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.

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 planepolarized 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.

Abstract. 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 (T e < 2 eV) plasma can be obtained in the downstream region. The density n and electron temperature T e 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.

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.

The dispersion relation for helicon waves in a cold plasma of radially varying density has been reduced to compact form, and the radial eigenmodes have been computed for different density profiles. The results show a marked asymmetry between the leftand right-hand circulariy polarized modes: the m = -1 iierij mode has a centrally peaked wave intensity and resonates with a much higher central density than the m = + 1 mode. Positive feedback is therefore possible, leading to nonlinear channelling of the discharge. At a radius where the density falls to a certain value, a singularity arises in the coefficients of the wave equation; care must be taken in integrating through this point. This singularity has no physical significance. The marked difference between the m = + 1 and -1 modes in a non-uniform plasma is caused by a difference in sign of the electron drifl along the density gradient. Energy deposition is peaked near the radius of the peak in BL. so that broad, uniform density profiles can be obtained by using the m = + 1 mode and narrow, dense columns by using the m = -1 mode. These results explain many features observed by various groups over the past two years.

Decay waves have been observed in the megahertz range in the helium plasma generated by the variable specific impulse magnetoplasma rocket magnetoplasma thruster. They are measured using one of the tips of a triple probe connected to a 50 Ω input of a spectrum analyzer via a dc block (a small capacitor). The maximum amplitude of all waves is in the center of the plasma and does not appear correlated to the radial electron density or temperature profiles. The waves seem to be generated close to the helicon antenna that was 91 cm “upstream” from the measuring Langmuir probe. A possible explanation is parametric decay of the large amplitude helicon wave that also generates the plasma.

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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.