David Darnell Blackwell

Two-dimensional imaging of a helicon discharge

Two-dimensional images of the ionized argon light of a helicon discharge are made both along and across the magnetic field with various antenna configurations. Two antennas, a Nagoya type III and a helical antenna, are used to create a magnetized RF plasma with density in the range . A CCD camera with a 488 nm bandpass filter is used to image the plasma as magnetic field and power are changed. By inserting a Faraday shield, it is demonstrated that the inductive component of the antenna coupling is responsible for producing high-density plasmas. Asymmetries in the plasma profile are shown to be caused primarily by capacitive coupling, with the purely inductively coupled plasmas being symmetric and centrally peaked. Numerical calculations of antenna coupling show that the configurations having the largest antenna loading correspond to the brightest plasmas observed in the experiment, with the m=+1 mode being the most strongly coupled.

Whistler wave propagation and whistler wave antenna radiation resistance measurements

Whistler waves are a common feature of ionospheric and magnetospheric plasmas. While the linear behavior of these waves is generally well understood, a number of interesting observations indicate that much remains to be learned about the nonlinear characteristics of the mode. For example, in space, very low frequency (VLF) emissions triggered by whistler modes launched from ground-based transmitters have been observed. Emission is assumed to come from transverse currents formed by counterstreaming electrons that are phase bunched by the triggering signal. In the laboratory, it has been shown that with increasing amplitude of the driving signal applied to an antenna, the whistler mode radiation pattern forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. These observations have prompted an Naval Research Laboratorys (NRL) Space Physics Simulation Chamber study of whistler wave dynamics. The goals are to investigate whistler wave ducting, self-focusing, and amplification, and to study nonlinear whistler-plasma interactions.

2D imaging of a helicon discharge

Summary form only given, as follows. Two dimensional images from the ionized argon light of a helicon discharge are made both along and across the magnetic field with various antenna configurations. It is shown that even at high densities where the discharge is said to be operating predominantly in a wave-mode, or at least primarily inductively coupled, capacitive coupling from the antenna can still have a significant effect on the plasma profile. Transverse images also show that with all other conditions being the same a helical antenna produces brighter plasmas than a straight Nagoya Type III antenna, and with the helical antenna a configuration corresponding to excitation of the m=+1 helicon mode produces a longer brighter plasma than the m=-1 configuration. A connection between these results and simplified helicon wave theory with the calculated power spectra of the antennas is discussed.

Characteristics of the Plasma Impedance Probe with Applied DC Bias

Summary form only given. The impedance of a small spherical probe immersed in a uniform plasma is measured by recording the reflection coefficient of an applied signal using a network analyzer. This impedance has a resonance at the plasma frequency where the imaginary part goes to zero, a feature that has made this measurement a good way of determining electron density. When the sphere is biased negatively with respect to the plasma potential a second resonance occurs at omega<omegap due to the depleted electron density in the sheath around the probe. A greatly increased power deposition occurs at this lower resonance, whose frequency can be controlled by changing the sheath width using a DC bias. As the bias is increased the value of this frequency becomes smaller until the resonance disappears completely at Vprobe=Vplasma. As the bias is further increased past the plasma potential an electron sheath forms with its own resonance which is at a lower frequency than the resonance associated with the ion sheath. The impedance of the electron sheath can be approximated using the Llewellyn-Peterson formulas for a space charge limited diode. As with the ion sheath resonance, the largest energy deposition occurs at the lower of the two frequencies.

Detection of Trivelpiece-Gould modes in helicon discharges

Summary form only given. That helicon discharges produce higher plasma densities than unmagnetized inductively coupled plasmas (ICPs) is well known in the plasma processing community. A reason for the high RF coupling efficiency in these discharges has been suggested by Shamrai et al. (1997); namely, that helicon waves, though weakly damped themselves, can transfer their energy at the boundary to electrostatic electron cyclotron waves [called Trivelpiece-Gould (TG) modes in confined cylinders], and these, in turn, are rapidly damped as they propagate inward toward the axis. To test this hypothesis, we have performed an experiment to detect the TG waves directly.

Fast time resolved measurements of the EEDF in a helicon wave plasma

Summary form only given. One of the biggest problems encountered when using electric probe diagnostics in RF plasmas is the high frequency fluctuation of the plasma potential. This fluctuation makes the probe sheath voltage drop an unknown quantity and in general makes the analysis of the probe characteristic impossible. In addition, the entire integrated volume of the fluctuating sheath will add displacement current to the probe signal causing further distortion. In the past, we have used high impedance mini-inductors built into the probe tip to reduce the RF sheath voltage drop. This scheme is easy to implement but has two major drawbacks: (1) the effectiveness of this filtering method requites a low ratio of sheath to circuit impedance, which limits the range of plasma parameters over which the probe is reliable, and (2) this method cannot measure fast variations of the EEDF, which have been inferred from fast fluctuations of the plasma optical emission. For these reasons, we have used a gridded energy analyzer with a 250-MHz digitizing oscilloscope to measure the instantaneous electron current over an entire RF period. Each signal can be recorded, and by varying the DC grid bias the current-voltage characteristic can be constructed at any phase of the RF cycle. To test this system, an electron gun has been constructed to generate known RF-modulated EEDFs. Special considerations in the analyzer design, measured EEDFs under various discharge conditions, and comparison with results from RF-compensated Langmuir probes will be discussed.

Probe detection of time-varying electron tails

Acceleration of electrons by wave-particle interactions has been proposed as an ionization mechanism in helicon wave plasma sources, and a number of authors have shown distribution functions with a fast electron tail. Using a carefully RF-compensated Langmuir probe, we have reported the absence of such fast electrons. We find that the discrepancy lies in the delicate nature of RF compensation and in the time variation of the fast electrons, which appear only in the acceleration phase of the RF cycle. When the probe is made to follow RF fluctuations in the floating potential (by use of an auxiliary electrode), the electron tail cannot be seen on the time-averaged I-V trace because the floating potential itself shifts whenever the tail is present. On the other hand, if the probe is inadequately RF-compensated, the tail is masked by the apparent tail that is seen in the de average in the presence of RF. These effects are shown in I-V curves computed with model distribution functions. A properly designed probe should follow the fluctuations in space potential, not floating potential.