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A seven channel polychromator, utilizing high performance interference filters (transmission ... 80%, rejection at laser wavelength > 10[sup 5]), has been tested for use in the multi-Nd:YAG laser Thomson scattering system for the DIII-D tokamak. Unique features of this polychromator are the combination of high throughput, easy alignment, flexibility, compact size, and Iow cost when compared with other alternatives. Light is introduced to the polychromator (f/1.75) via a fiber optic bundle which permits the use of small (3.0 cm diam) optics and leads to a compact (44 × 24 × 8 cm) design, an important design consideration for multiple polychromator systems. The light is cascaded through a series of different bandpass interference filters and relay lenses which are mounted on two precision parallel rails in such a way that alignment is trivial. The relay lenses are positioned directly in front of the filters so that light reflected from the filter passes through the lens twice. This leads to an efficient, compact design and reduces the angle of incidence (4°) and the cone angle of light (4.5°) seen by the filter, an important factor for narrowband (3.0 nm) filters. The transmission (average 70%) was optimized for 700-1100 nm by using broadband coatings throughout. The output images of each channel (2.3 mm. diam) can be directly coupled to large format (3 mm diam) RCA silicon avalanche photodiode detectors, avoiding the losses caused by fiber optic coupling. [ABSTRACT FROM AUTHOR]

A multilaser system is being developed for the DIII-D Thomson scattering diagnostic. This system combines the beams from up to 8 Nd:YAG lasers onto a common beamline in which the beams are nearly parallel and are all focused into a small, common area within the desired scattering volume. Each laser can be fired at a constant rate (20 Hz per laser) for a high average repetition rate, or together in a "burst," which will give very high sampling rates (10-20 kHz) for short periods. The burst mode will be triggerable by plasma events, which will allow for study of transient phenomena, but will require nonperiodic firing of the lasers. Beamline diagnostics include position sensitive detectors for computer controlled feedback alignment of the 35 m beamline, an image position detection system for monitoring the alignment of the collection lens to the scattering volume, and a 1-D reticon camera for divergence monitoring. The effects of the nonperiodic firing of the lasers will be monitored with the reticon camera. [ABSTRACT FROM AUTHOR]

A silicon avalanche photodiode with an internal gain of about 50 to 100 is used in a temperature-controlled environment to measure the Nd:YAG laser Thomson scattered spectrum in the wavelength range from 700 to 1150 nm. A charge-sensitive preamplifier has been developed for minimizing the noise contribution from the detector electronics. Signal levels as low as 20 photoelectrons (S/N = 1) can be detected. Measurements show that both the signal and the variance of the signal vary linearly with the input light level over the range of interest, indicating Poisson statistics. The signal is processed using a 100 ns delay line and a differential amplifier which subtracts the low-frequency background light component. The background signal is amplified with a computer-controlled variable gain amplifier and is used for an estimate of the measurement error, calibration, and Z[sub eff] measurements of the plasma. The signal processing has been analyzed using a theoretical model to aid the system design and establish the procedure for data error analysis. [ABSTRACT FROM AUTHOR]

A VME-based real-time computer system for laser control, data acquisition, and analysis for the DIII-D multipulse Thomson scattering diagnostic is described. The laser control task requires precise timing of ap to eight Nd:YAG lasers, each with an average firing rate of 20 Hz. A cpu module in a real-time multiprocessing computer system will operate the lasers with evenly staggered laser pulses or in a "burst mode," where all available (fully charged) lasers can be fired at 50-100 µs intervals upon receipt of an external event trigger signal. One or more cpu modules, along with a LeCroy FERA (fast encoding and readout ADC) system, will perform real-time data acquisition and analysis. Partial electron temperature and density profiles will be available for plasma feedback control within 1 ms following each laser pulse. The VME-based computer system consists of two or more target processor modules (25 MHz Motorola 68030) running the VMEexec real-time operating system connected to a Unix-based host system (also a 68030). All real-time software is fully interrupt driven to maximize system efficiency. Operator interaction and (non-real-time) data analysis takes place on a MicroVAX 3400 connected via DECnet. [ABSTRACT FROM AUTHOR]