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Two-dimensional electron temperature and density profiles in the plume region of 300 W-class Hall thruster Ar plasmas were obtained using tomographically reconstructed optical emission intensity profiles combined with a collisional-radiative (CR) model. A total of 1242 lines of sight were used by rotating the thruster to apply inverse Radon transform-based tomographic reconstructions and Abel inversion. The reconstruction accuracy of the developed diagnostic system was evaluated using a priori images derived from plasma pictures, and the reconstruction error was less than 1% in the region of interest, exhibiting higher accuracy than the Abel inversion. From the Ar Hall thruster plasma, more than 12 different two-dimensional profiles of Ar I emission intensity within a spectral range of 600–1000 nm were obtained 6 mm from the exit plane of the thruster. A CR model using 31 allowed transitions at 15 different states from the ground state to the 2p i states was incorporated with the tomographically reconstructed emission intensity sets. Consequently, two-dimensional electron temperature and density profiles in the range of 5–18 eV and 2.0 × 1016–4.7 × 1017 m−3 were obtained, respectively, exhibiting reasonable agreement with the double Langmuir probe measurements.

A diagnostic system was developed for spectrally resolved, three-dimensional tomographic reconstruction of Hall thruster plasmas, and local intensity profiles of Xe I and Xe II emissions were reconstructed. In this diagnostic system, 28 virtual cameras were generated using a single, fixed charge-coupled device camera by rotating the Hall thruster to form a sufficient number of lines of sight. The Phillips–Tikhonov regularization algorithm was used to reconstruct local emission profiles from the line-integrated emission signals. The reconstruction performance was evaluated using both azimuthally symmetric and asymmetric synthetic phantom images including 5% Gaussian white noise, which resulted in a root-mean-square error of the reconstruction within an order of 10−3 even for a 1% difference in the azimuthal intensity distribution. Using the developed system, three-dimensional local profiles of Xe II emission (541.9 nm) from radiative decay of the excited state 5p4(3P2)6p2[3]°5/2 and Xe I emission (881.9 nm) from 5p5(2P°3/2)6p2[5/2]3 were obtained, and two different shapes were found depending on the wavelength and the distance from the thruster exit plane. In particular, a stretched central jet structure was distinctively observed in the Xe II emission profile beyond 10 mm from the thruster exit, while gradual broadening was found in the Xe I emission. Approximately 10% azimuthal nonuniformities were observed in the local Xe I and Xe II intensity profiles in the near-plume region (

Two distinct discharge modes were observed in a 50 W-class micro-Hall thruster plasma under different operating conditions. A ball-shaped plasma with a broad plume (mode A) was observed at low mass flow rates (less than 0.37 mg s−1) over the entire operational anode voltage range (160–280 V). Raising the anode voltage beyond 200 V with the mass flow rate fixed (larger than 0.37 mg s−1) produced a narrow plume and stretched jet-like structure (mode B). In mode B, the thruster showed performance improvements in terms of thrust (3.8 mN vs 3.3 mN), specific impulse (913 s vs 800 s), and anode efficiency (28% vs 22%), with only a 2 W difference in the anode power (61 W in mode B and 59 W in mode A). This suggests that operation is more advantageous in mode B than in mode A for the utilization of such low-power Hall thrusters. Unique plume properties were observed in the two modes and considerable differences were measured in the Xe II ion acceleration structure, beam angle, and ionization rate as the mode changes, which were not reported in previous studies. Mode A exhibits an axially extended ion acceleration structure outside the discharge channel, where 75% of the final ion velocity is achieved at approximately 40 mm from the thruster exit, while most of the ion acceleration occurs within 10 mm from the thruster exit in mode B. Measurements show that the full width at half maximum of the Xe II ion energy distribution function, electron temperature, and Xe II emission intensity decreased after the plasma transitioned from mode A to mode B. Based on the optical emission spectroscopy, the ionization rate in the plasma plume decreased by 30%–41% after the mode change, which is likely related to the reduction of the beam angle and electron current by 24% and 30%, respectively.

A new low temperature multidipole plasma device with a magnetic X-point is developed. With a usual multidipole configuration generated by permanent neodymium magnets, a pair of axially flowing electrical currents up to 1.0 kA in the chamber creates figure-eight shaped poloidal magnetic fields with the X-point which separates plasmas into three distinct regions of core, edge and private regions. This new device, magnetic X-point simulator system (MAXIMUS), is equipped with end-plate wall filaments, core filaments and a LaB6 cathode as DC plasma sources. A wide range of plasma densities from 108 to 1012 cm−3 with electron temperatures of 0.4 to 3 eV is achieved. Plasmas in MAXIMUS are highly correlated with the shape of the magnetic fields as electrons are magnetized. Furthermore, electron velocity distribution functions can be significantly modified from usual Maxwellian distributions due to the strong grad-B and curvature drifts of electrons, resulting in high skewness and excess kurtosis. Such a capability of controlling the distribution function as well as having closed circular magnetic fields will allow us to systematically investigate effects of non-Maxwellian distribution functions and curved magnetic fields on various physical phenomena such as cross-field diffusion process, plasma waves and many nonlinear physics including solitons, shock waves and three-wave interactions. Tokamak edge physics correlated with neutral particles is also to be investigated with MAXIMUS.

Optical emission spectral tomography diagnostics have been developed to measure the spatial uniformity of meter-sized processing plasmas. The lines of sight and detector location are selected based on tomographic reconstruction tests using synthetic (phantom) images. The developed collection optics system is composed of slits, a plano-convex lens, optical interference filters and photodiode arrays. Using the collection optics, the line-integrated emission intensity is acquired from a meter-sized rectangular argon plasma (inductively coupled plasma at a frequency of 13.56 MHz), which was developed for display panel manufacturing. From the measured intensity, two-dimensional (2D) spatial distributions of argon atomic line intensity (675.3 nm and 852.1 nm) are obtained via the tomographic reconstruction technique based on the Phillips–Tikhonov regularization. In addition, 2D profiles of the electron excitation temperature (T exc) are obtained from those of Ar intensity at the wavelengths 675.3 nm and 852.1 nm via two line method (in essence, a Boltzmann plot). The 2D argon plasma emission and T exc profiles match well with the shape of the electrode and etch rate profile.

Due to the lack of convincing experimental evidence for electron information, there are still unclearly understood discharge phenomena in atmospheric pressure radio-frequency (rf) capacitive discharge, e.g. the electron heating, discharge structures, and the alpha–gamma mode transition. Thus, to perceive basic and meaningful principles with an unambiguous interpretation, simple and reliable electron diagnostics are required. Since bremsstrahlung emitted through electron-neutral atom interaction depends on electron density (ne) and temperature (Te), their diagnostic is possible. In particular, Te is easily estimated from the ratio of bremsstrahlung emissivities at two different wavelengths or more. In this paper, 2D Te distribution in an argon atmospheric pressure capacitive discharge measured by using a digital camera and optical band pass filters is described. Time-averaged Te in the bulk region obtained by a digital camera is consistent with that measured by an absolutely calibrated spectrometer. In addition, time-resolved emission spectra and the corresponding ne and Te during one rf cycle of the argon capacitive discharge are discussed. The result shows that Te varied from 2.3 to 3.0 eV, while ne did not change significantly.

Electron diagnostics based on electron–neutral atom (e–a) bremsstrahlung in the UV and visible range emitted from atmospheric pressure plasmas is presented. Since the spectral emissivity of the e–a bremsstrahlung is determined by electron density (ne) and mean electron temperature (Te) representing the Maxwellian electron energy distribution, their diagnostics is possible. As an example, emission spectra measured from capacitive discharges are presented, which show good agreement with the theoretically calculated emissivity of the e–a bremsstrahlung. For a single pin electrode nanosecond-pulsed plasma jet (n-PPJ) in argon, we investigate the electron properties and the temporal behavior of the positive streamers. Streamers with many branches are clearly observed inside the dielectric tube, while a few main streamers propagate outside the tube along the jet axis. A two-dimensional (2D) measurement of the time-averaged Te distribution was developed using a commercial digital camera and optical band pass filters based on the emissivity ratio of two wavelengths of the e–a bremsstrahlung. The viable measurement range of Te is 0.5–7 eV for the choice of two wavelengths of 300s and 900s nm and 0.5–4 eV for two wavelengths of 400s and 900s nm, which are uncontaminated by the atomic and/or molecular spectra. The 2D Te distribution obtained using 514.5 and 632.8 nm emissions helps to reveal the role of electrons in streamer characteristics in the argon n-PPJ. Time-averaged Te of 2.0 eV and 1.0 eV inside and outside the tube, respectively, were measured. The streamer dynamics of the n-PPJ is shown to be dependent on Te.