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AIAA 19Th Fluid Dynamics, Plasma Dynamics and Lasers Conference June 8-10, 1987/Honolulu, Hawaii

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AIAA 19Th Fluid Dynamics, Plasma Dynamics and Lasers Conference June 8-10, 1987/Honolulu, Hawaii AIAA-87 -1407 Electron-Cyclotron-Resonance (ECR) Plasma Acceleration J. C. Sercel Jet Propulsion Laboratory California Institute of Technology Pasadena, California AIAA 19th Fluid Dynamics, Plasma Dynamics and Lasers Conference June 8-10, 1987/Honolulu, Hawaii For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 1633 Broadway, New York, NY 10019 AIAA-87-1407 ELECTRON-CYCLOTRON-RESONANCE (ECR) PLASMA ACCELERATION Joel C. Sercel* Jet Propulsion Laboratory California Institute of Technology Pasadena, California Abstract P power per unit volume, W/m3 R position vector, m A research effort directed at analytically v velocity, mls and experimentally investigating Electron­ U energy, J or eV Cyclotron-Resonance (ECR) plasma acceleration V electrostatic potential, volts is outlined. Relevant past research is reviewed. T temperature, Kelvin or eV The prospects for application of ECR plasma acceleration to spacecraft propulsion are described. It is shown that previously unexplained losses in converting microwave magnetic dipole moment power to directed kinetic power via ECR plasma reaction cross section, m2 acceleration can be understood in terms of diffusion of energized plasma to the physical time constant, s walls of the accelerator. It is argued that line radiation losses from electron-ion and electron­ SubscriPts atom inelastic collisions should be less than estimated in past research. Based on this new A acceleration understanding, the expectation now exists that B Bohm efficient ECR plasma accelerators can be e electron designed for application to high specific impulse ex excitation spacecraft propulsion. ionization summation variable refers to lowest energy level Acronyms and Abbreviations p perpendicular r relative D-He3 Deuterium Helium-Three sp space charge induced ECR Electron-Cyclotron-Resonance to t total GE General Electric JPL Jet Propulsion Laboratory LeRC Lewis Research Center I. Introduction NASA National Aeronautics and Space Administration ECR plasma acceleration is an TE Transverse-Electric electrodeless process which promises to efficiently use microwave frequency Nomenclature electromagnetic radiation to accelerate a tenuous plasma to velocities of use for high B magnetic induction vector, T specific impulse spacecraft propulsion. The E energy, J or eV advantages of high specific impulse propulsion e electron charge (1.602 x 10-19 coul) are well known in the aerospace community. If F force, N high specific impulse propulsion technology can K Boltzmann's constant be implemented, propellant mass savings and L length of accelerator, m associated cost savings can be obtained for m mass of electron (9.1 x 10-31 kg) many current missions, and a wide variety of M mass of ion, kg desirable new missions can be enabled. Electric n number density, m- 3 propulsion represents the most likely near-term possibility for achieving high specific impulse propulsion. More advanced systems such as * Member Jet Propulsion Laboratory Technical beamed energy propulsion or fusion propulsion Staff, California Instititute of Technology still remain futuristic concepts. This paper re­ Graduate Research Assistant, Member AIAA. examines ECR plasma acceleration for application to electric propulsion, beamed This p.... r is doc'.," • work of Ihe U.S. Govemmenl .nd is nol subJocI 10 coPyrilhl prolocUon in Ihe United Slllts. energy propulsion, or fusion propulsion. Figure 1 shows a conceptual schematic of These free electrons move in Larmor an EGR plasma accelerator. Transverse-Electric orbits producing a field of magnetic dipoles (T E) mode microwave radiation is introduced with axes aligned opposite to the axis of the through a dielectric window into a neutral gas solenoid magnetic field. The opposition between region via a waveguide. The neutral gas region the electron-Larmor-orbit dipole field and the can be in an extension of the waveguide, in a applied diverging solenoid field results in a net resonant cavity, or in a flared portion of the force on the electrons. As this force waveguide that has a different internal radius accelerates the electrons, a space-charge than the power transmission waveguide. A potential is induced which accelerates the ions solenoid coil co-axial with the waveguide, but with the electrons down-stream toward a larger in radius, surrounds the neutral gas vacuum region. The quasi-neutral plasma then region. This coil is designed to produce a separates from the magnetic field with minimal magnetic field which decreases in intensity loss and moves away from the device into the away from the dielectric window in the vacuum region. direction the microwave radiation propagates. Overall conversion of microwave power to jet power involves several possible loss mechanisms including: (1) reflection of the Propellant applied microwave power, (2) exhaust plume Injection Neutral Gas divergence, (3) nonuniformities in the exhaust Region velocity profile, (4) propellant ionization energy costs, (5) ion-electron recombinations, (6) -----...... =-~~~.... ---..- B Lines incomplete propellant ionization, (7) plasma Microwave .•.• --- __ ~ interactions with the walls of the accelerator, Waveguide ...- and (8) radiation from the plasma. .... ___ Plasma •.•• Trajectory ~,::-" '": .. " History and Related Research ~ ECR plasma acceleration was first Dielectric Energizing investigated both experimentally and Zone analytically for application to electric Window ~ propulsion in the 1960s. Three groups of vacuum. researchers were involved in this early work: (1) Solenoid a group at General Electric Corporation 1 funded by Lewis Research Center (LeRC), (2) a NASA group at LeRC,2 and (3) a group at the University Fig. 1 Schematic of ECR Plasma Accelerator. of Tokyo.3 Although the early research was conducted with somewhat primitive apparatus A propellant, such as argon, is introduced and theoretical models (by today's standards), into the neutral gas region via peripherally enough research was done to verify the basic located injection ports. The intensity of the concept of the process. At the time of the solenoid magnetic field in the energizing zone is investigation, however, microwave sources were adjusted so that the electron-cyclotron quite massive and inefficient. Meanwhile, frequency is equal to the frequency of the significant progress was being made on dc applied microwave radiation. This frequency electric propulsion devices which showed matching provides a resonance between the promise of acceptable performance using the microwave field and- the electron-cyclotron technology and analysis tools of the 1960s. By motion that enhances microwave-to-plasma the middle of that decade, these advances had coupling. Some ionization occurs in the neutral presaged the temporary end of research into the gas region, but coupling there is limited because physics of ECR plasma acceleration. the electron-cyclotron motion is not at resonance with the microwave radiation. The A review of the early research gas enters the energizing zone via free publications and communication with the molecular diffusion. In the energizing zone the investigators suggests that state-of-the-art gas atoms are ionized by the microwave microwave technology and plasma diagnostiC radiation through electron-atom collisions in a tools can be used to make ECR plasma cascading process. The microwave energy is acceleration an important part of modern coupled to the electron-cyclotron motion in the spacecraft propulsion technology. energizing zone, which results in electron energies of 200 to 1000 eV. 2 The majority of the early experimental of this analysis was that the plasma trajectory work on this type of plasma accelerator was does cross the magnetic field lines, even conducted at the Space Sciences Laboratory of without particle collisions, and that the plasma the General Electric Company (GE) and funded by can emerge as a well-defined beam. the Lewis Research Center (LeRC).1,4 In this experimental effort, several different ECR This conclusion was in contrast to the plasma thrusters were tested. Various earlier theoretical analysis of Kilpatrick.S arrangements of gas injection schemes, Kilpatrick's work indicated that a plasma beam waveguide designs, and window materials were ejected from a diverging magnetic field will evaluated. Thrust was measured for a variety of closely follow field lines; unless the plasma is configurations at power levels of up to a few initially well-collimated with high initial kilowatts using several propellants. Over 95 directed kinetic energy.S The critical difference percent of the incident microwave power was between Kilpatrick's analysis and that of observed to be deposited in the plasma stream.4 Kosmahl was that Kosmahl studied a non­ A maximum efficiency for conversion of equilibrium plasma in which most of the thermal microwave power to jet power of 0.4 was energy was stored in electron motion. reported at specific impulses of a few thousand Kilpatrick studied a plasma in which collisions seconds.1 However, the validity of many of the resulted in near thermal equilibrium of all early thrust measurements was questioned. It charged species. Furthermore, Kilpatrick's was suggested that entrainment of gas from the analysis did not fully account for the magnetic vacuum chamber into the energizing zone was dipole-moment force that is responsible for ECR increasing the effective mass flow rate and plasma acceleration. producing erroneously high thrust measurements.4 In measurements not subject
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