Low-pressure, high-density, and supersonic plasma flow generated by a helicon magnetoplasmadynamic thruster

Abstract

A high density magnetoplasmadynamic (MPD) plasma under a magnetic nozzle is produced with a low gas flow rate of argon by combining helicon and MPD plasma sources, where a cathode and an anode are located upstream and downstream of the helicon source, respectively. Once the high density helicon plasma is produced in the source tube, a pulsed current of a few kA is triggered between the cathode and anode. A plasma density above 10 20 m À3 and a supersonic plasma flow (Mach number of $1.8) are obtained at $10 cm downstream of the source exit. As the thrust efficiency estimated from the measured plasma parameters is much higher than that of the simple MPD thruster, the helicon MPD thruster being proposed and tested potentially provides more efficient high-power plasma thruster. V C 2014 AIP Publishing LLC.Plasma flow expansion in a magnetic nozzle and subsequent plasma acceleration include many aspects of physics relating to space plasma, 1 astrophysics, 2 plasma processing devices, 3 and electric propulsion devices. Some of the representative electric propulsion devices utilizing the magnetic nozzle are a helicon thruster (HPT), 4 a variable specific impulse magnetoplasma rocket (VASIMR), 5 and an appliedfield magnetoplasmadynamic thruster (MPDT). 6 HPT is typically operated with gas flow rate less than a few tens of mg/s and radiofrequency (rf) power less than a few kW; 7 then it can yield the thrust less than $20 mN at this moment, 8 where a number of experiments have shown electrostatic ion acceleration by a spontaneous formation of a potential drop. 9 Some of the results have also been discussed in the context of a solar coronal funnel. 10 Theoretical studies have shown that an additional thrust can be obtained by the magnetic nozzle. 11,12 The simple fluid model predicted that the thrust arising from the magnetic nozzle can be increased by increasing the plasma density in the nozzle. 13 This fact has also been experimentally confirmed by inhibiting the plasma cross field diffusion. 14 VASIMR has an additional ion cyclotron resonance heating section and the increased ion energy perpendicular to the magnetic field is converted into the axial energy by the magnetic nozzle. Above 200 kW rf power has been applied to the thruster head; then the thrust of $6 N has been obtained. 15 MPDT is often operated at the discharge power above several hundreds of kW. 6,16,17 The electric current is sustained by the arc discharge between the central cathode and the peripheral anode; hence, much flow rate (typically a few hundreds of mg/s) of the propellant gas has to be introduced into the thruster head to trigger and maintain the arc discharge. The thrust above several N has been obtained by the MPDT, where plasma acceleration mechanisms both by self-induced and applied magnetic fields have been suggested. 18 The plasma source similar to the MPDT has also been used to simulate the space phenomena such as solar flares. 19The above-mentioned thrusters are operated in a weakly ionized condition for HPT and with much gas flow rate ...

Helicon high-density plasma sources: physics and applications

“…With the same concept, the Helicon Hall thruster (HHT), an addition of a helicon source to a Hall thruster, is developed as a two-stage thruster to investigate an rf ionization stage (HP), which can improve the overall efficiency of a Hall thruster [214]. By combining helicon and MPD plasma sources, n e above 10 14 cm −3 with supersonic plasma flow (Mach number, which is the ion velocity normalized by the ion sound velocity, of ~1.8) was obtained, showing a higher thrust efficiency compared to the simple MPD thruster [215].…”

Measurement of plasma momentum exerted on target by a small helicon plasma thruster and comparison with direct thrust measurement

“…19 More recently, a new concept of magnetic nozzle plasma thruster called helicon MPD thruster is proposed and operated in pulsed mode with attaching to a small diffusion chamber, where a pulsed discharge current of a few kA is triggered between the electrodes when igniting the high density helicon plasma. 20 Although the measurement of the thrust imparted by such pulsed helicon and helicon MPD thrusters gives important insight into the thruster development and the relevant magnetic nozzle plasma physics, it is difficult to perform the measurement by the thrust balance because the displacement of the balance occurs due to the pulsed magnetic field and/or discharge current. Although one of the solutions would be the target technique, the validity of the target technique and the measurement of the impulse bit have to be verified at the beginning of such experiments.…”