Relationship Between Anode Spots and Onset Voltage Hash in Magnetoplasmadynamic Thrusters

Experimental results are presented which indicate a direct relationship between voltage transients in magnetoplasmadynamicthrustersoperatingaboveonsetandthetime-resolvedappearanceofdestructivelyreleased anode material in the thruster plume. Such a relationship gives support to previously discussed anode spotting theories. Langmuir probe measurements of plasma density fluctuations and measurements of argon and copper (anodematerial)ionluminosityarecomparedwiththevoltagetransients.Theonsetofspikesinthethrustervoltageis directly correlated with the onset of similar spikes in the plasma density at the probe location, and with a rise in the copper luminosity, but a fall in that of argon, in the plume. The voltage hash is categorized into two types: largeamplitudespikesatcurrentswell abovetheonsetcurrent, andlower-amplitude random fluctuations atcurrentsjust abovethe onsetcurrent. Itisshownthatthetwocategoriesofvoltagehashcan berelated totwoclasses ofdamageon the anode surface: pitlike damage of 10 to 100 � m extent, caused by explosive emission due to voltage spikes, and shallow surface melting due to the lower-amplitude random fluctuations, which may be responsible for observed density and luminosity oscillations at 600 kHz, and a dip in the voltage power spectrum at the same frequency. I. Introduction O NSET in a magnetoplasmadynamic thruster (MPDT) is a condition encountered in high-current operation, in which the thruster voltage (quiescent at lower currents) fluctuates with an amplitude that can be large compared with the mean voltage, and in which the anode sustains significant damage. Onset is a lifetimelimiting factor in the operation of the MPDT, and so must be understood and overcome to improve the performance of MPDTs as primary propulsion systems for long-duration space missions. The nature of the voltage fluctuations (“hash”) and the associated anode damage has been the subject of many studies, many of which pointoutarelationship between thepresence ofa fluctuating voltage and fluctuationsinotherthrusterparameters,suchasopticalemission [1], electric and magnetic fields [2], and plasma density [3]. Kuriki and Iida [1] measured the fluctuation of the total light output from a quasi-steady MPDT, observing, in particular, similarities in the power spectra of both the voltage and the luminosityatabout500kHz,showingarelationshipbetweenthetwo quantities. Ho [4] and Rudolph [5] measured the time-integrated optical spectrum of a quasi-steady MPDT, and discovered an increase in the luminosity of anode material in the thruster plume as the current rose above onset. However, Kuriki and Iida’s data [1] cannot identify which atomic species in the plume was responsible for the luminosity fluctuation, nor could Ho’s and Rudolph’s measurements[4,5]provideinformationconcerningwhenduringthe thruster pulse anode material appeared in the plume; therefore, the connection between hash and anode erosion was not completely elucidated. Hugel [6] has suggested that the presence of anode material in the plume can be attributed to the formation of anode spots, which he observed to form on his MPDT anode in high-speed photographs at the moments of maxima in the voltage hash. At high currents, the plasma density near the anode is small enough that the anode is starved of the charge carriers needed to conduct the current. Hugel suggests that the anode spots form in response to this starvation. Diamant et al. [3] have further demonstrated that spot damage on the anodesurfaceismuchgreateratcurrentsaboveonsetthanbelow,and that the plasma density, averaged over a quasi-steady firing, increases above onset. What Hugel’s [6] and Diamant’s [3] data togethercannotconclusivelydemonstrate,however,isthattheanode spots are the source of additional plasma density above onset, and that their density contribution is correlated with the voltage hash fluctuations. Previous modeling work [7], however, has suggested this relationship between anode damage and voltage hash. In this paper, we address key questions that remain from these studies; namely, we search out the time-resolved correlations between voltage hash and anode erosion lacking in the previous work. We do this using time- and wavelength-resolved plume luminosity measurements and time-resolved plasma density measurements. We correlate the measurements made by these two diagnosticswiththevoltagehash fluctuations,andshowthatallthree arecorrelatedinamannerconsistentwithwhatistobeexpectedfrom transient damage to the anode surface. Briefly, the paper is organized as follows. In Sec. II, we give relevantdetailsoftheexperimentalsetupusedtotakethedata,which we present in Sec. III. We defer major interpretation of the results to Sec.IV,wherewerelatetheexperimentalresultstothehypothesisof anode damage at high currents.