Christopher N. Davis

Development of an Annular Helicon Source for Electric Propulsion Applications

Abstract : The performance of typical electrostatic propulsion systems, such as the Hall thruster, is limited in part by inefficiencies in the electron bombardment ionization process. These limitations become especially pronounced at the operating conditions required to achieve high thrust-to-power ratios. One approach for achieving significant increases in efficiency at such operating conditions is to replace the typically-employed DC ionization mechanism with a helicon source, which is widely regarded as an efficient method for creating a high-density, low-temperature plasma. Standard cylindrical helicons, however, are not amenable to straightforward integration with annular Hall thrusters. A rigorous mathematical treatment of helicon wave physics has been completed to establish the boundary conditions required to create an annular helicon source for both the m=0 and m=1 azimuthal modes. This analysis reveals no fundamental barriers to creation of an annular helicon source so long as the radial boundary conditions are set appropriately.

Experimental results for communications blackout amelioration using crossed electric and magnetic fields

As a vehicle reenters the atmosphere or travels at hypersonic speeds within it, a bow shock forms around the leading edge of the vehicle. The air is superheated as it passes through the shock wave and becomes ionized. This plasma layer prevents the transmission of radio frequency communications to or from the vehicle, causing what is know as communications blackout. In this paper, we present results from experiments performed to evaluate the use of crossed electric and magnetic fields to lower the plasma density in a region surrounding an antenna. Plasma number density, plasma frequency, and signal attenuation measurements were made with a Langmuir probe, hairpin resonance probe, and S2-1 probe, respectively. The hairpin resonance probe and the S2-1 probe measured frequency responses for input frequencies ranging from 200 up to 4000 MHz. Results show that this approach is a viablemethod for communications blackout amelioration.We found that the plasma number density decreases by as much as 70%with the operating conditions used in this work, and the plasma frequency dropped by asmuch as 75%. The increased reduction in the plasma frequency, as compared to the plasmanumber density, was due to the addition of greater magnetic field strength when the frequency measurements were made. In addition, frequencies that were previously attenuated by more than 10 dB have almost no attenuation after the application of the electric and magnetic fields.

ELECTROMAGNETIC WAVE SCATTERING EXPERIMENTS IN HALL THRUSTER PLASMA PLUMES

The interactions of an electromagnetic (EM) wave propagating through plasma plumes of modern electric propulsion systems include both phase change and attenuation. Significant effects can exist even if the EM wave frequency is well above the local plasma frequency of the plume. Experimental measurements of spectral characteristics and theoretical analyses have shown that temporal responses modulate the EM wave by both amplitude and phase modulation. This paper will discuss the spectrums measured for Hall Thrusters (SPT-100, D55, and UM P-5). The paper compares the measured spectrums of these thrusters to a calculated spectrum using ray tracing based on an axial and azimuthal model of density oscillation. It will be shown that an azimuthal model provides a better match to measured data, though more work needs to be done to experimentally show the existence of an azimuthal perturbation within the plume of a Hall thruster.

Density and'spectral Measurements using a 34 GHz Interferometry System

Interferometry was used to investigate the impact of the plasma plume of the University of Michigan P5 thruster on microwave signals. Measurements were made at 34 GHz using a newly built Ka-band interferometer and also at 17 GHz similar to other measurements made on Hall thrusters [2,4]. Very close to the thruster’s exit plane (.05 m) a peak phase shift of 32 and 60 degrees at 34 and 17 GHz, respectively, was measured. At 0.5 m from the P5 exit plane, the peak phase shift measured was 16 degrees at 34 GHz and 9 degrees at 17 GHz. Signal attenuation measurements howed at .05 m from the thruster’s exit plane around 1.2 dB of signal attenuation for 17 GHz and about 0.5 dB of attenuation for 34 GHz on centerline of the thruster. Electron densities profiles were calculated using Abel inversion on the phase shift measurements. At a discharge voltage of 300 V and a discharge current of 10 A, a peak electron density of about 4elO cm” was found .05 m from the exit plane. At 300 V, 5 A, and .05. m from the exit plane, a peak density of 2.2elO cmm3 was found. Spectral measurements of a signal propagating through the plasma plume were also made showing that the presence of specific harmonics is strongly dependent on operating conditions. tic= r= R= i= n= 8= Nomenclature (f/8.98)*, Critical or cutoff density Radial distance from center of the plume Maximum plume radius Wavelength of a wave Electron density Phase of a wave

Hypersonic or Re-Entry Plasma Communication

Plasma generated around a vehicle traveling at hypersonic velocities can be a significant impediment to fixed-frequency communications, particularly for GPS navigation signal reception. Under two complementary U.S. Air Force Phase II SBIR programs, we have been developing technology for improving communication through plasmas. ReComm (ReEntry and Hypersonic Vehicle Plasma Communication System) creates a physical communication window utilizing plasma density reduction and HyPASS (Hypersonic Plasma Adaptive Sensor System) improves the communication channel through that window by implementing an antenna matching system. The HyPASS system uses two diagnostic probes attached to frequency-adaptive vector network analyzer hardware to measure plasma parameters, and can use these parameters to adjust a matchbox, reducing the reflection at the signal antenna due to the plasma mismatch. The system also provides real-time information about local plasma parameters. When used in concert, ReComm and HyPASS are expected to provide a 25 dB or more increase in signal reception, thus enabling more bandwidth and broader communication envelopes for hypersonic vehicles.

Simulating hypersonic atmospheric conditions in a laboratory setting using a 15-cm-diameter helicon source

Summary form only given. While a spacecraft is reentering the atmosphere or a hypersonic vehicle is in flight, low-frequency electromagnetic radiation cannot penetrate the plasma layer that forms around the high speed vehicle. This interferes with real-time telemetry from hypersonic vehicles and interrupts spacecraft communications during atmospheric reentry. Hypersonic atmospheric plasmas are difficult to simulate in a laboratory setting because they are high density (~109 - 1011 cm-3 depending on altitude) and low temperature (~2 - 5 eV). A 6-cm-diameter helicon source capable of creating plasma with these requirements has been designed, fabricated and tested at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL). We present Langmuir probe, retarding potential analyzer and residual gas analyzer data from helicon source operation with argon, nitrogen and air.

Communication through Hypersonic or Re-Entry Plasmas

Plasma generated around a vehicle traveling at hypersonic velocities can be a significant impediment to fixed-frequency communications, particularly for GPS navigation signal reception. Under two complementary U.S. Air Force Phase II SBIR programs, we have been developing technology for improving communication through plasmas. ReComm (ReEntry and Hypersonic Vehicle Plasma Communication System) creates a physical communication window utilizing plasma density reduction and HyPASS (Hypersonic Plasma Adaptive Sensor System) improves the communication channel through that window by implementing an antenna matching system. The HyPASS system uses two diagnostic probes attached to frequency-adaptive vector network analyzer hardware to measure plasma parameters, and can use these parameters to adjust a matchbox, reducing the reflection at the signal antenna due to the plasma mismatch. The system also provides real-time information about local plasma parameters. When used in concert, ReComm and HyPASS are expected to provide a 25 dB or more increase in signal reception, thus enabling more bandwidth and broader communication envelopes for hypersonic vehicles.

Characterization of Ion Cyclotron Resonance Acceleration for Electric Propulsion with Interferometery

This paper describes the experimental characterization of single-pass ion cyclotron resonance heating as applied to acceleration of ions for electric propulsion. A millimeter-wave interferometer system has shown to be a clear and simple method of quantifying ion acceleration due to ion cyclotron resonance heating. The experimental work was done on the VX-10 experiment of the variable specific impulse magnetoplasma rocket concept. The perpendicular velocity of the ions generated by ion cyclotron resonance heating was converted into axial velocity by the decreasing gradientoftheaxialmagnetic fieldattheexhaustofthepropulsionsystemfromconservationofthemagnetmoment. Thisincreaseinaxialvelocityispredictedtocauseadecreaseindensityduetoconservationofcurrentintheplasma. InterferometerdensitymeasurementsweretakenatthreedifferentlocationsontheVX-10experimentupstreamand downstream of the ion acceleration zone. A clear measurement of a 25% density drop for helium and a 40% density drop for deuterium was measured downstream of the ion resonance zone characteristic of ion acceleration.

Understanding the discharge current distribution and upper operational limit of a high power, gridded ion thruster

Summary form only given. Past studies suggest that for a given ion thruster magnetic circuit configuration, there exists an upper discharge operational limit, above which the discharge is either unstable or operates in a reduced efficiency mode. A 50 cm, ring cusp ion thruster discharge chamber was operated at elevated discharge currents in an attempt to identify its operational upper limit. The discharge plasma properties near the anode and in the bulk of the discharge were monitored using Langmuir probes as a function of discharge current for currents up to 85 A. The current distribution to the magnet rings and the intervening anode shell was also monitored. It was found that the discharge was essentially resistive up to 85 A. The limits of the ring cusp magnetic circuit was also probed by floating collection electrodes placed at the various magnet rings. By floating various magnet rings, operational upper limits for a given collection electrode combination could be determined. Finally it should be pointed out that discharge operation without collection at any magnet ring was demonstrated. Variation in discharge efficiency for the various ring configurations is also reported.

Simulated beam extraction performance characterization of a 50-cm ion thruster discharge

A 50 cm ion thruster is being developed to operate at >65 percent total efficiency at 11 kW, 2700 s Isp and over 25 kW, 4500 s Isp at a total efficiency of >75 percent. The engine is being developed to address the need for a multimode system that can provide a range of thrust-to- power to service national and commercial near-earth onboard propulsion needs such as station-keeping and orbit transfer. Operating characteristics of the 50 cm ion thruster were measured under simulated beam extraction. The discharge current distribution at the various magnet rings was measured over a range of operating conditions. The relationship between the anode current distribution and the resulting plasma uniformity and ion flux measured at the thruster exit plane is discussed. The thermal envelope will also be investigated through the monitoring of magnet temperatures over the range of discharge powers investigated. Discharge losses as a function of propellant utilization was also characterized at multiple simulated beam currents. Bulk plasma conditions such as electron temperature and electron density near engine centerline was measured over a range of operating conditions using an internal Langmuir probe. Sensitivity of discharge performance to chamber length is also discussed. This data acquired from this discharge study will be used in the refinement of a throttle table in anticipation for eventual beam extraction testing.