levels with a few cm diameter(compared to about 0. 1 N for a 30 cm ion engine, or for a 10 cm Hall thruster). Applied field mPd thrusters Here currents are less strong so the main part of the b field is external Still steady or quasi-steady Pulsed Plasma Thrusters(PPT) Pulsed Plasma Thrusters(PPt)are very similar in principle to self-field MPD, but they use a solid propellant (Teflon)which is ablated during each ise of operation. These pulses last 10-20 us only but are just long aB enough that induced emf fields(from at=VXE)are still weak Because of various practical (mostly thermal)issues, PPt thrusters are not very efficient <10%, but they are simple and robust. Pulsed Inductive Thrusters (PIT) Here the emphasis is on very fast magnetic risetime(1-10 us )and the induced emf is used to break down the gas, ionize it and drive a closed current loop that exerts the desired magnetic force. They can be thought of as a one-turn transformer in which the secondary is a plasma ring; the repulsion between primary and secondary accelerates the plasma away and pushes the primary coil forward. To avoid dissipating most of the power in Ohmic losses, the device must be fairly large>0.5m and powerful (MW to GW of instantaneous power) In the following few lectures we will have time only to explore the self-field MPD type. We begin with some basic Physics Electromagnetic Forces on Plasmas- MPD Thrusters For a charge g, moving at velocity v in an electric field e and magnetic field B, the So-called Lorentz force is F=QE+VX B (1) Now, F cannot depend on the rectilinear motion of the observer For non-relativistic velocities, B is also independent of motion, and so is the scalar q. therefore, the field E must be the different as viewed from different frames of reference let e be the field in the laboratory frame and ethat in another frame moving at u relative to the Then we must have 16.522, Space Propulsion Lecture 21 Prof. Manuel martinez-s Page 2 of 2116.522, Space Propulsion Lecture 21 Prof. Manuel Martinez-Sanchez Page 2 of 21 levels with a few cm. diameter (compared to about 0.1 N for a 30 cm ion engine, or for a 10 cm Hall thruster). - Applied field MPD thrusters Here currents are less strong, so the main part of the B field is external. Still steady or quasi-steady. - Pulsed Plasma Thrusters (PPT) Pulsed Plasma Thrusters (PPT) are very similar in principle to self-field MPD, but they use a solid propellant (Teflon) which is ablated during each pulse of operation. These pulses last ∼ 10-20 sµ only, but are just long enough that induced emf fields (from B = ×E t ∂ ∇ ∂ JG G ) are still weak. Because of various practical (mostly thermal) issues, PPT thrusters are not very efficient <10% ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ∼ , but they are simple and robust. - Pulsed Inductive Thrusters (PIT) Here the emphasis is on very fast magnetic risetime ( ∼ 1 - 10 sµ ) and the induced emf is used to break down the gas, ionize it, and drive a closed current loop that exerts the desired magnetic force. They can be thought of as a one-turn transformer in which the secondary is a plasma ring; the repulsion between primary and secondary accelerates the plasma away and pushes the primary coil forward. To avoid dissipating most of the power in Ohmic losses, the device must be fairly large >0.5m ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ∼ and powerful (MW to GW of instantaneous power). In the following few lectures we will have time only to explore the self-field MPD type. We begin with some basic Physics. Electromagnetic Forces on Plasmas - MPD Thrusters For a charge q, moving at velocity v JG in an electric field E G and magnetic field B, JG the so-called Lorentz force is F = q E + v x B ( ) G GJG JG (1) Now, F G cannot depend on the rectilinear motion of the observer. For non-relativistic velocities, B JG is also independent of motion, and so is the scalar q. Therefore, the field E G must be the different as viewed from different frames of reference. Let E G be the field in the laboratory frame, and E' JJG that in another frame moving at u G relative to the first. Then we must have