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

Charged aerodynamics: Ionospheric plasma drag on objects in low-Earth orbit

Objects in Low-Earth Orbit (LEO) experience a range of environmental conditions that influence their trajectories. Aside from gravitational effects and solar radiation pressure, drag due to the residual background atmosphere is conventionally viewed as the primary perturbing factor. The ionosphere is an additional background environment composed of charged particles that is well-known to result in spacecraft charging, and in the worst case, cause arcing or unwanted electrostatic discharges. While the ionospheric plasma density is typically an order of magnitude (or more) lower than the atmospheric neutral gas density, electrostatic charging can lead to the formation of plasma sheath and wake structures around an object that artificially increase its effective collecting area. Direct charged particle collection and indirect charged particle deflection gives rise to a drag force that can exceed that due to atmospheric neutral gas alone at some altitudes and charging potentials. Here, we present a model accounting for charged particle flow effects (i.e. charged aerodynamics) around objects in LEO, and validate this model with previous particle-in-cell simulations and experiments. Using atmospheric properties from the Mass Spectrometer and Incoherent Scatter radar (MSIS) model and ionospheric properties from the International Reference Ionosphere (IRI), we show that plasma-induced drag in LEO can be significant and may have important orbit prediction implications for space domain awareness and space traffic management. Through differential spacecraft biasing, ionospheric plasma drag can also be used as a propellantless and “solid-state” mechanism to achieve in-orbit mobility for precision maneuvers, formation flying, deorbiting, and even partial attitude control

Particle-in-cell simulations of ambipolar and nonambipolar diffusion in magnetized plasmas

Using a two-dimensional particle-in-cell simulation, we investigate cross-field diffusion in low-pressure magnetized plasmas both in the presence and absence of conducting axial boundaries. With no axial boundary, the cross-field diffusion is observed to be ambipolar, as expected. However, when axial boundaries are added, the diffusion becomes distinctly nonambipolar. Electrons are prevented from escaping to the transverse walls and are preferentially removed from the discharge along the magnetic field lines, thus allowing quasi-neutrality to be maintained via a short-circuit effect at the axial boundaries

Particle-in-cell simulations of hollow cathode enhanced capacitively coupled radio frequency discharges

A two-dimensional particle-in-cell simulation has been developed to study density enhancement of capacitively coupled rf discharges with multi-slit electrodes. The observed density increase is shown to result from a hollow cathode effect that takes place within the multi-slit electrode configuration, which forms as a result of secondary electron emission due to ion bombardment. By investigating the ionization and power deposition profiles, it is found that rfsheathheating is too weak to sustain the discharge, and that secondary electron acceleration within the sheath is the primary heating mechanism. Due to a capacitive voltage divider formed by the rfsheaths at each electrode, the area ratio of the powered and ground electrodes is observed to have a strong effect on the resulting discharge, and if the ground electrode area is too small, the voltage drop at the powered electrode is too low to sustain a hollow cathodedischarge.The authors gratefully acknowledge financial support from the Lam Research Corporation

On the selection of propellants for cold/warm gas propulsion systems

Cold/warm gas spacecraft propulsion systems generate thrust by accelerating a cold or moderately heated gas through a nozzle. Despite their simplicity compared with typical chemical or electric propulsion systems, the choice of propellant nonetheless represents an important consideration that affects the design, operation, and performance of such thrusters. Here we review different propellants that have been used or proposed for cold/warm gas propulsion systems, and we perform a study investigating almost 5000 other possible alternatives. Propellants include organic and inorganic substances that can be stored in solid, liquid, gaseous, and multi-phase states at ambient conditions, and which undergo a phase change to a pure gaseous state (where needed) by heating before being ejected from the thruster. The different propellants are assessed by considering important evaluation factors, such as propulsive performance, storage requirements and storage density, mission suitability, safety, and cost. While conventional thruster performance metrics like the specific impulse are low for some propellants, the design and operational advantages they offer, together with a higher total impulse mass density (i.e. the impulse per propulsion system wet mass), make them superior choices for many missions

Ion beam formation in a very low magnetic field expanding helicon discharge

An ion beam has been measured emerging from a low pressure (0.04 Pa) helicon plasma reactor over a narrow range of magnetic field values (1 mT<B0<3 mT). The presence of the ion beam occurs simultaneously with a large increase in the plasma density for the same applied magnetic field, produced using a single solenoid half the length of the m=1 rfantenna. The peak central plasma density of 1.5×10¹⁷ m⁻³ is measured to be almost 15 times larger than that occurring before or after the increase, and is associated with a steep axial density gradient which follows the gradient of the magnetic field. During this low magnetic field transition the antenna power transfer efficiency is measured to increase from less than 10% to 50%, suggesting some form of localized bulk electron heating in the source region associated with the helicon wave