The CNEN Helium-Caesium Blow-Down MPD Facility and Experiments with a Prototype Duct

The CNEN blow-down loop has been designed to study a helium-caesium MPD generator with particular regard to non-equilibrium ionization effects. An operating condition of the loop is: gas mass flow 0.2 kg/sec, seed fraction 1 at, wt.%, useful pulse duration 20 sec, stagnation temperature 2000°K, stagnation pressure 5 atm abs, thermal power 1.6 MW, Mach number 0.6, magnetic field 4 Wb/m2, total impurity level less than 100 ppm. A sufficiently wide range of the stagnation conditions can be obtained with the present arrangement of the loop (temperature up to 2000*K, pressure from slightly sub-atmospheric to 6atmabs, gas mass flow from 50 g/sec to 400 g/sec, seed fraction from 0.1 to 2 at. wt.%. The storage heater is an alumina pebble bed electrically heated with tungsten elements and thermally insulated with zirconia fibre; the thermal capacity at 2000°K is about 1000 MJ. Pure helium is obtained by evaporation of liquid helium at between 4.5 and 5°K; liquid caesium is injected into a limited section of the pebble bed to provide a mixture of the two gases uniform in density and temperature. The duct is made of boron nitride (5 cm x 3 cm x 22 cm) with 25 pairs of tantalum electrodes whose geometry (electrode width 3 mm, segmentation pitch 9 mm) should prevent current leakage between adjacent electrodes; the duct walls and transfer can be pre-heated up to 1700°K. A magnetic field of 4 Wb/m2 is obtained with a pulsed cryogenic magnet with pulse duration of 6 sec. Two series of experiments have been completed to assess the feasibility of the helium-caesium heating system and the generator duct. Heating system experiments, (a) Compatibility of alumina with tungsten, tantalum and caesium, with thermal cycling at 2000°K; (b) Purification of zirconia fibre and its behaviour at high temperature, with thermal cycling at 2000°K; (c) Capability of an alumina pebble bed of evaporating, heating and mixing caesium with flowing helium at 2000°K, with blow-down tests. Generator duct experiments, (a) Wall duct heating system, with long duration tests at 1800°K using tantalum heating elements and boron nitride walls; (b) Feasibility of windows in the duct for spectroscopic and microwave diagnostics. A third series of experiments concerning the behaviour of the working channel as an MPD generator has been performed with the IRD closed-loop facility using a scale constant cross-section duct. Over 60 hours of high-temperature operation with helium, and 63 individual caesium-seeded runs (duration 30 sec to 2 min) were performed. The results, with temperatures from 1460°to 1740°K. magnetic field from 0.35 to 1.03 Wb/m2, seed fraction from 0.009 to 0.178 at % indicated the satisfactory operation of the channel in the following respects: Disassembly after the experiments showed the materials not appreciably changed from their initial condition (no significant transverse inter-electrode leakage was observed during the experiments); Complete reproducibility of the plasma conditions indicated excellent reliability of the electrical results; The electrical results obtained from the generated voltage-current characteristics of the plasma indicated thermal equilibrium ionization, as expected from the conditions of the experiments; The varying of the inner channel wall temperature by use of the wall heaters was reflected in the increased electrical conductivity of the plasma. This is a clear indication of the importance of wall temperature conditions in performing sensible MPD experiments. At present the loop is under the final assembly and the pebble bed storage heater has been successfully thermally cycled at 2000°K under vacuum. (author)