Paola Rossetti

Numerical model of thermoelectric phenomena leading to cathode-spot ignition

This paper deals with the results of a numerical model of the predischarge heating process encountered by a microprotrusion on the cathode surface of a vacuum gap, due to the field-effect current density flowing throughout the protrusion. The model is one-dimensional and nonstationary. The protrusion is sketched as a truncated cone and the material considered is tungsten, whose physical and thermal properties have been assumed temperature dependent. Electron current density emitted by the tip and the Nottingham effect have been calculated solving the integral of the energy distribution of the emitted electrons. In all other existing models, these two quantities are approximated by algebraic equations, valid in a limited temperature range. Results seem to confirm the experimental evidence that breakdown starts from the explosion of microprotrusions in a time of the order of 1-10 ns. In order to induce the explosion, current densities could be as high as 10/sup 13/ A/m/sup 2/, while the corresponding electric field at the tip can reach the value of 10/sup 10/ V/m, slightly higher than the value found by others. Numerical results confirm that the more slender the protrusion, the more likely its explosion. Investigation of the role of the surface work function shows that decreasing its value at the cathode surface favors the explosion of a larger number of protrusions, inducing the distribution of the arc current among more spots, with a cathode damage reduction, especially on electrodes operating at high current and low temperature.

Advanced scaling model for simplified thrust and power scaling of an applied-field magnetoplasmadynamic thruster

IRS and Alta are currently involved in ESA and EU (HIPER) programs aiming for the development of high power electric propulsion systems. In the framework of these programs, two 100 kW applied-field magnetoplasmadynamic thrusters (AF-MDPT) are to be developed and tested within 2011: The steady state gas-fed AF-MPD ZT2 and Alta gas-fed pulsed AFMPD thruster. Such devices present some of the best propulsion systems if it comes to high thrust density and high specific impulse requirements, for example in orbit raising or Mars transfer mission scenarios. However, up to this day, there exists no adequate simple scaling model for AF-MPDT available to mission analysis and engine design which is addressed in this work: The classic Tikhonov model is extended and improved for steady state gas fed AFMPDT. A simplified law of energy conservation is introduced, which can be extended with an improved anode loss model. Both models were fitted using data of existing thrusters. Further, a common envelope of both thrusters has been achieved by overlapping of operational conditions to allow a comparison of both designs. Also, power scaling and performance estimation of the 100 kW IRS AF-MPD ZT1 and the 100 kW Alta AF-MPD thruster are presented in this paper. The knowledge obtained is going to enable IRS and Alta to define the specifications of a future ESA/European AF-MPDT.

Experimental study of a Multichannel Hollow Cathode for High Power MPD Thrusters

The paper deals with the characterization test of a multichannel hollow cathode performed at Alta. The cathode operated at currents from 20 to 150 A, mass flow rates of Ar ranging between 1 and 5 mg/s, reaching a maximum tip temperature measured on the external cathode wall of about 2500◦C. The external wall temperature has been measured by a bi-color video pyrometer mounted on a 1-axis slit to record the temperature profile along the cathode axis. More than 200 ignitions were carried out for a total of 30 accumulated hours of operation. Besides, the post-test destructive analysis gave evidences of the plasma penetration and surface modification due to the high operative temperature along with the erosion pattern. Data including cathode temperature and electrical characteristics are presented and analyzed and some operative conditions are compared with the results of a simple algebraic model of arccathode interaction.

Characterization test and partial-endurance of HT100

The paper deals with the preliminary results of several tests carried out at Alta during the last months on different low-power Hall thruster models, with nominal discharge power < 200 W. All configurations have been obtained by scaling from the HT-100 design, a 100-W Hall thruster with permanent magnets, under development at Alta since 2003. The new configurations include HT-100b with the same nominal power of HT-100, but with a modified magnetic circuit for achieving higher lifetime and HT-100c, scaled up to a nominal power of 175 W. The different models have been tested with the KR the results available until now seem to give interesting indications on magnetic field pattern and cathode coupling in low power Hall thrusters.