Institute of Fundamental Technological Research

Hall-effect thruster plasma oscillations recorded by means of probes located at the channel exit are analyzed using the empirical mode decomposition (EMD) method. This self-adaptive technique permits to decompose a nonstationary signal into a set of intrinsic modes, and acts as a very efficient filter allowing to separate contributions of different underlying physical mechanisms. Applying the Hilbert transform to the whole set of modes allows to identify peculiar events and to assign them a range of instantaneous frequency and power. In addition to 25kHz breathing-type oscillations which are unambiguously identified, the EMD approach confirms the existence of oscillations with instantaneous frequencies in the range of 100–500kHz typical for ion transit-time oscillations. Modeling of high-frequency modes (ν∼10MHz) resulting from EMD of measured wave forms supports the idea that high-frequency plasma oscillations originate from electron-density perturbations propagating azimuthally with the electron drift velocity.

Plasma oscillations, Interplanetary magnetic fields, Plasma waves, Wavelets, High frequency discharges

An analysis is presented of the oscillations of keyhole pressure and plasma radiation emitted during welding with a continuous wave (CW) CO2 laser. Welding was done with a CW CO2 laser, Photon Sources VFA 2500, operating at the power of 1.75 kW. The welded materials were mild and stainless steel sheets, 0.8–2 mm thick. The shielding gas was argon or helium. Oscillations of plasma radiation were registered in monochromatic or broad band radiation with the use of a photomultiplier or photodiode and pressure variations with a microphone in the frequency range of 20–2 × 104 Hz. It has been found that the optical signal from the plasma plume is closely connected with the acoustic signal and that the source of the acoustic signal is the pulsating movement of the plasma plume. Spectral analysis of the measured oscillations shows differences in power spectra depending on the welding conditions. Generally, two intrinsic frequency peaks in the range of 0.5–4 kHz are always present but the amplitude, frequency and width of the peaks depend on the material and welding conditions. The results show that the optical and acoustic signals emitted during the welding process can be useful for process monitoring. The behaviour of the observed oscillations is characteristic for deterministic chaos. Considerable regularization of the process was observed as an effect of modulation of the laser beam. The modulation factor (Pmax−Pmin)/Pmax was equal to 0.2 and the modulation frequency was 2 kHz. In this case, the intense peak corresponding to the modulation frequency was observed in the power spectrum together with smaller peaks corresponding to the harmonic frequencies.

A spectroscopic study of a low-pressure supersonic jet of laser-heated argon plasma is presented. The experimental set-up consisted of a high-pressure convergent nozzle and a supersonic nozzle. The supersonic nozzle was placed just behind the convergent nozzle and was connected to a low-pressure chamber. A continuous wave laser with output power of 2 kW was used to maintain the plasma in the stream of argon gas flowing from the convergent nozzle. The plasma then expanded through the supersonic nozzle. Emission spectra from the laser-sustained plasma and supersonic jet were measured with a 1.3 m focal length spectrograph and 1254 silicon intensified target (SIT) detector connected to an EG&G PARC optical multichannel analyser (OMA) III. We found that the supersonic stream of argon plasma had an electron density of - and a temperature of 6 - 7 kK.

The boundaries of the existence of a laser-sustained argon plasma in forced convective flow were investigated experimentally and numerically. The plasma was maintained by a cw CO2 laser with an output power of 2.5 kW and burnt in free space at atmospheric pressure. Flow velocities changed from 2.3 m s-1 to 8 m s-1. The f number was 8.8. The results show that a quasi-2D model in which the axial flow is given by the relation rho u= rho 0u0( rho / rho 0)12/, where rho 0 and u0 are the density and velocity of the cold gas respectively, describes the observed phenomena fairly well.

The decay of the high pressure (7, 19 bar) optical discharge argon plasma is studied experimentally and theoretically. Time resolved spatial distributions of electron temperature and density have been measured spectroscopically giving the maximum values Te ≈ 1500 and Ne ≈ 8 × 1017 cm-3. The characteristic time of the decay is of the order of 10-4s. The experimental results are compared with the calculations based on the solution of the simplified energy equation in the spherically symmetric case. The kinematic and diffusion terms are neglected in the energy balance. It is found that radiation and thermal conduction are the dominant energy loss mechanisms during the decay process.

Discharge current and local plasma oscillations are studied in a high voltage Hall effect thruster PPS®-X000. Characteristic time scales that appear in different operating conditions are resolved with the use of Hilbert-Huang spectra (HHS) which display time dependenc of instantaneous frequency and power. Sets of intrinsic mode functions (imfs) that are used for HHS calculation result due to application of empirical mode decomposition method (EMD) to nonstationary multicomponent signals. In the experiment the signals are captured in the electric circuit of the thruster as well locally, in the vicinity of the thruster exhaust region. Classical electric probes spaced along the azimuth and/or thruster axis let us study correlations of signals which were captured in different locations. In this way azimuthal and axial propagation of disturbances is inspected. The discharge voltage is varied in the range of 200÷900 V while xenon mas flow rate of 5÷9 mg/s. LF, MF, and HF characteristic bands that are known from previous studies of PPS®-100 thruster have been also detected here. However, expanding discharge current onto a set of intrinsic modes we can resolve MF mode more reliably than before. Moreover, for higher discharge voltages, this irregular mode turns into more regular waveform and tends to dominate in the discharge current masking almost completely the breathing mode (LF oscillations of the discharge current). In such a case triggering of HF oscillations is correlated with the phase of MF mode while in the case of PPS®-100 thruster it was correlated with the appropriate phase of the breathing mode (LF band). Regular HF emission that can be unambiguously interpreted as azimuthal electrostatic wave now is observed only in the specific operating conditions of the thruster. However, even if irregular HF emission is observed the time delay of cross-correlated signals which are captured in different azimuthal locations corresponds to the velocity of azimuthal electron drift in the field of magnetic barrier.

High voltage direct current transmission, Electrical circuits, Electrostatic discharges, Electrostatic waves, Plasma oscillations

Using the fluid equations of Hall thruster plasma we analyze the influence of the electron energy balance on the stability of ion sound modes. For frequencies lower than ωc = 107 s−1 the gains and losses in the source term are approximately equal, thus the temperature can be in principle determined in terms of other dependent variables. This permits to reduce the number of equations. It appears however, that the new system can have complex characteristics in some regions. This in turn implies instability of certain modes with frequencies lower than ωc.

Energy balance, Acoustic analysis, Fluid equations, Instability analysis, Plasma instabilities