Examining Ion Ejection from a Spacecraft Thruster Using Impedans Semion RFEA

Hall thrusters and electrostatic ion thrusters are two types of spacecraft thrusters that are often used. These two techniques have been demonstrated to be more beneficial than conventional MHD or chemical thrusters that were used in the earlier days of space exploration.

In the electrostatic technique, the ion thrusters produce plasma whereby voltage is applied between an anode grid and a thermionic cathode. The ions traveling through that grid are accelerated to a high velocity by a second grid. This creates an ion beam that propels the spacecraft forward through the principle of momentum conservation.

Xenon is a chemical element that is often used as it has a huge mass and is inert (non-reactive). This technique needs a source of cool electrons that work to neutralize the ion beam’s space charge.

Hall thrusters produce denser plasma by holding back the electrons with a magnetic field (or the B-field) while the ions are accelerated by an electrostatic voltage. An electron source is also required in this technique to neutralize the space charge. Equation 1 shows the formula to achieve a specific impulse (Isp).

This is one among many ways to quantify the performance, effectiveness, and the capability of the thruster. The speed achieved by the spacecraft relies on the amount of xenon available for ejection.

Helicon sources are capable of generating higher plasma density when compared to most other discharges at the same power; hence, this method is considered for use in thrusters. As the plasma is not fully ionized, neutral atoms entrained by the ions will also be discharged, increasing the effective Isp.

(1)

 

Where νex stands for the ion exhaust velocity and g denotes the acceleration caused by gravity.

Generally, helicons need a DC magnetic field, and virtually all the experiments performed to date have created the B-field with the use of heavy copper coils powered by a DC power supply.

This study proposes a thruster whose weight and size are considerably reduced by developing a small but dense helicon source with the help of permanent magnets. Furthermore, the helicon thruster design has been described and characterized using a Semion ion energy analyzer from Impedans Ltd.

Method

A. Tube Design

Generally, helicon designs are complex as they need magnetic fields as well as unique antenna designs. Luckily, the experimental design in this study has been streamlined by program HELIC as illustrated in Figure 1.

The final design of the discharge tube.

Figure 1. The final design of the discharge tube. Image Credit: Impedans.

B. Magnet Design

Annular magnets with vertical polarization produce a powerful field within the tube, but the plasma that is produced will leak sideways along the field lines rather than downward, as illustrated in Figure 2.

Shows the field lines of an annular permanent magnet and the possible positions of the discharge tube indicated by the rectangle.

Figure 2. Shows the field lines of an annular permanent magnet and the possible positions of the discharge tube indicated by the rectangle. Image Credit: Impedans.

Therefore, the discharge is located below the stagnation point, where the B-field is not stronger but is fairly even and extends endlessly. The magnet depicted in Figure 2 is a neodymium (NdFeB) magnet that has an inner diameter of 3″ (7.6 cm), an outer diameter of 5″ (12.7 cm), and a thickness of 1″ (2.54 cm).

C. Plasma Chamber

As shown in Figure 3, the helicon source is connected to the large chamber and the retarding-field energy analyzer (RFEA) is supported on a vertical shaft that can place it at varying distances z beneath the source. Three ports located on the side allow the insertion of Langmuir probes for radial measurements at three z positions.

Diagram of experimental chamber.

Figure 3. Diagram of experimental chamber. Image Credit: Impedans.

D. Ion Analyzer

The RFEA system has been configured to be used in a powerful RF environment. Covered with aluminum oxide, the aluminum case has a thickness of about 5 mm and a diameter of 70 mm. It has a small ground capacitance, so that the whole grid assembly, illustrated in Figure 4, follows the RF at the local floating potential.

Diagram of RFEA.

Figure 4. Diagram of RFEA. Image Credit: Impedans.

While the grid G1 repels electrons, the grid G2 offers the swept retarding voltage to retard the ions, and C serves as the ion collector. The insulated cable conducts and transfers the voltages to a filter box, which passes the DC voltages through a vacuum seal to the control unit but discards most of the RF. Using an oscilloscope probe, G0’s DC voltage was cautiously measured to achieve the floating potential to relate the ion energy distribution functions (IEDFs) to machine ground.

Findings

The quantified IEDFs are summed up in Figure 5. The voltage denotes the ion energy in eV.

Ion energy distributions with the large magnet.

Figure 5. Ion energy distributions with the large magnet. Image Credit: Impedans.

Figure 5 shows the variation in IEDF as a function of distance from the source for RF power of 400 W. Neutral collisions cause the ions to slow down and lose density as a result of diffusion, as predicted. The resulting Isp values, with and without the bias applied to the endplate of the helicon source, are shown in Figure 6.

A specific impulse from a PM helicon thruster at 15 mTorr: intrinsic and with +24 V bias.

Figure 6. A specific impulse from a PM helicon thruster at 15 mTorr: intrinsic and with +24 V bias. Image Credit: Impedans.

The value of 1200 seconds is similar to what is presently possible with Hall thrusters. But this can be realized at relatively lower voltages with a helicon thruster, which is a significant advantage.

Conclusion

Helicon thrusters have attracted the interest of the spacecraft community, because of their large inherent thrust as well as the circumvention of electron neutralizers. Since the B-field needed is just 30-60 G, both weaker and stable magnets can also be used.

It is not essential to use advanced magnets. The use of simple annular permanent magnets has considerably reduced the weight and size of the helicon device. The helicon can be controlled by a small 200–400 W RF power supply and its corresponding network. The ions flowing from the source have an intrinsic specific impulse of around 700 seconds.

This can be increased permanently by biasing the endplates of the helicon discharge positively, drawing just milliamperes of current. Therefore, Isp can be brought up to that of Hall effect thrusters using bias of just 50 V.

References

*Francis F. C. “Ion ejection from a permanent-magnet mini helicon thruster.” Physics of plasma. doi:10.1063/1.4896238. Published on 26 September 2014.

This information has been sourced, reviewed and adapted from materials provided by Impedans Ltd.

For more information on this source, please visit Impedans Ltd.

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