement, i.e., the measurement of a small scattered signal (≲10−9 W) in the presence of a high-power source laser (≳106 W) at small scattering angles (≲1°). The goals of this test were to eliminate stray laser light which would produce a background signal at the receiver and to maintain alignment between the pulsed laser and the receiver beams, which was set at 0.86°, while performing scattering measurements on a plasma. The proof of the diagnostic method is the measurement of the electron plasma frequency resonance feature in the scattering. Research sponsored by the Office of Fusion Energy, U.S. Department of Energy under Contract No. DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

High rejection ruby filter for laser light scattering experiments

A rejection filter against stray ruby laser light in scattering experiments is described. It consists of an array of sixty-four ruby crystal cubes with a doping of up to 2.3 wt. % of chromium oxide. The filter is 15 mm thick. Suppression factors of more than 10(6) are achieved with individual cubes (5 x 5 x15 mm(3)), the effective overall suppression of the assembled array being 10(4). The filter facilitates Raman calibration of the lidar Thomson scattering system of the JET tokamak.

Hα laser fluorescence diagnostic on the Tara Tandem Mirror experiment

A laser fluorescence diagnostic has been used for measuring the neutral hydrogen density in the central cell of the Tara thermal barrier tandem mirror. Experiments have been performed using laser-induced, resonance fluorescence detection of Hα (6563-Å) radiation. Measurements were made at a number of radial positions with 1-cm resolution, from the magnetic axis to near the plasma limiter. Stray laser light contributions to the signal were eliminated with a double-pulse technique. For comparison, the chord-averaged plasma Hα radiation was analyzed under the identical conditions for which laser fluorescence data were taken.

TFTR edge Thomson scattering system

A system is under construction which will make use of the laser and collection optics of an existing multichannel Thomson scattering system on the tokamak fusion test reactor (TFTR). A scanning sampling mechanism will divert signal from a 10-cm segment of the radial-beam path to a spectrometer–detector system which is optimized for low temperatures and densities. In the absence of stray laser light and background plasma light the system will be capable of measuring temperatures down to 3–5 eV at 1–3×1011 cm−3 minimum densities.

First results from Thomson scattering on JET (abstract)

The Single Point Thomson Scattering System uses a multipulse ruby laser to measure Te and ne up to twenty times during a JET plasma pulse. The scattering volume is on the equatorial plane and, between plasma pulses, can be relocated to any one of seven specific points on a major radius. The collection system employs multimirror arrays and a triple prism spectrometer is used to eliminate stray laser light and disperse the scattered light. Novel features include two optoelectronic feedback systems which maintain alignment within the laser input and the collection systems. Sensitive components are located outside the biological shield, and those inside are compatible with remote handling procedures. The entire diagnostic can be remotely controlled via the JET computer system. Initial results from the diagnostic are presented.