A research group has demonstrated that spontaneously excited plasma waves may be the solution to a long-associated problem with magnetic nozzle plasma thrusters, turning conventional thinking on its head.
In magnetic nozzle radio frequency thrusters, sometimes referred to as helicon thrusters, magnetic nozzles channel and accelerate plasma to allow spacecraft to generate thrust. The technology, which harnesses electric propulsion, shows great potential for ushering in a new era of space travel. Yet the so-called "plasma detachment" problem has hampered further development.
Since magnetic field lines always form closed loops, the ones inside magnetic nozzles inevitably turn back to the thruster structure. For this reason, the plasma flow has to be detached from the magnetic nozzle. Ions, having a large gyro radius, detach easily from the magnetic nozzle. But electrons, with their small mass and small gyro radius, are tied to the field lines, generating an electric field that pulls the ions back and renders a net thrust of zero.
When plasma expands, it can gain or lose energy and momentum due to waves, turbulence or electromagnetic forces. Plasma transport and loss due to the wave and turbulence have been a major issue for confining plasma in thermonuclear fusion reactors.
Abstract - Plasma flows in divergent magnetic fields resembling a magnetic nozzle can be found over wide scales ranging from astrophysical objects to terrestrial plasma devices. Plasma detachment from a magnetic nozzle is a frequent occurrence in natural plasmas, e.g., plasma ejection from the Sun and release from the Sun’s magnetic field, forming the solar wind. Plasma detachment has also been a challenging problem relating to space propulsion devices utilizing a magnetic nozzle, especially the detachment of the magnetized electrons having a gyro-radius smaller than the system’s scale is required to maintain zero net current exhausted from the system. Here we experimentally demonstrate that a cross-field transport of the electrons toward the main nozzle axis, which contributes to neutralizing the ions detached from the nozzle, is induced by the spontaneously excited magnetosonic wave having the frequency considerably higher than the ion cyclotron frequency and close to the lower hybrid frequency, driving an E × B drift that only effects the electrons. Wave-induced transport and loss have been one of many important issues in plasma physics over the past several decades. Conversely, the presently observed electron inward transport has a beneficial effect on the detachment by reducing the divergence of the expanding plasma beam; this finding will open a new perspective for the role of waves and instabilities in plasmas.
A plasma thruster is a type of propulsion system that uses electric and magnetic fields to ionize and accelerate a gas, typically a noble gas like xenon, to produce thrust. The gas is heated to extremely high temperatures, creating a plasma state, and then ejected out of a nozzle to create thrust.
Plasma instability refers to the tendency of a plasma to become unstable and chaotic due to the interaction between the plasma particles and the surrounding magnetic fields. In a magnetic nozzle, the plasma particles can become trapped and collide with each other, leading to instabilities and fluctuations in the plasma flow.
Plasma instability can have a significant impact on the performance of a plasma thruster. It can cause fluctuations in the plasma flow, which can lead to variations in thrust and efficiency. It can also cause erosion and damage to the nozzle, reducing its lifespan.
To mitigate plasma instability in a magnetic nozzle, various design and operational measures can be taken. These include using a higher magnetic field strength, optimizing the shape and size of the nozzle, and adjusting the gas flow rate and temperature. Additionally, active control techniques can be used to stabilize the plasma and reduce instabilities.
Plasma thrusters with magnetic nozzles have a wide range of potential applications, including space propulsion for satellites and spacecraft, as well as in experimental and research settings. They are also being explored for potential use in future deep space exploration missions, as they are more efficient and have a higher specific impulse compared to traditional chemical thrusters.