Velikhov Electrothermal Instability Cancellation by Modification of Electrical Conductivity Value in a Streamer by Magnetic Confinement

Acta Polytechnica April 2013 Volume 53 num 2 page 219-222

J.P. Petit and J.C. Dore

When a two-temperature, cold plasma, is submitted to a transverse magnetic field, the electrothermal instability takes place very rapidly, if the local value of the Hall parameter is greater than a critical value, which depends on the local plasma parameter’s values. The characteristic growth time of this instability is close to the ionization time. The plasma becomes very inhomogeneous, which increases the global electric resistance of the streamer. Pictures show the characteristic light streaks. The cancellation method is inspired by the one presented by the author in 1983 at the international MHD meeting of Moscow. The spatial distribution of the steady state magnetic field is designed in order to reduce its value where a streamer, linking electrode and cathode, is supposed to take place. Thus, the corresponding local value of the Hall parameter is weakened, which rises the local value of the electrical conductivity. We get a favourable non-linear positive feedback. To a strongly non-linear well-known phenomenon we give a strong non-linear remedy. The subsequent rise of the electrical conductivity enhances the electrical current density and, subsequently, the electron temperature. The local rise of the degree of ionization and the ion density makes the plasma to shift it into locally local coulomb dominated regime. This produces a collapse of the Hall parameter, under the critical value, corresponding to a fully ionized plasma regime, which is close to 2.

As a result the Velikhov instability cannot take place, which is successfully confirmed by the perfect homogeneity of the plasma in the streamer, as shown in the pictures. By the way, the weakened B-field lane behaves somewhat like a rail, giving a possibility of electric current guidance.

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Wall Confinement Technique by Magnetic Gradient Inversion

Acta Physica Polonica A March 2012 Volume 121 num 3 page 611

J.P. Petit and J.C. Dore

When a plasma is subjected to a transversal magnetic field and its Hall parameter is non-negligible, it weakens the local electric conductivity value. If an electric discharge is created near a wall and the magnetic field decreases with distance, the electric discharge will follow a path that minimises the streamer’s electric resistance, which could expel it far from the wall. One solution to ensure that it remains up against the wall is by inversion of the magnetic field’s gradient by arranging that field B be minimal at the wall. In the experiment we are presenting, effected in a low-density gas, in order to obtain a high value for the Hall parameter using simple permanent magnets, we will show the remarkable efficiency of this parietal confinement method and present the main lines of the programme of which this experiment forms part and whose successful realization will be the demonstration of the feasibility of the displacement of disk-shaped MHD aerodynes at supersonic speed without creating either shock waves or turbulence, an approach that we have already set out in numerous publications.

PACS numbers: 47.65.-d, 52.30.Cv, 52.55.-s, 52.80.Pi, 52.75.Di

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Wall Confinement Technique by Magnetic Gradient Inversion. Accelerators Combining Induction Effect and Pulsed Ionization. Applications

Acta Physica Polonica A June 2009 Volume 115 (6):  page 1162-1163

J.P. Petit J. Geffray

We published the first description of a disk-shaped MHD aerodyne, combining pulsed ionization and induction systems. Magnetic pressure tends to push away electric discharge, in region of low magnetic field. In the seventies the efficiency of the wall confinement, due to the inversion of the magnetic pressure gradient was demonstrated. It is time now for ambitious MHD projects. We have planned experimental studies of MHD acceleration, combining pulsed ionization, wall confinement and induction system. We intend to study pulsed ionization systems based on wall micro discharges or microwaves action. We present the different features of the project.

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Non-Equilibrium Plasma Instabilities

Acta Physica Polonica A June 2009 Volume 115 (6):  page1170-1172

J.P. Petit J. Geffray

Low magnetic Reynolds non-thermal plasmas are sensitive to the Velikhov electrothermal instability when the Hall parameter exceeds the local critical value. Then the instability quickly grows and highly reduces the performance of MHD converters. After operating (International MHD Meeting, Warsaw 1968) the first high power density two-temperature – but short duration – MHD Faraday generator, using a first stabilization method, we presented a better one in the beginning of the eighties that was demonstrated through low pressure experiments (French Academy of Sciences, 1981; and 8th International MHD meeting, Moscow 1983). This second method was based on a non-uniform B-field and electrical tensor conductivity. It is time now to rebuild MHD activities all over the world as deserved, corresponding to progress of today’s techniques. Different applications of this instability cancellation are presented.

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MHD flow-control for hypersonic flight

Acta Physica Polonica A June 2009 Volume 115 (6):  page1149–1513

J.P. Petit J. Geffray

At the beginning of the seventies, experimental research using short duration supersonic shock driven wind tunnel were conducted and showed the efficiency of Lorentz force action on supersonic (Mach 1.4) dense (1 bar) low magnetic Reynolds number hot (10 000 K) argon flows. When working as a generator, the linear Faraday MHD converter was efficient enough, due to high electrical conductivity (3000 S/m) to create a front shock wave at the intake of the constant cross-section MHD converter. Accelerations up to 5000 m/s were obtained in 10 cm long channels. This suggested the possibility to achieve complete shock wave and turbulent wake cancellation, through MHD bypass concept. Such work gave publications in peer-reviewed journals and presentation in international MHD meetings (French Academy of Sciences 1970, Moscow 1983, Tsukuba 1986, Beijing 1992). The work was conducted both through computational simulations (based on the method of characteristics) and hydraulic simulation experiments. We present the synthesis of such studies through a project of a hypersonic vehicle based on an MHD bypass concept, landing under its own steam, using classical turbojets. Then, at Mach 3 and high altitude, MHD controlled inlets are opened. A wall converter slows down the hypersonic incoming air flow, without excessive heating, feeding a ramjet system. The subsequent electric power provides an additional impulse to the flow in the exhaust section. Cruise Mach number: 12. Local Hall parameter regime produces high voltage that, sent to the leading edges, creates a plasma cushion which prevents too high thermal flux. Lift is provided by wave riding technique. Additional rocket propeller could transform it as a reusable space launcher.

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