Specrrochrmtca Arm. Vol 43B. Nos 6,7. pp 851-865. 1988 OS‘-8547;88 13 nn+ no Prmted ,n Great Brltam Pergamon Press plc zyxwvutsrqpon Thomson scattering for determining electron concentrations and temperatures in an inductively coupled plasma-II. Description and evaluation of a multichannel instrument K. A. MARSHALL* and G. M. HIEFTJE+ Department of Chemistry, Indiana University, Bloomington, IN 47405, U.S.A. zyxwvutsrqponmlkjihgfe (Received 11 June 1987; in revised form 15 January 1988) Abstract-A new instrument has been assembled to measure Thomson scattering from an inductively coupled plasma. Unlike previous designs, the new system employs a fiber-optic array to measure simultaneously up to 12 channels on a Thomson-scattering spectrum. Shutters enable either side of the nominally symmetrical Thomson spectrum to be examined, or permit summing equally spaced channels on opposite sides of the spectrum from the incident laser wavelength. A central channel, used to monitor Rayleigh scattering, enables a simultaneous determination of true gas-kinetic temperatures. Photodetector fatigue is avoided through a rotating mirror that serves as an optical shutter while individual gated integrators attached to each detector minimize the detection of background radiation from the plasma. The new instrument oITers high stray-light rejection capability and provides measurements whose precision is photon-hmited. Results obtained with the new instrument are presented in a companion paper. 1. INTR~DUCTI~N THOMSON scattering has been shown to be capable of measuring, on a spatially and temporally resolved basis, both electron concentrations and temperatures in plasmas of various kinds [l, 21. In a companion paper [3], we reveal how a new multichannel instrument for performing Thomson-scattering measurements can help unravel the many and complex processes that occur in the analytical inductively coupled plasma (ICP). Here, we will outline in detail the considerations that led to the development of the multichannel system, describe its design and construction, and assess its performance. The new instrument was designed when the shortcomings of an earlier single-channel system [4, 51 were realized. This earlier unit was proven capable of minimizing stray-light problems and of producing usable Thomson-scattering spectra. However, the use of a relatively low repetition-rate laser source required data-collection times that were excessively long. During the long periods required for acquisition of a single Thomson spectrum, ICP drift occurred and rendered the resulting data interpretation suspect. As a consequence, values for both electron number density (concentration) and temperature could be considered only approximate. The new instrument to be described here alleviates these difficulties. It possesses a multichannel read-out system so a complete Thomson-scattering spectrum can be acquired at once. Also, photodetector fatigue is overcome through use of a rotating-mirrored chopper which permits each detector in the multichannel array to view the ICP for only the brief period during which the Thomson signal is generated. Finally, a bank of gated integrators samples the signals from individual photodetectors for just that time when the detectors are receiving Thomson-scattered light. At the heart of the new device is a fiber-optic array placed at the focal plane of the Thomson spectrometer. Specially fabricated for this application, the array simultaneously collects radiation from 25 discrete wavelengths across the Thomson spectrum and carries the light to a group of 13 individual photomultipliers. One of the detectors views the incident laser wavelength and thus records Rayleigh scattering; the other 12 channels register one half of a Thomson spectrum. Through manipulation of a set of shutters incorporated into the *Current address. Instruments SA, Inc., 173 Essex Avenue, Metuchen, NJ 08840, U.S.A. *Author to whom correspondence should be addressed. 851