Laser Diagnostics: Species identification, velocity, temperature, velocity/energy distribution, and relative/absolute density

Traditional measurement tools used in plasma research such as Langmuir probes and retarding potential analyzers are by their nature intrusive.  Intrusive techniques sometimes change the properties of the very plasma they are studying.  Intrusive techniques run the risk of affecting the local plasma in high heat flux environments such as the interior of Hall thrusters.  High heat flux environments also reduce probe life; e.g., through probe melt or vaporizing.  Probe melting/vaporization  products contaminate the plasma and calls into question all data collected by the probe.  Thus, some plasma environments are either inaccessible or difficult to interrogate with intrusive techniques.  Laser-induced fluorescence (LIF) is a non-intrusive optical method for measuring the properties of gases and plasmas.  LIF is particularly useful for probing plasmas that are otherwise inaccessible with intrusive techniques.  The beam from a tunable laser is passed through the medium, exciting an electronic or vibrational transition of one of the species. The fluorescence light is then collected.  The velocity is determined by the Doppler shifted fluorescence peak emanating from the interrogated medium with respect to the commensurate lab-frame reference spectrum.  The temperature is determined by the fluorescence spectra shape (width in particular), the relative or absolute density is determined by the intensity of the fluorescence signal.  Absolute density measurements typically require a calibration method and/or the use of ground-state excitation.  Because of the energetic photons required for ground state LIF for most species of interest (e.g., ground state Xe II LIF requires vacuum ultraviolet laser frequencies), most LIF experiments are conducted at weakly populated excited states.  A recent comparison made between Hall thruster plume data collected via an intrusive system (MBMS) and LIF showed excellent agreement in bulk velocity predictions but poor agreement in temperature (Ti =6500 K for LIF vs. 2500 K for MBMS)[1].  This result is not surprising since it suggest while the ions are accelerated through the same potential field regardless of excitation state, the plasma is not in equilibrium.  For example, electron temperatures at the MBMS/LIF interrogation zone routinely exceed 1 eV (Te> 11,600 K).

PEPL Laser Diagnostic Facility:

PEPL Laser Diagnostics Facility: PEPL operates a number of lasers for Laser-induced fluorescence (LIF) measurements. PEPL’s Coherent model 899-29 Autoscan II ring-dye laser is capable of outputting light within a spectral range of 375 nm - 900 nm (with appropriate dyes and Ti:Sa system) with a linewidth of less than 500 kHz rms. The single frequency ring laser is computer controlled and has a wavelength meter attached to it that measures the laser frequency to within ±200 MHz (0.0067 cm-1). The dye laser is pumped by a Coherent Sabre R 20/4 argon ion laser. This UV-capable pump laser produces up to 20 W of power and enables the ring dye laser to reach the UV with the proper dye. This laser enables the dye laser to generate several tens of mW to over 1 W of power over the spectral range of interest. PEPL can also operate the Coherent 899-29 laser a Ta:Sa ring laser for high-power (>1 W) access in the near-infrared. Not only does the Ti:Sa conversion kit for the 899-29 allow for more powerful access to the near-IR, the kit has a greater tuning range than a dye system and the ability to interrogate boron nitride (the principal erosion product of stationary plasma thrusters), Xe, Xe+, and any other (as yet unspecified) transition within the 800-900 nm range.

PEPL also operates a high-power, single-frequency, continuously-tunable TA 100 diode laser by TUIOPTICS of Martinsried, Germany.  This diode laser can interrogate Xe, Xe⁺  and any as yet unspecified transitions between 825 and 875 nm. PEPL typically uses the diode laser system to scan for Xe or Xe⁺ and the 899-29 dye/Ti:Sa-configured ring laser to scan for a second species.

A “multiplex” LIF technique is used at PEPL that allows simultaneous measurement of two or more velocity components at a given location. A small fraction of the laser beam (~1 percent) is split from the main beam and passed into a wave meter. This wave meter monitors the wavelength of the laser, the mode of the laser, and the linearity of fine scale tuning. The main beam is split into multiple parallel beams (Fig. 2) by a coated prism. The beams are then chopped at different frequencies to permit discrimination of the three fluorescence signals. A small fraction of one of these beams is passed through a reference cell. The reference cell is actually an opto-galvanic cell filled with xenon gas. However, since xenon ions and neutrals are of primary interest, the cell serves as a zero velocity LIF reference for both. 

The fluorescence signal resulting from the laser beams are monitored via a monochromator coupled to a lock-in amplifier that is in phase with the chopping frequency. A reference cell also enables a rough analysis of laser beam linewidth and quality. The parallel beams are then directed to the optical port of the vacuum chamber. The parallel beams that enter the chamber are directed to a spot of interrogation by a series of mirrors and lenses mounted to a fixed breadboard. The thruster is moved about via a high-fidelity positioning system to create the spatial map. The fluorescence signal is collected via a large (8-cm-diameter) lens and mirror positioned on the same breadboard as the optics guiding the laser beams. The collected signal exits the chamber via a second port and is monitored by a monochromator that is coupled to three lock-in amplifiers, one in phase with each of the chopping frequencies. The system is aligned via optics external to the vacuum chamber and a pin (to determine the interrogation spot). 

PEPL also uses a custom periscope laser delivery system for axial interrogation. Single-beam axial injection provides more laser power for interrogation and is more accurate in determining axial velocity and temperature. The test article is often placed on a rotation stage to allow LIF interrogation along multiple angles from centerline. 

Figure 4 shows the LVTF beam handling setup for axial injection. A three-prism periscope system, shown in Fig. 4(b), sends the beam through a focusing telescope parallel to the thruster axis, reducing the beam diameter (which grows to approximately 2.0 cm over the 12 m path length) to less than 1 mm. An enclosure with anti-reflection (AR) coated windows protects the beamturning prisms and focusing telescope from sputtering deposition and erosion. A focus tube between the telescope elements provides axial adjustment of the laser focus. A 1-mm-diameter steel T-pin, centered on the downstream face of the thruster, facilitates laser alignment. Two separate AR windows protect the 100 mm-diameter, f/2.5 collection lens. The collimated fluorescence from the thruster plume is focused by a 100 mm-diameter, f/2.5 lens onto a Spex H-10 monochromator with a Hamamatsu 928 PMT. Stanford SR810 and SR850 DSP lock-in amplifiers, using a 1-second time constant, isolate the fluorescence components of these signals.

Figure 1:  Schematic of the Large Vacuum Test Facility and PEPL’s Laser Diagnostics Facility.

Figure 2:  Illustrations of the multiplex interrogation scheme for both Hall (P5 and ion (NSTAR-FMT2) thrusters.

Figure 3:  Illustration of the multiplex injection optical delivery/interrogation system.  Note: interior optics boxes are encased in graphite with quartz windows during testing.

Figure 4:  Illustration of the axial injection interrogation scheme.  Note rotation stage below thruster.

1. Williams, G. J., Smith, T. B., Gulczinski, F. S., Gallimore, A. D., "Correlating Laser Induced Fluorescence and Molecular Beam Mass Spectrometry Ion Energy Distributions," Journal of Propulsion and Power, Vol. 18, No. 2, pp. 489-491, March-April 2002.
2. Williams, G.J., Smith, T.B., Gulczinski, F.S., Beal, B.E., Gallimore, A.D., and Drake, R.P., "Laser Induced Fluorescence Measurement of Ion Velocities in the Plume of a Hall Effect Thruster," AIAA-99-2424, 35th Joint Propulsion Conference, Los Angeles, CA, June 1999.
3. Williams, G.J., "The Use of Laser-Induced Fluorescence to Characterize Discharge Cathode Erosion in a 30cm Ring-Cusp Ion Thruster," Ph.D. Dissertation, University of Michigan, 2000.
4. Smith, T.B., Herman, D.A., Gallimore, A.D., and Drake, R.P., "Deconvolution of Axial Velocity Distributions from Hall Thruster LIF Spectra," IEPC-01-019, 27th International Electric Propulsion Conference, Pasadena, CA, October 15-19, 2001.
5. Smith, T. B., “Deconvolution of Ion Velocity Distributions from Laser Induced Fluorescence Spectra of Xenon Electrostatic Thruster Plumes,” Ph.D. Dissertation, University of Michigan, 2002.