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Title: Transport of ion beam in an annular magnetically expanding helicon double layer thruster

Abstract

An ion beam generated by an annular double layer has been measured in a helicon thruster, which sustains a magnetised low-pressure (5.0 × 10{sup −4} Torr) argon plasma at a constant radio-frequency (13.56 MHz) power of 300 W. After the ion beam exits the annular structure, it merges into a solid centrally peaked structure in the diffusion chamber. As the annular ion beam moves towards the inner region in the diffusion chamber, a reversed-cone plasma wake (with a half opening angle of about 30°) is formed. This process is verified by measuring both the radial and axial distributions of the beam potential and beam current. The beam potential changes from a two-peak radial profile (maximum value ∼ 30 V, minimum value ∼ 22.5 V) to a flat (∼28 V) along the axial direction; similarly, the beam current changes from a two-peak to one-peak radial profile and the maximum value decreases by half. The inward cross-magnetic-field motion of the beam ions is caused by a divergent electric field in the source. Cross-field diffusion of electrons is also observed in the inner plume and is determined as being of non-ambipolar origin.

Authors:
; ;  [1]
  1. Space Plasma, Power and Propulsion Laboratory, Research School of Physics and Engineering, The Australian National University, Bldg 60, Mills Road, ACT 0200 (Australia)
Publication Date:
OSTI Identifier:
22304059
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 6; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ARGON; BEAM CURRENTS; DIFFUSION; DIFFUSION CHAMBERS; ELECTRONS; IONS; MAGNETIC FIELDS; MHZ RANGE 01-100; PEAKS; PLASMA; POTENTIALS; PRESSURE RANGE MILLI PA; RADIOWAVE RADIATION; THRUSTERS

Citation Formats

Zhang, Yunchao, E-mail: yunchao.zhang@anu.edu.au, Charles, Christine, and Boswell, Rod. Transport of ion beam in an annular magnetically expanding helicon double layer thruster. United States: N. p., 2014. Web. doi:10.1063/1.4885350.
Zhang, Yunchao, E-mail: yunchao.zhang@anu.edu.au, Charles, Christine, & Boswell, Rod. Transport of ion beam in an annular magnetically expanding helicon double layer thruster. United States. doi:10.1063/1.4885350.
Zhang, Yunchao, E-mail: yunchao.zhang@anu.edu.au, Charles, Christine, and Boswell, Rod. Sun . "Transport of ion beam in an annular magnetically expanding helicon double layer thruster". United States. doi:10.1063/1.4885350.
@article{osti_22304059,
title = {Transport of ion beam in an annular magnetically expanding helicon double layer thruster},
author = {Zhang, Yunchao, E-mail: yunchao.zhang@anu.edu.au and Charles, Christine and Boswell, Rod},
abstractNote = {An ion beam generated by an annular double layer has been measured in a helicon thruster, which sustains a magnetised low-pressure (5.0 × 10{sup −4} Torr) argon plasma at a constant radio-frequency (13.56 MHz) power of 300 W. After the ion beam exits the annular structure, it merges into a solid centrally peaked structure in the diffusion chamber. As the annular ion beam moves towards the inner region in the diffusion chamber, a reversed-cone plasma wake (with a half opening angle of about 30°) is formed. This process is verified by measuring both the radial and axial distributions of the beam potential and beam current. The beam potential changes from a two-peak radial profile (maximum value ∼ 30 V, minimum value ∼ 22.5 V) to a flat (∼28 V) along the axial direction; similarly, the beam current changes from a two-peak to one-peak radial profile and the maximum value decreases by half. The inward cross-magnetic-field motion of the beam ions is caused by a divergent electric field in the source. Cross-field diffusion of electrons is also observed in the inner plume and is determined as being of non-ambipolar origin.},
doi = {10.1063/1.4885350},
journal = {Physics of Plasmas},
number = 6,
volume = 21,
place = {United States},
year = {Sun Jun 15 00:00:00 EDT 2014},
month = {Sun Jun 15 00:00:00 EDT 2014}
}
  • Peripheral magnetic field lines extending from the plasma source into the diffusion chamber are found to separate two regions of Maxwellian electron energy probability functions: the central, ion-beam containing region with an electron temperature of 5 eV, and region near the chamber walls with electrons at 3 eV. Along the peripheral field lines a bi-Maxwellian population with a hot tail at 9 eV is shown to both originate from electrons in the source traveling downstream across the double layer and correspond to a local maximum in ion and electron densities.
  • A current-free electric double layer is created in a helicon double layer thruster operating with xenon and compared to a recently developed theory. The Xe{sup +} ion beam formed by acceleration through the potential drop of the double layer is characterized radially using an electrostatic ion energy analyzer. For operating conditions of 500 W rf power, 0.07 mTorr gas pressure, and a maximum magnetic field of 125 G, the measured beam velocity is about 6 km s{sup -1}, the beam area is about 150 cm{sup 2}, and the measured beam divergence is less than 6 deg.
  • Electrons neutralizing an ion beam are additionally supplied to a magnetically expanding double layer (DL) plasma from the downstream side of the DL. The rf power and the argon gas pressure are maintained at 200 W and 55 mPa, respectively, and the source magnetic field is varied in the range of about 70-550 G. It is observed that the ion beam energy corresponding to the DL potential drop increases up to 30 eV with an increase in the magnetic field when supplying the additional electrons, while it saturates at 20 eV for the case of the absence of the additionalmore » electrons. The supplied electrons are believed to be an energy source for the DL such that increasing the magnetic field is able to increase the potential drop beyond the limit found in the absence of the supplied electrons.« less
  • The ion beam generated by a helicon double layer has been electrically steered up to 20 deg. off axis by using a solenoid placed normal to the two axial solenoids of the helicon plasma source without significantly changing the beam exhaust velocity.
  • Direct thrust measurements of a permanent magnet helicon double layer thruster have been made using a pendulum thrust balance and a high sensitivity laser displacement sensor. At the low pressures used (0.08 Pa) an ion beam is detected downstream of the thruster exit, and a maximum thrust force of about 3 mN is measured for argon with an rf input power of about 700 W. The measured thrust is proportional to the upstream plasma density and is in good agreement with the theoretical thrust based on the maximum upstream electron pressure.