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# PowerPedia:Pulsed Plasma Thruster

Pulsed plasma thrusters are a method of spacecraft propulsion which use an arc of electric current adjacent to a solid propellant (almost always teflon), to produce a quick and repeatable burst of impulse.

## Description

PPTs are great for altitude control, and for main propulsion on particularly small spacecraft with a surplus of electricity (those in the hundred-kilogram or less category). However they are also one of the least efficient electric propulsion systems, with a thrust efficiency of less than 10%. Pulsed plasma thrusters were the first electric propulsion system to be deployed in space, on the Soviet probes Zond-2 in 1964 and Zond-3 in 1965. Used as an experimental system for spacecraft orientation control, Soviet engineers subsequently returned to the use of high-pressure nitrogen jets. Pulsed plasma thrusters were flown in November, 2000 as a flight experiment on the NASA Goddard Space Flight Center EO-1 spacecraft. The thrusters successfully demonstrated the ability to perform roll control on the spacecraft and also demostrated that the electromagnetic interference from the pulsed plasma did not affect other spacecraft systems.

Electrostatic plasma propulsion engine built in USSR in 1962-1965 to propel spacecrafts to Solar system planets. Have much higher exhaust velocity than chemical propulsion engines. According to Tsiolkovsky equation this results in proportionally higher final velocity of propelled craft. First plasma propulsion engine was successfully used to propel Zond-2 vehicle from parking at Earth orbit to Mars in Nov 30, 1964. The principle of operation is acceleration of ions in strong electric field to the velocities of the order of hundreds km/s - which is much higher than thermal velocity of chemical engines. The active gas used in russian plasma propulsion engines is argon and helium. Chemical propulsion engines with their limited by rate of chemical reaction exaust velocity (which is in the range of 2-3 km/s) become exponentially ineffective (see Tsiolkovsky equation) to achieve high interplanetary speeds (which in Solar system is in 20-70 km/s range).

## Plasma acceleration

Plasma acceleration is a technique for accelerating charged particles, such as electrons and ions, using an electric field associated with an electron plasma wave. When a laser pulse or an electron bunch goes through a plasma, an electron plasma wave arises in its trace, like a wake following a ship on the ocean. Since an electron plasma wave is a longitudinal wave, high-density and low-density regions of electrons appear periodically in which the quasineutrality of the plasma is broken. An electric field originating from the breakdown of the neutrality, which is referred to as a "wakefield," accelerates charged particles inside the plasma in the direction parallel to the propagation direction of the electron plasma wave.

Plasma acceleration is categorized into several types according to how the electron plasma wave is formed: plasma wakefield acceleration (PWFA), laser wakefield acceleration (LWFA), laser beat-wave acceleration (LBWA), and self-modulated laser wakefield acceleration (SMLWFA). In PWFA, an electron plasma wave is formed by an electron bunch. In LWFA, a laser pulse is introduced to form an electron plasma wave. In LBWA, an electron plasma wave arises based on different frequency generation of two laser pulses. And in SMLWFA, the formation of an electron plasma wave is achieved by a laser pulse modulated by stimulated Raman forward scattering instability. The concept of plasma acceleration was first proposed by Toshiki Tajima and John Dawson in a theoretical article published in 1979. The first experimental demonstration of wakefield acceleration, which was performed with PWFA, was reported by a research group at Argonne National Laboratory in 1988.

The advantage of plasma acceleration is that its acceleration field is much stronger than that of conventional radio-frequency (RF) accelerators. In RF accelerators, the acceleration field has an upper limit determined by the threshold for dielectric breakdown of the acceleration tube. Therefore, to obtain high-energy electrons or ions, a long acceleration length is inevitably required, resulting in huge sizes for accelerator facilities. On the other hand, plasma acceleration, in which the acceleration field is generated in plasma, achieves an acceleration field several orders of magnitude stronger than with RF accelerators. It is hoped that a compact particle accelerator can be created based on plasma acceleration techniques or accelerators for much higher energy can be built, if long accelerators are realizable with 10TeV/m.

According to icfa-usa.jlab.org the acceleration gradient for a linear plasma wave is:

$E = C \cdot \sqrt{\frac{m_e \cdot \rho}{\epsilon_0}}$

In this equation, E is the electric field, C is the speed of light in vacuum, me is the mass of the electron, ρ is the plasma density, and ε0 is the permittivity of free space.

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