HY-Tech Laser Diagnostic Capabilities

At HY-Tech numerous laser diagnostics have been employed and some unique applications have been pioneered.  These techniques can be broadly classified into the three categories of laser scattering, laser absorption and laser interferometry.  Laser scattering diagnostics include Mie scattering, Rayleigh scattering, Resonant Rayleigh scattering and Laser Induced Fluorescence (LIF).  Absorption diagnostics include resonant and non-resonant absorption.  Interferometric diagnostics include 2D resonant and non-resonant interferometry, 2D resonant and non-resonant holographic interferometry, and single-beam time resolved heterodyne laser interferometry.

Laser Scattering Measurements

Mie and Rayleigh scattering have been employed to measure particle size and velocity distributions of a beam of dust grains used in a laboratory simulation of the charging of space-borne dust [1] and image dust grains trapped in a dc plasma discharge used in the processing of semiconductors.[2]  Particles in sizes from 40 to 1000 nm have been detected.

LIF has been used to study a number of different parameters in various kinds of plasmas.  N⁺₂ ion densities were measured in a spark discharge channel by tuning to the 391.4 nm ground state transition of the ion.  The system simulated the channel produced by a relativistic electron beam traveling through air.  This technique allowed the measurement of channel densities on the order of 10 ¹² cm^-3, well below the sensitivity of Thomson scattering or conventional interferometry.[3]  Ground state barium ion and neutral densities were measured in plasmas produced by the PHAROS laser (Naval Research Laboratories) interacting with a solid barium target in a moderate density background gas.  The overall program simulated the effects of high altitude nuclear explosion (HANE).[4]  Local ion densities and electron temperatures were measured in plasma opening switches (POS), devices which are used in pulsed power generators to shorten pulse lengths and multiply voltages delivered to x-ray simulator loads.[5]

HY-Tech pioneered a method to use LIF to measure magnetic field profiles in a pulsed glow discharge used to simulate a REB channel, from which the current profile in the channel can be derived.  The technique involves the localization of emission from Zeeman shifted atoms in the plasma channel.  For Zeeman shifts greater than the intrinsic line width (natural plus Doppler) of the probed atomic transition, tuning the laser off line center results in direct visualization of regions Zeeman shifted into resonance.  Scanning the laser wavelength gives a full radial magnetic field profile.[6]  For shifts less than the intrinsic linewidth a different method was used.  When a 1 3 transition splits one term is right circularly polarized (RCP) and one is left circularly polarized (LCP).  By tuning a narrow band laser off center frequency but still within the intrinsic linewidth, the entire channel will fluoresce.  The two components were separated using an imaging polarimeter and recorded with a gated image intensifier.  The ratio of the component intensities are then analyzed to give the radial field profile .[7]

Laser Absorption

2D absorption images of a POS plasma were generated using a pulsed dye laser tuned to the 493.4 nm transition of singly ionized barium and an intensified framing camera.  A series of absorptiongrams were used to track the density profiles through the conduction phase of the switch .[5]  Ion temperatures in a similar switch were measured by using a broad linewidth (0.3 nm) dye laser to interact with the entire 493.4 nm spectrum.  The absorption linewidth was measured with a detection system consisting of a 1 meter monochromator, a Fabry-Perot etalon and a gated, intensified optical multichannel analyzer.[8]  Temperatures up to 25 eV were measured. 2D measurements of magnetic fields in a POS were made by measuring the effect of Zeeman shifts on the RCP and LCP components of a laser beam tuned to a barium ion resonance. [9]  Finally, non-resonant absorption measurements were used to track electron temperatures and densities in aluminum wire z-pinches[10] and exploding foils.  Using a long pulse (700ns) dye laser, streaked and framed photos demonstrated the evolution of these plasmas.

Laser Interferometry

The phase of a laser beam passing through a plasma is shifted relative to vacuum by changes in the index of refraction caused by the presence of electrons, ions, and neutrals.  In general the electron contribution to the index in a plasma is dominant, except in cases where the laser wavelength is close to a resonant transition in an atom to ion.  HY-Tech scientists have exploited both resonant and non-resonant regimes to study plasmas.

Electron densities have been measured in aluminum z-pinch plasmas by holographic interferometry using both ruby and NdYAG lasers.  The densities measured ranged from 10¹⁷ to 10²⁰ cm^-3.  Phase shifts were read out graphically and represent a minimum resolution of about 70 degrees.  Single beam heterodyne interferometry was used to measure time resolved electron densities in a POS plasma and neutral densities in a gas puff used as an x-ray simulator load.  A phase resolution of 0.5 degrees translates into minimum resolutions 5x10¹³ and 1x10¹⁷ cm^-3 for the electrons and neutrals respectively.

The holographic interferometry offers the advantages of 2D resolution of densities and the convenience of being able to analyze the data at one's leisure with the disadvantage of lower resolution when conventionally analyzed.  Heterodyne interferometry offers the advantages of high sensitivity and the ability to temporally resolve the density.  The disadvantage is that measurements can be taken only at one position in the plasma.

 For low density plasmas HY-Tech has developed resonant holographic interferometry where holograms of a plasma are taken with a dye laser tuned near a resonant transition of a particular species in the plasma.  Initial measurements to prove the feasibility of the process were made in a spark discharge in neon.  With the laser tuned to the 585.2 nm transition radial density profiles of neutral neon were mapped. [11] Densities on the order of 10¹³ cm^-3 were detected.  In the barium POS measurements (described earlier) resonant holograms were used to benchmark the barium density profiles with densities of up to 10¹³ cm^-3 .  In both cases conventional analysis procedures were used.

HY-Tech Laser Diagnostic Capabilities:

1. E.J. Yadlowsky and R.C. Hazelton, "Laser induced fluorescence measurements of magnetic field and air chemistry species in relativistic electron beam propagation experiments," final report contract #N60921-S8-0227,Dec. (1990).

2. G.G. Spanjers, E.J. Yadlowsky, R.C. Hazelton and J.J. Moschella, "Ion mass effects on plasma opening switches," final report, contract DNA001-92-0034, Aug. (1994).

3. E.J. Yadlowsky, T.B. Settersten, R.C. Hazelton, J.J. Moschella and G.G. Spanjers,"Streaked laser absorption measurements of density and temperature profiles in high density z-pinch," Rev. Sci. Instrum., 66, 652 (1995).

4. G.G. Spanjers, E.J.Yadlowsky, J.J. Moschella and T.B. Settersten," Zeeman absorption measurements of 2-dimensional magnetic field structures," Rev. Sci. Instrum., 66, 1189 (1995).

5. R.C. Hazelton, E.J Yadlowsky, J.J. Moschella and G.G. Spanjers, "Resonant Laser diagnostics of a planar plasma opening switch," IEEE Trans. Plas. Sci., 23, 113 (1995).

6. R.C. Hazelton and E.J. Yadlowsky, "Direct measurement of PEOS ion density distribution using resonant holographic interferometry, " 1988 IEEE ICOPS, #88ch2559_3, 127 (1988).

7. C.C. Klepper, J.T. Hogan, S.J. Tobin et.al. J.Nucl. Mater.  220-222, 521 (1995)

8. C.C. Klepper, W.R. Hess, J.T. Hogan, D. Guilhem, Article in AIP Conference Proceedings 322, ATOMIC PROCESSES IN PLASMAS, William L. Rowan, Ed.,  AIP Press (New York 1995), pp. 55-66

9. C.C. Klepper, J.T. Hogan, et.al., Nucl. Fusion 33 (4), 533 (1993)

10. David V. Tsu , ..C.C. Klepper, L.A. Berry. "Ion and Neutral Argon Temperatures in Microwave ECR Plasmas by Doppler Broadened Emission Spectroscopy" J. Vac. Sci.& Technol.-A , 13 (3), 935 (1995)

11. E.J. Yadlowsky and R.C. Hazelton, "Laser induced fluorescence study," NRL order #N00173-87-M-8792, Feb. (1988).