Timothy Ziemba

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...

High Power Helicon Thruster

The High Power Helicon (HPH) plasma thruster under development at MSNW and the University of Washington is an emerging electrode-less in-space thruster that is potentially capable of high thrust level (1-2 Newtons) at moderate power levels of 20 to 50 kW. Unlike previous lower power (2 to 5 kW) helicon thruster schemes, which have been shown to produce moderate temperature of 5 to 10 eV thermal plasmas, the HPH axial and radial plasma characteristics show that the plasma is created in the helicon coil and is then accelerated in the axial direction downstream away from the HPH. The bulk acceleration of the plasma is believed to be due to a directional coupling of the plasma electrons with the helicon wave field, which in turn transfers energy to the ions via an ambipolar electric field. Downstream energy distribution functions obtained with an ion energy retarding field analyzer show a highly non-thermal or beamlike supersonic ion flow away from the HPH thruster. The system is very versatile and is capable of operation at variable input power levels in either pulsed or DC modes. Additionally, the HPH system has been shown to operate utilizing different propellants with hydrogen, nitrogen, argon and xenon having been tested to date. Baseline Isp levels for argon, nitrogen and hydrogen are 1500, 3000 and 5000 respectfully, giving some variability in Isp and thrust by the choice of propellants or propellant mixtures. Current work focuses on the optimization of the system and increasing output plasma power levels.

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.

Wave propagation downstream of a high power helicon in a dipolelike magnetic field

The wave propagating downstream of a high power helicon source in a diverging magnetic field was investigated experimentally. The magnetic field of the wave has been measured both axially and radially. The three-dimensional structure of the propagating wave is observed and its wavelength and phase velocity are determined. The measurements are compared to predictions from helicon theory and that of a freely propagating whistler wave. The implications of this work on the helicon as a thruster are also discussed.

Magnetic inflation produced by the Mini-Magnetospheric Plasma Propulsion (M2P2) prototype

Mini-Magnetospheric Plasma Propulsion (M2P2) seeks the creation of a magnetic wall or bubble (i.e. a magnetosphere) attached to a spacecraft that will intercept the solar wind and thereby provide high-speed propulsion with little expenditure of propellant. The prototype uses a helicon source embedded asymmetrically in a dipole-like magnetic field. Breakdown of the plasma can be produced at high densities 1012−1013 cm−3 with a temperature of several eV. The plasma pressure is sufficient to cause the outward expansion or inflation of the mini-magnetosphere. This expansion has now been measured directly by magnetic field probes. Computer simulations of the laboratory geometry show the presence of magnetic field perturbations that have similar magnitude and temporal variations as seen in the experiments. The field line mapping from the model has similar features to the optical images taken during laboratory prototype. The agreement between the laboratory experiments and the computer simulations provide quanti...

High Power Helicon Propulsion Experiments

The High Power Helicon (HPH) under development at the University of Washington may have an attractive application as an electrode‐less in‐space thruster. Output plasma characteristics show that plasma is created in and near the helicon coil and is accelerated by a helicon induced axial potential downstream away from the HPH. The bulk acceleration of the plasma is believed to be due to a coupling of the plasma electrons to the helicon field, which in turn transfers energy to the ions via an ambipolar electric field. Downstream electric potentials of greater than 150 volts having been measured with the amplitude of the electric field being dependent on experimentally controlled parameters. Time of flight measurements of the plasma transiting downstream show specific impulses (Isp) near 2000 seconds for Argon with calculated thrust levels near 1 Newton for input powers to the plasma in the tens of kilowatts. The system is capable of using different neutral gases as propellants with nitrogen and hydrogen havi...

Reduction of plasma density in the Helicity Injected Torus with Steady Inductance experiment by using a helicon pre-ionization source

A helicon based pre-ionization source has been developed and installed on the Helicity Injected Torus with Steady Inductance (HIT-SI) spheromak. The source initiates plasma breakdown by injecting impurity-free, unmagnetized plasma into the HIT-SI confinement volume. Typical helium spheromaks have electron density reduced from (2-3) × 10(19) m(-3) to 1 × 10(19) m(-3). Deuterium spheromak formation is possible with density as low as 2 × 10(18) m(-3). The source also enables HIT-SI to be operated with only one helicity injector at injector frequencies above 14.5 kHz. A theory explaining the physical mechanism driving the reduction of breakdown density is presented.

Performance Enhancement and Modeling of the High Power Helicon Plasma Thruster

The High Power Helicon (HPH) plasma thruster under development at MSNW and the University of Washington is an emerging electrode-less in-space thruster that is capable of high thrust level (1-2 Newtons) and efficiency (50-60%) at moderate to high average power levels 5 to 50 kW). The HPH represents a significant departure from other electric thrusters in that the momentum is transferred to the plasma through plasma wave fields. By launching only one mode of the helicon wave, the bulk acceleration of the plasma is found to be in only one direction due to due to the directional coupling of the plasma electrons with the helicon wave field. This in turn transfers energy to the ions via an ambipolar electric field. Downstream energy distribution functions obtained with an ion energy retarding field analyzer show a highly non-thermal or beamlike supersonic ion flow away from the HPH thruster. Recent work has concentrated on achieving higher efficiency at increased peak power. This achieved by operating the thruster with a spatial as well as temporal propagating m=1 mode. The modified device is capable of operating at peak powers in the 100s of kW range. Efficiency increases with increased peak power. The device can be operated in a pulsed manner with shot durations ranging from 30 μs to several milliseconds, with ambient magnetic field strengths (B0) ranging from 6 to 50 mT. Measured source plasma densities in Argon are near 2-5x10 m with electron temperatures of 5-8 eV. Langmuir probe measurements, at the exit of the source and further downstream, show a peaked spatial profile. The ion energy distributions show a dual peaked population flowing downstream from the discharge. Maximum sustained directed ion energies using Argon ~ 60 eV. Preliminary results of the 3D MHD modeling of the Helicon thruster support the above interpretation and results.

Laboratory testing of the Mini-Magnetospheric Plasma Propulsion (M2P2) prototype

Mini-Magnetospheric Plasma Propulsion (M2P2) seeks the creation of a magnetic wall or bubble (i.e., a magnetosphere) attached to a spacecraft that will intercept the solar wind and thereby provide high-speed propulsion with little expenditure of propellant. Results from a laboratory prototype that demonstrate the basic formation and expansion of a mini-magnetosphere are presented. The prototype uses a helicon source embedded asymmetrically in a dipole-like magnetic field. Breakdown of the plasma can be produced at neutral pressures of between about 0.25 to 1 mTorr to produce plasma densities of the order of 1011–1012 cm−3 with a temperature of a few eV. The plasma pressure is sufficient to cause the outward expansion or inflation of the mini-magnetosphere. The motion of both open and closed field lines within the vacuum chamber is demonstrated through the optical emissions from the helicon plasma. Inflation of the magnetosphere to several feet away from the magnetic coil and the equatorial confinement of ...

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.