Measurements of Spectral Emission and Absorption of a High
`Pressure Xenon Arc in the Stationary and the Flashed Modes
`
`Lothar Klein
`
`Concurrently with emission measurements of a high pressure xenon arc in the spectral range 3000 A to 2 pt,
`its absorption in the ir was measured by a technique based on modulating the less intense radiation of a
`carbon arc used as background source. The emission measurements were repeated with a rapid scanning
`spectrometer while flashing the xenon are for 0.1 sec at 10 kW, which is five times the normal power input.
`The are showed excellent stability and reproducibility both in the stationary and the flashed modes. The
`intensity increase of the continuum was proportional to the increase of power input during the flash. A
`simple expression was derived connecting the spectral radiance of the continuum directly with the tem-
`perature and pressure of the arc. The temperature profile of the xenon arc was obtained using this ex-
`pression and also by applying the Planck-Kirchhoff method to the Abel inverted emission and absorption of
`an ir xenon line. Both approaches show fair agreement at the arc center. The wavelength dependence
`of the correction factor for departures from hydrogenic behavior of the xenon continuum was derived f1'om
`the measured spectral radiances and compared with theoretical calculations.
`
`I.
`
`Introduction
`
`Emission spectra are extensively used in plasma
`diagnostics, but only rarely is the absorption of arc
`plasmas measured quantitatively. One reason seems
`to be the widely held opinion that are plasmas at
`atmospheric pressure viewed over short optical paths
`are always optically thin, except for the resonance lines.
`Another reason is the difficulty of finding a suitable
`background source
`for
`absorption measurements.
`Laboratory plasmas,
`typically at 10,000°K are so
`much brighter than the brightest light sources com-
`monly available (e.g., carbon arc or tungsten strip
`lamp),
`that the latter are unsuitable. Finally,
`the
`methods for deriving the radial distribution of emission
`and absorption coefficients of optically thick arc
`plasmas from line—of—sight measurements have been
`slow to develop, and this has limited the value of ab-
`sorption spectra for plasma diagnostics. Freeman and
`Katz were the first to obtain a practical solution for the
`Abel inversion of plasmas with self-absorption} but a
`more general approach was only recently found by
`Elder at al.2 Elder et al. also derived the radial tem-
`
`perature profile by the Planck-Kirchhoff method from
`the Abel inverted emission—absorption measurements.
`To demonstrate their method, Elder et al. used a plasma
`seeded with sodium and determined the temperature
`from a sodium resonance line. The peak temperature
`
`The author is with the Warner & Swasey Company, Control
`Instrument Division, Flushing, New York 11354.
`Received 10 October 1967.
`
`was below 3000°K, which is not typical for a laboratory
`plasma.
`Tourin*" found that the strong ir lines of argon can
`become optically thick (self-absorbing) at atmospheric
`pressure. He measured the absorption by a technique
`based on modulating the radiation from the back-
`ground source;
`thus, absorption measurements can be
`made even when the intensity of the plasma is higher
`than the intensity of the background source. For the
`measurements reported in Ref. 3 a tungsten strip lamp
`could be used, because the mismatch of intensities in
`their is less than at shorter wavelengths.
`If absorption
`measurements of plasmas are to be extended into the
`Visible or uv regions of the spectrum, however, a much
`brighter light source has to be used.
`The high pressure xenon arc appears to have the
`desired characteristics. Since the early measurements
`of Baum and Dunkelman,4 its strong continuum in the
`uv and visible part of the spectrum is known to be
`considerably more intense than the radiation from the
`carbon arc. Goncz and l\'ewell’s5 recent Work with
`
`stationary and flashed xenon arcs covers a more ex-
`tended spectral
`range. However,
`their data were
`obtained by measuring the spectral
`irradiance from
`these arcs and are therefore of limited value for a back-
`
`ground source evaluation, where the spectral radiance
`of the brightest part of the arc is the parameter of
`interest. The peak temperature of a high pressure
`xenon arc has been determined by Kopec° from wave-
`length scans of two ir lines, using Bartels’ method’ to
`estimate the peak temperature of an inhomogeneous
`plasma from line—of-sight emission measurements of
`lines showing self-reversal.
`
`April 1958 / Vol. 7, No.4 / APPLIED OPTICS 677
`
`ASML 1332
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`ASML 1332