Magnetic steering of the ion beam in the Helicon Double Layer Thruster
Creators: name: Cox, Wesley
Publisher: name: The Australian National University
Year: 2010
Subjects: subject: QC718.5.M3 C69 2010
subject: Plasma (Ionized gases)
subject: Electric discharges through gases
subject: Ion bombardment
subject: Magnetic fields

description: Experiments are performed in the CHI KUNG plasma reactor to study the effect of introducing transverse components (aligned along the x-axis) to the cylindrically symmetric magnetic field (aligned along the z-axis) to produce magnetic steering of the ion beam in the Helicon Double Layer Thruster (HDLT). CHI KUNG is a cylindrical helicon plasma reactor, on which the first HDLT prototype is based, consisting of a 15 cm diameter, 31 cm long Pyrex source tube attached contiguously to a 32 cm diameter, 30 cm long aluminium diffusion chamber. Coaxially surrounding the source tube is an 18 cm long double saddle antenna connected to an L-matching network/generator system, fed with 13.56 MHz of radio-frequency power. Enclosing the source tube and antenna are two coaxial solenoids, referred to as the source and exit solenoids, which generate a diverging magnetic field. The peak magnetic field magnitudes produced by the source and exit solenoids are 132 G and 118 G, respectively, when a current of 3 A is flowing through each solenoid, pointing north into the diffusion chamber. Argon gas is fed in through the diffusion chamber and pumped to an operational pressure of 0.3 mTorr by a turbomolecular/rotary pumping system. Under these conditions, a double layer is formed ~ 5 cm inside the exit of the source tube, which accelerates a population of ions to form an ion beam that can be detected in the diffusion chamber. In this thesis, spatial measurement techniques are developed to measure the properties of the ion beam in the diffusion chamber, downstream of the double layer. Of particular interest is the ion beam density, and an analysis is developed to determine this parameter from the spatial data. Because the ion beam in the diffusion chamber is surrounded by ambient plasma, a retarding field energy analyzer (RFEA) is used to measure the flux of ions as a function of their energy, to separate the fast moving beam from the population of thermal background ions. A spatial map of the ion beam density can be obtained by sweeping the RFEA across the exit of the source tube at a number of different z{u00AC}axial positions, downstream of the double layer. As the ion beam travels into the diffusion chamber, a 5.70 {u00B1}0.4{u00B0} angle is observedbetween the ion beam reactor and the z-axis. This asymmetry has not before been observed in the largely cylindrically symmetric CHI KUNG reactor and the possible causes for this asymmetry are discussed. By introducing either one or two transverse solenoids to the CHI KUNG source region, asymmetric components of the magnetic field may be introduced and the resulting effect on the ion beam vector is observed. Experiments are broken up into two categories, with the effect of a single transverse solenoid initially investigated to demonstrate the phenomenon of magnetic steering in the HDLT. Subsequent experiments explore the use of an additional second transverse solenoid to better understand the magnetic steering phenomenon. The effect of varying the magnitude and polarity of the single transverse magnetic field (changing the solenoidal current) is observed to cause the ion beam to deflect away from the main axis (the z-axis), with larger transverse solenoidal currents resulting in greater angles of deflection. The direction of ion beam deviation is determined by the polarity of the transverse solenoid, with the ion beam deflecting in the direction of the transverse solenoid polarity. Deflection of the ion beam is measured up to -26.1{u00B0} {u00B1} 0.8{u00B0} and 13.5{u00B0} {u00B1} 0.7{u00B0} for single transverse solenoidal currents of 4 A north-out and 3 A north{u00AC}in, where north-out and north-in indicate the polarity of the transverse solenoid pointing in-to or out-of the source tube. Adding a second transverse solenoid, and orienting the two coaxial transverse solenoids in the same direction (in the negative x-direction) results in deflections of the ion beam for lower solenoidal currents. A current of 1.25 A flowing through both the transverse solenoids is observed to produce deflection of the ion beam up to 18.0{u00B0} {u00B1} 0.5{u00B0}, with the direction of deflection parallel to the transverse solenoidal polarity. By obtaining, for the first time, a spatial map of the ion beam downstream of a double layer in CHI KUNG, other properties of the ion beam can be obtained. The divergence of the ion beam is considered for a number of different measurement geometries and analyses, and reveals divergences similar to the more widely developed gridded ion engines. The experimental investigation of magnetic steering in the HDLT is uncommon in most electric thrusters, and the magnetic steering capabilities demonstrated in this thesis represent a significant advantage for the Helicon Double Layer Thruster as an alternative for electric propulsion missions.