Abstract.A hanging thrust balance has been designed, manufactured and tested at the University of Southampton. The current design allows for direct steady thrust measurements ranging from 0.1 mN to 3 mN but this can be easily extended to measure thrust in a different range. Moreover the chosen balance design and the thrust measurement procedure allow for the cancellation of thermal drifts. The thrust balance was tested with a T6 hollow cathode thruster providing measurements with an uncertainty of about 9.7%. The thrust data were compared to those obtained with another direct thrust balance, and they are in quantitative agreement being the maximum difference only 6%.

Hollow cathodes have been used as electron sources for gridded ion engines and HallEffect thrusters, but in recent years the possibility of using them as standalone thrusters has been extensively investigated at the University of Southampton. Recently an ESA ITT study, involving the University of Southampton, QinetiQ and Mars Space Ltd, kicked off aiming to investigate the thrust production mechanisms of a hollow cathode thruster in order to design an optimized HCT. A one dimensional model has been developed in order to predict the behavior of the plasma in the HCT orifice; it has been compared to experimental data collected from a direct thrust balance and to experimental measurements on the NSTAR cathode found in the literature. Preliminary results shown in this paper seem encouraging: the thrust values predicted by the model are in good agreement with the values measured for a T5 HCT at different current levels performed with a direct thrust balance at Aerospazio Tecnologie. A = Cross sectional area A or = Orifice cross sectional area B = Magnetic field c p = Specific heat at constant pressure d/dx = Derivative in coordinate x E = Electric field F = Thrust F H gas = Heavy particles gasdynamic thrust F e gas = Electrons gasdynamic thrust F em = Electromagnetic thrust H = Energy h 0 = Total enthalpy h 0H = Heavy particles total enthalpy h 0e = Electrons total enthalpy I sp = Specific impulse I D = Discharge current j = Total current density k = Boltzmann's constant M = Mach number M H = Heavy particles Mach number m = Heavy particles mass m e = Electrons mass ṁ = Mass flow rate n = Plasma density n i = Ions density n e = Electrons density n n = Neutrals density ṅ = Ionization rate n in = Plasma density at the orifice entrance n n,in = Neutrals density at the orifice entrance p i = Ions pressure p e = Electrons pressure p n = Neutrals pressure P H = Power transferred to heavy particles P D = Discharge power Q αβ = Energy exchanged per unit time q = Electric charge R a = Anode radius R c = Cathode radius R αβ = Mean change in momentum T e = Electrons temperature T e,in = Electrons temperature at the orifice entrance T H = Heavy particles temperature T H,in = Heavy particles temperature at the orifice entrance T 0H = Heavy particles total temperature T 0H,in = Heavy particles total temperature at the orifice entrance T W = Wall temperature u = Velocity u i = Ions velocity u e = Electrons velocity u n = Neutrals velocity u H = Heavy particles velocity V or = Voltage drop in the orifice ∆V = Change in velocity W = Mass source term W H = Heavy particles mass source term X = Momentum source term X H = Heavy particles momentum source term X e = Electrons momentum source term Y H = Heavy particles energy source term Y e = Electrons energy source term Y i = Ions energy source term Y n = Neutrals energy source term α = Ionization fraction γ = Ratio of specific heats δ = Factor γ/(γ-1) ε = Ionization potential η = Resistivity ζ = Power efficiency µ 0 = Vacuum permeability υ αβ = Collision frequency <σ i u e > = Ionization r...

Hollow Cathode Thrusters are small electric propulsion devices developed starting form hollow cathodes, used already in the '60s as sources of electrons for GIEs and HETs. A T6 HCT has been tested with a direct thrust balance with and without an external magnetic field in the orifice region. The discharge voltage, thrust and Isp increase with the application of the magnetic field while the thrust efficiency decreases. NOMENCLATURE E = Specific Energy ṁ = Mass Flow Rate HC = Hollow Cathode P m = Power needed by the coil HCT = Hollow Cathode Thruster T = Thrust I D = Discharge Current UoS = University of Southampton Isp = Specific Impulse V = Discharge Voltage MPD = Magneto Plasma Dynamic η = Thrust Efficiency

A new direct thrust balance has been designed, manufactured, assembled and tested at the University of Southampton. The balance design allows for reliable direct thrust measurements in steady state conditions on light, small, low power thrusters. The measurements procedure allows for the cancellation of the thermal drifts. Measurements have been carried out in a range 0.1 mN to 3 mN on a T6 hollow cathode thruster with an uncertainty of 9.7%. The same device was tested at Aerospazio Tecnologie with their thrust balance: the comparison is in good agreement being only 6% the maximum difference