Fabrizio Paganucci

A NEW HYPERSONIC HIGH ENTHALPY WIND TUNNEL

A hypersonic high enthalpy, arc heated, blow-down wind tunnel has been recently developed at Centrospazio in order to perform experiments in the low to medium Reynolds number range (10-^10). The tunnel is capable of delivering Mach 6 flows up to 2.5 MJ/kg in specific total enthalpy on an effective test section 60 mm in diameter. It operates in pulsed quasi-steady mode, with running times ranging from 10 to 30 ms. The uniformity of the potential core of the flowfield has been experimentally assessed in different operational points of the facility, demonstrating a good steadiness and repeatability of the test conditions. Activities are presently ongoing aimed at defining the wind tunnel overall capabilities by investigating in detail its characteristics as well as at expanding its operational envelope.

Critical regimes and magnetohydrodynamic instabilities in a magneto-plasma-dynamic thruster

An extensive experimental investigation of magnetic and electrostatic fluctuations in a magneto-plasma-dynamic (MPD) thruster, with and without the application of an external magnetic field, has shown that gross magnetohydrodynamic instabilities develop whenever the current rises beyond a threshold value. These instabilities are helical kink modes with azimuthal m and axial n periodicity m/n=1/1 and their occurrence can be described by the Kruskal–Shafranov stability criterion. The presence of these modes is found to be the cause of the loss of efficiency observed in MPD thrusters at high current.

A Reduced-Order Model for Thermionic Hollow Cathodes

A reduced-order numerical model describing the plasma in an orificed hollow cathode is presented as a quick tool for the design of thermionic cathodes. A time-independent, volume-averaged model is developed to determine plasma properties, wall temperatures, and cathode lifetime without requiring experimental data as input. A system of particle and energy balance equations is numerically solved without invoking a Saha-type equilibrium under the hypothesis of a direct-impact ionization process. Further, a lumped-parameter thermal model is coupled with the plasma model to estimate the temperature profile along the cathode axis and the emitter lifetime. The obtained results capture most of the characteristic features of this class of hollow cathodes as compared with the available experimental data. In addition, the model gives insight into the most important power deposition processes affecting the emitter and orifice regions. The effect of the geometry on both plasma parameters and cathode performance is discussed to suggest design guidelines for the development of state-of-the-art hollow cathodes.

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.

Compact soft x-ray multichord camera: Design and initial operation

A compact and low cost diagnostic for spatially resolved measurements of soft x-ray or total radiation emission has been designed and realized to be flexibly applied to different plasma physics experiments. Its reduced size (outer diameter=35 mm) makes it suited to a variety of devices. The line integrated emissivity (brightness) has been measured along up to 20 lines of sight, using an array of miniaturized silicon photodiodes. Preliminary prototypes of the diagnostic have been installed in the Madison Symmetric Torus reversed field pinch (RFP) device at University of Wisconsin and in the EXTRAP T2 RFP device at the Royal Institute of Technology in Stockholm. Application of the diagnostic to a gas-fed (argon, helium) magnetoplasma dynamic thruster (MPDT) with an external magnetic field will also be discussed.

Performance of Centrospazio Hypersonic High Enthalpy Wind Tunnel

A new High Enthalpy Arc-heated Tunnel (HEAT) for aerothermodynamic research on hypersonic air flows has been operating at Centrospazio since 1996. Numerical simulation of the flow-field and classical semi-empirical correlations for electric discharges in high pressure gases have been used to extrapolate the available experimental data and predict the performance envelope of the facility in terms of total pressure and specific enthalpy in the test section as functions of the operational constraints imposed by the electric power supply and gas feeding systems. The results show that, with just minor modifications of the experimental apparatus, the Reynolds number per unit length can be increased by an order of magnitude above the value attained in the present configuration. Performance maps of the facility are shown and a comparison with other European hypersonic facilities is provided. C 0 ,k ,m arc characteristic coefficients I arc current intensity L characteristic length M Mach number p arc region static pressure R B ballast resistance R gas constant Re Reynolds number s electrodes distance T static temperature v flow velocity W thermal power input V arc voltage V 0 power supply voltage γ gas specific heat ratio η energy exchange overall efficiency µ viscosity coefficient ρ density ζ binary scaling factor

MHD instabilities in magneto-plasma-dynamic thrusters

Magneto-plasma-dynamic thrusters (MPDTs) act as electromagnetic plasma accelerators and represent a high power, electric propulsion option for primary space missions. One of the major problems facing MPDT operation is the onset of a critical regime, which is found when the power is increased beyond a threshold value, depending mainly on the thruster geometry, the type and mass flow rate of the propellant and the intensity of the magnetic field applied. In this regime, large fluctuations in the electrode voltage signals, damage to the anode and decreased efficiency are observed. Since 2000, several test campaigns have been carried out to investigate the electrostatic and magnetic properties of plasma fluctuations using electromagnetic and optic probes and ultraviolet tomography. Results obtained have shown a strong relation between the onset phenomena and the growth of a large scale magnetohydrodynamic (MHD) instability, with helical kink mode features. On the basis of the experimental observations, a passive method to suppress instability is proposed and has been partially tested, with encouraging results.

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.

Ultraviolet tomography of kink dynamics in a magnetoplasmadynamic thruster

In this paper the results of a project concerning the ultraviolet (UV) imaging of a plasma for space applications, produced in a magneto-plasmadynamic (MPD) thruster, are presented. MPD are a class of high-power electric space propulsion devices that accelerate a plasma to high velocities (>10km∕s), by exploiting the Lorentz force. This force arises from the interaction between the discharge electrical current and a self induced and externally applied magnetic field. The imaging system has been realized by inserting three arrays of UV-enhanced photodiodes directly into the plastic structure of the anode. The amplifiers are miniaturized and built into the detector. This advanced diagnostic design allows for a detailed tomographic reconstruction of the emissivity spatial structure, both in the axial direction z (corresponding to a wave number n) and azimuthal direction (wave number m) with high time resolution. A magneto-hydrodynamic instability, with mode numbers m=1 and n=1 has been observed, which might ...

Experimental characterization for hypersonic testing

The preliminary experimental characterization of the flow field developing in the test section of a small high enthalpy arc-heated Mach 6 tunnel has been carried out in terms of Mach number and local flow velocity angle. Measurements have mainly been obtained in cold flow conditions (no arc), though a limited number of tests has also been performed in hot flow conditions (Ttot=1800 K). To this purpose Pitot and yawmeter probes have been employed: in particular a theoretical model for using uncalibrated conical yawmeters in hypersonic flow has been developed. Total temperature measurements could not be obtained with the necessary accuracy due to problems of response time and survivability of thermocouples. The test campaign is presently underway and will include total temperature measurements performed by means of a fast-response total temperature probe based on transient thin-film heat flux gauge technology.