Kr II laser-induced fluorescence for measuring plasma acceleration

Abstract

We present the application of laser-induced fluorescence of singly ionized krypton as a diagnostic technique for quantifying the electrostatic acceleration within the discharge of a laboratory cross-field plasma accelerator also known as a Hall effect thruster, which has heritage as spacecraft propulsion. The 728.98 nm Kr II transition from the metastable 5d(4)D(7/2) to the 5p(4)P(5/2)(∘) state was used for the measurement of laser-induced fluorescence within the plasma discharge. From these measurements, it is possible to measure velocity as krypton ions are accelerated from near rest to approximately 21 km/s (190 eV). Ion temperature and the ion velocity distributions may also be extracted from the fluorescence data since available hyperfine splitting data allow for the Kr II 5d(4)D(7/2)-5p(4)P(5/2)(∘) transition lineshape to be modeled. From the analysis, the fluorescence lineshape appears to be a reasonable estimate for the relatively broad ion velocity distributions. However, due to an apparent overlap of the ion creation and acceleration regions within the discharge, the distributed velocity distributions increase ion temperature determination uncertainty significantly. Using the most probable ion velocity as a representative, or characteristic, measure of the ion acceleration, overall propellant energy deposition, and effective electric fields may be calculated. With this diagnostic technique, it is possible to nonintrusively characterize the ion acceleration both within the discharge and in the plume.

Measurements of the ion drift velocities in the presheaths of plasmas with multiple ion species

“…The problem then is one of deconvolution, where one aims at finding the frequency space line spreading function (the ivdf) that attaches to the line spectrum (which the isotopes compose) from the measured LIF signal. The quantum physics of the energy levels involved in the excitation transition is well described in Hargus, et al [28]. A key feature in the deconvolution process is the ratio of the thermal Doppler broadening and to the isotope shifts between the central most prevalent isotope ( 84 Kr 36 ) and its satellite lines.…”

“…Krypton had been a focus of attention in the Hall Thruster community as a reasonable propellant to substitute for xenon, which is much more expensive. Recently, some excellent work done with diode laser based KrII LIF measurement acquired in the plume of a 200 W laboratory BHT-200-X3 Hall effect thruster has been published by Hargus et al [28] using the LIF scheme that we had proposed, exciting from the metastable state 4p 4 4d 4 D 7/2 → 4d 4 5p 4 P 0 5/2 → 4d 4 4d 4 P 5/2 , with an excitation wavelength of 728.98 nm (air), and with a fluorescence wavelength of 473.90 nm. The unfolding of the ivdf from the LIF signal is complicated by the presence of isotope shifts, along with the hyperfine splitting of an odd A isotope ( 83 Kr 36 ).…”

Laser induced fluorescence diagnostic for velocity distribution functions: applications, physics, methods and developments

“…These isotope-shifted lines combine with the Doppler broadening from the IVDF to produce a complex convoluted spectrum. The quantum physics of the energy levels involved in the excitation transition has been well described by Hargus et al [115]. One of the key features of deconvolution is the ratio of thermal Doppler broadening and isotope shift between the most common isotope in the center ( 84 Kr 36 ) and its satellite line.…”

Recent development of LIF diagnostics in ASIPP

“…LIF was thus established to be the better diagnostics for IVDFs/NVDFs and were widely used as a tool to measure ion flow [4], ion temperature [5], magnetic field strength [6], and ion density [7]. It is becoming an increasingly popular diagnostic for the studies of basic plasma physics [8][9][10][11][12][13], plasma processing [14][15][16], plasma propulsion [17][18][19][20], among other applications.…”