Civil and Industrial Engineering Department

ABSTRACT. The principal advantage of introducing some percentages of crumb rubber in production of asphalt mixtures is related to pavement environmental sustainability, since this process allows to recycle a significant amount of an industrial waste and, in some specific cases, to reduce tire/road noise emissions; there are some other not unanimously recognized advantages related to this process that are the improvement of asphalt mixture mechanical properties and durability as well as friction on pavement surface.

An experimental investigation of the operating characteristics of a heaterless LaB 6 hollow cathode was performed to examine low-current and low flow rate operation. The cathode has been continuously operated at Xe mass flow rates between 0.08 mg/s and 1 mg/s and discharge currents ranging from 0.5 to 3 A in diode mode for two different orifice diameters. The minimum power consumption was of about 25 W while the minimum mass flow rate required for spot-mode emission was approximately 0.08 mg/s Xe. The experimental data gathered during the test campaign were used to refine the theoretical model developed by Alta for the performance assessment of rare-earth thermionic hollow cathodes. * Research Engineer

A model to predict the plasma properties inside a thermionic hollow cathode as a function of operational conditions and geometry is presented. The hollow cathode features a lanthanum hexaboride (LaB6) insert, which is capable of emitting current densities as high as 10^5 Am^-2 at temperatures of about 1900 K, along with a tantalum orifice plate located at the downstream end of the cathode tube. The model self-consistently computes the plasma parameters in both the emitter and orice regions. A simple semi-empirical relation is suggested to evaluate the plasma penetration depth in the cathode interior, which is of primary importance to establish the plasma conditions. The heat transfer mechanisms and the related temperature gradients along the cathode are evaluated with the aid of a dedicated thermal model, which is coupled to the plasma model and accounts for temperature-dependent material properties. A parametric study of the cathode performance was conducted to assess the dependence of the power consumption and operational lifetime on discharge current and mass flow rate, as well as on the geometry. The results are in good agreement with both theoretical and experimental trends found in the literature as well as with experimental data collected by Alta. Further developments will include a deeper investigation into the cathode erosion phenomena, along with a broader comparison with empirical data.

Efficient,  long-life  and  low-power  Hall  thrusters  would  be  attractive  for  a wide  variety  of  missions  ranging  from  drag  compensation  in  LEO  and  VLEO  to accurate
final  orbit  insertion  and  spacecraft  end-of-life  disposal.  The  primary  challenges  for  Hall thrusters at small scale (<500 W) concern the thruster lifetime  and the  low thrust efficiency, due to the inherently higher surface-to-volume ratio. In the last few years, Sitael has devoted significant efforts in designing and testing low-power Hall thrusters providing thrust levels in the 4-40 mN range, specific impulses up to 1600s and estimated lifetimes in the order of
2000  hours.  Besides,  extensive  research  and development  activities  have  been  carried  out with  the  aim  of  designing  and  testing  low-current  hollow cathodes  that  can  be  effectively coupled with such thrusters.