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February 23, 2009

MIT Mini Helicon Plasma Space Propulsion



Mini-Helicon plasma thrusters are the first three parts of a Vasimr plasma propulsion unit. The Mini-Helicon is the first rocket to run on nitrogen, the most abundant gas in our atmosphere. Nitrogen is cheaper than Xenon and other gases used in other electric propulsion.

The new system, called the Mini-Helicon Plasma Thruster, is much smaller than other rockets of its kind and runs on gases that are much less expensive than conventional propellants. As a result, it could slash fuel consumption by 10 times that of conventional systems used for the same applications.

The Mini-Helicon has three general parts: a quartz tube wrapped by a coiled antenna, with magnets surrounding both. The gas of interest is pumped into the quartz tube, where radio frequency power transmitted to the gas from the antenna turns the gas into a plasma, or electrically charged gas.

The magnets not only help produce the plasma, but also confine, guide, and accelerate it through the system.

The exhaust velocity (ISP) from the new rocket is some 10 times higher than the velocity from the average chemical rocket.


European Space Agency feasibility study of helicon thruster

Helicon thrusters have been recently considered a possible new electric propulsion system thanks to the acceleration mechanism called “current free double layer “ which allows specific impulse up to 1300 s applying Argon and 4000 s applying hydrogen.


MIT presentation on fusion and plasma thrusters including the helicon thruster






Chemical rockets have about 400 ISP.




Summarizing:
* The system presents a thrust lower than Hall thruster systems (SPT-70 - 40 mN, SNECMA PPS 1350 - 85 mN), but it uses less power and the mass and dimensions are significantly reduced compared to Hall Thruster (their chamber diameter is around 50-100 mm or more), moreover hall thruster works greatly at higher power.

* The FEEPs have a specific impulse higher (10000s) than HPH.com system, but the thrust is limited to 0.1-0.01 mN

* PPTs (PPT-4, EOS-1) presents similar characteristic (with a smaller thrust) but the mass is higher (up to 9 kg)

As a result, the following conclusions were drawn:

* The helicon based plasma thruster propulsion system in plasma configuration presents advantages respects other propulsion system

* The system presents a compact volume and a light mass.

* The required electric power is very low (about 50W)



FURTHER READING
111 page theses paper on Helicon thruster power balance

Helicon Double Layer Thruster at wikipedia

A Helicon Double Layer thruster is a type of plasma thruster, which ejects high velocity ionized gas to provide thrust to a spacecraft. In this thruster design, gas is injected into a tubular chamber (the source tube) with one open end. Radio frequency AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped antenna wrapped around the chamber. The electromagnetic wave emitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a Helicon wave in the plasma, which further heats the plasma. The device has a roughly constant magnetic field in the source tube (supplied by Solenoids in the prototype), but the magnetic field diverges and rapidly decreases in magnitude away from the source region, and might be thought of as a kind of magnetic nozzle. In operation, there is a sharp boundary between the high density plasma inside the source region, and the low density plasma in the exhaust, which is associated with a sharp change in electrical potential. The plasma properties change rapidly across this boundary, which is known as a current free electric double layer. The electrical potential is much higher inside the source region than in the exhaust, and this serves both to confine most of the electrons, and to accelerate the ions away from the source region. Enough electrons escape the source region to ensure that the plasma in the exhaust is neutral overall.

The Helicon Double Layer Thruster has two main advantages over most other ion thruster designs; first, it creates an accelerating electric field without inserting unreliable components like high voltage grids into the plasma (the only plasma facing component is the robust plasma vessel). Secondly, a neutralizer isn't needed, since there are equal numbers of electrons and (singly-charged) positive ions emitted.


Australian National University looked Helicon thrusters for a Mars mission

The Helicon thruster has the edge on rival technologies as it is simpler and has been proven to work with many propellants including hydrogen, a waste product of human habitation.




the Australian Helicon double layer thruster

This double layer can be thought of as a thin standing shock wave across which there exists a strong electric field gradient. It is this electric field that accelerates ions from the source plasma to very high exhaust velocities creating thrust. Because the double layer is purely the result of plasma density, system and magnetic field geometry, no accelerating grids are required. Also, because there is equal flux of electrons and positive ions from the thruster there is no need for a neutraliser. It is in this sense that the HDLT is a "true" plasma thruster as it ejects equal numbers of both positive ions and negative electrons.

Power is required only for the maintenance of plasma and the creation of the magnetic field. In our current bench top prototype, 250W is sufficient to create several milli-Newtons of thrust. In space the solenoids that generate the 250 Gauss of magnetic field this requires we estimate could be cooled to 200K, reducing the resistance in the coils by a factor of 5 and representing a power consumption of a few 10s of Watts. Relative to other existing systems this constitutes quite a power saving and is well with-in the capabilities of solar panels. The 0.5sccm of feed gas represents a mass consumption of 160 mg/hr, so that a typical 5 hour burn would use 0.8g of propellant.



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