Boltzmann expansion in a radiofrequency conical helicon thruster operating in xenon and argon

A low pressure (~ 0.5 mTorr in xenon and ~ 1 mTorr in argon) Boltzmann expansion is experimentally observed on axis within a magnetized (60 to 180 G) radiofrequency (13.56 MHz) conical helicon thruster for input powers up to 900 W using plasma parameters measured with a Langmuir probe. The axial forces, respectively, resulting from the electron and magnetic field pressures are directly measured using a thrust balance for constant maximum plasma pressure and show a higher fuel efficiency for argon compared to xenon

Interface creation and stress dynamics in plasma-deposited silicon dioxide films

The stress in amorphous silicon dioxide filmgrown by plasma-assisted deposition was investigated both during and after film growth for continuously and intermittently depositedfilms. It is shown that an intermittent deposition leads to the creation of interfacial regions during film growth, but also causes dynamical structural change in already-deposited film which results in a significantly different stress-thickness profile measured after deposition.Film growth in the continuously depositedfilm was also monitored using an in situ laser reflection technique, and a strong change in stress was detected at about 145nm which was attributed to the onset of island coalescence

Plasma control by modification of helicon wave propagation in low magnetic fields

By making use of nonuniform magnetic fields, it is shown experimentally that control of helicon wave propagation can be achieved in a low pressure (0.08 Pa) expanding plasma. The m=1 helicon waves are formed during a direct capacitive to wave mode transition that occurs in a low diverging magnetic field(B₀<3 mT). In this initial configuration, waves are prevented from reaching the downstream region, but slight modifications to the magnetic field allows the axial distance over which waves can propagate to be controlled. By changing the effective propagation distance in this way, significant modification of the density and plasma potential profiles can be achieved, showing that the rf power deposition can be spatially controlled as well. Critical to the modification of the wave propagation behavior is the magnetic field strength (and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56 MHz

Detailed plasma potential measurements in a radio-frequency expanding plasma obtained from various electrostatic probes

On-axis plasma potential measurements have been made with an emissive probe in a low pressure (0.044 Pa) rf expanding plasma containing an ion beam. The beam is detected with a retarding field energy analyzer (RFEA), and is seen to disappear at high pressure (0.39 Pa). The emissive probe measurements are in very good agreement with corresponding measurements made with two separate RFEAs, and the results indicate that the floating potential of the strongly emitting probe gives an accurate measure of the plasma potential under the present conditions

Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster

The temporal evolution of neutral gas temperature over the first 5 min of operation for an electrothermal radio-frequency micro-thruster with nitrogen (N2) propellant was measured using rovibrational band matching of the second positive N2 system. Three distinct periods of gas heating were identified with time constants of τ 1 = 8 × 10⁻⁵ s, τ 2 = 8 s, and τ 3 = 100 s. The fast heating (τ 1) is attributed to volumetric heating processes within the discharge driven by ion-neutral collisions. The slow heating (τ 3) is from ion neutralization and vibrational de-excitation on the walls creating wall heating. The intermediate heating mechanism (τ 2) is yet to be fully identified although some theories are suggested.This research was partially funded by the Australian Space Research Program (APT project) and the Australian Research Council Discovery Project (No. DP140100571)

The magnetic-field-induced transition from an expanding plasma to a double layer containing expanding plasma

The magnetic-field-induced transition from a simple expansion to a double layer is experimentally investigated in an argon low pressure radio frequency helicon sourceplasma. When the magnetic field is increased from 30to140G in the plasma source, an abrupt increase in the plasma density and upstream potential is measured at 50G. In the downstream plasma, the plasma density and potential show a small decrease with increasing magnetic field and no abrupt change. When the upstream jump is measured, simultaneous measurements in the downstream plasma show an ion beam characteristic of a double layer near the source exit

Time development of a current-free double-layer

The time development of a supersonic (∼2.3Cs) argon ion beam generated by a current-free double-layer (DL) is obtained by pulsing a “helicon” discharge (13.56 MHz) and measuring the total ion current and the ion beam current during the first few milliseconds using a retarding field energy analyzer. The ion beam current is detected during the plasma breakdown phase (60–250 μs) and is stable thereafter (⩾250 μs). Temporal measurements of the floating potential upstream and downstream of the DL show evidence of wall charging in the plasma source during the first 250 μs of the discharge which appear to be related to the appearance of the double-layer. A comparison between the DL case and a somewhat higher pressure “non DL” case suggests that the double-layer is formed at ∼100 μs

Particle in cell simulation of a radiofrequency plasma jet expanding in vacuum

The effect of a pressure gradient (∼133 Pa–0.133 Pa) on electron and ion energy distributions in a radiofrequency (rf at 13.56 MHz) argon plasma jet is studied using a 1D-3v Particle In Cell (PIC) simulation. The PIC domain is three times that of the 0.018 m long plasma cavity and the total simulation time is 1 ms. Ion heating and acceleration up to a drift velocity about 2000 m s⁻¹ are measured along the jet's main expansion axis. Elastic and charge exchange ion-neutral collisions histograms computed at equilibrium during 0.74 ms show that charge exchange collisions act as the main neutral heating mechanism.This research was funded by the Australian Research Council Discovery Projects DP 1096653 and DP140100571

Nonlinear instability dynamics in a high-density, high-beta plasma

Entrainment and periodic pulling of an ion acoustic instability have been observed in the power spectra of a low-pressure high-beta plasma. The observed nonlinear phenomena can be modeled by using the van der Pol equation with a forcing term. Experimental results of the nonlinear processes are presented. Ion density fluctuations are detected on a negatively biased Langmuir probe for magnetic fields and input powers above 30 G and 900 W at 7.2 MHz respectively, and gas pressure below 1.5 mTorr. This low-frequency instability is observed in the central plasma blue core (argon II emission) and can be controlled by amplitude modulation of the radio frequency input power at frequencies close to the instability frequency