Max Light

Helicon wave excitation with helical antennas

Components of the wave magnetic field in a helicon discharge have been measured with a single‐turn, coaxial magnetic probe. Left‐ and right‐handed helical antennas, as well as plane‐polarized antennas, were used; and the results were compared with the field patterns computed for a nonuniform plasma. The results show that the right‐hand circularly polarized mode is preferentially excited with all antennas, even those designed to excite the left‐hand mode. For right‐hand excitation, the radial amplitude profiles are in excellent agreement with computations.

Downstream physics of the helicon discharge

Measurements of the radial and axial profiles of both the plasma parameters and the wave properties in a long, thin helicon discharge show that most of the RF power is deposited near the antenna and that a dense, cool ( eV) plasma can be obtained in the downstream region. The density n and electron temperature profiles in that region can be explained quantitatively with classical collisional theory, and factor-of-two agreement can be obtained on total particle and energy balance. Spatial modulation of the helicon wave amplitude can be explained by the beating of two different radial modes launched simultaneously by the antenna. Though the helicon wave can be shown to be essential to the production of high densities, it plays little role in the downstream evolution of the plasma. These results indicate that helicon discharges can produce the cool plasmas normally associated with afterglows without the attendant loss of density.

Low frequency electrostatic instability in a helicon plasma

Recent discoveries in a helicon plasma show a decrease in equilibrium plasma density as magnetic field strength is increased. This can be explained in the framework of a low frequency electrostatic instability. However, quiescent plasma behavior in helicon sources has been hitherto accepted. To verify the existence of an instability, extensive measurements of fluctuating quantities and losses as a function of magnetic field were implemented. Furthermore, a theoretical model was developed to compare to the measurements. Theory and measurement show very good agreement; both verifying the existence of a low frequency instability and showing that it is indeed responsible for the observed density characteristic.

Axial propagation of helicon waves

Traveling‐ and standing‐wave characteristics of the wave fields have been measured in a helicon discharge using a five‐turn, balanced magnetic probe movable along the discharge axis z. Helical and plane‐polarized antennas were used, and the magnitude and direction of the static magnetic field were varied, yielding three primary results. (1) As the density varies along z, the local wavelength agrees with the local dispersion relation. (2) Beats in the z variation of the wave intensity do not indicate standing waves, but instead are caused by the simultaneous excitation of two radial eigenmodes. Quantitative agreement with theory is obtained. (3) The damping rate of the helicon wave is consistent with theoretical predictions based on collisions alone.

Experimental evidence of parametric decay processes in the variable specific impulse magnetoplasma rocket (VASIMR) helicon plasma source

This project was proudly supported by the International Science Linkages programme established under the Australian Government’s innovation statement Backing Australia’s Ability.

Quiescent and unstable regimes of a helicon plasma

Known for their ability to produce high densities at low power, helicon discharges have found many uses. However, it has been discovered that the plasma density saturates, and even falls for light ion gases, as the magnetic field is increased. Detailed measurements of fluctuations in plasma density reveal the onset of a strong, low-frequency electrostatic instability. This onset correlates well with density saturation and is predicted from a linear theory.

Broad-band pulse performance of short helices

Helical antennas are popular and well characterized for CW frequency domain performance. Renewed interest in time-domain applications of electromagnetics, such as impulse radar, makes accurate time-domain data on broad-band antennas desirable. Although the principal endfire helix radiation mode has been extensively studied in the frequency domain, other modes important in pulse operation are poorly characterized, making a total Fourier transform approach difficult. We have performed impulse tests on helices with two to five turns, establishing novel features of the response and confirming some aspects of frequency domain data. Quick comparisons to time and frequency-domain modeling codes indicate good correspondence of gross features. Successful octave band-pulse operation was achieved, and a few features of helix pulse response invite further investigation. >

Helicon Plasma Generator‐Assisted Negative Ion Source Project at Los Alamos Neutron Science Center

Helicon plasma generators are widely used for plasma processing applications due to their long life‐time and capability of creating high‐density plasmas efficiently. The aim of the helicon plasma generator‐assisted negative ion source project at Los Alamos Neutron Science Center (LANSCE) is to use these features for producing intense beams of H− ions. Our development work builds upon pioneering experiments previously conducted at Lawrence Berkeley National Laboratory (LBNL) with a 2.45 GHz electron cyclotron resonance plasma generator. In the new approach a helicon plasma generator is used as a plasma cathode injecting electrons into a multi‐cusp H− ion source. The secondary source can be operated without filaments or any other consumable parts and, consequently, the life‐time of the ion source can be extended significantly. The development of the ion source is aimed to meet the beam production goals of the LANSCE 800 MeV linear accelerator refurbishment project i.e. 20 mA of H− beam with normalized area emittance (95 % of the beam) less than 1.1 π⋅mm⋅mrad and a duty factor of 12 %. The operation principle of the source, the test stand design and the status of the development work will be presented in this article.Helicon plasma generators are widely used for plasma processing applications due to their long life‐time and capability of creating high‐density plasmas efficiently. The aim of the helicon plasma generator‐assisted negative ion source project at Los Alamos Neutron Science Center (LANSCE) is to use these features for producing intense beams of H− ions. Our development work builds upon pioneering experiments previously conducted at Lawrence Berkeley National Laboratory (LBNL) with a 2.45 GHz electron cyclotron resonance plasma generator. In the new approach a helicon plasma generator is used as a plasma cathode injecting electrons into a multi‐cusp H− ion source. The secondary source can be operated without filaments or any other consumable parts and, consequently, the life‐time of the ion source can be extended significantly. The development of the ion source is aimed to meet the beam production goals of the LANSCE 800 MeV linear accelerator refurbishment project i.e. 20 mA of H− beam with normalized area ...

Electron Beam Generation by an Electron Cyclotron Resonance Plasma

Electron guns based on a plasma, instead of a thermionic material cathode, are gaining more attention due to their ability to generate beams of a variety of sizes for both pulsed and steady state operation. These guns have a major advantage in that they have no material cathode, can drive current densities larger than their thermionic counterparts, and can operate reliably at relatively high pressures (for example, fore vacuum gas pressures). This paper presents initial results on the characterization of a plasma cathode electron source driven by a microwave electron cyclotron resonance plasma discharge. A negatively biased plasma chamber is electrically isolated from the downstream system, and electrons from the discharge are extracted through a small aperture. These electrons then interact with background gas in the main chamber. Electron beams of greater than 80 A have been calculated based on measurements in this configuration.

Generation and Diagnostics of an Electron Beam in a Plasma Cathode Electron (PCE) Source

Summary form only given. An electron beam source is under development at Los Alamos National Laboratory. We outline the generation of the electron beam in a plasma cathode electron (PCE) source. Our PCE source was created using 1.5 kW of microwave power at 2.45 GHz delivered in a static magnetic field of 875 Gauss. We were able to drive electron beams of greater than 100 A in our source with very high beam efficiencies by biasing the ECR source chamber to -140V. Diagnosis of the beam was done using spectroscopy and electric probes. We discuss both the generation method and the diagnostics we used in this experiment.