Some Examples of Small Plasma Devices
In this chapter, the principle of operation, experimental setup as well as some results obtained from research work carried out by the authors and collaborators based on several small plasma devices will be discussed briefly.
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Some Examples of Small Plasma Devices
Abstract In this chapter, the principle of operation, experimental setup as well as some results obtained from research work carried out by the authors and collaborators based on several small plasma devices will be discussed briefly. Keywords Cost effective plasma devices
4.1
The Electromagnetic Shock Tube [1]
The schematic diagram of an electromagnetic shock tube is shown in Fig. 4.1. The electrodes are of a coaxial configuration. The electrodes are insulated at the back-wall by a quartz cylinder. Discharge is initiated along the surface of the insulator as a surface discharge, forming a current sheet. This current sheet will be pushed away from the insulator surface by the J ∧ B force until it finally becomes perpendicular to the electrodes and ready to move in the axial direction. The initial phase of current initiation along the surface of the quartz insulator and lift-off is called the breakdown phase. During the breakdown phase, the gas is mainly heated by the joule heating effect and this is the pre-ionization phase where a sufficiently ionized plasma will be formed prior to the subsequent axial acceleration phase. This is essential since the effective shock heating of the plasma requires it to be sufficiently ionized. During the axial acceleration phase, the current sheet is driven by the electromagnetic force which is acting as the axial electromagnetic piston. The azimuthal magnetic field Bθ is generated by the coaxial discharge current itself and is given by Bh ¼
lI ; 2pr
which is a function of time and radial position r. This magnetic field is only present behind the current sheet. The magnetic field ahead of the current sheet is zero. It can be seen that its magnitude will be stronger at the surface of the inner electrode as compared to the inner surface of the outer electrode. This gives rise to the slanting © The Author(s) 2016 C.S. Wong and R. Mongkolnavin, Elements of Plasma Technology, SpringerBriefs in Applied Sciences and Technology, DOI 10.1007/978-981-10-0117-8_4
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4 Some Examples of Small Plasma Devices
Fig. 4.1 Schematic diagram of an electromagnetic shock tube
structure of the electromagnetic piston as shown in Fig. 4.1. The J ∧ B force in the direction downstream of the tube (in the z-direction) can be expressed as Zb
B2h 2prdr; 2l
a
where a is the radius of the inner electrode and b is the radius of the outer electrode. This force will drive the electromagnetic piston to supersonic speed so that a shock heated layer of plasma will be formed. In this way, with discharge current in the region of 100 kA, piston speed of up to more than 10 × 104 m/s may be achieved, which is sufficient to produce a fully ionized hydrogen plasma. There is another component of the J ∧ B force which is acting in the radial direction but no motion is possible due to the present of the solid inner electrode.
4.1.1
Numerical Modeling of the Electromagnetic Shock Tube Dynamics
The dynamics of the electromagnetic piston can be modeled by simplifying the geometry o
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