Plasma propulsion could become effective and useful for higher altitude aviation, airships and eventually space access
Berkant Giksel at the Technical University of Berlin and his team now want to fit plasma engines to planes. "We want to develop a system that can operate above an altitude of 30 kilometers where standard jet engines cannot go," he says. These could even take passengers to the edge of the atmosphere and beyond.
The challenge was to develop an air-breathing plasma propulsion engine that could be used for take-off as well as high-altitude flying.
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Published under licence by IOP Publishing Ltd. 14th High-Tech Plasma Processes Conference (HTPP 14) IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 825 (2017) 012005 doi:10.1088/1742-6596/825/1/012005
A new breakthrough in jet propulsion technology since the invention of the jet engine is achieved. The first critical tests for future air-breathing magneto-plasma propulsion systems have been successfully completed. In this regard, it is also the first time that a pinching dense plasma focus discharge could be ignited at one atmosphere and driven in pulse mode using very fast, nanosecond electrostatic excitations to induce self-organized plasma channels for ignition of the propulsive main discharge. Depending on the capacitor voltage (200-600 V) the energy input at one atmosphere varies from 52-320 Joules per pulse corresponding to impulse bits from 1.2-8.0 mNs. Such a new pulsed plasma propulsion system driven with one thousand pulses per second would already have thrust-to-area ratios (50-150 kN/m^2) of modern jet engines. An array of thrusters could enable future aircrafts and airships to start from ground and reach altitudes up to 50 km and beyond. The needed high power could be provided by future compact plasma fusion reactors already in development by aerospace companies. The magneto-plasma compressor itself was originally developed by Russian scientists as plasma fusion device and was later miniaturized for supersonic flow control applications. So the first breakthrough is based on a spin-off plasma fusion technology.
The team used a rapid stream of nanosecond-long electric discharges to fire up the propulsion mixture. A similar technique is used in pulse detonation combustion engines, making them more efficient than standard fuel-powered engines.
* Plasma propulsion was proved on an airship in 2005
* this is a pathway to far more powerful and effective plasma propulsion
* this could enable propulsion beyond the height of existing planes
* nearterm it could be for routine airships at 50-100 km
* it could eventually be used to get to orbit and transform aviation and space access
It's the first time anyone has applied pulse detonation to plasma thrusters. Jason Cassibry at the University of Alabama in Huntsville is impressed. "It could greatly extend the range of any aircraft and lower the operational cost," he says.
But there are several hurdles to overcome before the technology can propel an actual plane. For a start, the team tested mini thrusters 80 millimetres long, and a commercial airliner would need some 10,000 of them to fly, which makes the current design too complex for aircraft of that size. Giksel's team plans to target smaller planes and airships for now. Between 100 and 1000 thrusters would be enough for a small plane, which the team thinks is feasible.
In future experiments the plasma dynamics could be investigated using ultrafast cameras with up to 2 Mio frames per seconds which are available to the corresponding authors. In the present work the maximum voltage and power limits of the new MPC thruster were not tested. The main task was the first demonstration of a pulsed MPC-based plasma thruster with ns-internal excitation for a stabile operation at high atmospheric pressures up to 1 bar. In this regard, a first breakthrough and pulse operation with 4.7 Hz was demonstrated. In the next step, the pulse frequency of the main discharge will be increased up to 10 Hz. Furthermore, a new mobile power generator will be developed for the first flight demonstration onboard of the b-Ionic Airfish, which was the world's first airship propelled by plasma engines in 2005, Only 0.08 N or 8 g would be sufficient to propel this 7.5 m airship at low speeds up to 1 m/s. A 5 Hz thruster has
already about 0.02 N. So an array of four plasma pulse "detonation" thrusters with the present power level would make it fly. The available maximum weight for the power generator is about 5.1 kg plus 1.2 kg for LiPo batteries.
The general thrust of an array with 10 cells, each operating with a pulse frequency of 50 Hz, is 2.0 N. The total power required for a first high altitude (H=20 km) demonstrator mission using an array with 10 thruster is about 75 kW. With a solar battery effectiveness of 0.2-0.3, the required minimum surface area is 250 m^2.
In any case, the new propulsion technology is still away from being competitive but it has to be noted that the research, development and optimization is now at the very beginning. The impulse bit for each thruster unit can be essentially increased by using different ejector schemes and jet focusing nozzle structures. These are items of next investigations.
Furthermore, there are also a large amount of other possible technological applications in the field of aerodynamics, material sciences and power engineering. But a real flight demonstration is the next milestone goal towards new magneto-plasma flux compression thrusters for stratospheric airships or high altitude platform stations (HAPS) which are currently all limited to about 25 km altitude by using propellers. With future air-breathing magneto-plasma flux compression thrusters next generation solar, beamed or fusion energy powered airships could climb to altitudes up to 50 km and beyond. 
