Louis Roy Miller Giersch

Simulation and laboratory validation of magnetic nozzle effects for the high power helicon thruster

The efficiency of a plasma thruster can be improved if the plasma stream can be highly focused, so that there is maximum conversion of thermal energy to the directed energy. Such focusing can be potentially achieved through the use of magnetic nozzles, but this introduces the potential problem of detachment of plasma from the magnetic field lines tied to the nozzles. Simulations and laboratory testing are used to investigate these processes for the high power helicon (HPH) thruster, which has the capacity of producing a dense (1018−1020m−3) energetic (tens of eV) plasma stream which can be both supersonic and super-Alfvenic within a few antenna wavelengths. In its standard configuration, the plasma plume generated by this device has a large opening angle, due to relatively high thermal velocity and rapid divergence of the magnetic field. With the addition of a magnetic nozzle system, the plasma can be directed/collimated close to the pole of the nozzle system causing an increase in the axial velocity of t...

Plasma characteristics of a high power helicon discharge

A new high power helicon (HPH) plasma system has been designed to provide input powers of several tens of kilowatts to produce a large area (0.5?m2) of uniform high-density, of at least 5 ? 1017?m?3, plasma downstream from the helicon coil. Axial and radial plasma characteristics show that the plasma is to a lesser extent created in and near the helicon coil and then is accelerated into the axial and equatorial regions. The bulk acceleration of the plasma is believed to be due to a coupling of the bulk of the electrons to the helicon field, which in turn transfers energy to the ions via ambipolar diffusion. The plasma beta is near unity a few centimetres away from the HPH system and Bdot measurements show ?B perturbations in the order of the vacuum magnetic field magnitude. In the equatorial region, a magnetic separatrix is seen to develop roughly at the mid-point between the helicon and chamber wall. The magnetic perturbation develops on the time scale of the plasma flow speed and upon the plasma reaching the chamber wall decays to the vacuum magnetic field configuration within 200??s.

The Plasma Magnet

*Plasma sail propulsion based on the plasma magnet is a unique system that taps the ambient energy of the solar wind with minimal energy and mass requirements. The coupling to the solar wind is made through the generation of a large-scale (~> 30 km) dipolar magnetic field. Unlike the original magnetic sail concept, the coil currents are conducted in a plasma rather than a superconducting coil. In this way the mass of the sail is reduced by orders of magnitude for the same thrust power. The plasma magnet consists of a pair of polyphase coils that produce a rotating magnetic field (RMF) that drives the necessary currents in the plasma to inflate and maintain the large-scale magnetic structure. The plasma magnet is deployed by the Lorentz self-force on the plasma currents, expanding outward in a disk-like shape until the expansion is halted by the solar wind pressure. It is virtually propellantless as the intercepted solar wind replenishes the small amount of plasma required to carry the magnet currents. Unlike a solid magnet or sail, the plasma magnet expands with falling solar wind pressure to provide constant thrust.

Advances in High Power Beamed Plasma Propulsion

Magnetic nozzles offer the ability to provide highly collimated plasma streams that increase thruster efficiency by maximizing conversion of thermal energy into directed energy. However, in order to ensure that the plasma becomes detached from the field lines, the plasma must become super-Alfvenic as it traverses the nozzle. If the plasma is also supersonic, self-focusing of the plasma can occur due to the modification of the magnetic field by induce plasma currents that cause the magnetic field lines to be dragged outwards with the plasma. In so doing the subsequent plasma encounters a more convergent magnetic field configuration as it leaves the nozzle, enhancing the collimation. These processes are demonstrated through computer simulations and verified using a high power helicon for the thruster. Increase in transit times of a factor of 33% are demonstrated with the density being substantially enhanced along the axis of the magnetic nozzle. The plasma beam is used to beam power into a distant system at the end of the chamber. This remotely powered thruster is shown to be able to support very high densities and with excellent collimation, albeit at reduced specific impulse but without any onboard power. This experiment demonstrates the ability of using a beamed plasma system to power the propulsion of a remote spacecraft. Such systems could substantially reduce the cost of orbital transfers from low Earth orbit to geosynchronous orbit and even for planetary transfer orbits.

Experimental apparatus for studying rotating magnetic field current drive in plasmas

This article describes the design and operation of an experimental apparatus that was constructed for studying rotating magnetic field (RMF) current drive in plasmas formed in a metal vacuum chamber. The device was designed to enable the study of various RMF coil geometries that are fully enclosed inside the vacuum chamber. To date, the apparatus has been used with three distinct RMF coil geometries, one of which was fully immersed in the RMF-driven plasma.

Magnetic Dipole Inflation with Cascaded ARC and Applications to Mini-Magnetospheric Plasma Propulsion

Mini-Magnetospheric Plasma Propulsion (M2P2) seeks to create a plasma-inflated magnetic bubble capable of intercepting significant thrust from the solar wind for the purposes of high speed, high efficiency spacecraft propulsion. Previous laboratory experiments into the M2P2 concept have primarily used helicon plasma sources to inflate the dipole magnetic field. The work presented here uses an alternative plasma source, the cascaded arc, in a geometry similar to that used in previous helicon experiments. Time resolved measurements of the equatorial plasma density have been conducted and the results are discussed. The equatorial plasma density transitions from an initially asymmetric configuration early in the shot to a quasisymmetric configuration during plasma production, and then returns to an asymmetric configuration when the source is shut off. The exact reasons for these changes in configuration are unknown, but convection of the loaded flux tube is suspected. The diffusion time was found to be an order of magnitude longer than the Bohm diffusion time for the period of time after the plasma source was shut off. The data collected indicate the plasma has an electron temperature of approximately 11eV, an order of magnitude hotter than plasmas generated by cascaded arcs operating under different conditions. In addition, indirect evidence suggests that the plasma has a β of order unity in the source region. As an alternative, Mini-Magnetospheric Plasma Propulsion (M2P2) seeks to create a plasma-inflated magnetic bubble capable of intercepting significant thrust from the solar wind. One of the critical factors for plasma-inflation is the creation of high β plasma. β is the ratio of plasma pressure (proportional to density×temperature) to magnetic field pressure (proportional to the square of the magnetic field strength). In theory, a high β plasma with a large Magnetic Reynolds number injected into a modest (~1 m, 0.1 T) magnetic dipole will carry the magnetic field outward (“inflate”) as the plasma expands. The condition of a large Magnetic Reynolds number indicates that the plasma pushes the magnetic field outward faster than the magnetic field can diffuse inward through the plasma. The resulting magnetic field scales as a current sheet (B scales with 1/r), and it then becomes possible to produce the required magnetic field at the required distances to yield thrusts ~1N. The M2P2 system can thus be used as a high ∆V, high efficiency propulsion system for a modest (100s of kg) interplanetary spacecraft, using reasonable amounts of onboard power (few 10s of kW) and consumables.

Demonstration of current drive by a rotating magnetic dipole field

A dipole-like rotating magnetic field was produced by a pair of circular, orthogonal coils inside a metal vacuum chamber. When these coils were immersed in plasma, large currents were driven outside the coils: the currents in the plasma were generated and sustained by the rotating magnetic dipole (RMD) field. The peak RMD-driven current was at roughly two RMD coil radii, and this current (60 kA m -2 ) was sufficient to reverse the ambient magnetic field (33 G). Plasma density, electron temperature, magnetic field and current probes indicated that plasma formed inside the coils, then expanded outward until the plasma reached equilibrium. This equilibrium configuration was adequately described by single-fluid magnetohydrodynamic equilibrium, wherein the cross product of the driven current and magnetic filed was approximately equal to the pressure gradient. The ratio of plasma pressure to magnetic field pressure, β, was locally greater than unity.