Contamination control and plume assessment of low-energy thrusters

FIGURE 2. CROSS SECTION OF THE FLIGHTPLASMA GENERATOR (FROM ROBSON ET AL.(REF. 1), IBID.)THRUSTER PARAMETERSAn ion thruster that provides thrust for attitude control,orbit transfer, and other propulsive functions is character-ized by the mass rate, rh, of the propellant and the propellantexit velocity, v. In general, the thrust, F = rhv, from this typeof engine is in the microNewton range. The propellant maybe Xe or other suitable gas which can be ionized andconfined by magnets or grids. The efflux from the thruster iscomposed of neutrals, a cold plasma consisting of a low-energy mix of positive, negative and neutral atoms andelectrons, and a high-energy, hot plasma consisting of thesame components. The thruster also releases so-callednonpropulsive, sputtered materials.In general, the effluents will exit with a distribution thatmay be described with a power of the cosine function. Thecold plasma is affected by electromagnetic fields created inthe thruster. The particles of the plasma interact with thespacecraft, are neutralized on the surfaces, and then arereleased after a brief delay. The energies may be of about15 eV for the cold plasma, and about 28 eV for the hotplasma. The hot plasma may alter, erode, deposit upon, orsputter upon surfaces. The energetic particles are confinedwithin a narrow plume. The nonpropulsive materials are alsoemitted within a very small angle about the centerline of theexit plume. The nonpropulsive effluents may have energiesof 35--40 eV and may consist of molybdenum (Mo), rhenium(Re), barium-cobalt (BaCo3), and others, which can sputteron surfaces in their paths. The electromagnetic fields producedby the plasma flow and by the engine's intemal magnets maychange the trajectory of the contaminants. Also, they maycause interference at some nearby instrument, disrupt com-munication, and produce certain disturbing low frequencies.CONTAMINATION EVALUATION HAZARDSFor a gaseous source with a given volumetric flow rateq (cm3/s) and gas with mole mass M, the corresponding massflow rate is rh = QM/V (g/s) where V = 22.4 x 103 (cm3/mol).The adiabatic exit flow velocity of the gas expanding intovacuum is v = [27RT/M (1-7)] 1/2where T (K) is the tempera-ture acquired by the gas at the source, 7= Cp/Cv is the ratio ofthe gas' specific heat, which is 1.66 for monoatomic gas, and1.4 for diatomic gas, and R = 8.31x 107 (erg/mole/K) is thegas constant.The plume distribution of the emitted gas can be de-scribed approximately by a power of the cosine function.The function was derived using experimental data obtainedfrom measurements in the plume of alow-pressure vent froma pipe (ref. 3). The flux as a function of the angle 0 (°) fromthe plume axis, r (cm) the distance from the vent exit, is:_ rh (n+l) cosn0 (g/cm2/s)2zcr2where the exponent n (n = 1, 2, 3,...) represents the diver-gence of the plume shape from the cosine to the first powerdistribution.The value of n, as indicated in ref. 3, can be obtainedfrom a normalized plot of the flux or from any parameterdescribing the plume as a function of the angle from theplume centerline. The angle 01/2 corresponding to 50% ofthe maximum value at the centerline, is a measure of theplume spread. A graphical representation of cosn0 versusangle 0 with n as a parameter is shown in Figure 3 and canbe used to evaluate n. Representations of the fluxes in termsof normalized _/dl and location in an axisymmetrical planeare shown in Figures 5, 6, 7, 8, and 9, for cosine to the n =1, 2, 3, 4, and 5 power.