D. D. Blackwell

Measurement of absolute electron density with a plasma impedance probe

A small spherical probe is used in conjunction with a network analyzer to determine the impedance of the probe-plasma system over a wide frequency range. Impedance curves are in good agreement with accepted circuit models with plasma-sheath and electron plasma frequency resonances easily identifiable. Clear transitions between capacitive and inductive modes as predicted by the model are identified. Sheath thickness and absolute electron density are determined from the location of these transitions. The absolute electron density indicated by the location of the impedance resonance is compared to measurements using the plasma oscillation method.

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis ...

Characteristics of the plasma impedance probe with constant bias

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 plasma potential is positive with respect to the sphere—for example, if the sphere is electrically floating or grounded, a second resonance occurs at ω<ωpe due to the capacitance created by the depleted electron density in the sheath. A greatly increased power deposition occurs at this lower resonance, whose frequency can be controlled by applying a dc bias which changes the sheath width. 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 th...

Plasma diagnostics in large area plasma processing system

A series of plasma diagnostic have been carried out in our large area plasma processing system which is based on a modulated electron-beam produced plasma. These discharges were created in both electrically conducting and insulated vacuum chambers operated in 30–120 mTorr of pure gases (argon, oxygen, and nitrogen). Langmuir probes, microwave transmission, mass spectrometry, and external current sensors show robust discharges were made over fairly wide parameter ranges resulting in plasma densities of 1–4×1011 cm−3 and temperature ranging from 0.2 eV for the molecular gases and 1.4 eV for argon. The effects of various experimental techniques, parameters, and contamination issues are discussed in detail.

Time-resolved measurements of the electron energy distribution function in a helicon plasma

An energy analyser with the capability of making time-resolved measurements of the instantaneous electron current in a radiofrequency (RF) plasma has been designed and constructed. This current is then reconstructed into the instantaneous I-V characteristic at various phases of the RF cycle. Results are shown for a helicon wave discharge under various conditions. From the first derivative of the I-V characteristic, it is observed that there is an absence of high-energy electrons characteristic of strong Landau damping, suggesting that some other mechanism is responsible for the discharges high ionization efficiency.

Antenna impedance measurements in a magnetized plasma. II. Dipole antenna

This paper presents experimental impedance measurements of a dipole antenna immersed in a magnetized plasma. The impedance was derived from the magnitude and phase of the reflected power using a network analyzer over a frequency range of 1MHz–1GHz. The plasma density was varied between 107and1010cm−3 in weakly (ωce ωpe) magnetized plasmas in the Space Physics Simulation Chamber at the Naval Research Laboratory. Over this range of plasma conditions the wavelength in the plasma varies from the short dipole limit (λ≫L) to the long dipole limit (λ∼L). As with previous impedance measurements, there are two resonant frequencies observed as frequencies where the impedance of the antenna is real. Measurements have indicated that in the short dipole limit the majority of the power deposition takes place at the lower resonance frequency which lies between the cyclotron frequency and the upper hybrid frequency. These measured curves agree very well with the analytic theory for a short dipole i...

On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry

An investigation of the rf impedance characteristics of a small spherical probe immersed in a laboratory plasma is ongoing in the large Space Physics Simulation Chamber [D. N. Walker et al., Rev. Sci. Instrum. 65, 661 (1994)] at the Naval Research Laboratory. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level rf signal to the probe which is either near floating potential or negatively dc biased in a low pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes “resistive,” even though the plasma is effectively collisionless, at frequencies below the bulk plasma frequency, thus leading to collisionless energy absorption. This behavior comes directly from Maxwells equations along with the cold fluid equations. The solutions obtained indicate that this form of plasma res...

Ion energy distributions in a pulsed, electron beam-generated plasma

In this work, we investigate the ion flux at a grounded electrode located adjacent to a pulsed, argon plasma generated by a high-energy electron beam. The plasmas, produced in 100 mTorr, are characterized by high plasma densities (>1011 cm−3) and low electron temperatures (<1.5 eV). An energy selective mass spectrometer was used to measure temporally resolved ion kinetic-energy distributions at the electrode surface. In addition, ion energy distributions are presented for various electrode locations. The ion energy distributions correlate well with Langmuir probe measurements of the plasma potential.

Antenna impedance measurements in a magnetized plasma. I. Spherical antenna

The input impedance of a metal sphere immersed in a magnetized plasma is measured with a network analyzer at frequencies up to 1GHz. The experiments were done in the Space Physics Simulation Chamber at the Naval Research Laboratory. The hot-filament argon plasma was varied between weakly (ωce ωpe) magnetized plasma with electron densities in the range 107–1010cm−3. It is observed that the lower-frequency resonance of the impedance characteristic previously associated with series sheath resonance ωsh in the unmagnetized plasma occurs at a hybrid sheath frequency of ωr2=ωsh2+κωce2, where κ is a constant 0.5<κ<1. As seen in previous experiments, the higher frequency resonance associated with the electron plasma frequency ωpe in the unmagnetized plasma is relocated to the upper hybrid frequency ωuh2=ωpe2+ωce2. As with the unmagnetized plasma, the maximum power deposition occurs at the lower frequency resonance ωr.

Determining electron temperature for small spherical probes from network analyzer measurements of complex impedance

In earlier work, using a network analyzer, it was shown that collisionless resistance (CR) exists in the sheath of a spherical probe when driven by a small rf signal. The CR is inversely proportional to the plasma density gradient at the location where the applied angular frequency equals the plasma frequency ωpe. Recently, efforts have concentrated on a study of the low-to-intermediate frequency response of the probe to the rf signal. At sufficiently low frequencies, the CR is beyond cutoff, i.e., below the plasma frequency at the surface of the probe. Since the electron density at the probe surface decreases as a function of applied (negative) bias, the CR will extend to lower frequencies as the magnitude of negative bias increases. Therefore to eliminate both CR and ion current contributions, the frequencies presently being considered are much greater than the ion plasma frequency, ωpi, but less than the plasma frequency, ωpe(r0), where r0 is the probe radius. It is shown that, in this frequency regime...