James Prager

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.

Enhanced diamagnetic perturbations and electric currents observed downstream of the high power helicon

The high power helicon (HPH) is capable of producing a high density plasma (1017−1018 m−3) and directed ion energies greater than 20 eV that continue to increase tens of centimeters downstream of the thruster. In order to understand the coupling mechanism between the helicon antenna and the plasma outside the immediate source region, measurements were made in the plasma plume downstream from the thruster of the propagating wave magnetic field and the perturbation of the axial bulk field using a type ‘R’ helicon antenna. This magnetic field perturbation (ΔB) peaks at more than 15 G in strength downstream of the plasma source, and is 3–5 times larger than those previously reported from HPH. Taking the curl of this measured magnetic perturbation and assuming azimuthal symmetry suggests that this magnetic field is generated by a (predominantly) azimuthal current ring with a current density on the order of tens of kA m−2. At this current density the diamagnetic field is intense enough to cancel out the B0 axia...

Ion energy characteristics downstream of a high power helicon

The High Power Helicon eXperiment operates at higher powers (37 kW) and lower background neutral pressure than other helicon experiments. The ion velocity distribution function (IVDF) has been measured at multiple locations downstream of the helicon source and a mach 3–6 flowing plasma was observed. The helicon antenna has a direct effect in accelerating the plasma downstream of the source. Also, the IVDF is affected by the cloud of neutrals from the initial gas puff, which keeps the plasma speed low at early times near the source.

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.

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.

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.

High power microwave generation with nonlinear transmission lines

Summary form only given. Eagle Harbor Technologies, Inc. (EHT) is investigating the generation of high power microwaves using the EHT Nanosecond Pulser (NSP) and nonlinear transmission lines (NLTLs). The EHT NSP provides independent control of the output voltage (20 kV), pulse width (20-200 ns), and pulse repetition frequency (up to 100 kHz) and is used to drive two different NLTLs. The gyromagnetic NLTL produces RF around 2 GHz. EHT has constructed a test setup including solenoid for producing an axial field. Experimental results, including RF measurements with a D-dot probe, will be presented. The second NLTL is based on the nonlinear properties of high voltage, Schottky diodes and produces RF at a lower frequency. Rise time sharpening and RF experimental data and modeling results will be presented.

Isolated, high voltage arbirtary pulse generator

Eagle Harbor Technologies, Inc. has developed an Arbitrary Pulse Generator (APG) with isolated high voltage output. The EHT APG can produce output pulses with voltages up to 10 kV and fast rise time (100 ns) at high pulse repetition frequency (up to 100 kHz) with a user-adjustable duty cycle from 0-100%. The isolated output allows the pulse generator to be connected to loads that need to be biased. These pulser generators utilize modern silicon carbide (SiC) MOSFETs, which offer lower switching and conduction losses while allowing for higher switching frequency capabilities compared to IGBTs. This pulse generator has applications for RF plasma heating; inductive and arc plasma sources; magnetron driving; and generation of arbitrary pulses at high voltage, high current, and high pulse repetition frequency in the semiconductor processing, non-equilibrium plasma source, and material processing communities.

High voltage, fast rise nanosecond pulsers

Eagle Harbor Technologies, Inc. (EHT) has developed a series of high voltage nanosecond pulsers that allow the user to independently adjust the output voltage, pulse width, and pulse repetition frequency. Recent results demonstrate the ability to produce 80 kV pulses with rise times down to 18 ns and a full width half maximum (FWHM) pulse width down to 20 ns. Additionally, versions of these pulsers have the capability to drive low impedance loads (25 Ω) with 11 ns rise time. These pulsers have been used for non-equilibrium plasma production for surface sterilization, surface modification, nonlinear transmission driving for high power microwave production, as well as other aerospace and biomedical applications.